SECRETION OF GONADOTROPINS CHANGE DURING THE LUTEAL-PHASE OF THE BOVINE ESTROUS-CYCLE IN THE ABSENCE OF CORRESPONDING CHANGES IN PROGESTERONE OR 17-BETA-ESTRADIOL

The hypothesis in the present study was that changes in circulating lu teinizing hormone (LH) and follicle stimulating hormone (FSH) would oc cur during the luteal phase of the oestrous cycle (Days 4-19; Day 0, d ay of behaviourial oestrus) that were not related to corresponding cha nges in concentrations of progesterone and 17 beta-oestradiol. The sta ge of the oestrous cycle of cows (n = 18) was synchronised to obtain c ows that were on alternate days of the cycle. Blood samples were colle cted every other day at 15 min intervals for 12 h from all cows: Days 4, 6, 8, 10, 12, 14, 16, 18 (n=9) and Days 5, 7, 9, 11, 13, 15, 17, 19 (n = 9). Concentrations of LH, FSH, 17 beta-oestradiol and progestero ne were determined in these samples. Data were compared across days to determine when significant changes occurred in concentrations or patt erns of secretion of the gonadotrophins and ovarian steroid hormones d uring the oestrous cycle. There were significant changes in mean conce ntrations of FSH in circulation between Days 6 and 12. The most striki ng changes in secretion of gonadotrophins that could not be explained by changes in gonadal steroids were the fluctuations in amplitude of L H pulses between Days 7 and 12. Amplitude of LH pulses increased betwe en Days 7 and 11 and subsequently decreased between Days 11 and 12 of the oestrous cycle. Some changes in gonadotrophin secretion that occur red in the present study can be explained by fluctuations in concentra tions of progesterone and 17 beta-oestradiol in circulation. Other cha nges cannot be explained by fluctuations in circulating concentrations of these steroids. We accept our hypothesis because the concomitant c hanges in mean concentration of FSH between Days 6 and 11 and amplitud e of LH pulses between Days 7 and 12 of the bovine oestrous cycle cann ot be explained by changes in circulating concentrations of progestero ne and 17 beta-oestradiol.