Endocrine Physiology - Peptide, amine and steroid hormones

Question 1

Life history of a typical hormone.

Answer 1

• Produced by a cell or group of cells
• Secreted from those cells into the blood
• Transported via the blood to distant targets
• Exert their effects at very low concentrations
• Act by binding to receptors on target tissues
• Have their action terminated, often via negative feedback loops

Question 2

What are the 3 types of hormones?

Answer 2

• Peptide hormones - composed of chains of amino acids (most common)
• Amine hormones - all derived from one of two amino acids ( tryptophan or tyrosine)
• Steriod hormones - all derived from cholesterol

Question 3

Describe the stages of making peptide hormones.

Answer 3

• Preprohormone
  
  • produced by ribosomes
  • large and inactive
  • cleaved into smaller units in the endoplamic reticulum to form prohomrones
• Prohormones are packaged into vesicles in the golgi aparatus, along with proteolytic enzymes which break the prohormone down into active hormone and other fragments
• Hormones and fragments are stored in vesicles in the endocrine cells until release is triggered

Question 4

How can measuring inactive fragments in plasma be clinically useful?

Answer 4

Levels can be measured as an indication of endogenous production. For example, C-peptide for insulin. However because insulin is metabolised faster, levels of C-peptide are about 5 times higher than endogenous insulin.

Question 5

How are peptite hormones tranported?

Answer 5

Peptide hormones are hydrophilic so disolve easily in plasma. However, as they are lipophobic they cannot cross cell mambranes and so bind to membrane bound receptors on the target cell.

Question 6

How do peptide hormones produce biological responses?

Answer 6

• G protein coupled receptor - activates 2nd messenger system and/or ion channels leading to modification of existing proteins, rapid response.
• Tyrosine kinase linked receptor - alters gene expression, slower longer lasting activity.

Question 7

Which two types of hormones are formed from tyrosine?

Answer 7

• Catecholamines (hydrophilic - similar mechanism of action to peptide hormones)
• Thyroid hormones (lipophilic - similar mechanism of action to steroid hormones)

Question 8

What are the catecholamines?

Answer 8

• Dopamine (CNS neurotransmitter)
• Norepinephrine (neurotransmitter)
• Epinephrine (hormone released by adrenal medulla)

Question 9

What are the thyroid hormones?

Answer 9

• Thyroxin (Tetraiodothyronine, T4)
• Triiodothyronine (T3)

Question 10

Which amine hormone is dervied from tryptophan?

Answer 10

Melatonin

Question 11

How are steroid hormones synthesised?

Answer 11

• All steroid hormones are derived from cholesterol
• This common ancestry explains why excess steroids have crossover effects

Question 12

Unlike other types of hormones, steroid hormones are synthesised directly as needed, why is this?

Answer 12

Steroid hormones are highly lipophilic so cannot be retained within lipid membranes. Once synthesised they diffuse across the membrane into the ISF and the blood.

Question 13

How are steroid hormones transported?

Answer 13

Steroid hormones are transported bound to carrier proteins such as albumin. This stabilises their transport through the plasma and protects them from enzymatic degredation, phenomenally incresing their half life (60-90 minutes vs 2 minutes for amine hormones).

Question 14

Steroid hormones are produced by:

Answer 14

• Gonads ( testes and ovaries) - sex steroids
• Placenta - hCG, sex steroids
• Kidney - Vitamin D3
• Adrenal cortex - corticosteroids

Question 15

Mechanism of action of steroid hormone.

Answer 15

• There is a small amount of unbound free steroid/thyroid hormone in the plasma and this is the physiologically important fraction.
• They trigger activation or repression of gene function within the nucleus = genomic effect.

Question 16

When normal hormone concentraion is disrupted, how do homeostatic responses reduce the impact on end function?

Answer 16

• Prolonged exposure to low [hormone] plama often leads to up-regulation of receptor number (increases tissue sensitivity to hormone)
• Conversely, prolonged exposure to high [hormone] plama typically leads to down-regulation of receptor number (decreases tissue sensitivity to hormone)

Question 17

What are permissive effects of hormones?

Answer 17

The presence of one hormone enhances the effect of another. eg. epinephrine causes only modest lipolysis in adipose tissue, but when thyroid hormones are also present, greatly increased lipolysis occurs. TH incresases receptors for epinephrine on adiposites.

Question 18

What are antagonistic effects of hormones?

Answer 18

The presence of one hormone reduces the effect of another. eg. growth hormone impairs the response to insulin by decreasing the number of receptors for insulin on tissues.