PROLACTIN DOES NOT ENHANCE GLUCOSE-STIMULATED INSULIN RELEASE IN RED DEER STAGS

Red deer stags have a seasonal pattern of insulin secretion that is ch aracterized by both elevated basal and glucose-stimulated insulin rele ase in summer compared with winter. Since the seasonal timing of this pattern is similar to that of prolactin and growth rate, the objective s of this study were: first, to determine whether prolactin is associa ted with the enhanced secretion of insulin during the summer growth pe riod, and second, to determine whether a chronic reduction in plasma p rolactin levels would alter body composition. Prolactin was suppressed in plasma using a long-acting form of the dopamine agonist bromocript ine (parlodel LA), which was administered at one of four doses (0-0.3 mg/kg) to each of four groups of castrate stags. Bromocriptine was adm inistered during two 6-wk periods; the first in winter and the second in summer. During the sixth wk of each period, each animal was given t hree IVGTT at the following glucose doses (10 mg/kg, 70 mg/kg, and 200 mg/kg). Two d later, ovine prolactin was administered to each animal (0.08 mg/kg) and a single IVGTT (70 mg/kg) was given 2 hr later. Body composition was determined by the tritriated water dilution method at the beginning and end of each 6-wk treatment. Chronic suppression of p rolactin during winter or summer did not significantly alter the amoun t of insulin released after each IVGTT, nor did it significantly alter body composition. Furthermore, acute administration of prolactin did not significantly enhance the release of insulin following an IVGTT, d uring winter or summer treatment periods. It is concluded that elevate d levels of prolactin in summer do not enhance the release of insulin to glucose in red deer. Furthermore, a reduction in growth rate follow ing a reduction in plasma prolactin is not associated with a change in body composition. (C) Elsevier Science Inc. 1997.