Endocrinology
Part Two
Spring 2013
The information given out in all the handouts for Endocrinology this
semester is to help you understand better this complex subject. There are a couple
of general websites that are excellent sources of information for this course of
lectures. The addresses are below. I will also direct you to specific web sites during
the lectures.
http://www.endotext.org/index.htm
http://www.endocrinesurgeon.co.uk
http://www.merck.com/mrkshared/mmanual/section2/sec2.jsp
Prepared by: Dr. Sean Holroyd
Lecturer:
Dr. Vijaya Chellapilla
1
Section 4
The Hormones of the Adrenal Cortex
Learning Objectives
1. Discuss the hypothalamic-pituitary-adrenal axis
2. Describe the synthesis, storage, release and breakdown of CRH
3. Explain the mechanism of action of CRH at its target cells
4. Describe the synthesis, storage, release and breakdown of ACTH
5. Explain the mechanism of action of ACTH at its target cells
6. Discuss the physiological actions of ACTH, especially as it pertains to the adrenal
cortex and the adrenal hormones
7. Describe the synthesis of the adrenal cortical hormones.
8. Describe aldosterone synthesis and release
9. Explain how aldosterone synthesis and release is regulated
10. List and discuss the physiological importance of other mineralocorticoids released
from the adrenal cortex
11. Explain the mechanism of action of aldosterone at its target cells
12. Discuss the physiological actions of aldosterone
13. Describe the transport and metabolism of aldosterone
14. Explain how negative feedback systems control aldosterone release
15. Describe the underlying physiology of medical problems associated with altered
aldosterone secretion.
16. Describe cortisol synthesis and release
17. Explain how cortisol synthesis and release is regulated
18. Explain the mechanism of action of cortisol at its target cells
19. Discuss the physiological actions of cortisol
20. Describe the transport and metabolism of cortisol
21. Explain how negative feedback systems control cortisol release
22. Describe the underlying physiology of medical problems associated with altered
cortisol secretion.
23. Describe adrenal androgen synthesis and release
24. Explain how adrenal androgen synthesis and release is regulated
25. Explain the mechanism of action of adrenal androgens at their target cells
26. Discuss the physiological actions of adrenal androgens
27. Explain the importance of a lack of specific negative feedback regulation of adrenal
androgen release
28. Describe the underlying physiology of medical problems associated with altered
adrenal androgen secretion
2
Readings
Chapter 33 of your textbook
Introduction
The major function of the adrenal cortex is to synthesize and secrete aldosterone, the
glucocorticoids and androgenic hormones. These hormones have a variety of important
roles in maintaining whole body homeostasis. Aldosterone is important in the
maintenance of blood volume; glucocorticoids have many roles within the body including
the maintenance of plasma glucose levels during fasting while the androgenic hormones
are important in the development of the female.
The synthesis and release of the glucocorticoids and the androgens are under direct
control of the hypothalamic-pituitary-adrenal axis. The role of this axis in aldosterone
synthesis is controversial.
Hypothalamus
CRH
Anterior
Pituitary
ACTH
Adrenal Cortex
(Zona Fasciculata
Zona Reticularis)
Cortisol / Androgens
Figure 1.
The hypothalamic-pituitary-adrenal axis and how it regulates
glucocorticoid and androgen secretion by the adrenal gland. The hypothalamus releases
CRH into portal blood vessels where it then binds to the corticotrophic cells of the
anterior pituitary. This increases the synthesis and causes the release of ACTH into the
circulation. This ACTH then binds to receptors of cells of the adrenal cortex to initiate
the synthesis of the glucocorticoids (eg cortisol) and androgens.
3
Hypothalamus
CRH
Anterior
Pituitary
ACTH (minor)
Angiotensin II
Adrenal Cortex
K+
(Zona Glomerulosa)
Aldosterone
Figure 2.
Diagram showing the minimal effect of the hypothalamic-pituitary-adrenal
axis on aldosterone release by the adrenal gland. The cells of the Zona glomerulosa have
ACTH receptors but the importance of ACTH on aldosterone synthesis is arguable. The
important message here is that the major regulators of aldosterone synthesis are
angiotensin II and potassium. These work directly on the cells of the Zona glomerulosa
to stimulate aldosterone synthesis.
Corticotropin Releasing Hormone (CRH; Corticotropin Releasing Factor)
A 41 AA peptide that is synthesized by neurons in the in the anterior portion of the
paraventricular nucleus and stored in the median eminence. It is released into the portal
system where it binds to receptors on corticotrophs of the anterior pituitary. Its release is
both diurnal and pulsatile and may be stimulated by stressors (physical, emotional,
chemical), catecholamines, and inhibit via negative feedback mechanisms (see below).
In the corticotrophic cells it stimulates pro-opiomelanocorticotrophin (POMC)
transcription and ACTH synthesis and release. It does this via elevation of cAMP and
protein kinase A activity. It has a half-life of ~ 4 minutes in plasma.
4
Corticotroph
CRH
Vesicles
containing
ACTH
cAMP
Release of
ACTH via
exocytosis
Via PKA
Ca2+
Figure 3. Diagram showing the mechanism of action of CRH at the corticotroph to lead
to ACTH release. As you can see the second messenger is cAMP and the elevation of
intracellular calcium causes the release of the vesicles containing ACTH.
Adrenocorticotrophic Hormone (ACTH)
A 39 AA peptide that is synthesized and secreted by the corticotrophic cells of the
anterior pituitary.
Figure 4. ACTH is processed from a large precursor molecule POMC (Boron and
Bolpaep, 2005). CRH activating its receptor on the corticotrophic cell stimulates both the
production and cleaving of POMC. This of course increases ACTH production.
5
At the adrenal gland
ACTH
cAMP
Protein Kinase A
Phosphoylation
of proteins
Figure 5. Diagram showing the action of ACTH at the cells of the adrenal cortex.
ACTH binds to its receptor and activates cAMP. It should be pointed out that although
ACTH does not have a major role in aldosterone release it is required to be present.
When ACTH binds to its receptor on the adrenal cortical cell it produces many reactions
within the target cell that facilitate the conversion of cholesterol into pregnenolone in the
mitochondria. This includes increasing cholesterol uptake, transporting cholesterol into
the mitochondria and the activation of cholesterol esterases. It has been suggested that
ACTH activates the synthesis of Steroidal Acute Regulatory protein (StAR) on the
mitochondrial membrane. This StAR is said to be transient, this means that is only
present during hormone activation, once hormone levels decrease StAR is rapidly broken
down. StAR activates a cholesterol channel in the mitochondria allowing it to enter and
be converted into pregnenolone. This is shown in the next diagram.
6
At the adrenal gland
ACTH
LDL
Cholesterol
LDL
StAR
Lysosomes
Figure 6.
Diagram showing how cholesterol gets to the mitochondria in order to be
converted into the various adrenocortical hormones. Remember this is what occurs in the
Zona fasciculata and reticularis. In the Zona glomerulosa it is either angiotensin II or K+
that activate StAR.
The cholesterol then follows the processes described in Figure 8 leading to the synthesis
of the various adrenal cortex hormones. Longer term it will increase gene transcription
of the enzymes involved in hormone synthesis and the number of LDL receptors on the
cell. Even longer term will be an increase in the size and number of cells due to the
stimulation of growth factors. Therefore it causes the synthesis and release of the
glucocorticoids and adrenal androgens from the adrenal cortex. This is summarized in
the next slide.
7
Short-term
Conversion of
cholesterol into
pregnenolone
ACTH
Transcription
of synthesis
enzymes
Number of
LDL
receptors
Long-term
Figure 7. Some of the short and long-term actions of ACTH on the cells of the adrenal
cortex. This is discussed in detail on the previous two pages.
The release of ACTH is stimulated mainly by CRH. Its release is pulsatile and diurnal.
It has a half-life of ~ 15 minutes and plasma levels (at 6am) range from 20-100pg/ml.
Note that due to the diurnal nature of its release it is important to know the time the
sample was taken.
8
Synthesis Pathway of the adrenal cortical hormones
Figure 8. Overview of the synthesis of the adrenal cortex hormones. It is important
that you know the various enzymes involved in these reactions as deficiencies in these
can have dramatic effects on hormone synthesis. Taken from your text (Figure 33.5).
9
Aldosterone
Hypothalamus
CRH
Anterior
Pituitary
ACTH (minor)
Angiotensin II
Adrenal Cortex
K+
(Zona Glomerulosa)
Aldosterone
Figure 9. Diagram showing the role of angiotensin II and K+ on the synthesis and
release of aldosterone from the cells of the Zona glomerulosa. This is the same as Figure
2.
Synthesis
Occurs only in the Zona glomerulosa as the cells of this area have aldosterone synthase.
There are two main regulatory steps in its synthesis:
- the conversion of cholesterol to pregnenolone
- the conversion of deoxycorticosterone to aldosterone
Regulation of synthesis and release
There is some controversy over the role of ACTH in the synthesis of aldosterone.
It is important to stimulate the synthesis of StAR (steroidogenic acute regulatory protein)
that allows for cholesterol to be transformed into pregnenolone. It is believed that
Angiotensin II and K+ also stimulate StAR synthesis by increasing [Ca2+] inside the cells
of the Zona glomerulosa. Once formed aldosterone is free to leave the cell.
10
At the adrenal gland
AII
K+
DAG IP3
Activation of
PKC
Depolarization
Ca2+
Activation of
aldosterone
synthesis
Figure 10. Diagram showing the role of angiotensin II and high levels of K+ on the
activation of aldosterone synthesis. Angiotensin II acts via the IP3 / DAG system
elevating calcium (activating StAR) and hence promotes cholesterol entry into the
mitochondria. Potassium also elevates intracellular calcium by depolarizing the cell
membrane increasing the probability of opening of calcium channels and so also may
activate StAR. The other enzymes associated with aldosterone synthesis (see Figure 8)
are also stimulated by angiotensin II and potassium.
Other mineralocorticoids
11-deoxycorticosterone is a mineralocorticoid secreted in the main from the Zona
reticularis and fascicula. It is as potent as aldosterone however under normal
conditions it provides little mineralocorticoid function in the body.
Transport and half-life of Aldosterone
Aldosterone has a half-life of ~ 15 minutes, with approximately half bound to plasma
proteins.
11
Aldosterone Action
Intracellular Actions
Being a steroid hormone aldosterone binds to intracellular receptors and activates
changes in the RNA and DNA activity of the cell.
At the target tissues
Aldosterone
Modulation of gene
expression
Enzymes
Transport
proteins
Figure 11. Diagram showing the mechanism of action by which aldosterone acts with its
receptor to affect target cell function. So like any steroid hormone aldosterone modifies
genetic expression changing protein synthesis and activity of the cell.
This leads to the following in the late distal tubule and collecting duct of the nephron
in the number of Na+ and ROMK channels on the apical membrane
’s the activity of Na+-K+ ATPase of basal membrane
Physiological Actions
Aldosterone increases Na+ reabsorption and K+ secretion by the renal tubule. Therefore it
helps to maintain ionic homeostasis and water balance in the body. It has a similar action
in sweat and perspiration with regards to Na+ retention and K+ loss, as well as increasing
Na+ reabsorption in the colon and K+ excretion in feces.
12
Distal tubule and collecting duct
Plus
Aldosterone
Na+
K+
Na+
K+
Lumen
Principal Cell
Interstitial
Fluid
Fig 12. Diagram showing the affect of aldosterone on Na+ reabsorption and K+ secretion
by the kidney. The left hand side of the diagram is the lumen of the nephron the right
hand side the interstitial fluid. The action of aldosterone is to increase the synthesis of
the Na+ and K+ channels on the luminal membrane of the principal cells of the nephron.
This increases Na+ reabsorption and K+ secretion by the kidney. Note that aldosterone
also increases the number of Na+-K+ pumps on the basolateral membrane therefore
maintaining the gradient to favor both Na+ reabsorption and K+ secretion.
13
Hypothalamus
Blood volume
GFR
renin release
CRH
Anterior
Pituitary
K+ secretion
ACTH (minor)
Angiotensin II
Adrenal Cortex
K+
(Zona Glomerulosa)
Aldosterone
Figure 13. Aldosterone directly decreases K+ levels via secretion by the kidney and so
negatively feedbacks on the K+ stimulus. The negative feedback associated with
Angiotensin II is indirect and involves a few steps. (NB: You really need to follow the
animation in lecture to fully appreciate this diagram)
Disorders of Aldosterone secretion
Hypersecretion
This is usually caused by a tumor in the Zona glomerulosa or increased renin secretion.
Elevation of aldosterone levels can lead to hypertension, slightly expanded extracellular
fluid volume, hypokalemia and slight hypernatremia. It may be diagnosed by finding
high levels of aldosterone in plasma and urine even after a Na+ load. (Note: If due to a
tumor in the Zona glomerulosa you would observe a low renin concentration).
Hyposecretion
Addison’s Disease, complete destruction of adrenal cortex, is the major cause of
hyposecretion of aldosterone. Decreased levels of aldosterone may lead to polyuria,
dehydration, hypotension, hyperkalemia and hyponatremia. Not surprisingly it may be
diagnosed by the patient having low levels of aldosterone in both the plasma and urine
accompanied by high levels of renin and AII.
NB. Addison’s Disease will also affect levels of the other adrenal cortical hormones,
these will be discussed later.
14
Adrenal Androgens and Glucocorticoids
Synthesis
The glucocorticoids and adrenal androgens are synthesized in the Zona fasciculata and
reticularis.
DHEA is synthesized mainly in the Zona reticularis
Androstenedione in both
Glucocorticoids in both
Regulation of synthesis and release
Most of the glucocorticoid and adrenal androgen synthesis and release is regulated by the
hypothalamic-pituitary adrenal axis. Therefore their release follows a diurnal pattern but
can also be altered by various stimuli on the axis. Like all steroid hormones they pass out
of the cell after being synthesized and are not stored.
Hypothalamus
CRH
Anterior
Pituitary
ACTH
Adrenal Cortex
(Zona Fasciculata
Zona Reticularis)
Cortisol
Figure 14. Diagram showing the hypothalamic-pituitary-adrenal axis with respect to
cortisol synthesis and secretion.
15
Figure 15.
The diurnal rhythm that is important in regulating cortisol release. As you
can see cortisol levels rise dramatically prior to waking and then slowly decrease over the
day reaching their lowest levels in the late evening. This data taken from one person
shows how the release of ACTH is quite pulsatile however it corresponds with the
changes in cortisol levels. From your textbook (Figure 31-7).
Transport and half-life
Cortisol is mainly found bound to plasma proteins (the major protein being cortisol
binding globulin). Most adrenal androgens are bound to albumin. Most glucocorticoids
are conjugated by the liver and excreted by the kidney. Adrenal androgens are either
degraded or metabolized into more active byproducts.
16
Glucocorticoid Action
Intracellular Actions
Glucocorticoid receptors are found in most tissues of the body.
NB. Most natural glucocorticoids also have some mineralocorticoid (ie aldosterone-like)
effect.
At the target tissues
Cortisol
Modulation of gene
expression
Many
effects
Figure 16. Diagram showing the mechanism of action of cortisol after binding to its
receptor in the cytoplasm of its target cell. Like all steroid hormones it modulates gene
expression in the nucleus of its target cell causing the production of many proteins that
change the activity/function of the cell.
Physiological Actions
Not surprisingly the glucocorticoids have many actions in the body. Many of these
actions are permissive. By permissive it is meant that the glucocorticoid has to be present
in order for another hormone to act or for an action to take place. Without the
glucocorticoid present the hormone will not act as well or the action will not be as
pronounced.
In the liver the effects of cortisol are quite confusing. In times of hypoglycemia it will
stimulate gluconeogenic enzymes (phosphoenolpyruvate carboxykinase, glucose 6phosphatase) and increase the response of the liver to glucagon, overall increasing liver
glucose production. In times of hyperglycemia it will work with insulin to increase
glycogen synthesis and inhibit glycogen breakdown therefore stimulating liver glucose
storage.
17
Therefore cortisol enhances both the effects of glucagon and insulin. This seems
counterintuitive but this is what happens. Note however that insulin is required for
cortisol to act as a glucose storer. In other tissues cortisol inhibits glucose uptake by
muscle and adipose tissue. It also stimulates lipolysis in adipose tissue and protein
breakdown in muscle.
Energy
glucose
Action of
Insulin
Inhibited by
Cortisol??
General cell
Fatty acids
glycerol
glucose
Amino Acids
glycogen
Liver
lipolysis
Adipose Cells
Plasma
proteins
Protein
breakdown
Muscle
Figure 17. This diagram shows the mechanisms by which cortisol effects metabolism in
the body. In the liver cortisol stimulates the production of glucose. This seems
contraindicated by the elevated glycogen but this requires the presence of insulin. In
muscle cortisol breaks down proteins elevating blood amino acids, these are taken up into
the liver to form more glucose and also stimulate glucagon release. Cortisol is believed
to inhibit the action of insulin on most cells and hence decreases glucose uptake by these
cells elevating blood glucose. These cells use the breakdown products of fat metabolism
in the adipose tissue to provide them with energy.
The following is a list of the actions of glucocorticoids in the body.
Growth/Development
accelerates the development of many fetal systems (unknown mechanism)
Cardiovascular System
increases contractility of the heart
increases vascular reactivity to catecholamines and angiotensin II
Mammary gland
must be present for lactation to occur
18
Lung
stimulates surfactant production (important for fetal lung development)
Immune System
inhibits many steps of the immune response including production of
interleukin-1 (hence fever), interleukin-2 and 6 as well as inhibiting Tcell proliferation. OVERALL inhibits !!!
Inflammatory response
inhibits many steps of the inflammatory response (NO and platelet
activating factor production; inhibits phospholipase, cyclooxygenase
pg and AA production.
CNS effects
maintains emotional balance
increases appetite
Elevated cortisol levels also lead to specific effects in the body. These effects may be
used to diagnose hypercortisolism. In connective tissue you have loss of collagen and
connective tissue, and the inhibition of fibroblasts, you observe easy bruising, thin skin
and poor wound healing. In bone you have inhibition of the formation of bone and the
promotion of resorption, you observe ease in the breaking of bones. It also causes
negative calcium balance (see bone resorption above) due to a reduced intestinal calcium
reabsorption and increased calcium urinary excretion.
Hypothalamus
CRH
Anterior
Pituitary
ACTH
Adrenal Cortex
(Zona Fasciculata
Zona Reticularis)
Cortisol
Figure 18. Flow diagram showing the negative feedback control of cortisol secretion.
The major controller of cortisol secretion is negative feedback directly on the
hypothalamus by cortisol decreasing CRH release.
19
Disorders of cortisol secretion
Many adrenal cortical disease states are associated with altered secretion of cortisol. In
some of these there will also be altered mineralocorticoid and androgen secretion.
Hypersecretion
Don’t get the following mixed up.
Cushing’s Syndrome – a myriad of problems associated with cortisol excess
Cushing’s Disease – cortisol excess due to hypersecretion of pituitary ACTH
Most cases of hypercortisolism are either secondary (ACTH-dependent) or primary
(problem with the adrenal gland itself). Secondary may be due to excessive ACTH
secretion from the anterior pituitary or ACTH secreted from non-pituitary tumor (eg a
lung tumor). Primary is usually due to adrenal tumors producing uncontrolled cortisol
release.
NB. A lot of patients that present with all the symptoms of hypersecretion of
cortisol are overusing their corticosteroid medication
As the glucocorticoids have many effects throughout the body there are many symptoms
associated with hypersecretion. These include a moon-face, truncal obesity, buffalo
hump. This goes against the grain as you know from above that glucocorticoids promote
lipolysis. So why the preferential fat deposition in these areas??? We just don’t know
why … but we know it happens. You also observe muscle wasting and weakness, the
formation of stretch marks on the skin, poor wound healing and easy bruising, menstrual
irregularities (maybe due to elevated androgens), glucose intolerance and hypertension
(due to the mineralocorticoid action of glucocorticoids).
The diagnosis of hypercortisolism and the underlying cause involves numerous tests.
Initially you should observe elevated plasma/urinary cortisol levels and if it is secondary
hypercortisolism you should observe elevated plasma/urinary ACTH levels. In primary
hypercortisolism ACTH levels will be decreased due to negative feedback.
Hyposecretion
Hyposecretion of cortisol is also known as Addison’s Disease. It is mainly due to
autoimmune destruction of the adrenal gland or by granuloma (as seen in tuberculosis).
In most cases the patient has low levels of ALL adrenal hormones
Like hypercortisolism many symptoms may be observed. You will find fasting
hypoglycemia, anorexia, weight loss and muscle weakness, darkening of the skin, the
inability to handle stress, hypoaldosteronism effects and in females low adrenal androgen
effects.
In diagnosing hypocortisolism you would normal record low cortisol levels and you will
usually have elevated ACTH levels (lack of negative feedback).
20
Adrenal Androgens
Hypothalamus
CRH
Anterior
Pituitary
ACTH
Adrenal Cortex
(Zona Fasciculata
Zona Reticularis)
Androgens
Figure 19. Diagram showing the hypothalamic-pituitary-adrenal axis with respect to
androgen synthesis and secretion. Like cortisol the androgens are under the influence of
CRH from the hypothalamus elevating ACTH levels. But importantly the androgens
have no negative feedback action on either CRH or ACTH secretion. The secretion of
androgens is therefore coupled to the glucocorticoid regulation of these two hormones.
Intracellular Action
The adrenal androgens have very little effect directly on tissues. They are mainly
converted into either testosterone or dihydrotestosterone in the peripheral tissues.
Physiological Action
In females the adrenal androgens are important in the growth of axillary hair and they
contribute substantially to female testosterone and dihydrotestosterone production. In
males they have very little physiological action.
21
Hypothalamus
CRH
NO
NEGATIVE
FEEDBACK
Anterior
Pituitary
ACTH
Adrenal Cortex
(Zona Fasciculata
Zona Reticularis)
Androgens
Figure 20. Flow chart showing the lack of negative feedback of androgens on their own
release. Under normal circumstances the cortisol negative feedback will control
androgen release.
Disorders of adrenal androgen action
Hypersecretion
There are two major instances that lead to hypersecretion of adrenal androgens. Firstly a
lack of 21 - or 11 -hydroxylase activity in the adrenal cortex leads to the preferential
formation of adrenal androgens. The lack of cortisol production means ACTH levels rise
(due to lack of negative feedback). Secondly adrenal hyperplasia (will also observe
elevated cortisol levels) will cause androgen hypersecrtion.
In females hypersecretion of adrenal androgens will lead to masculinization of the fetus
in utero and in adults loss of menses, regression of breast tissue, body hair, acne,
deepening of voice, enlargement of clitoris and the formation of more muscle.
See http://jcem.endojournals.org/cgi/content/full/84/12/4431 for a good review of
hyperandrogenism in children.
Hyposecretion
No major effect in males.
In females may see lack of body hair.
In many cases hyposecretion may be associated with hyposecretion of cortisol.
22
Section 5
Hormones of the Adrenal Medulla
Learning Objectives
1. Discuss the role of the autonomic nervous system in catecholamine release from the
adrenal medulla
2. Describe the synthesis of norepinephrine and epinephrine in the adrenal medulla
3. Describe the physiological process involved in the storage and release of the
catecholamines of the adrenal medulla
4. Discuss the transport and metabolism of the medullary hormones
5. * Discuss the mechanism of action of the medullary hormones
6. * Describe the effects of the medullary hormones on various organs of the body
7. Explain the physiology underlying some selected medical problems associated with
altered medullary hormone secretion
8. Describe and explain the stress response, including the physiological role of the
medullary and cortical hormones.
* Already discussed in detail in the Autonomic Pharmacology Section
Introduction
The adrenal medulla is an important source of circulating catecholamines. It is
considered a modified sympathetic nervous system and is stimulated by a pre-ganglionic
neurons secreting ACh. The adrenal medulla has an important role in metabolism and the
flight/fright response. It also coordinates with the adrenal cortex in the body’s response
to stress.
95 % of chromaffin cells secreted epinephrine
5 % secrete norepinephrine
Please review your Autonomic Nervous System lectures prior to attending these lectures.
23
Autonomic Nervous
System
Pre-ganglionic
neuron
Adrenal Medulla
Epinephrine/Norepinephrine
Figure 1. Diagram showing the relationship of the adrenal medulla with the Autonomic
Nervous System. The adrenal medulla receives input from the preganglionic neuron of
the autonomic nervous system. It is the neurotransmitter acetylcholine released from this
neuron that stimulates both the synthesis and release of the catecholamines
(epinephrine/norepinephrine) from the adrenal medulla.
24
Synthesis of the medullary hormones
ACTH
Cytoplasm
Tyrosine
Epinephrine
Tyrosine hydroxylase
Dopa
PNMT
Dopamine
Norepinephrine
Epinephrine
Secretory granule
Figure 2.
Diagram showing the various steps of the synthesis of the two major
adrenal medulla hormones norepinephrine and epinephrine. The synthesis of both
follows the same pathway starting with tyrosine conversion into dopa. ACh stimulates
the activity of tyrosine hydroxylase which sets the pathway in motion. Dopamine is
moved from the cytosol of the medullary cell into the secretory granule where the
enzyme dopamine β-hydroxylase is present to convert it into norepinephrine. This
norepinephrine is either stored ready for release or may be converted in epinephrine. The
conversion into epinephrine requires the norepinephrine to leave the secretory
granule to the cytoplasm where phenylethanolamine-N-methyltransferase is found. This
epinephrine is then taken back up in the granule waiting for release.
Modulators of Synthesis
ACh released from preganglionic sympathetic nervous system is a major controller of
catecholamine synthesis by the adrenal medulla by increasing the activity of tyrosine
hydroxylase and long term stimulation upregulates transcription and translation of
tyrosine hydroxylase and dopamine -hydroxylase.
Cortisol increases the synthesis and activity of phenylethanolamine-Nmethyltransferase (PMNT). This increases the epinephrine:norepinephrine ratio
Catecholamines in general negatively feedback on the activity of tyrosine hydroxylase
25
As can be seen in Figure 2 most catecholamine synthesis occurs the cytoplasm of the
chromaffin cell. Dopamine must be transported into the secretory vesicle before being
converted into norepinephrine. This transport is via vesicular monoamine transporters
(VMAT). Note also that epinephrine is also taken back up by VMAT after
norepinephrine has leaked out and been converted by PNMT.
Storage of Medullary Hormones
Epinephrine and norepinephrine are stored in the secretory vesicles of the chromaffin
cells of the adrenal medulla. An electrical and pH gradient is maintained across the
membrane of the vesicle by an H+-ATPase that works with the electrogenic VMAT. For
every catecholamine transported into the vesicle 2 protons are pumped out.
Release of Medullary Hormones
Chromaffin Cell
ACh
Vm
Vesicles
containing
Epinephrine
and
Norepinephrine
Release of
Epi and
Norepi via
exocytosis
N
Na+
Ca2+
Figure 3. Diagram showing the control of epinephrine and norepinephrine release by
acetylcholine. ACh released from the sympathetic preganglionic neurons depolarizes the
chromaffin cells activating voltage-dependent Ca2+ channels, the associated influx of
Ca2+ leads to exocytosis of the vesicle and the deposition of its contents into the
extracellular fluid.
Transport and Circulation of Medullary Hormones
~ 50% travel loosely bound to albumin
Half-life of between 10-100 seconds, very short
26
Mechanism of Action of Norepinephrine and Epinephrine
The main task of epinephrine is to mobilize stored chemical energy through lipolysis and
glycogenolysis. Epinephrine enhances the uptake of glucose into skeletal muscle, and
activates enzymes that activate lipolysis and lactate formation
To enhance blood flow to the muscles involved, it increases cardiac output and decreases
gastrointestinal blood flow and activity.
Epinephrine and norepinephrine stimulate hormones that replenish energy reserves while
alarm reaction is still in process.
Effects of the Medullary Hormones on their Target Organs
It is most useful to first look at the affect of the whole autonomic nervous system on
target organs before looking specifically at the medullary hormones. Figure 6-4 of your
textbook is a useful guide.
The medullary hormones are released in response to increased physical activity (such as
exercise), stress, emergencies, and exposure to cold or severe hypoglycemia.
Severe hypoglycemia releases all stress hormones (cortisol, growth hormone, glucogan
and epinephrine).
The medullary hormones have a major effect on metabolism as summarized in the
following table.
Receptor
2
3
2
2
2
2
Action
gluconeogenesis, glycogenolysis
lipolysis
glycogenolysis
insulin secretion
glucagon secretion
insulin secretion
Organ
Liver
Adipose tissue
Muscle
Pancreas
Pancreas
Pancreas
The overall effect of the catecholamines is to increase glucose production and inhibit
glucose use. Therefore you observe a rise in plasma glucose, free fatty acids and
ketoacids (diabetogenic). It is important to note the dual role of catecholamines on
insulin secretion, during mass activation of the sympathetic system (flight/fright) the
effect is to decrease insulin secretion via norepinephrine release from nerves.
27
insulin
glucose
General cell
glucagon
Pancreas
Fatty acids
glycerol
glucose
Lactate
glycogen
Liver
lipolysis
glycogenolysis
Adipose Cells
Muscle
Figure 12. This diagram puts together all the effects of the catecholamines on the major
organs of metabolism. In the liver the breakdown of glycogen into glucose is stimulated.
This glucose production by the liver is further elevated by the catecholamines stimulating
glycogenolysis in muscle, the glucose produced is used for energy by the muscle leading
to the release of lactate into the blood. This lactate is then taken up by the liver to
produce more glucose. On top of this the adipose tissue is stimulated to breakdown fats
leading to the release of fatty acids and glycerol into the circulation to be taken up and
converted into more glucose by the liver. Glucagon release is stimulated over insulin
release further elevating glucose production by the liver. So overall the effect is to
increase blood glucose levels.
During exercise there is also an increase in secretion of hormones by the adrenal medulla.
This increase leads to the following responses in target cells:
Organ
Heart
Vasculature
Lungs
Metabolism
Action
contractility of muscle
heart rate
Vasodilation in muscle
Vasoconstriction in kidney and
splanchnic areas
Relaxes bronchial smooth muscle
energy mobilization
Receptor
1
1
2
1
2
2
3???
28
Breakdown of Catecholamines
Although we are talking only about adrenal medulla hormones it is prudent to discuss the
breakdown of catecholamines as a whole. There are two major mechanisms by which
catecholamines are broken down.
In the CNS and synaptic clefts reuptake into nerve terminals via Uptake-1. They are then
broken down by monoamine oxidase (MAO) and taken back into the vesicles by VMAT
In the target cells there is uptake via Uptake-2, where the hormones are broken down by
MAO and catechol-o-methyltransferase (COMT).
Medical Problems Associated with Altered Medullary Hormone
Secretion
Hyposecretion
Does not lead to any physiological changes. Removal of medulla leads to low levels of
epinephrine but does not alter norepinephrine levels.
Because many actions of epinephrine is mediated by norepinephrine, adrenal medulla is
not essential for life
Hypersecretion
PHEOCHROMOCYTOMA is due to tumors of the chromaffin cells. Most release
norepinephrine, but epinephrine & dopamine can be released.
Hypertension is episodic, is usually present.
Other symptoms, include, severe sudden headache, chest pains, palpitations, extreme
anxiety, cold perspiration and high blood pressure during the period of high
catecholamine release.
Hyperglycemia and glucosuria may be present.
29
The Stress Response
Both the adrenal medulla and adrenal cortex are important in the body’s response to
stress. When we talk about stress it means things such as trauma, surgery, infection,
extreme heat and cold etc. Stress is perceived throughout the brain and leads to CRH
release from the PVN and activation of the adrenergic neurons of the hypothalamus. This
releases catecholamines from the adrenal medulla as well as from adrenergic neurons
throughout the body. There is an overall shifting of glucose utilization to the CNS and
general increase in glucose production and with the increase in cardiac output improved
delivery of substrates to organs required for the defense of the person. Cortisol is
important as a regulator of the cytokines released at the site of the injury. The cytokines
stimulate cellular and humoral defenses; however it is important that their response is
limited to the area of need.
The flow chart on the next page shows the integration of the ANS and the Adrenal cortex
in response to a stressor.
30
STRESS
Hypothalamus
ANS/ Adrenal
Medulla
PituitaryAdrenal Cortex
cortisol
Release of
catecholamines
HR & BP
Blood glucose
Metabolic rate
Bronchodilation
Short-term response
glucocorticoids
Protein b/down
Fat b/down
mineralocorticoids
BP
Immune supression
Long-term response
Figure 13. Basic diagram showing the major players in the stress reaction. Classically
the release of catecholamines from the adrenal medulla leads to the “short-term” response
to stress. The release of adrenocortical hormones leads to the “long-term” response. It is
now more accepted to look at the response overall rather than separate them due to the
integration of the catecholamines and adrenocortical hormones in the stress response. In
the stress response you observe an increased heart rate and blood pressure, an increase in
glycogenolysis by the liver, bronchodilation, an increase in metabolic rate, sodium and
water retention by kidneys, protein and fat breakdown (for glucose or energy) and
immune system suppression.
31
Section 6
Hormones of the Endocrine Pancreas
Learning Objectives
1. Describe insulin synthesis and release
2. Explain the mechanism of action of glucose in insulin release
3. Discuss other regulators of insulin release
4. Explain the mechanism of action of insulin at its target cells
5. Discuss the physiological actions of insulin (esp. in liver, muscle and adipose tissue)
6. Describe the transport and metabolism of insulin
7. Describe glucagon synthesis and release
8. Explain the mechanism of action of glucose and insulin in glucagon release
9. Explain the mechanism of action of glucagon at its target cells
10. Discuss the physiological actions of glucagon (esp. in liver)
11. Describe the transport and metabolism of glucagon
12. Discuss somatostatin
13. Explain the effects of hypo- and hypersecretion of the pancreatic hormones
14. Explain the physiology of diabetes mellitus.
Readings
Chapter 34 of your textbook
Introduction
The pancreas is an important endocrine organ that has a major role in the maintenance of
blood glucose levels. It synthesizes and releases a number of hormones from the cells of
the pancreatic islets. Like all hormone systems those of the endocrine pancreas are
regulated in a variety of manners and in fact in some cases are self-regulated.
The major hormones released by the pancreatic islet cells are insulin and glucagon,
though other hormones and products are released by this organ. Unlike other glands that
we have studied the pancreas is not under the direct control of the hypothalamic-pituitary
axis.
32
Release of Insulin
Glucose in
blood
Cell of Pancreas
Insulin
Glucose
GLUT2
Insulin synthesis
Ca2+
Insulin release
Figure 1 and 2. Simplified diagrams showing how the release of insulin is regulated by
blood glucose levels. An increase in blood glucose leads to more glucose entering the
cell. This glucose then leads to both the synthesis and release of insulin via elevation of
calcium levels within the cell. This insulin will then decrease blood glucose. Please note
that the system of blood glucose regulation is much more complicated and will be
discussed later.
33
Basic Information
Insulin is a 51AA peptide that is synthesized in the cells of the pancreas and stored in
secretory granules. Once released into the blood it has a half-life of 3-5 minutes and is
broken down by the liver and cleared by the kidney.
Synthesis and Storage
Figure 3. Diagram showing insulin synthesis in the pancreatic cell. Note that for each
insulin molecule formed you get a companion C peptide. Therefore C peptide
concentration may be measured as an indicator of insulin release. Useful clinically to
distinguish between exogenous (factitious) versus endogenous insulin secretion.
34
Mechanism of Insulin release in Pancreatic Beta cell
Glucose
Depolarization
of cell
membrane
GLUT2
Vesicles
containing
insulin
ATP
K+
Sulfonureas
can be used to
bind to K-ATP
receptors, in
DM 2 patients
K+
Ca2+
Release of
insulin via
exocytosis
Figure 4.
Diagram showing how glucose stimulates insulin release. It is also
important to know that this elevated intracellular calcium concentration is also believed
to lead to increased insulin synthesis. Note that there are other postulated mechanisms
for this including that glucose may increase the half-life of the mRNA for insulin via
cAMP.
Glucose is required for insulin synthesis to take place and there is no other proven
stimulator of synthesis (although leucine has also been postulated).
Basically glucose enters the cell via the insulin-independent GLUT-2 transporter. This
glucose is metabolized to produce ATP.
The elevated ATP blocks the ATP-sensitive K+ channel. This leads to the build up of
positive K+ ions inside the cell depolarizing the cell membrane. This activates voltagedependent calcium channels, allowing Ca2+ entry into the cells & trigger Ca2+ induced
calcium release. This elevated calcium causes insulin release.
Looking at the above diagram it should be obvious that the major controller of insulin
synthesis and release is glucose. Elevation of glucose levels causing increased synthesis
and release, just as important is that decreased glucose levels decrease synthesis and
release. There are some factors that may amplify or inhibit the release of insulin however
these will be looked at in more detail in Pathophysiology.
35
Intracellular Actions of Insulin
At the target tissues
Insulin
TK
Activation of
various proteins
Insulin
Effects
Figure 5. Simplistic diagram showing insulin acting via tyrosine kinase receptors. See
Figure 34-5 in your text for a much more detailed account.
Insulin acts via tyrosine kinase to recruit and activate kinases/phosphotases other
enzymes as well as move GLUT4-containing vesicles to the cell membrane
Physiological Function of Insulin
In most tissue type’s insulin will increase glucose and amino acid uptake and stimulate
protein and DNA synthesis. It will inhibit protein breakdown and regulate the expression
of many genes.
The role of insulin in the liver, muscle and adipose tissue is of special interest and will be
discussed in the next few pages.
36
Action of I nsulin in Liver
Indirectly
Via Glut-2
Transporters
Glucose
Glycogen
Therefore decreasing
Blood glucose
levels
Glucose
Pyruvate
Triglycerides
VLDL
Proteins
AA
Liver
Figure 6. In the liver insulin increases glucose storage as glycogen
(activating/inhibiting enzymes) and also stimulates the conversion of glucose into
triglycerides which are released into the circulation as VLDL (very low density
lipoproteins).
Insulin also decreases glycogenolysis and moves both fatty acids and amino acid away
from producing glucose. For example amino acids are converted into proteins rather than
glucose by the liver. Therefore overall glucose production by the liver is decreased.
Note that insulin does not increase glucose uptake by the liver via increasing the number
of glucose transporters on the membranes of hepatocytes (insulin does not effect GLUT2 transporters)
37
Action of Insulin on Muscle
Increases
No. of Glut-4
Transporters
On membrane
Glycogen
Glucose
Glucose
Pyruvate
Triglycerides
AA
AA
Proteins
Muscle
Figure 7. This diagram shows the effect on insulin on muscle.
Insulin increases glucose uptake by increasing the number of GLUT-4 transporters on
the muscle membrane.
It stimulates glycogen synthesis (by increasing glycogen synthase activity and decreasing
glycogen phosphorylase activity).
It also stimulates the conversion of glucose in triglycerides and increases protein
synthesis (by increasing amino acid transport and ribosomal protein synthesis)
38
Action of Insulin on Adipose Tissue
Increases
No. of Glut-4
Transporters
On membrane
Glucose
Glucose
-glycerol
phosphate
Pyruvate
Triglycerides
FFA
FFA
Increases lipoprotein
Lipase which breaks
Down TGs in VLDL
And chylomicrons
Adipose Tissue
Figure 8. This diagram shows the effect of insulin on adipose tissue.
Insulin increases triglyceride storage by increasing glucose entry into the cell (by
increasing the number of GLUT-4 transporters on the adipocyte cell membrane)
providing a source of triglycerides via glycerol phosphate and pyruvate.
It also induces the production of lipoprotein lipase which hydrolyzes triglycerides from
circulating lipoproteins providing a source of fatty acids to produce triglycerides as well
as decreasing intracellular lipase activity.
Overall insulin promotes triglyceride production by adipose tissue.
It is important to understand the various effects of insulin on different target tissues.
Then put these together to understand its overall role in the body.
NB. Insulin can only directly influence glucose entry into cells which have GLUT-4
transporters.
39
Summary – Mechanism of Insulin action on various target tissues
Many
effects
Insulin
glucose
General cell
glucagon
Pancreas
Glycogen
Triglycerides
FFA
AA
Liver
Triglycerides
Adipose Cells
glycogen
proteins
Muscle
Figure 9. This diagram really puts together the last three diagrams and adds some further
relevant information. An increase in blood glucose stimulates the release of insulin from
the cells of the pancreas, this insulin itself acts in a paracrine fashion to inhibit the
release of glucagon from the α cells.
This elevated insulin stimulates the conversion of glucose into glycogen and triglycerides
by the liver.
In adipose tissue it stimulates the uptake and conversion of fatty acids and glucose into
triglycerides.
In muscle tissue it stimulates the uptake of glucose and its conversion into glycogen as
well as the uptake and conversion of amino acids into protein.
It also increases the uptake of glucose in many other cells of the body. Therefore the
overall role of insulin is to decrease blood glucose levels.
Insulin also decreases the serum potassium levels.
Mechanism is by increased uptake of K+ by the cells. Clinically significant can
administer insulin and glucose (to prevent insulin induced hypoglycemia) in
hyperkalemic patients.
40
Glucagon
Basic Information
Glucagon is a 29AA polypeptide that is synthesized in the cells of the islets of
Langerhans and stored in secretory granules. When released into the blood it has a halflife of 3-6 minutes.
Amino acids in
blood
Cell of Pancreas
Glucagon
Figure 10. Surprisingly the major stimulator of glucagon secretion is amino acid
concentration in plasma. See the next slide for clarification on glucagon and blood
glucose.
Release
The release of glucagon is inhibited by high levels of blood glucose most likely via the
action of insulin. Insulin has a direct action on the α cells of the pancreas inhibiting
glucagon release.
Therefore decreasing blood glucose will decrease insulin release and its inhibition on the
cell leading to an increase in glucagon release. Its release is also inhibited by high
levels of circulating fatty acids.
41
Blood
Glucose
Amino acids in
blood
Insulin
Cell of Pancreas
Glucagon
Exact
Mechanisms
Remain
Unknown
Figure 11. Now Figure 10 should make more sense. High levels of glucose and insulin
inhibit glucagon release. Therefore decreasing either or both of these two will increase
glucagon levels in the plasma. As stated previously most now agree it is the decrease in
insulin levels that lead to an elevation of glucagon release.
42
Intracellular mechanism of action
At the target tissues
Glucagon
cAMP
Activation of
protein kinase A
Glucagon
Effects
Figure 12. Diagram showing the mechanism of action of glucagon. Glucagon acts via
the cAMP second messenger system.
Physiological Function of Glucagon
Glucagon has two direct target tissues (liver and adipose tissue). At physiological levels
it is believed to have its major effect on liver. Glucagon has no direct effect on muscle.
43
Actions of glucogan
Fuel for Brain
During fasting
state
glycogenolysis
gluconeogenesis
Glucose
Fat
Oxidation
Liver
Energy
Ketone
Bodies
Ketone
Bodies
Figure 13. In the liver glucagon increases glycogenolysis and gluconeogenesis while
decreasing glycogen synthesis and glycolysis; all these actions increase glucose
production by the liver.
As glucagon is trying to conserve glucose use for energy in the liver it stimulates the oxidation of fatty acids to produce energy and also releases ketone bodies into the
circulation as an energy source.
In adipose tissue glucagon stimulates the activity of hormone-sensitive lipase (increasing
lipolysis)
44
Diabetes Mellitus
.
Type I Diabetes Mellitus
This is primary deficiency of insulin as a consequence of selective cell destruction,
usually due to an autoimmune process. This lack of insulin means that the liver produces
glucose with less storing, the efficiency of peripheral glucose use is decreased, protein is
broken down to produce glucose and you have uninhibited breakdown of fats.
This leads to elevated levels of levels of ketone bodies and other substances in the blood.
These are initially buffered by HCO3- and hyperventilation BUT eventually pH drops so
much that if untreated death occurs.
If untreated patients may present with
- hyperglycemia
glucose in urine
excretion of water and salts
- Symptoms are polyuria and polydipsia
- loss of glucose caloric drain
- Symptoms are polyphagia
eating more
Therefore you observe a patient who is polyuric, polydipsic, polyphagic and
hyperglycemic and has a loss of lean body mass, adipose tissue and body fluids.
Type II Diabetes
This is mainly due to an impaired ability of target tissues to respond to insulin. It may be
induced for short periods of time (for example during pregnancy). It may be due to a
desensitization of insulin receptors and may eventually lead to problems with the cell as
well. There are many theories for the actual cause of Type II diabetes eg. obesity and/or
a genetic influence. Insulin levels may be normal, or elevated, BUT you do not see the
same effect on glucose levels when compared to normal.
In Type II diabetes there is a gradual increase in glucose levels, this may cause damage to
tissues before glucose is entering the urine meaning that it is difficult to catch in time. In
some cases it might only be discovered on irreversible damage to retina, heart, kidneys or
nervous system. Therefore regular testing of those at risk is important.
Diagnosis
A random blood glucose level of > 11.1mM (on two occasions)
A fasting blood glucose level of > 7.0mM
Treatment
In milder forms dietary restriction
Sulfonylurea class of drugs.
weight loss may be enough.
45
Section 7
Endocrine Regulation of Calcium and Phosphate
Balance
Learning Objectives
1. Explain the physiological importance of calcium balance
2. Describe calcium distribution in the body
3. Describe phosphate distribution in the body
4. Discuss the synthesis and release of parathyroid hormone (include the importance of
ionized calcium levels)
5. Explain the mechanism of action of parathyroid hormone at its target cells
6. Discuss the physiological actions of parathyroid hormone
7. Describe the transport and metabolism of parathyroid hormone
8. Discuss the synthesis and release of calcitriol
9. Explain the mechanism of action of calcitriol at its target cells
10. Discuss the physiological actions of calcitriol
11. Describe the transport and metabolism of calcitriol
12. Be able to compare the effect of calcitriol and PTH on calcium and phosphate levels
and explain the underlying physiological mechanisms
13. Describe the underlying physiology of medical problems associated with altered
PTH and/or calcitriol secretion
Readings
Chapter 35 of your textbook
Introduction
The maintenance of calcium homeostasis within the body is of the utmost importance.
Extracellular calcium levels must be tightly regulated as they are important in the
functioning of many physiological processes. Intracellular calcium levels must also be
tightly controlled.
Calcium in the extracellular fluid may be present in its ionized form, bound to proteins
(albumin) or complexed with anions (phosphate, citrate). The latter two are
metabolically inert and only the ionized calcium serum levels are important with respect
to physiological function. Therefore it is the levels of ionized calcium that are
maintained by the hormones associated with calcium homeostasis. The two major
hormones we will be discussing are Parathyroid hormone and calcitriol (Vitamin D). We
will also briefly discuss calcitonin, however it has a minor role in calcium homeostasis.
The plasma levels of phosphate do not need to be as closely regulated as those of
calcium, however it is still important in providing ATP and the activation/deactivation of
enzymes. Plasma phosphate levels are also controlled by PTH and calcitriol.
46
Calcium and Phosphate Distribution
Figure 1. This diagram shows the distribution of calcium throughout the body. Note
that some of these values may vary however the extracellular value must be closely
regulated (Figure 35-1 from your text). It is important to see here that the major sources
of calcium are from the diet and bone. Therefore if dietary calcium is insufficient
calcium will be taken from the bone. The kidney acts as an important regulator of blood
calcium levels with the bone. All three of these, gut absorption of calcium, bone uptake
and release of calcium and calcium reabsorption by the kidney are all tightly regulated in
order to maintain blood calcium levels within a tight physiologically safe range.
47
Figure 2. This diagram shows the distribution of phosphate in the body (Figure 35-2
from your text). As you can see the 3 major organs associated with phosphate regulation
are the same as for calcium. This is not surprising as calcium and phosphate bind
together in the formation of bone. As stated above phosphate regulation is not as
physiologically important as calcium regulation but there are some subtle differences in
how phosphate is handled and knowledge of this will allow you as physicians to pinpoint
medical problems associated with altered calcium homeostasis.
48
Parathyroid Hormone
Ionized calcium
in blood
Parathyroid Gland
PTH
Figure 3. Diagram showing the reaction of the parathyroid glands to hypocalcemia.
PTH is released in greater amounts increasing its effects on elevating calcium levels and
maintaining homeostasis. In other words parathyroid hormone elevates blood calcium
levels.
Basic Information
PTH is a 84 AA peptide that is synthesized in chief cells of the parathyroid gland and
stored in secretory granules. It is released into the blood and has a half-life of 2-4
minutes.
49
Regulation of Synthesis and Release
Ionized Calcium
Ca - R
DAG
IP3
PKC
Ca2+
Inhibition of
synthesis
and release
of PTH
Figure 4. Diagram showing the regulation of PTH synthesis and secretion via calcium
binding to a receptor on the membrane of the chief cell. We are looking at this
backwards as it is easier to understand this way. Elevated levels of ionized calcium in the
blood mean more Ca2+ receptors are bound on the membrane of the chief cells of the
parathyroid gland. The calcium binding to the receptor leads to the activation of IP3 and
DAG. IP3 causes the release of calcium from internal stores that is followed by a
sustained influx of extracellular calcium. This increase in intracellular calcium prevents
secretory granule fusion and PTH release as well as inhibiting PTH synthesis. This is
totally opposite to everything we have seen previous in my lectures, this is the
mechanism however. A decrease in ionized calcium levels therefore will decrease the
activity of the IP3 and DAG and hence more PTH will be synthesized and released.
Intracellular Mechanism of Action
PTH binds to a receptor on the target cell membrane activating two different G proteins,
one that activates adenylate cyclase and hence cAMP, the other activating phospholipase
C. Therefore activation may occur due to increased cAMP or calcium levels inside the
target cell.
50
Physiological function of PTH
The major role of PTH is to prevent hypocalcemia from occurring. It does this through a
number of mechanisms (both direct and indirect).
1.
Increasing calcium availability from bone.
PTH and bone
PTH
M-CSF
PTH-R
Osteoclast
development
RANK
IP3 & cAMP
?
IL-6
Stimulates
Osteoclast
Action
Osteoblast
Bone
resorption
Figure 5. Diagram showing how PTH increases bone resorption and hence elevates the
levels of free ionized calcium in the plasma. PTH binds to receptors on on stromal
osteoblasts in bone marrow causing them to release cytokines. These cytokines both
stimulate the development of osteoclasts and increases their action leading to
demineralization of the bone and the release of calcium
2.
Actions on (or in) the kidney
- ~ 90% of filtered Ca2+ is reabsorbed with Na+ in the proximal tubule.
Therefore ~ 10% reaches the distal tubule where it is reabsorbed by a
saturable active transporter that is PTH-regulated. It is important to note
that this is a low capacity system. In other words PTH regulates the
reabsorption of last 10% of the filtered calcium and so can finally tune the
amount of calcium reabsorbed.
- PTH stimulates the synthesis of calcitriol by increasing the activity of the
1 -hydroxylase enzyme (see Figure 7)
51
PTH also has an effect on phosphate homeostasis.
1.
PTH increases phosphate availability from bone. As shown in Figure 2 most
body phosphate is found in bone. We have just seen that PTH breaks down bone to
release calcium into the blood, this breakdown will also release phosphate into the blood.
2.
PTH inhibits the reabsorption of phosphate by the kidney. Therefore it increases
the excretion of phosphate in the urine. It does this by moving the Na+-phosphate
transporter away from the apical membrane of the proximal tubule.
Therefore the overall role of PTH is to decrease phosphate levels in the blood (and the
body) by moving the phosphate from the bone to the blood and then from the blood into
the urine.
PTH
Decreased
Phosphate
Levels
Increased
Free Calcium
Levels
Figure 6. This diagram summarizes the effects of PTH on free calcium and phosphate
levels in the blood. PTH elevates free calcium levels in many ways described, for
example by increasing release from bone, increasing reabsorption by the kidney and
indirectly by increasing gut absorption of calcium by stimulating calcitriol synthesis (see
next section). PTH decreases phosphate levels because even though it causes its release
from bone it also inhibits phosphate reabsorption by the kidney meaning most of the
release phosphate is quickly excreted in the person’s urine.
52
Calcitriol (Vitamin D, 1,25-dihydrocholecalciferol)
Calcitriol is a steroid hormone that may be either synthesized in the body or ingested (as
a precursor).
Cutaneous synthesis of calcitriol
7-Dehydrocholesterol
UV light
Cholecalciferol
25-hydroxylase
liver
25-Hydroxycholecalciferol
1-hydroxylase
kidney
1,25-Hydroxycholecalciferol
Figure 7.
Diagram showing the conversion to Vitamin D3 from its cholesterol-based
precursor in the skin (7-dehydrocholesterol). Ultraviolet light is required to cleave the B
ring of 7-hydrocholesterol to begin the synthesis of Vitamin D3. There is currently some
controversy but it is believed that ~15 minutes of exposure to UVB light is required (this
varies amongst people) to produce the maximum amount of Vitamin D3 possible. The
thermal isomerization of 7-dehydrocholesterol into cholecalciferol is dependent upon
timing – if exposure to UVB is too long it is further changed into tachysterol and
lumisterol. Cholecalciferol is quickly removed from the skin and travels in the plasma
bound to DBP (Vitamin D binding protein). This cholecalciferol is the same as the
Vitamin D in your diet. In the liver cholecalciferol is hydroxylated in both mitochondria
and microsomes by a 25-hydroxylase enzyme, this forms 25-hydroxycholecalciferol.
Most of the conversion into 1,25-hydroxycholecalciferol (calcitriol) takes place in the
kidney. This conversion is catalized by the enzyme 1-hydroxylase (the production of 1hydroxylase is stimulated by by PTH and is inhibited by high levels of Ca and
phosphate as well as high levels of calcitriol itself). In cases of production stimulation
the enzyme 1-hydroxylase is activated, in cases of inhibition the 24-hydroxylase enzyme
is activated. The 24-hydroxylase enzyme converts the 25-hydroxycholecalciferol into the
relatively inert 24.25-hydroxycholecalciferol.
53
Dietary Sources of Vitamin D
Important if there is not enough exposure to UVB. Found in high levels in fish oils, fish
liver and eggs. Absorbed from GIT with help of bile salts. Enters either the lymph in
chylomicrons or into portal system. It then follows the conversion into calcitriol as
described above.
Intracellular mechanism of action of calcitriol
At the target tissues
Vitamin D
Modulation of gene
expression
Formation
of proteins
Figure 8. Diagram showing the intracellular actions of calcitriol. In most cases calcitriol
will bind to a Vitamin D receptor (VDR) found within the nucleus of target cells. This
leads to the activation of mRNA and the formation of proteins.
It has also been suggested that calcitriol may increase intracellular calcium levels by
activating the IP3 / DAG system.
54
Physiological Functions of Calcitriol
Calcitriol increases the levels of calcium in the blood. It is currently accepted that it has
roles in the intestine, bone and kidney and recent research suggest roles in other tissues
including those not associated with calcium homeostasis.
1.
Increasing calcium reabsorption from the small intestine
Vit D and the GIT
lumen
Ca2+
Interstitial
fluid
Duodenal cell
Na+
Ca2+
Ca2+
H+
calbindin
Ca2+
Na+
PO4
PO4
?
Figure 9. Diagram showing the transport of calcium through the intestinal epithelial cell
and how calcitriol may affect this. Calcitriol increases the production of calcium
channels on the luminal side of the duodenal cell, this allows more calcium to enter the
duodenal cell down there concentration gradient. This concentration gradient is helped
by increased production of calbindin a cellular protein that binds to the calcium in the cell
decreasing its free concentration and hence increasing the concentration gradient.
Calcitriol also increase the production of two membrane proteins found on the basolateral
side of the duodenal cell (Na+-Ca2+ exchanger and a Ca2+-ATPase) both which increase
the movement of the calcium out of the duodenal cell into the interstitial fluid. Note that
calcitriol also increases the expression of the Na+-PO4-cotransporter increasing phosphate
absorption, whether or not calcitriol effects phosphate movement out of the cell into the
interstitial fluid remains unknown. So overall calcitriol increases both calcium and
phosphate absorption by the gut.
55
2.
Calcitriol) is necessary for normal bone formation. The mechanism of its
function and how it affects calcium levels is unknown.
3.
Believed to increase calcium reabsorption by the distal tubule. May require
PTH for this to take place.
So the overall role is for there to be an increase in blood calcium levels.
It is interesting to note that this increase in blood calcium will stimulate the formation of
bone (an indirect action of Vit D). The direct effect of Vit D on bone is to promote the
breakdown of bone. Calcitriol also increases the levels of phosphate in blood. The
mechanism of action of some of these processes still remain unclear.
1.
Calcitriol increases phosphate aborption by the GIT. It is believed to increase the
number of Na-phosphate transporters on the luminal membrane.
2.
Calcitriol increases phosphate reabsorption by the kidney. Again the mechanism
is unclear.
CALCITRIOL
Increased
Phosphate
Levels
Increased
Free Calcium
Levels
Figure 10. This digram summarizes the action of calcitriol on phosphate and calcium
levels. As you can see the overall effect of calcitriol is the increase both the levels of
phosphate and calcium in the blood. Is this the same as PTH???
56
Calcitonin
Basic Information
Calcitonin is a 32AA peptide that is synthesized and secreted by C cells of the thyroid
gland. It is released into blood and has a half-life close to 5 minutes.
Physiological function of Calcitonin in calcium homeostasis
None really
- removal of thyroid has no effect on calcium handling
- excess calcitonin (malignant C cells) does not effect calcium handling
Control of Release
Increase in blood calcium levels increase release
Actions of Calcitonin
Inhibits osteoclastic bone resorption. Therefore it is important in the formation of bone.
This is especially the case in the fetus and neonate.
57
Overview of Calcium Homeostasis
proximal tubule
Kidney
1-hydroxylase
activity
PTH
Cholecalciferol
Distal tubule
Kidney
calcium
reabsorption
liver
bone
resorption
Calcifediol
INCREASE
CALCIUM
LEVELS
calcium
reabsorption
Calcitriol
Gastro-intestinal
tract
Figure 11. Flow diagram showing the hormonal regulation of calcium levels. You must
understand this. This diagram is just to help you put together calcium homeostasis it will
not be presented in the lecture.
58
Medical Problems Associated with altered Hormone Synthesis
Hyperparathyroidism
This is where the parathyroid glands are secreting excess parathyroid hormone.
It may be primary, due to inherent problem with parathyroid gland or secondary, excess
release due to another problem decreasing calcium levels
Primary Hyperparathyroidism
This is usually due to a benign parathyroid tumor and in many cases no symptoms are
observed for a long time. It causes hypercalcemia (see above for symptoms) and a
reduction of bone density. In many cases the formation of either kidney stones or a
broken bone will lead you to a diagnosis.
Secondary Hyperparathyroidism
In most cases you observe elevated PTH levels that are there to maintain calcium levels
due to some other factor decreasing these levels (eg Vitamin D deficiency, chronic renal
disease). Interestingly is also common during pregnancy and lactation. You can treat it
either with calcium supplements or by treating the underlying disease.
In other words you see high levels of PTH without the high calcium levels.
Hypoparathyroidism
Usually due to removal of the parathyroid glands during surgery or it is sometimes due to
autoimmune problems. It leads to hypocalcemia.
Vitamin D deficiency
Rickets
Vitamin D deficiency due to inadequate exposure to sun, inadequate Vitamin D in diet or
problems in absorbing Vitamin D, leading to hypocalcemia, weak bones and dental
deformities. It is treated with supplementation of calcium or Vitamin D
Osteomalacia
Softening of bones occurring due to the factors described above for rickets.
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