LHRH-receptor-regulated Ca²⁺-ATPase activity in murine pituitary gland

Abstract

Addition of luteinizing hormone releasing hormone (LHRH) in vitro (10⁻⁵−5×10⁻⁹ M) to murine pituitary membranes resulted in a dose-related decrease in Ca²⁺-ATPase activity within 15 min. Inhibitory effects of LHRH (10⁻⁷ M) occurred after 90 sec, and appeared maximal by 120 sec. Eadie-Hofstee analysis at 10⁻⁷ M LHRH, at varying [Ca²⁺]_free, resulted in aK _m=0.89±0.06 μM and aV _max=18.8±0.71 nmol/mg per 2 min, compared to aK _m=0.69±0.06 μM and aV _max=32.8±1.21 nmol/mg per 2 min for controls. Pre-incubation for 5 min with LHRH antagonist (10⁻⁸ M) significantly attenuated (50%) the inhibitory effects of 10⁻⁷ M LHRH on pituitary Ca²⁺ ATPase activity with aK _m=0.97±0.24 μM and aV _max=28.1±2.8 nmol/mg per 2 min. The addition of LHRH (10⁻⁷ M) to pituitary homogenates significantly increased luteinizing hormone (LH) release already at 10 and up to 40 sec compared to basal LH release. Systemic administration of 50 ng LHRH (i.p.), significantly (P<0.05) reduced pituitary Ca²⁺-ATPase after 30, 60 and 90 min, with a return to control levels by 120 min. Pituitary LH content was reduced slightly at 15 min, but was increased significantly at 90 and 120 min post-treatment. Plasma LH levels were elevated by 5 min, reached a peak by 15 min and returned to control within 60 min. The present findings indicate that LHRH receptor activation may influence cytosolic Ca²⁺ transport through effects on membrane Ca²⁺-ATPase activity. These actions may regulate LHRH-induced synthesis, storage and release of LH from pituitary gonadotropes.