THE DYNAMIC OF PLASMA LUTEINIZING-HORMONE SURGE AND ITS RELATIONSHIPSTO THE TIME OF OVULATION IN GILTS FOLLOWING EXOGENOUS GNRH

The LH response after 75 mu g GnRH (Gonavet ''Berlin-Chemie''(R)) and the time of ovulation were analyzed in 16 gilts following estrous sync hronization with Regumate(R) and 800 IU PMSG. Blood samples were colle cted for radioimmunological determination of LH via intravenous cathet ers 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16 and 22 hours after GnRH. Repea ted laparoscopic observations of the ovaries started 24 h after GnRH. A difference in LH response after GnRH was found. According to the dyn amic of LH surge gilts were classified into three groups: LH peak 1-3 h (group I, n=6), LH peak 4-6 h (group II, n=6) and LH peak 7-10 h aft er GnRH (group III, n=4). The onset of LH peak sifnificantly differed between groups. Gilts of group I had a high responsiveness of LH to ex ogenous GnRH compared to slower and later increase of LH in gilts of g roup II. The late LH peak of group III seems not to be related to GnRH injection but to endogenous LH surge. Related to the LH dynamic there were differences in onset and completion of ovulation between groups. Gilts showing an early LH output (groups I and II) had a significantl y earlier onset and completion of ovulation compared to gilts exhibiti ng later LH peak after GnRH (group III). But the interval between LH p eak and onset of ovulation was delayed in gilts of group I. A differen t sensitivity of the hypophyseal-ovarian axis is postulated. The durat ion of ovulation was not influenced by different LH dynamic. Irrespect ive of the individual LH release and the variations in time of ovulati on the application of exogenous GnRH stimulates ovulation in gilts fol lowing estrous synchronization in a precise time interval (36.4+/-3.3 - 39.0+/-2.8 h after GnRH). Such a relatively constant interval is a p rerequisite for a successful fix-time insemination.