Constancy of Interval Between Luteinizing Hormone Release and Ovulation in the Ewe

To investigate the time relationships between the preovulatory luteinizing hormone (LH) peak and ovulation in the ewe a "short" radioimmunoassay for measuring plasma LH concentration was developed. The technique is described. This short assay permitted the determination of LH concentration within 6 hours of sampling. The interval from LH peak to ovulation was then defined by a single laparotomy on each ewe at a known interval after the start of the LH surge. Blood samples were taken immediately after onset of estrus and then at hourly intervals until the LH peak had been identified by 2 consecutive samples. An indwelling silicate cannula had been placed in the jugular vein. Vasectomized rams were used in determining estrus. Using this method the time interval was investigated in 47 ewes experiencing normal estrous cycles in 11 young and 16 old ewes experiencing their second estrus after withdrawal of Cronolone-impregnated sponges and in 19 ewes experiencing their first estrus after withdrawl of such sponges. The results indicated that normally cycling ewes ovulated between 21-26 hours after the LH peak while the young and the old ewes and the Cronolone-treated ewes ovulated 22-26 hours after. At 23 hours 16.6% of ewes from all four groups had ovulated whereas by 24 hours 66.6% of ewes had ovulated indicating that a high proportion of ewes ovulated between 23-24 hours after the start of the LH peak. No correlation was noted between the onset of LH release or the first signs of estrus and the time of day. It is concluded that the constancy of the interval between LH release and ovulation in these experiments confirms the importance of the role of LH in the ovulatory physiology. The present procedure is too cumbersome and too costly for commercial use.