LHRH antagonist decreases LH and progesterone secretion but does not alterlength of estrous cycle in heifers

The objective of this study was to evaluate the suppressive effect of an LH RH antagonist, Cetrorelix SB-75 (SB-75), on secretion of LH, FSH and ovaria n function in beef heifers. In Exp. 1, heifers were treated with a single d ose of 10 mug/kg body weight intramuscularly on d 3 of the estrous cycle. I n Exp. 2, heifers received either a single injection (100 mug/kg) of SB-75 on d 3 of the estrous cycle or multiple injections of 20 mug/kg on d 3, 4, 5, 6, and 7. Serum LH, but not FSH, was suppressed from one to several days . However, neither FSH nor progesterone was significantly altered. In Exp. 3, heifers received an injection vehicle (5% mannitol) or 100 mug/kg BW of SB-75 on d 1 of the estrous cycle (30 h after first observed standing estru s). Injection of SB-75 suppressed LH pulse frequency on d 3, 5, and 7 (P < 0.001). The mean LH concentrations in the SB-75 treatment groups were lower on d 3 (P < 0.01) and 5 (P < 0.05). There were no differences (P > 0.1) be tween the two groups in the mean concentrations of LH on d 1, 7, or 14. Tre atment did not affect the secretion pattern or concentration of FSH. Inject ion of SB-75 did not alter estradiol-17 beta concentrations (P > 0.1). Trea tment reduced corpus luteum (CL) function as indicated by lower progesteron e production. However, the length of the estrous cycle was not shortened. T hese data show that the CL can form and survive in the face of depressed LH concentrations during the early stages of the estrous cycle.