Regulatory Role of Estradiol in Pituitary Responsiveness to Luteinizing Hormone-Releasing Hormone on Proestrus in the Rat

Abstract

Pituitary LH [luteinizing hormone] release in response to LHRH [luteinizing hormone-releasing hormone] was examined following acute ovariectomy (OVX) at 1300 h on proestrus in rats that had received Nembutal to block the LH surge. When LHRH (250 ng i.v.) was administered 1 h after OVX, the plasma LH peak (374 .+-. 34 ng/ml) was significantly greater than that observed in intact (155 .+-. 13 ng/ml), sham-OVX (168 .+-. 25 ng/ml) or intact progesterone-treated (156 .+-. 19 ng/ml) rats. When the pulse of LHRH was delayed until 2 h after OVX, the plasma LH peak was 676 .+-. 69 ng/ml. In controls not treated with LHRH, the 1st significant increase over baseline LH was not detected until 3 h after OVX. In intact proestrous rats, serum estradiol-17.beta. (E2) was 56.1 .+-. 4.95 pg/ml at 1300 h. One hour following OVX serum E2 had fallen to 10.2 .+-. 2.0 pg/ml; by 2 h the concentration was 6.6 .+-. 0.66 pg/ml which was not significantly different from that found in diestrus-1 rats. OVX of such diestrus-1 animals had no effect on pituitary LH response to LHRH. In order to duplicate the LHRH-induced pattern of LH release found in intact rats, 100 ng E2 was injected i.v. at 1300 h and again at 1400 h in OVX rats. The peak in plasma LH following LHRH at 1500 h did not differ from that obtained in intact rats. These studies indicate a reversible increase in the pituitary LH response to LHRH following a sudden decrease in circulating estradiol secondary to ovarian removal on proestrous afternoon.