Serum Levels of Prolactin and Luteinizing Hormone (LH) in the Ewe at Various Stages of the Estrous Cycle 1

Abstract

Summary

Estrus activity was checked daily in three whitefaced, cross-bred, yearling ewes and jugular blood samples were collected daily during five estrous cycles. Serum prolactin and LH levels were determined by double radioimmunoassays. Prolactin levels were significantly higher during proestrus, (49 mμg/ml) and Day 1 estrus (40 mμg/ml) than during Day 2 estrus (11 mμg/ml), metestrus (15 mμg/ml) and diestrus (14 mμg/ml). LH levels were elevated on Day 1 estrus in only two of the five estrous cycles observed. The mean LH level for all five of the Day 1 estrus observations was 56 mμg/ml. This level was significantly higher than that during Day 2 estrus (1.9 mμg/ml), metestrus (1.6 mμg/ml), diestrus (1.1 mμg/ml) and proestrus (1.4 mμg/ml). These data suggest that the duration of the rise in prolactin levels is longer and precedes the elevated LH value at estrus. Serum prolactin levels did not seem to be correlated with the rise in LH, even though the increase in LH occurs at a time when serum prolactin levels are still high.