Chapter 20
Mechanisms of Hormonal Regulation
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Endocrine System

Functions





Differentiation of the reproductive and central nervous
systems in the developing fetus
Stimulation of sequential growth and development
during childhood and adolescence
Coordination of the male and female reproductive
systems
Maintenance of an optimal internal environment
Initiation of corrective and adaptive responses when
emergency demands occur
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Hormones

General characteristics

Specific rates and rhythms of secretion
• Diurnal, pulsatile and cyclic, and patterns depending on
circulating substances



Operate within feedback systems
Affect only cells with appropriate receptors
The liver inactivates hormones, rendering the
hormones more water soluble for renal excretion
OR hormones are excreted by kidneys directly
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The Endocrine System
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Regulation of Hormone Release

Hormones are released:

In response to an alteration in the cellular
environment
 To maintain a regulated level of certain
substances or other hormones



Hormones are regulated by chemical,
hormonal, or neural factors
Negative feedback
Positive feedback
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Feedback
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Hormone Transport

Hormones are released into the circulatory
system by endocrine glands


Water-soluble hormones circulate in free, unbound
forms
Lipid soluble hormones are primarily circulating
bound to a carrier
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Cellular Mechanism of
Hormone Action


Target cell
Up-regulation


Down-regulation


Low concentrations of hormone increase the
number of receptors per cell
High concentrations of hormone decrease the
number of receptors
Hormone effects


Direct effects
Permissive effects
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Cellular Mechanism of
Hormone Action

Hormone receptors


Water-soluble hormones



Located in or on the plasma membrane or in the
intracellular compartment of target cell
High molecular weight
Cannot diffuse across the plasma membrane
Lipid-soluble hormones

Easily diffuse across the plasma membrane and
bind to cytosolic or nuclear receptors
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Cellular Mechanism of
Hormone Action

Water-soluble hormones



First messenger
Signal transduction
Second-messenger molecules
• Calcium
• Cyclic adenosine monophosphate (cAMP)
• Cyclic guanosine monophosphate (cGMP)
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Cellular Mechanism of
Hormone Action

Lipid-soluble hormones

Steroid hormones
• Androgens, estrogens, progestins, glucocorticoids,
mineralocorticoids, thyroid


Diffuse across the plasma membrane
• Bind to cytoplasmic or nuclear receptors
Activate
• RNA polymerase
• DNA transcription and translation
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Structure and Function of the
Endocrine Glands

Hypothalamic-pituitary axis


Hypothalamus
Pituitary gland
• Anterior pituitary (adenohypophysis)
• Posterior pituitary (neurohypophysis)
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The Pituitary Gland
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Posterior Pituitary Hormones


Synthesized with binding proteins in
supraoptic and paraventricular nuclei of the
hypothalamus
Secreted by the posterior pituitary

Antidiuretic hormone (ADH, formerly called
vasopressin)
• Controls plasma osmolality
 Oxytocin
• Uterine contractions and milk ejection in lactating women
• Suggested: reduces the brain’s responsiveness to
stressful stimuli, especially in the pregnant and
postpartum states

Treatment of anxiety disorders and autism being explored
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Posterior Pituitary Hormones
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Anterior Pituitary Hormones









Adrenocorticotropic hormone
Melanocyte-stimulating hormone
Growth hormone
Prolactin
Thyroid-stimulating hormone
Luteinizing hormone
Follicle-stimulating hormone
β-lipotropin
β-endorphins
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Anterior Pituitary Hormones
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Thyroid and Parathyroid Glands

Thyroid gland





Two lobes lateral to the trachea
Isthmus
Follicles (follicle cells surrounding colloid)
Parafollicular cells (C cells)
• Secrete calcitonin
Regulation of thyroid hormone secretion
• Thyrotropin-releasing hormone and thyroid-stimulating
hormone
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Thyroid and Parathyroid Glands

Thyroid hormones



90% T4 and 10% T3
Bound to thyroxine-binding globulin, thyroxinebinding prealbumin, or albumin
Affect growth and maturation of tissues, cell
metabolism, heat production, and oxygen
consumption
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Thyroid and Parathyroid Glands
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Thyroid and Parathyroid Glands

Parathyroid glands


Small glands located behind the upper and lower
poles of the thyroid gland
Produce parathyroid hormone
• Regulator of serum calcium
• Antagonist of calcitonin
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Parathyroid Glands
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Endocrine Pancreas


The pancreas is both an endocrine
(producing hormones glucagon, insulin) and
an exocrine gland (producing digestive
enzymes)
Houses the islets of Langerhans

Secretion of glucagon and insulin
 Cells
• Alpha—glucagon
• Beta—insulin
• Delta—somatostatin and gastrin
• F cells—pancreatic polypeptide
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Endocrine Pancreas
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Endocrine Pancreas
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Endocrine Pancreas

Insulin

Regulated by chemical, hormonal, neural mechanisms
 Synthesized from proinsulin
 Secretion promoted by increased blood glucose levels
 Facilitates rate of glucose uptake into body’s cells
• Sensitivity of the insulin receptor is a key component in
maintaining normal cellular function (insulin resistance)

Facilitates the intracellular transport of potassium
 Anabolic hormone
• Synthesis of proteins, lipids, and nucleic acids
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Endocrine Pancreas

Glucagon



Insulin antagonist
Secretion promoted by decreased blood glucose
levels
Stimulates glycogenolysis, gluconeogenesis, and
lipolysis
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Endocrine Pancreas

Somatostatin


Produced by delta cells of the pancreas
Little is known
• In animal studies found to be involved in the regulation of
alpha cell and beta cell function
• May inhibit glucagon and insulin secretion
• May prevent excess secretion of insulin
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Endocrine Pancreas

Gastrin


Likely controls the secretion of glucagon
Grehlin



Stimulates GH secretion
Controls appetite
Regulation of insulin sensitivity
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Endocrine Pancreas

Pancreatic polypeptide





Released by F cells in response to hypoglycemia and
protein-rich meals
Signals satiety
Inhibits gallbladder contraction and exocrine pancreas
secretion
Increases gastric acid secretion
Frequently increased in pancreatic tumors and in
diabetes
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Endocrine Pancreas
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Adrenal Glands

Adrenal cortex





80% of an adrenal gland’s total weight
Zona glomerulosa
Zona fasciculata
Zona reticularis
Adrenal medulla

Innervated by the sympathetic and
parasympathetic nervous systems
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Adrenal Glands

Adrenal cortex


Stimulated by adrenocorticotropic hormone
(ACTH)
Glucocorticoid hormones
• Direct effects on carbohydrate metabolism
• Anti-inflammatory, growth-suppressing effects
• Influence awareness and sleep habits
• Most potent naturally occurring glucocorticoid is cortisol
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Adrenal Glands

Adrenal cortex

Mineralocorticoid hormones
• Affect ion transport by epithelial cells


Increase the activity of the sodium pump of the epithelial
cells
Cause sodium retention and potassium and hydrogen loss
• Most potent naturally occurring mineralocorticoid is
aldosterone

Regulated by the renin-angiotensin-aldosterone system
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Aldosterone
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Adrenal Glands

Adrenal cortex

Adrenal estrogens and androgens
• Estrogen secretion by the adrenal cortex is minimal
• The adrenal cortex secretes weak androgens

Androgens converted by peripheral tissues to stronger
androgens such as testosterone
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Adrenal Glands

Adrenal medulla

Chromaffin cells (pheochromocytes)
• Chromaffin cells secrete the catecholamines epinephrine
(majority) and norepinephrine


Release of catecholamines has been
characterized as a “fight or flight” response
Catecholamines promote hyperglycemia
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Catecholamines
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Neuroendocrine Response to
Stress



The endocrine system reacts with the
nervous system to respond to stressors
The stress response also involves the
immune system
Influenced by corticotropin-releasing hormone
from the hypothalamus
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Tests of Endocrine Function



Radioimmunoassay
Enzyme-linked immunosorbent assay
(ELISA)
Bioassay
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Aging and the Endocrine System


Endocrine changes: consequence or cause?
Thyroid gland


Parathyroid glands


Glandular atrophy, fibrosis, nodularity, and
increased inflammatory infiltrates
Related to alterations in calcium balance
• Inadequate intake, malabsorption, or renal changes
Adrenal glands

Decreased clearance of cortisol
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