Uterine estrogen and progesterone receptors in prepubertal ewes: distribution in myometrium, endometrium and caruncles.

In order to study the expression of uterine steroid receptors during animal development, estrogen and progesterone binding sites (E2R and P4R) were investigated in 9-month-old Corriedale ewe-lambs during the non-breeding season. E2R and P4R were identified by binding assays and Scatchard analysis in all uterine layers. The dissociation constant (Kd) values were: 0.63 +/- 0.14 nM (n = 24) for E2R and 2.10 +/- 0.32 nM (n = 24) for P4R. The E2R distribution was: 188 +/- 32, 367 +/- 83 (NS) and 494 +/- 72 fmol/mg protein (P < 0.02) for myometrium, caruncles and intercaruncular endometrium, respectively. Interestingly, high levels of P4R (fmol/mg protein) were found. By layer the P4R content was: caruncles, 1 123 +/- 198; intercaruncular endometrium, 1 283 +/- 187 (NS); and myometrium, 502 +/- 76 (P < 0.002). A positive correlation existed between both receptors (r = 0.772, P < 0.0001, n = 24). E2 and P4 circulating levels (measured by radioimmunoassay), were similar to the adult ewe basal levels. These findings suggest that ewes have sensitive and functional uterine estrogen and progesterone receptors even before cyclic ovarian activity.