EFFECT OF DIET ON THE CLEARANCE OF ESTRADIOL-17-BETA IN THE EWE

The twinning rate of ewes is poorly related to plasma concentrations o f gonadotropins. In this study, we tested an alternative hypothesis fo r the control of twinning rate, by testing whether nutritional treatme nt sufficient to affect twinning could alter estrogen metabolism, with a particular focus on the enterohepatic recirculation. Groups of 5 ov ariectomized ewes were fed either above maintenance (supplemented) or below maintenance (restricted). The metabolism of estradiol-17 beta wa s examined by following the fate of a single i.v. injection of 1.45 mu g [H-3]estradiol-17 beta. In both groups, 74% of the radioactivity wa s recovered from the feces within 10 days, predominantly as free estra diol-17 alpha, but excretion was slower in the nutritionally restricte d ewes. A further 6% of injected radioactivity was excreted in the uri ne, mostly within 24 h, with no effect of dietary group. Radioactivity in plasma was characterized by ion-exchange chromatography and HPLC. Within 30 min of injection, the main circulating radioactive compound was estradiol-17 alpha sulfate. This remained at a greater concentrati on than free steroid for the next 48 h, and was greater after 16 h (p < 0.05) in plasma of nutritionally restricted ewes than in the supplem ented group. At 0.5 and 2 h, the free steroid was almost entirely estr adiol-17 beta, but a polar compound, which appeared by 4 h and probabl y arose by recirculation from the intestine, remained the major unconj ugated metabolite in plasma for the next 24 h. Plasma concentrations o f this compound were higher (p < 0.05) in the restricted ewes than in the supplemented ewes during this period. The slower passage of digest a in the nutritionally restricted ewes was associated with a slower ex cretion of radioactivity in the feces and higher plasma concentrations of metabolites. Thus, a decreased rate of passage of digesta has the potential to increase ovarian feedback in the ewe.