HORMONE SYNTHESIS, TRANSPORT, BINDING, AND EFFECTS.

Three classes of hormones:

• Peptides and proteins (P&P)
• Steroids (S)
• Amines (A)
  – Catecholamines (C)
  – Thyroid hormones (T)

4 hormone physiology features:

Synthesis

• Hormones can be made in advance and stored prior to secretion.
  – The peptide and protein hormones, and amines follow this model.
• Alternatively, they can be synthesized and secreted on demand.
  – The steroids follow this model.

Transport

• This generally depends on chemical structure.
• Hormones can either:
  – Dissolve and travel freely in the blood – the peptides and proteins and also the catecholamines: they are water soluble (aka, hydrophilic)
  – Bind carrier proteins – the steroid and thyroid hormones are less water soluble and travel bound to carrier proteins.

Receptor binding

• The chemical relationship between hormones and target cell membranes determines whether hormones:
  – Bind surface membrane receptors – the peptide and protein hormones and the catecholamines, and in some cases, steroid hormones where they have non-genomic effects on the cell.
  – Bind intracellular receptors — because they are lipophilic (lipid soluble), steroid hormones and thyroid hormones readily slip past the cell membrane to bind with cytoplasmic and/or nuclear receptors.

Mechanism of Action

• Hormones can modify existing proteins within a cell – the peptide and protein hormones and the catecholamines.
• Or they can trigger protein synthesis – the peptide and protein hormones do this, as well, as do the steroid and thyroid hormones.

Comments on the Amines

• The peptides and proteins almost NEVER behave like the steroids.
• The amines divide into the catecholamines and thyroid hormones:
  – Catecholamines act most like the peptides and proteins.
  – The thyroid hormones act most like the steroids.

Peptide and protein hormones

Synthesis:

• Synthesized and then stored in secretory vesicles.
• First, within the nucleus of a cell, the gene for a hormone is transcribed as mRNA
• The mRNA moves to the ribosomes, where it is translated to create a preprohormone;
• The preprohormone moves to the endoplasmic reticulum, where it is converted to a prohormone;
• Finally, the prohormone is transported to the golgi apparatus to be packaged into secretory vesicles; within these vesicles, peptide cleavage produces the final hormone product.
• The hormone is stored until its release is triggered.

Transport:

• Travels freely in the blood; recall that this is because it is water soluble, and readily dissolves.

Mechanisms of Action:

• Peptide hormone binds cell membrane surface receptor and the hormone-receptor complex activates second messenger systems to initiate protein modification and synthesis.

Steroid hormones

Synthesis:

• Steroid hormones are synthesized in the endoplasmic reticulum and secreted on demand; they are not stored in the cell.
• Cholesterol is the parent of steroid hormones.
  • In the adrenal cortex, Mineralocorticoids, Glucocorticoids, or Androgens are produced.
  • In the testes and ovaries, aka, the gonads, testosterone and estrogen are produced.
  • DHEA (Dehydroepiandrosterone) and Progesterone are important intermediate steroids.

Transport:

• Steroid hormones travel in the blood bound to carrier proteins. Only a small portion of steroid hormones travel freely, or unbound.

Mechanisms:

• When steroid hormones bind intracellular receptors, they activate or repress transcription
• Testosterone can pass through the cell membrane to bind with these intracellular receptors.
• When steroid hormones bind surface membrane receptors, they initiate non-genomic effects via second messenger systems are activated.