Peripheral plasma levels of l7-hydroxyprogesterone and progesterone in cyclic pluriparous cows during the preovulatory, estrous and ovulatory periods

Vita.Peripheral plasma levels of 17-hydroxyprogesterone (17-OHP), progesterone (P) and luteinizing hormone (LH) were determined by radioimmunoassay procedures in cyclic pluriparous cows from 36 hr before estrus until time of confirmed ovulation. Blood sample collection and estrous detection were performed every 6 hours. Palpation per rectum was conducted daily for evidence of ovarian activity and ovulation. Five cyclic non-lactating pluriparous cows constituted the final test group. Four cyclic, non-lactating pluriparous cows were initially sampled via jugular cannulation and housed in total confinement. These plasma samples were not analyzed because of abnormal estrous periods due to the supposed physiological stress of confinement and/or cannulation. The five cyclic cows constituting the final test group remained on pasture and were gathered for sampling. All cows experienced normal estrous periods of 9.6 to 15.6 hr but ovulation was delayed in four of the five cows with ovulation occurring over a range of 64.5 to 84 hours. Plasma levels of LH were quantitated for all samples by Dr. P. G. Harms. All five cows exhibited sharp LH peaks at or within 6 hr after the onset of estrus. Luteinizing hormones remained at baseline levels at all other times. At approximately 18 hr preceding the onset of estrus, a significant peak of 17-OHP occurred. The peak was of short duration with levels rising and falling back to baseline within 6 to 12 hours. In four of the five cows, P levels began dropping at or shortly before the 17-OHP peak. The rise in 17-OHP before onset of estrus preceded by approximately 18 to 24 hr the plasma LH surge. The 17-OHP peak occurred in all five cows before the onset of estrus. This is the first time that 17-OHP has been identified and quantitated in the blood of cyclic pluriparous cows