Relationships between the Luteinizing Hormone Response to Gonadotropin Releasing Hormone and Endogenous Steroids

Twenty-four yearling heifers, exhibiting regular estrous cycles, were assigned at random to groups of four and received 5,40 or 320 µg of synthetic gonadotropin releasing hormone (GnRH) via jugular cannula on day 14 of the estrous cycle or on the day following corpus luteum regression which occurred on the average on day 20 of the cycle. Serum estradiol (pg/ml), estrone (pg/ml) and progesterone (ng/ml) prior to GnRH treatment averaged 5.1, 6.9 and 3.9 on day 15 and 10.8,7.5 and 0.9 on the day following luteal regression and differences between days were significant (P< 0.01) for estradiol and progesterone. Peak serum LH (ng/ml) after 5,40 and 320 µg GnRH was 1.8, 7.7 and 13.9 on day 15 and 1.0, 13.6 and 8.3 on day 20 but differences between days of the cycle were not significant. However analysis revealed a significant (P< .01) relationship between serum estradiol and estrone concentration at the time GnRH was administered, and the magnitude of LH release in response to GnRH. Serum estradiol measured at 4 hr. post-GnRH was increased on day 15 but decreased on day 20 of the cycle (P< .02), while serum estrone and progesterone remained unchanged (P< .05). It appears that the LH response to GnRH is related to serum estrogen concentration.