Protein and peptide hormones are commonly synthesised in a precursor form which is processed before incorporation into secretory granules within secretory vesicles.
Following stimulation by circulatory or neural signals, the secretory cell releases the stored hormones by exocytosis of the secretory vesicles.
There are often two pools of hormone:
Peptide hormones circulate unbound in the plasma at low concentrations. The relatively large size of protein and peptide hormones prevents rapid renal excretion.
Peptide hormones bind receptors on the surface of target cells and modulate the levels of secondary messengers within those cells.
Examples of secondary messengers include:
Secondary messengers trigger further events which may, for example, modulate protein expression or alter membrane ion transport.
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