Lactose synthesis in the mammary gland is dependent on the hormonally controlled synthesis of the two protein components of lactose synthetase, alpha-lactalbumin and a galactosyltransferase. Prolactin induces the synthesis of both proteins in mammary gland explants treated with insulin and hydrocortisone, but the induction kinetics cannot account for the asynchronous synthesis of the two proteins that are observed in vivo. Progesterone appears to take part in the control of lactose synthesis and acts to repress the formation of alpha-lactalbumin throughout pregnancy. At parturition, when the concentration of progesterone in the plasma decreases, the rate of alpha-lactalbumin synthesis increases.

A B S T R A C T Previous studiesThese results support the concept that the effect of L-tryptophan on the secretion of human prolactin is mediated through its conversion to serotonin and are consistent with reported experimental observations that serotonin may participate in the reciprocal regulation of prolactin and gonadotropins. INTRODUCTIONRecent studies on human prolactin have indicated the primary importance of hypothalamic mechanisms in the regulation of secretion of this hormone by the pituitary (1-3). A large body of experimental observations in animals and of pharmacological studies in the human has indicated the presence of catecholaminergic mechanisms in the hypothalamic regulation of prolactin secretion. In these studies we report an additional substance that can reciprocally regulate the secretion of prolactin and gonadotropins, the amino acid L-tryptophan. Effects observed with the intravenous infusion of this amino acid appear to relate to the presence of a serotonergic pathway mediating neuroendocrine regulation of prolactin secretion (4, 5) and provide an additional means of probing the human hypothalamic-pituitary axis.

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