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

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

Water soluble hormones –

mechanism of action

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 cAMPin 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