Hormonal regulation of hormone release involves a hormone binding
to its receptor on an endocrine cell to regulate hormonal
secretion. A hormone that regulates hormone secretion is called a
tropic hormone. Tropic hormones may also stimulate
proliferation of endocrine cells.
For a nonpolar hormone, tropic hormones stimulate endocrine secretion by stimulating hormone synthesis in the endocrine cell. (Recall that endocrine secretion is whatever occurs to increase the amount of hormone in the circulation). An example is the hormone adrenocorticotropic hormone (ACTH) that stimulates secretion of the steroid hormone cortisol by cells in the zona fasciculata of the adrenal cortex.
The figure schematizes how this occurs. ACTH is secreted by cells in the adenohypophysis (anterior pituitary), and travels via the circulation to the adrenal cortex, where it binds to its receptor on cells in the zona fasciculata. The ACTH receptor is a G-protein coupled receptor (seven-transmembrane domain receptor). ACTH binding ultimately leads to stimulation of adenylyl cyclase and the production of the second messenger cyclic AMP (cAMP). cAMP causes cellular changes that ultimately lead to the activation of the enzymes involved in cortisol synthesis.
Other examples of polypeptide hormones that stimulate nonpolar
hormone release are the gonadotropins (FSH and LH) that
control gonadal steroid secretion, and thyroid stimulating
hormone (TSH) that controls thyroid hormone secretion. (We
will focus on the gonadotropin hormones in Conjoint 403, Spring
For polar hormones, tropic hormones stimulate endocrine secretion by stimulating cell secretion; that is, exocytosis. An example is the regulation of growth hormone (GH) secretion by the tropic hormone growth hormone releasing hormone (GHRH). GHRH is a hypophysiotropic hormone. Hypophysiotropic hormones are produced by endocrine cells in the hypothalamus, and released at a capillary bed called the median eminence. They are conveyed directly to the adenohypophysis (anterior pituitary) via the hypophyseal portal vessels. The sole function of the hypophysiotropic hormones is to regulate hormone release by the adenohypophysis.
As in the previous example, the receptor for the peptide hormone GHRH is a G-protein coupled receptor, and GHRH binding leads to an increase in cAMP. But in this case, the increase in cAMP leads to exocytosis of secretion vesicles containing the hormone GH.
In some cases, hormones can inhibit hormone secretion. One key example is the peptide hormone somatostatin, which is a hypothalamic hormone that inhibits GH secretion. Another important example is negative feedback regulation (see web page), in which hormones negatively regulate the secretion of their own tropic hormones.