Regulation of GH and Prolactin Secretion
The hypothalamus controls the production of both hormones using specific neuropeptides. For growth hormone (GH), the main stimulus is growth hormone-releasing hormone (GHRH, somatoliberin), while somatostatin inhibits this process. For prolactin, a similar system operates: prolactin-releasing factors stimulate synthesis, while prolactin-inhibiting factor (dopamine) serves as the primary inhibitor. Notably, dopamine can also suppress GH secretion.
Somatotropin production follows a strict circadian rhythm. Basal blood concentrations are maintained by pulsatile bursts, with frequency and amplitude increasing at night. For prolactin, synthesis intensity depends directly on physiological status: it rises sharply during pregnancy and breastfeeding, and is stimulated reflexively by suckling.
Effects of Growth Hormone on Growth and Metabolism
About half of the circulating GH is bound to specific carrier proteins, forming a hormonal reserve. Unlike many other peptide hormones, it exhibits pronounced species specificity.
The hormone's primary role is a potent anabolic effect. It stimulates amino acid transport into cells and activates protein synthesis. However, linear bone growth (most active during youth) is not mediated directly by GH, but rather through somatomedins—insulin-like growth factors. Stimulated by GH, the liver produces IGF-I, with which the hormone forms a functional complex.
The metabolic effects of GH on carbohydrate and lipid metabolism occur in two phases:
- Insulin-like phase (short-term): The hormone helps muscle and adipose tissue take up glucose and blocks lipolysis. It may also directly stimulate $\beta$-cells of the pancreatic islets to secrete insulin.
- Anti-insulin phase (long-term): Develops after several minutes. Tissues stop utilizing glucose, and lipolysis is activated with the release of fatty acids. Concurrently, GH stimulates glucagon production by $\alpha$-cells and promotes insulin degradation by the hepatic enzyme insulinase. The net result is a sustained hyperglycemic effect.
Functions of Prolactin in the Body
Prolactin regulates not only milk production but also numerous other physiological mechanisms. Its effects depend on the presence of estrogens, glucocorticoids, and thyroid hormones in the blood.
Key directions of action:
- Reproductive system: Participates in puberty, maintains a normal ovarian-menstrual cycle, stimulates corpus luteum activity and progesterone production. Together with GH and estrogens, it induces mammary duct proliferation.
- Metabolism: Activates protein synthesis and stimulates the conversion of carbohydrates into fats (partly explaining the tendency toward postpartum weight gain).
- Water-electrolyte balance: Works synergistically with vasopressin and aldosterone to increase the reabsorption of water, sodium, and chloride ions in the renal tubules.