Growth hormone-releasing factor effects on pituitary function, growth, and lactation

Growth and lactation are complex processes controlled by several metabolic hormones such as insulin, glucagon, and thyroid hormones but most notably growth hormone. Growth hormone secretion is regulated by two hypothalamic hormones, somatostatin, an inhibitory regulatory factor, and growth hormone-releasing factor. The quantity and pattern of growth hormone secretion is ultimately regulated in concert by the secretion of both regulatory factors from the hypothalamus through the hypothalamo-hypophyseal portal system, where they exert their actions at unique pituitary receptors. Because the potential use of exogenous growth hormone administration for the stimulation of growth efficiency and lactation has been demonstrated, recent efforts have been directed toward the enhancement of production through manipulation of endogenous growth hormone secretion via its releasing factor. Thus far, releasing factor-stimulated growth and lactation has not been achieved to the same extent as that of exogenous growth hormone. Growth hormone-releasing factor has stimulated growth in two growth hormone-deficient children, as well as female, but not male, rats. Although all food-producing species tested to date respond to growth hormone-releasing factor with the appropriate growth hormone response, continuous or pulsatile administration of releasing factor has not resulted in increased growth rate in sheep, chicks, or hogs. Despite levels of circulating growth hormone that would be expected to produce a 30% increase in milk production if given exogenously to dairy cows, releasing factor-stimulated growth hormone secretion has resulted in only a 3 to 9% increase. It is clear from these studies that further developments are necessary to demonstrate the practical application of growth hormone-releasing factor.