EFFECT OF OXYTOCIN ON ESTROGEN AND PROGESTERONE RECEPTORS IN THE RAT UTERUS

The effect of oxytocin on uterine estrogen and progesterone receptors (ER and PSR) was investigated in vive in groups of immature female rats that were treated subcutaneously with oxytocin, 0.5 or 5 IU (1 and 10 mu g) for 5 and 3 d, respectively. Receptor concentrations and affinities were estimated by Scatchard analysis, using radioactive hormones as ligands. Statistical analysis was performed by Student's ttest and by ANOVA. Oxytocin did not alter the receptor affinity for either steroid hormone, but the lower dose significantly decreased the concentrations of receptors: ER = 486 +/- 76 fmol/mg vs 346 +/- 105 fmol/mg (P < 0.001) and PgR = 686 +/- 237 fmol/mg vs 433 +/- 236 fmol/mg (P < 0.01) (mean +/- SD values for control and treated animals, respectively). There were no significant effects on the plasma 17 beta-E and Pg concentrations. In in vitro studies with mature rats, uterine specific binding of estradiol and progesterone in the presence or the absence of oxytocin showed no modification. Oxytocin could be a negative modulator of ER and PgR in the uterus, even though the mechanism of its action remains unknown. It could have potential implications on reproductive capacity and fertilization.