Temporal effects of estradiol (E) on luteinizing hormone-releasing hormone(LHRH) and LH release in castrated male sheep

We tested the hypothesis that rapidly expressed inhibitory effects of estra diol (E) on luteinizing hormone (LH) release in the male are attributable, in part, to suppression of luteinizing hormone-releasing hormone (LHRH) rel ease. Hypophyseal-portal cannulated, castrated male sheep were infused with E (15 ng/kg/hr) or vehicle. Portal and jugular blood samples were collecte d at 10-min intervals for 4 hr before, and for either 12 hr (E, n = 4; vehi cle, n = 4) or 24 hr (E, n = 8; vehicle, n = 3) after the start of infusion . In animals sampled for 16 hr, temporal changes in both LHRH and LH were a ssessed. In animals sampled for 28 hr, only LH data were analyzed. Before e ither the 12-hr or 24-hr infusion, LHRH and/or LH mean concentrations, pois e amplitude and interpulse interval (IPI) did not differ between E- and veh icle-infused animals. In animals sampled for 16 hr, no effects of time or s teroid X time interactions were detected for mean LHRH and LHRH pulse ampli tude; however, both were greater (P< 0.01) in vehicle-infused than in E-inf used males. LHRH IPI was unaffected by infusion. In contrast, both mean LH and LH pulse amplitude declined (P < 0.01) within 4-8 hr after the start of E infusion, whereas mean LH IPI was unaffected. In animals sampled for 28 hr, an effect of time (P < 0.01) and a steroid x time interaction CP < 0.01 ) was detected for mean LH, and there was an effect of time (P < 0.01) on L H pulse amplitude. Mean LH IPI was not affected. Our results show that in m ale sheep E rapidly reduces LH release in the absence of a detectable chang e in LHRH release. (C) Elsevier Science Inc. 1998.