The Ovarian Cycle of the Bitch: Plasma Estrogen, LH and Progesterone

Radioimmunoassays of estrogen and LH, and radioligand assays of progesterone were conducted on plasma samples collected serially and frequently during pregnant and nonpregnant cycles of Beagle bitches. Measurements of LH daily throughout proestrus and estrus in 20 cycles yielded mean (± SEM) basal levels of 1.4 ± 0.1 ng/ml, preovulatory elevations for 1–3 days, and peaks (7.5 ± 0.8 ng/ml) on the first day of estrus, or on the previous day. The day of the LH peak was designated as Day 0 for each cycle. Preovulatory increases in plasma progesterone could not be temporally dissociated from elevations in LH. Mean levels on Days −1, 0 and +1 were 0.8 ± 0.1, 1.6 ± 0.2 and 2.6 ± 0.2 ng/ml, both increases being significant (P<.001). Progesterone increased rapidly throughout estrus, reached 19.1 ± 2.5 ng/ml on Day 10 and a maximum of 22.9 ± 2.7 ng/ml on Day 25, and remained elevated until Day 30 at 19.9 ± 2.7 ng/ml. Mean plasma estrogen rose during proestrus; levels for Days 10, 3 and 1 before the LH peak were 26 ± 4, 43 ± 4, and 62 ± 4 pg/ml, respectively. Peak levels ranged from 46 to 113 ng/ml and occurred 0, 1, or 2 days prior to the LH peak. Estrogen levels fell rapidly coincident with or immediately following the LH peak. No differences were found during the luteal phases of 12 pregnant and 10 nonpregnant cycles in mean maximum progesterone levels (29.1 ± 3.8 vs 26.8 ± 2.8 ng/ml), the ranges of individual maximum levels, nor the time of their occurrence. Levels declined gradually after Day 30 in both groups. However, the time at which plasma progesterone fell below 1 ng/ml was more variable (P<.00l) in nonpregnant (51–82 days) than in pregnant (61–65 days) cycles. Plasma progesterone invariably declined during the two days prepartum; mean levels for Days 2, 1 and 0 prior to parturition were 3.3 ± 0.4, 2.1 ± 0.4 and 1.0 ± 0.1, each change being significant (P<.05). Secondary increases in plasma progesterone were found between Days 20 and 40 in 9 of 12 pregnant cycles. No comparable increases were seen in nonpregnant cycles. Plasma estrogen levels during the luteal phase remained constant (9–15 pg/ml) in nonpregnant cycles (n=5), but were elevated (P<.01) in pregnant cycles (n=8) during the three weeks prepartum and fell at parturition. Mean estrogen levels on Days 28, 4 and 0 prepartum were 27 ± 3, 19 ± 2 and 11 ± 2 pg/ml, respectively. The data suggest that in the bitch: (1) late proestrus increases in plasma estrogen initiate and/or potentiate the preovulatory release of LH; (2) early preovulatory increases in plasma progesterone may facilitate the release of peak levels of LH and the onset of behavioral estrus; (3) pregnancy-specific mechanisms exist ensuring both a maintenance of elevated plasma progesterone levels throughout gestation and their decline to less than 1 ng/ml at parturition.