ENDOCRINE AND OVARIAN RESPONSES TO EXOGENOUS ESTRADIOL-17-BETA IN 6-MONTH-OLD HEIFERS PREVIOUSLY IMMUNIZED AGAINST GROWTH HORMONE-RELEASINGFACTOR

We hypothesized that estradiol-17 beta (E(2)) was capable of stimulati ng an LH surge in 6-mo-old-heifers immunized against growth hormone-re leasing factor (GRFi). To test this hypothesis Angus X Simmental heife rs were immunized against GRF (n = 11) or human serum albumin (HSAi) ( n = 6) at 113 d of age. Heifers were injected i.m. with 500 mu g of E( 2) at 172 d of age. To characterize the LH response, samples were coll ected at frequent intervals for 1 h before and 32 h after E(2). To ass ess ovarian responses, real-time ultrasonography was performed from d 9 to 20, and serum progesterone was evaluated from d 6 to 25 after E(2 ), then weekly for 13 wk. Exogenous E(2) stimulated (P <.001) LH surge s, preceded by negative E(2) feedback that decreased serum LH, in all GRFi and HSAi heifers. During the negative feedback period, serum LH w as greater (P <.001), and the change in LH from before to after E(2) w as lower (P <.05), in GRFi than in HSAi heifers. Puberty occurred in 7 /17 heifers within 66 d after receiving E(2). We conclude that GRFi do es not impair the release of an E(2)-induced LH surge, but it does dec rease hypothalamo-hypophyseal sensitivity to the inhibitory effect of E(2).