Influence of Exogenous Estradiol-17β on Endometrial Progesterone and Estrogen Receptors During the Luteal Phase of the Ovine Estrous Cycle1

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Abstract
The effect of exogenous estradiol-17.beta. [E2] on cytoplasmic progesterone [P] and estrogen [E] receptors [R] in the ovine endometrium during the midluteal phase of the estrous cycle was studied. Mature ewes received a single i.m. injection of 500 .mu.g estradiol-17.beta. on day 11 and/or day 12 of the cycle and were necropsied on day 13. Total cytoplasmic PR and ER were determined by exchange assay using a synthetic [3H]-progestin and [3H]-E2. Jugular blood serum was quantified for P by radioimmunoassay, and in vitro incorporation of [3H]-thymidine into DNA was used as a corollary measure of the endometrial response to exogenous E2. The equilibrium (Kd) for the PR complex ranged from 1.30-1.87 nM and the maximum receptor concentration in endometrial cytosol of ewes was estimated to be 1130 f[femto]mol/mg DNA. Although a single injection of E2 provoked an increase (P < 0.01) in cytoplasmic ER by 48 h post-treatment, 2 sequential injections of hormone were necessary to stimulate increases (P < 0.01) in cytoplasmic PR and DNA synthesis. No change in cytoplasmic steroid receptors was detected by 24 h after a single injection of E2. Serum concentrations of P did not differ among control and treated ewes. Apparently exogenous E2 can provoke significant increases in endometrial cytoplasmic ER and PR in the presence of relatively high concentrations of endogenous P but not within 24 h after initial treatment.