LH and progesterone were assayed in plasma collected at 0800, 1600 and 2400 h daily from each of 6 Beagles during proestrus and estrus. The bitches’ sexual behavior in response to males was assessed daily. Preovulatory LH surges lasting 24-40 h peaked (4.4-11.1 ng/ml) at 2400 or 0800 h between three days before and one day after the onset of estrus. Estrous behavior in each bitch was observed only after an increase in progesterone. The initial increase in progesterone occurred either just prior to, or concomitant with, the initiation of the LH surge. Mean (± SEM) progesterone at -56, -40, -24, -16, -8, 0, 24 and 48 h from the LH peak was 0.7 ± 0.1, 0.9 ± 0.1, 1.2 ± 0.1, 1.4± 0.2, 1.5 ± 0.2, 2.6 ± 0.3, 3.2 ± 0.4 and 5.4 ± 0.6 ng/ ml. LH at the same times was 1.7 ± 0.3, 1.1 ± 0.2, 1.5 ± 0.3, 1.8 ± 0.3, 4.9 ± 1.0, 7.3 ± 1.0, 2.0 ± 0.3 and 1.4 ± 0.4 ng/ml. Ovulation time in the bitch was estimated by correlating results of gross and histological examination of ovaries removed from each of 10 Beagles during proestrus or estrus with LH and progesterone measured in plasma collected two to three times a day prior to obtaining the ovaries. Ovaries were removed from 6 of these bitches at approximately 4, 8, 20, 38, 44 and 50 h after the LH peak, when plasma progesterone was 1.3, 2.8, 4.2, 3.3, 4.4 and 6.9 ng/ml, respectively. The respective ovulated:nonovulated mature follicle ratios were 0:10, 0:6, 0:7, 0:8, 4:3 and 11:0. The sexual behavior of 6 ovariectomized bitches was observed daily for 5 weeks during which 16 days of estrogen treatment was initiated either 8 days before or 8 days after initiation of a 16 day progesterone treatment. Initial progesterone (4.8 ± 0.4 ng/ml plasma) had no effect. Induction of full sexual receptivity occurred 8 days after superimposing estrogen (65 ± 5 pg/ml plasma) on progesterone. Initial estrogen caused bitches to be attractive to males but failed to induce any acceptance of the males. Full sexual receptivity was induced within 18 h of superimposing progesterone on estrogen, was not affected by removal of estrogen 8 days later but was lost entirely within 18 h of progesterone removal after another 8 days.
The results suggest that, in the bitch: 1) ovulation occurs between 1 day before and 5 days after the onset of estrus and approximately 38-44 h following the LH peak; 2) follicle luteinization and increased progesterone secretion begins just prior to or concomitant with initiation of the LH surge some 60-70 h prior to ovulation and 3) the initial preovulatory rise in progesterone is synergistic with the preceding late-proestrous estrogen peak in eliciting estrous behavior.
