Changes in the responsiveness of perifused rat adenohypophysial cells to repeated stimulation with luteinizing hormone releasing hormone

Abstract

Abstract. The ability of luteinizing hormone releasing hormone (LRH) to stimulate the release of luteinizing hormone (LH) from columns of enzymatically dispersed perifused adenohypophysial cells is being used to study the mechanisms controlling the secretion of LH. LRH stimulated the release in vitro of LH from columns of rat pituitary cells. However, when exposed repeatedly (1 pulse every 12 min) to the same submaximal dose (8 nmol/l) of LRH the cells always exhibited a marked progressive increase and subsequent decrease in their responsiveness. Similar effects occurred when the interval between pulses was extended to 20, 30 or 45 min. The enhanced responsiveness of the cells was prevented by the inclusion of protein synthesis inhibitors, cycloheximide or puromycin, in the perifusion fluid. Cells removed from rats ovariectomized 14 days previously also failed to exhibit increased responsiveness when stimulated repeatedly with LRH. LH secretion was also elicited by K+ (50 nmol/l), 8-bromoadenosine 3′-5′-cyclic monophosphate (8-Br-cAMP, 6 nmol/l), 8-bromoguanosine 3′-5′-cyclic monophosphate (8-Br-cGMP, 6 nmol/l) and a calcium ionophore (A23187, 40 μmol/l) but the responses to these secretagogues differed markedly from those to LRH for the tachyphylaxis which resulted from repeated exposure was not preceded by an increase in responsiveness. The decreased responsiveness to K+ developed in parallel with that to LRH. Diminished responses to the cyclic nucleotides and the Ca++ ionophore developed more rapidly, but the refractory cells responded readily to stimulation with LRH or K+. The results suggest that the increased responsiveness of the perifused pituitary cells induced by LRH is associated with steroid-dependent protein synthesis and that the secretion of LH elicited by its releasing hormone does not involve cAMP or cGMP.