Restoration of estrogen and progesterone uterine receptors during the ovine postpartum period

Estrogen and progesterone uterine cytosolic receptors (E(2)R and P(4)R) were investigated in postpartum ewes that lambed during the breeding season. Reproductive organs of 10 multiparous Corriedale ewes were studied after ovarihysterectomies at Day 1 (n=2), 5 (n=4), 17 (n=2) or 30 (n=2) post partum. E(2)P and P(4)R were determined by binding assays with tritiated hormones, dextran-charcoal separation and inverse Scatchard analysis. Follicles present at ovarian surface at time of ovarihysterectomy were recorded and progesterone profiles were determined to evaluate ovarian activity restoration. High-affinity binding sites for both receptors were found in all uterine tissue samples; Kd values (Mean +/- SEM) were 0.42 +/- 0.04 nM (n=10) for E(2)R and 0.76 +/- 0.10 nM (n=10) for P(4)R. Binding affinity did not differ in the different postpartum days. Both E(2)R and P(4)R levels in early post partum (Days 1 to 5) were significantly lower than late postpartum (Days 17 to 30). Mean values (+/- SD) expressed as fmol/mg protein for E(2)R were 173 +/- 54 (n=6) and 322 +/- 77 (n=4; P < 0.05), and for P(4)R they were 253 +/- 84 (n=6) and 604 +/- 356 (n=4; P < 0.05). There was an inverse relationship between uterine weights and E(2)R contents, (r=-0.64, n=10; P < 0.05) and P(4)R contents, (r=-0.75, n=9; P < 0.05). While only 1 ovarihysterectomized ewe (1/6) at Days 1 to Shad a follicle larger than 4 mm (presumptive estrogen-active follicle) all the ewes (4/4) ovarihysterectomized at Days 17 to 30 presented follicles of this size (P < 0.05). This data suggests that the postpartum restoration of uterine receptor levels was probably due to the recovery of the ovarian activity and the subsequent upregulation exerted by estrogen of follicular origin. The P(4)R values were positively correlated with E(2)R concentrations, (r=0.78; P < 0.05) and support this hypothesis. In conclusion, we have shown that the uterine tissue recovers its steroid receptor contents as uterine involution and reassumption of ovarian activity takes places.