ENDOCRINE SYSTEM
OVERVIEW
Objectives
• Understanding the common aspects of neural and
endocrinal regulations.
• Describing the chemical nature of hormones.
• Describing the types of hormones.
• Understanding
the
different
mechanisms
of
hormonal action & concept of second messenger
system.
Introduction
• Body systems work to maintain homeostasis (internal
environment of the body as blood pressure, pH, water and
electrolyte balance, temperature, etc.)
• Major regulatory systems of body:
1- Nervous system
2- Endocrine system
• Nervous and Endocrine systems act together to coordinate
functions of all body systems.
• These systems communicate by way of electrical and/or
chemical signals
Regulatory Systems
 Act together to coordinate functions of all body systems.
1- Nervous system:
 Nerve impulses / Neurotransmitters.
 Faster responses, briefer effects, acts on specific targets.
2- Endocrine system:
 Its information is conveyed via hormones released
directly into blood and diffuse to the tissues to regulate
their functions.
 Slower responses, effects last longer, broader influence.
Fig. 13.2
Role of endocrine system
1) Regulating the body functions. Along with autonomic
nervous system, controlling activities of both circulatory
and digestive systems.
2) Controlling cellular metabolism as growth & secretion.
3) Controlling the rate of chemical reactions in cells.
4) Controlling
transport
membrane.
5) Controlling Reproduction.
of
substances
through
cell
Features of Endocrinology
• The endocrine system consists of the ductless
endocrine glands that are scattered throughout the
body.
• Its information is conveyed via hormones released
directly into blood and diffuse to tissues to regulate
their functions.
• Its speed of response is relatively slow (some
hormonal effects occur within seconds while others
require days to start).
• It is largely a self-regulating system.
RECEPTORS
 A target cell responds to a hormone because it bears receptors
for the hormone.
 It is a protein made by the target cell (protein synthesis after
gene expression).
 The protein is made, then inserted into plasma membrane, or
found in cytoplasm or nucleus.
 The active site on the protein “fits” the hormone.
 Acts to convert the signal into a response.
 What would happen if there was a gene defect in the DNA code
for a receptor?
 What would happen if the receptor protein was denatured?
Human Endocrine System (major glands)
Hormones
 Chemical substances released from endocrine glands
or specialized cells (e.g., neuroendocrine cells) directly
into blood to produce a systemic physiological
response.
 They slow down or speed up the biological functions
according to body needs (they do not initiate
activities).
Types of Hormones
• According to the distance the hormone travels:
– Endocrine
– Paracrine
– Autocrine
• According to solubility:
– Hydrophilic e.g. Peptide hormones.
– Lipophilic e.g. Steroid hormones.
Types of Hormones
1- Endocrine hormones: circulate in blood throughout body.
2- Local hormones:
• Paracrines: tissue hormones act on nearby cells only,
include cytokines, neurotrophins, and prostaglandins.
• Autocrines:
act on the same cells
that secrete them.
Types of Hormones
1) Protein hormones: more than 100 amino acids (Growth
Hormone & Prolactin).
2) Polypeptides: less than 100 amino acids (insulin).
3) Glycoprotein: > 100 amino acids with carbohydrate (FSH & LH).
4) Amino acid derivatives: derived from tyrosine (epinephrine,
norepinephrine & thyroxine) or from tryptophan (melatonin).
5) Steroids: derived from cholesterol as:
 Sex steroids: testosterone, estradiol, progesterone are
secreted by testes, ovaries, placenta and adrenal cortex.
 Corticosteroids: cortisol and aldosterone are secreted by
adrenal cortex.
 Calcitriol (1, 25 dihydroxy cholecalciferol).
MECHANISM OF HORMONAL ACTION
• Response depends on both hormone and target cells.
1) Lipophilic (Lipid-soluble) hormones: pass through the
plasma membrane of target cells and bind to intracellular
receptors, thereby regulating target cell genes.
1) Hydrophilic (Water-soluble) hormones: cannot pass
through plasma membranes, and thus use second
messengers within the target cells, such as cyclic AMP, IP3,
and Ca++, for mediation.
1- Hormones That Do Not Enter Cells
• Hormones that are too large or too polar to cross
plasma membranes include all peptides, glycoprotein
hormones, as well as catecholamines.
• They bind to receptors located on the outer surface of
the plasma membrane triggering reactions inside cells
through second messenger mechanisms.
• Many of these hormones induce the release of
intracellular second messengers that transmit the
hormone signal inside the cell.
Second messengers
• The polar hormones bind to receptors located on the outer surface of the cell
membrane. This activates enzymes that enlist second-messenger molecules.
 Many hormones activate adenylate cyclase when they bind to their
receptors. This enzyme produces cyclic AMP (cAMP), which activates
protein kinase enzymes within the cell cytoplasm.
 Other hormones may activate phospholipase C when they bind to their
receptors. This leads to the release of inositol triphosphate (IP3), which
stimulates the endoplasmic reticulum to release Ca2+ into the cytoplasm,
activating calmodulin.
 The membrane receptors for insulin and various growth factors are
tyrosine kinase enzymes that are activated by binding to the hormone.
Once activated, the receptor kinase phosphorylates signaling molecules in
the cytoplasm that can have many effects.
Second messengers
Second messengers include:
1) cyclic AMP
2) cyclic GMP (e.g. Atrial natriuretic peptide)
2+
3) Intracellular Ca
4) phospholipase C which catalyzes phosphoinositide
turnover producing inositol triphosphates (IP3) &
diacyl glycerol (DAG).
Cell mechanism & Second messengers
G Protein–Linked Hormone Receptors:
• Many hormones activate receptors that indirectly regulate the
activity of target proteins (e.g., enzymes or ion channels) by
coupling with groups of cell membrane proteins called
heterotrimeric GTP-binding proteins (G proteins)
2- Hormones That Enter Cells
Lipophilic hormones
 Lipophilic hormones (steroids and thyroid hormones) bind to
intracellular receptors, which function as ligand-dependant
transcription factors.
 Some steroid hormones bind to cytoplasmic receptors, which
then move into the nucleus. Other steroids and thyroxine bind
to receptors already in the nucleus.
 Each receptor binds to both the hormone and to a region of
DNA called a hormone-response element.
Steroids action
Free hormone
Blood capillary
1 Lipid-soluble
Transport
protein
hormone
diffuses into cell
2 Activated
Nucleus
Receptor
receptor-hormone
complex alters
gene expression
DNA
Cytosol
mRNA
3 Newly formed
mRNA directs
synthesis of
specific proteins
on ribosomes
Ribosome
New
protein
4 New proteins alter
cell's activity
Target cell
Table 18-3, p. 674
Feedback control
• The effective plasma conc. of a hormone is regulated by
Feedback control:
• -ve feedback mech.:
 Anterior pituitary hormones
stimulate target gland.
(TSH from Anterior pituitary
  Thyroxine from thyroid
gland).
 E.g. ↓ Thyroxine  ↑ TSH release from Anterior pituitary
↑ Thyroxine  ↓ TSH.
Endocrine disorders
Result mainly from
1. Hormone excess (hypersecretion)or
2. Deficiency (hyposecretion)or
3. Decreased target-cell responsiveness
‫"وهللا في عون العبد مادام العبد فى عون أخيه"‬