Glucoregulation of prolactin secretion

ABSTRACT

In order to assess the influence of glucose on prolactin secretion and differences in prolactin responses between diabetic and normal subjects, we studied a total of 18 normal and 13 stable insulin-dependent diabetic subjects. All were males and within 10% of ideal body weight. Seven of the normal subjects underwent standard insulin-induced hypoglycemia, with a significant [P < 0.01] prolactin response to this stimulus. Six each of the normal and diabetic groups underwent five hour oral glucose tolerance tests. No change in prolactin levels was noted in the diabetic group, but the normal group had a suppression in prolactin levels significantly below baseline [P < 0.001] and significantly different from the diabetic group [P < 0.01]. Seven of the normal and eight of the diabetic group received L-Dopa, 500 mg orally, with a resultant significant suppression of prolactin in each group [P< 0.01], while no difference was noted between the groups. These results suggest that hypoglycemia stimulates prolactin release and hyperglycemia inhibits prolactin release in normal subjects, while the latter response is absent in the diabetic. L-Dopa-induced suppression obtained in both the normal and diabetic group, suggests that this presumed hypothalamic derived prolactin suppressing mechanism is intact in the diabetic. Possible mechanisms for the absent glucose suppression of prolactin in diabetics are discussed. The effects of gestation and suckling on prolactin secretion, as well as the lactogenic properties of prolactin, are well established in the human[1,2]. In addition, an osmoregulatory function of prolactin exists in lower animals[3], and recent reports have suggested that changes in plasma osmolarity modulate prolactin secretion in man[4] and prolactin has been shown to alter water and electrolyte metabolism in man[5]. A glucoregulatory function for prolactin has not been explored, and the effect of glucose on prolactin secretion is not clear. Furthermore, the dynamics of prolactin secretion in diabetics has not been investigated. It is the purpose of this communication to report a series of studies designed to assess the effect of glucose on prolactin secretion and to compare prolactin responses in normal and diabetic subjects: