Serum Hormone Concentrations and Their Relationships to Sexual Behavior at the First and Second Estrous Cycles of the Labrador Bitch

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Abstract
Serum was analyzed for luteinizing hormone (LH), estrone (E1), estradiol-17.beta. (E2) and progesterone (P) concentrations. All hormonal data were normalized to the day of preovulatory peak LH (day 0) for both cycles. Mean E1 concentration during the week preceding day 0 was greater (P < 0.05) in the 2nd cycle compared with that at the 1st cycle (42.0 .+-. 3.0 vs. 29.0 .+-. 2.0 pg/ml), but the mean peak E1 levels were not different (52.0 .+-. 16.0 vs. 34.0 + 6.3 pg/ml). Both the mean peak E2 concentration (54.0 .+-. 8.0 vs. 28.0 .+-. 7.0 pg/ml) and the mean level during the week before peak LH (47.0 .+-. 4.0 vs. 22.0 .+-. 4.0 pg/ml) were greater (P < 0.05) in the 2nd cycle compared with the 1st cycle. Mean serum LH concentration during the week before day 0 (4.8 .+-. 0.9 ng/ml) in the 2nd cycle was higher (P < 0.05) compared with that of the 1st cycle (2.5 .+-. 0.7 ng/ml). The peak preovulatory LH (day 0) for the 2nd cycle was also higher (P < 0.01) than that of the 1st cycle (51.6 .+-. 6.9 vs. 23.1 .+-. 4.2 ng/ml, respectively). The mean serum P concentration during the week before day 0 was greater (P < 0.05) in the 2nd cycle (1.6 .+-. 0.1 vs. 1.0 .+-. 0.3 ng/ml) but the mean P level during the 2nd wk following day 0 was higher (P < 0.05) in the 1st cycle (15.8 .+-. 2.0 vs. 9.4 .+-. 0.8 ng/ml). The durations of proestrus (7.7 .+-. 0.5 vs. 3.9 .+-. 0.7 days) and estrus (11.0 .+-. 1.1 vs. 5.4 .+-. 0.4 days) were longer (P < 0.01) in the 2nd estrous cycle. In general, serum hormone concentrations were elevated during the 2nd cycle compared with the 1st cycle and durations of proestrus and estrus were longer during the 2nd cycle. The shorter duration of sexual receptivity at the 1st behavioral estrous cycle in the bitch may have a hormonal basis.