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.
