The ovarian cycle of the bitch: plasma estrogen, LH and progesterone.

Changes in LH, progesterone and sexual behavior associated with preovulatory luteinization in the bitch.

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.

Effects of hypophysectomy and of LH administration on luteal phase plasma progesterone levels in the beagle bitch.

Luteal-phase progesterone levels in 5 beagle bitches fell to 0·3–0·4 ng/ml within 3–17 days after hypophysectomies on Day 10–50 of the cycle. Plasma progesterone in 6 bitches increased (P < 0·01) within 3 h after LH (100 µg/kg) injection between Days 35 and 55 of the cycle. Normal luteal function in the bitch appears to require continuous pituitary trophic support perhaps involving LH.

Relationship of serum estrone, estradiol-17b and progesterone to LH, sexual behavior and time of ovulation in the bitch.

Serum estrone, estradiol-17β and progesterone were measured in bitches in which LH, sexual behavior and laparoscopically confirmed time of ovulation had been previously studied. Steroid values from 25 estrous periods were normalized to day of peak LH (Day 0). Estrone, estradiol-17β and progesterone concentration did not differ significantly in either dogs subjected to laparoscopy at 48 h intervals or those bitches undergoing no laparoscopy.

During the preovulatory interval, mean peripheral estrone concentrations were more variable than estradiol-17β; however, both estrogens tended to rise gradually throughout this time period. Overall maximal estradiol-17β concentration was detected on Day -2 with estrone peaking later (Day -0.5). Both estrogens declined (P<0.05) coincidentally with the LH surge. Progesterone from Days -14.0 through -1.5 was maintained at basal concentrations. An increase (P<0.05) in mean progesterone occurred on the day of the preovulatory LH peak which correlated with direct observation of apparent preovulatory follicle luteinization. Progesterone increased gradually from Day -0.5 through 8 and then varied for the remainder of the sampling period.

Analysis of individual cycles indicated that circulating serum concentrations of reproductive steroids fluctuated from day to day or within days of the proestrous-estrous interval. In dogs subjected to laparoscopy, initial evidence of the presence of visible preovulatory follicles was closely related to the first detectable elevations in both estrone and estradiol-17β. Either estrone or estradiol-17β concentrations or both surged sharply in 24 of the 25 cycles examined; however, only titers of estradiol-17β were elevated above baseline in all cycles prior to, or at the time of, the LH peak. In 16 of 25 cycles, estradiol-17β concentrations declined by 20 pg/ml or more 12-24 h prior to the onset of sexual receptivity. Bitches remained in estrus during periods of declining estrone and estradiol-17β and increasing progesterone levels.

These data integrate the hormonal, ovarian and behavioral events of the estrous cycle of the bitch and suggest that: 1) a preovulatory estradiol-17β surge exists and is likely responsible for triggering LH release; 2) estrone may play a supportive role to estradiol-17β in the endocrine control of LH secretion; 3) preovulatory changes in follicular morphology are distinct and can be correlated with a shift from estrone and estradiol-17β to progesterone secretion; 4) prolonged sexual receptivity in this species is supported in the presence of declining estrogen and continuously rising progesterone concentrations.