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

Abstract

Plasma levels of luteinizing hormone (LH) progesterone and estrogen were determined in pregnant and nonpregnant cycles of female beagle dogs. Mean basal LH levels during estrus and proestrus were 1.4 + or -.1 ng/ml and peaked at 7.5 + or -.8 ng/ml on the 1st day of estru s. Preovulatory increases in progesterone and LH appeared to be temporally correlated and were significantly elevated by the 2nd day of estrus (p less than .001). Progesterone levels reached their maximum on the 25th day of estrus (22.9 + or -2.7). Estrogen levles increased from 26 + or -4 on Day 10 prior to the LH peak to 62 + or -4 pg/ml 1 day before the LH peak. Once the LH peak was established estrogen levels fell rapidly. In the luteal phases of 12 pregnant and 10 nonpregnant cycles there were no marked differences in mean maximum progesterone levels the ranges of individual maximum levels or the time of their occurrence. After Day 30 progesterone levels declined gradually and were significantly (p less than .001) variable in nonpregnant cycles compared with pregnant cycles once progesterone levels dropped below 1 ng/ml. Progesterone levels showed significant (p less than .05) daily changes during the 2 days prior to parturition. Plasma Estrogen levels were constant in nonpregnant cycles but significantly (p less than .01) increased during the 3 weeks prior to parturition. The results suggest that plasma estrogen initiates or potentiates the preovulatory release of LH that the early preovulatory release of progesterone may encourage the LH peak surge and the onset of behavioral estrus and that mechanisms exist that maintain elevated levels of plasma progesterone throughout pregnancy.