LH Release in Ovariectomized Dogs in Response to Estrogen Withdrawal and its Facilitation by Progesterone

Abstract

The effects of various estrogen treatments on the profiles of circulating LH in ovariectomized Beagle bitches were determined in 3 separate studies. In the first study, 13 bitches received 1-4 Silastic tubing (4.7 mm o.d.) implants of estradiol-17 3 providing doses of 0.1-0.8 cm/kg during a 72 h period after which the accumulated treatment dose (0.1-1.2 cm/kg) was removed or reduced. In the second study, 16 bitches received an initial estradiol benzoate (EB) injection of 2 pg/kg and 16 h later received an injection of either EB (2 pg/kg) or oil vehicle. In the third study, bitches received EB injections every 6 h for 30 h (n = 4) or 66 h (n = 10) in increasing doses of 0.4-3.6 or 0.4-13.8 pg/kg, respectively, 4 days after receiving small implants of estradiol-17 l (0.15 cm/kg). Five bitches receiving EB for 66 h received progesterone implants (1.6 cm/kg) 6 h after the last EB injection. Each estrogen treatment depressed LH levels within 8-24 h. Preovulatory-like LH surges were consistently observed 26 ± 2 h following removal of estradiol implants (n = 12) and 36 ± 4 h following a single or final EB injection (n = 25). Distinct increases in LH were seldom observed during continued estrogen treatment and were infrequent and sporadic even in bitches in which estradiol levels were elevated above those observed prior to ovulation in intact bitches. Progesterone administered 6 h after 66 h of EB treatment caused higher, earlier and more synchronous LH peaks than did EB treatment alone. The results suggest that the preovulatory surge release of LH in the bitch may be a response to withdrawal of the negative feedback effect of rising estrogen levels and that this withdrawal effect is facilitated by the preovulatory increase in progesterone.