Hormones of the adenohypophysis: Growth hormone and prolactin

Regulation of GH and prolactin secretion

The hypothalamus controls the production of both hormones using specific neuropeptides. For growth hormone, the primary stimulus is growth hormone-releasing hormone (somatoliberin), while somatostatin inhibits this process. A similar system operates for prolactin: prolactin-releasing hormone triggers synthesis, and prolactin-inhibiting hormone (known as dopamine) acts as the main inhibitor. Notably, dopamine can also suppress GH secretion.

The production of somatotropin follows a strict circadian rhythm. Baseline blood concentrations are maintained by pulsatile releases, the frequency and amplitude of which increase at night. For prolactin, the intensity of synthesis depends directly on physiological status: it increases sharply during pregnancy and breastfeeding, and is also stimulated reflexively (by suckling).

How somatotropin affects growth and metabolism

About half of the circulating GH in the blood is bound to specific proteins, forming a hormonal reserve. Unlike many other peptides, it exhibits marked species specificity.

The hormone's primary task is a potent anabolic effect. It stimulates the transport of amino acids into cells and activates protein synthesis. However, linear growth of bones (most active in youth) is achieved not directly, but via somatomedins—insulin-like growth factors. Under the influence of GH, the liver produces IGF-I, with which the hormone forms a working complex.

The effect of GH on carbohydrate and lipid metabolism occurs in two stages:

1. Insulin-like phase (short-term): the hormone helps muscle and adipose tissue take up glucose and blocks fat breakdown. It may also directly stimulate $\beta$-cells of the islets of Langerhans to produce insulin.
2. Counter-insular phase (long-term): develops after a few minutes. Tissues stop utilizing glucose, and lipolysis is activated with the release of fatty acids. Concurrently, GH stimulates glucagon production by $\alpha$-cells and triggers insulin destruction by the enzyme insulinase in the liver. The net result is a persistent hyperglycemic effect.

Functions of prolactin in the body

Prolactin regulates not only milk production but also influences numerous other physiological mechanisms. Its effects depend on the presence of estrogens, glucocorticoids, and thyroid hormones in the blood.

Key areas of action:

• Reproductive system: the hormone participates in puberty, maintains a normal ovarian-menstrual cycle, stimulates corpus luteum activity and progesterone production. Together with GH and estrogens, it causes proliferation of the mammary ducts.
• Metabolism: activates protein synthesis and stimulates the conversion of carbohydrates into fats (which partly explains the predisposition of women to postpartum obesity).
• Water-salt balance: works in synergy with vasopressin and aldosterone, increasing the reabsorption of water, sodium, and chloride ions in the renal tubules.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The effect of free fatty acid and amino acid concentrations on pulsatile GH secretion.
• The role of additional humoral stimulators of GH (vasopressin, endorphin, thyrotropin-releasing hormone).
• The specific actions of insulin-like growth factor II (IGF-II) during the embryonic period.
• Mechanisms of the direct impact of GH on pancreatic $\beta$-cells in the first phase.

Other topics in the section «Hormonal regulation of physiological functions»

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