Effects of negative energy balance on endogenous and on LHRH-induced LH release in ovariectomized heifers

Abstract

Three experiments were conducted to investigate the effects of negative energy balance (NEB) on reproductive activity in cattle. In experiment I the interrelationships of body weight (BW), retained energy (RE), plasma levels of insulin (IN), growth hormone (GH), glucose (GLC), free fatty acids (FFA), luteinizing hormone (LH) and LHRH-induced LH release response curve area (LHAR) were determined in 8 ovariectomized heifers. Fisher's paired t test revealed differences in GH (P < .01), IN (P < .10), FFA (P < .01) and LH (P < .05) mean levels between groups losing (L) and maintaining (M) BW. Both GLC and LHAR did not differ between groups L and M. Differences in mean LH levels were related primarily to a reduction (P < .05) in the height of LH peaks of heifers under NEB. Multiple regression analyses (MRA) demonstrated that LH levels were associated with differences in plasma levels of IN and GH. Moreover, MRA showed that LH levels were also associated with differences in FFA levels. Conversely, MRA revealed that LHAR was not associated with differences in plasma levels of IN, GH or FFA. In experiment II, heifers under NEB during experiment I were infused with IN in order to mimic the plasma IN pattern found in animals under positive energy balance (PEB) during experiment I. Similarly, heifers in PEB received GH infusions in order to simulate the plasma GH pattern observed in heifers in NEB during experiment I. The only alteration in the LH pattern caused by those treatments was an augmented frequency of LH peaks in heifers receiving IN infusions (P < .10). In addition, MRA did not provide adequate models relating LH release to differences in levels of GH and IN or FFA as observed in experiment I. This adverse result probably was caused by the fact that neither GH nor IN infusion properly mimicked endogenous patterns observed during experiment I. Conversely, it appears that infusion of GH in heifers undergoing a PEB reeducated the LHRH-induced LH release when compared to heifers in NEB receiving IN infusions, and MRA also appears to indicate that FFA levels are related to LHRH-induced LH release. In experiment III, all animals were sacrificed for in vitro comparisons of LH and LHRH release and total content in anterior pituitary (AP) and median eminence (ME) of heifers in NEB and PEB conditions. Results demonstrate that ME from heifers in NEB had higher LHRH content (P < .05) and concentration (P < .01), however, no differences were found regarding K+-induced LHRH release between heifers experiencing NEB and PEB...