Decreased LH pulsatility during initiation of gonadotropin superovulation treatment in the cow: Evidence for negative feedback other than estradiol and progesterone

LH pulse secretion is suppressed during superovulation of cattle. The objec tive of this study was to determine how soon after initiation of superovula tion treatments this suppressive effect occurs, and to test the hypothesis that decreased LH pulsatility is not related to changes in circulating estr adiol or progesterone. Heifers (n = 7/group) were injected with eCG (FOLLIG ON(R): a single injection of 2500 IU) or twice daily injections of decreasi ng doses of FOLLTROPIN(R)-V (total equivalent of 280 mg of NIH-FSH-P1) or F .S.H.-P(R) (total equivalent of 28 mg of Armour standard) or saline (time c ontrols), starting on Day 10 (Day 0 = estrus). Blood samples were taken eve ry 10 min for 12 h intervals on the day prior to first injection, at 8 to 2 0 h and 32 to 44 h after initiation of gonadotropin treatment, and also dur ing prostaglandin (PG)-induced luteolysis. A simple method based on robust statistics and on graphical representations of time series was developed to characterize LH pulses. There was a significant interaction between time a nd treatment for mean LH, estradiol and progesterone when control and treat ed groups were analyzed together, and no interaction when only the gonadotr opin groups were analyzed together. When compared to pretreatment values, p ulse frequency of LH was significantly reduced (P<0.05) in each treatment g roup, 8 to 20 h and 32 to 44 h following initiation of gonadotropin treatme nt. Mean LH concentrations were also reduced 32 to 44 h following initiatio n of treatments (P<0.05). Mean estradiol concentrations increased two to th reefold at 8 to 20 h following initiation of superovulation treatments (P<0 .05). Progesterone concentrations also increased by 20 or 44 h. There was n o significant correlation between estradiol or progesterone and LH pulse fr equency, amplitude and mean concentrations at any time in control or supero vulated animals. This study demonstrates that superovulation treatment in t he cow causes a rapid decrease in pulsatile release of LH and suggests that this effect is not mediated through the negative feedback actions of estra diol and progesterone. (C) 2000 by Elsevier Science Inc.