Adenohypophyseal Hormones: Physiological Effects and Control of Their Secretion.
Adenohypophyseal hormones are the hormones of the anterior lobe of the pituitary gland. The anterior pituitary secretes six peptide hormones: thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone,
prolactin, and adrenocorticotropic hormone (ACTH).
- Each hormone is secreted by a different cell type (except FSH and LH, which are secreted by the same cell type).
- The cell types are denoted by the suffix "troph," meaning nutritive.
- TSH is secreted by thyrotrophs
- FSH and LH are secreted by gonadotrophs
- ACTH is secreted by corticotrophs
- Growth hormone is secreted by somatotrophs
- Prolactin is secreted by lactotrophs
Each of the anterior pituitary hormones is a peptide or polypeptide. When the anterior pituitary is stimulated by a hypothalamic-releasing hormone or release-inhibiting hormone, there is exocytosis of the secretory granules, and the anterior pituitary hormone enters capillary blood and is delivered by the systemic circulation to the target tissue.
Control of the Secretion of Anterior Pituitary Hormones
The hypothalamus and anterior pituitary are linked directly by the hypothalamic-hypophysial portal blood vessels, which provide most of the blood supply of the anterior lobe. Hypothalamic-releasing hormones and release-inhibiting hormones are synthesized in the cell bodies of hypothalamic neurons and travel down the axons of these neurons to the median eminence of the hypothalamus. Upon stimulation of these neurons, the hormones are secreted into the surrounding hypothalamic tissue and enter the nearby capillary plexus, which drains into the hypophysial portal vessels and is delivered directly to the anterior lobe of the pituitary. There, the hypothalamic hormones act on the cells of the anterior lobe, where they stimulate or inhibit the release of the anterior pituitary hormones.
Growth Hormone
Growth hormone is the most important hormone for normal growth to adult stature. It has large effects on:
- protein, carbohydrate, and fat metabolism
- bone growth
Growth hormone is secreted in bursts, and the largest burst occurs during sleep.
Regulation of Growth Hormone Secretion
Growth hormone secretion is controlled by two pathways from the hypothalamus, one stimulatory (GHRH) and the other inhibitory (somatostatin):
- Growth hormone-releasing hormone (GHRH) is released by the hypothalamus and stimulates the somatotrophs in the anterior pituitary gland to both synthesize and secrete growth hormone.
- Somatostatin acts on the somatotrophs to inhibit growth hormone secretion by binding to membrane receptors and inhibiting the generation of cAMP.