Endocrine and endometrial secretory protein changes associated with uterine receptivity in sheep

Crossbred ovariectomized ewes were treated with steroid therapies determined previously to be adequate (progesterone-primed) or inadequate (unprimed) for embryonic development in order to determine actual serum concentrations of replaced steroid hormones achieved by such treatments and to identify secreted endometrial proteins that might mediate uterine receptivity. Ewes received estradiol-17 beta on day 0, and on days 1-4, either vehicle (unprimed; N = 16) or progesterone (primed; N = 16) daily. All ewes then received "estrus estradiol" (at 8 hr-intervals), followed by "maintenance progesterone" (at 12 hr-intervals), to mimic endocrine profiles of intact ewes at and following estrus. Jugular blood samples were obtained at 4-hr intervals from 6 ewes/treatment on day 0-15 to determine serum progesterone, estradiol, and PGFM concentrations. Endometrium from two ewes/treatment on days 11-15 was cultured in vitro with [3H]leucine and radiolabeled proteins in media were analyzed electrophoretically. Results demonstrated that 1) treatments generated transient serum estradiol levels slightly greater than those reported in intact animals at estrus, 2) serum progesterone concentrations due to treatments were similar to those reported in the luteal phase of intact ewes, 3) progesterone-priming was specifically associated with a small, sustained (24-36 hr) elevation in serum PGFM, and that 4) priming was not associated with the presence or absence of major, secreted endometrial proteins that might act either as factors required for development or as embryotoxins. These results suggest that positive effects of progesterone-priming on embryo survival are not due to pharmacological doses of exogenously administered hormones, nor are due to changes in secretion of hormonally-regulated, major endometrial proteins.