Hormonal coordination 1
•Introduction to hormones,
•Chemical classes of hormones
•Basic mechanisms of hormone action
At the end of the lecture you will be able to
• explain how hormones can identify the
target cells.
• compare mechanism of action of a lipid
soluble hormone with that of a water
soluble hormone.
Regulation of homeostasis by
endocrine system
• Hormones delivered to tissues throughout
the body by blood
• All body cells affected
• Resulting action –changes in metabolism
• Slow response, action longer
Endocrine glands
release hormones to
blood
target tissue
(target cell)
Effects
(change in structure and/or function)
Why only some cells respond to a
particular hormone?
• Although a hormone travels throughout the
body in the blood, it affects only specific
cells (target cells)
• Hormones influence the target cells by
binding to specific proteins (receptors) in
the target cell
• Only target cells for a certain hormone
have receptors that bind & recognize the
particular hormone
Main classes of hormones
1. Steroids
2. Amines
3. Peptides & proteins
4. Eicosanoids
1. Steroids : aldosterone, testosteron,
estrogen, progesterone, cortisol etc.
2. Amines : adrenaline, noradrenaline,
thyroxine, T3
3. Peptides & proteins: all hypothalamic
releasing and inhibiting hormones,
oxytocin, ADH, insulin, glucagon,
parathyroid hormone etc.
4. Eicosanoids ;e.g. prostaglandins,
leucotrienes (produced by all cells except
RBCs)
Hormone transport in blood
• Soluble in plasma
Peptide & protein hormones, adrenaline,
noradrenaline
• Bind with transport proteins in the
blood
Thyroxine, T3, steroid hormoness
hormone action
• Response to a hormone depends on
hormone & target cell
• Various target cells respond differently to
the same hormone
e.g insulin stimulates synthesis of
– glycogen in liver cells
– Triglycerides in adipose cells
Response to a hormone may be
• Synthesis of new molecules
• Change in permeability of plasma
membrane
• Alter rate of metabolic reactions
• Cause contraction of smooth & cardiac
muscles etc.
• Only the target cells for a certain hormone
have receptors that bind and recognize
that hormone, (TSH binds on receptors of
the cells of thyroid gland, not on ovarian
cells)
• Lipid soluble hormones –receptors
inside the target cell
• Water soluble hormones – receptors on
the plasma membrane of the target cell
Lipid soluble hormones –
mechanism of action
1. Hormone diffuse from blood through
interstitial fluid & enter into cells
through plasma membrane
2. If the cell is the target cell, the
hormone binds to receptors in the
cytosol
cytosol
3. Activated receptor alter gene expression : it
turns specific genes of nuclear DNA on or off
4. DNA is transcribed, new mRNA forms, leaves
the nucleus & enter cytosol
5. New mRNA direct the synthesis of new
proteins on ribosomes
6. New proteins alter cell’s activity & cause
typical physiological response of that
hormone
Water soluble hormones –
mechanism of action
They cannot diffuse through phospholipid
bilayer of the plasma membrane to
attach the receptor inside the cells.
Receptor for water soluble hormones
present on the plasma membrane of the
target cell
G protein
1. Hormone diffuses from blood through
interstitial fluid, & binds to receptors on
the plasma membrane of the target
cell
2. This binding activates another
membrane protein (G protein) which
acts to turn on adenyl cyclase
3. Adenyl cyclase converts ATP to cAMP
in the cytosol of the cell
4. cAMP (second messenger) activates
one or several protein kinases (an
enzyme that phosphorylate cell proteins)
5. Activated protein kinases phosphorylate
enzymes (Activate or inactivate some
enzymes)
6. This could regulate other enzymes,
secretion, protein synthesis, & changes
in membrane permeability.
• After a short period, phophodiesterase
enzyme inactivate cAMP, the cell’s
response is turned off until new hormone
molecule binds to the receptor.
• Several other substances also serve as
second messenger
Summary
• Hormones, receptors, target cells
• Four chemical classes of hormones
• Mechanism of action of a lipid soluble
hormone
• Mechanism of action of a water soluble
hormone