The synthesis and subsequent secretion of GH is regulated directly by two peptide hormones :
GHRH and somatostatin are synthesised and secreted by neurones in the hypothalamus and reach the somatotrophs of the anterior pituitary gland through a direct local capillaries known as hypophyseal portal vesselsGrowth Hormone-Releasing Hormone (GHRH).
GHRH is a 43 amino acid peptide hormone secreted by neurones in the arcuate nucleus of the hypothalamus. GHRH binds to a membrane-associated receptor on somatotrophs and stimulates GH release from the anterior pituitary.
Somatostatin
There are two forms of somatostatin in the human body but it is the 14 amino acid peptide (SS-14) that is involved in the control of GH secretion. SS-14 is synthesised and released from neurones in the periventricular region of the hypothalamus and reaches the anterior pituitary through the hypophyseal portal vessels. Like GHRH, SS-14 binds to a membrane-associated receptor on somatotrophs but in this case produces inhibition of GH release.
From this brief summary it should be clear that the level of GH secretion is controlled by a balance between the excitatory effects of GHRH and the inhibitory effects of SS-14. However, like many endocrine control systems, the secretion of GH is further regulated by a series of other influences:
GH itself has an inhibitory effect on somatotrophs and on the arcuate nucleus neurones of the hypothalamus. The net effect of this is a direct inhibition of GH secretion and an indirect inhibition through the reduction in the stimulatory effects of GHRH.
The IGF-I that is released from the liver in response to GH also has an inhibitory effect on GH secretion in three different ways:
1. A direct inhibitory effect on GH secretion by the somatotrophs of the pituitary gland.
2. An indirect inhibition of GH secretion by inhibition of GHRH release from the hypothalamus.
3. An indirect inhibition of GH by stimulation of SS-14 secretion by the hypothalamus.
Another hormone that is involved in the regulation of GH secretion is the 28-amino acid peptide Ghrelin. Ghrelin is synthesised by the epithelial cells lining the stomach. Ghrelin is thought to be partly responsible for the sensation of hunger but is also a potent stimulator of GH secretion.
The major excitatory (green) and inhibitory (red) influences that control growth hormone secretion are summarised in the figure opposite.