Tamoxifen and progesterone effects in target tissues during the perinatal period

The biological effects of tamoxifen (TAM), progesterone (P), or a combination of TAM + P were investigated in the uterus and vagina of newborn guinea pigs after short (2 days) and long (12 days) treatments. In both tissues, tamoxifen provoked a significant trophic effect which is indicated by the increase in weight, protein and DNA content. In the uterus, progesterone also provoked an increase in weight, protein and DNA content, but much less than that provoked by tamoxifen. In contrast, in the vagina progesterone had no effect on the weight, protein and DNA content, but progesterone did not block the agonistic effect provoked by tamoxifen. The situation was different when progesterone receptor was concerned. Tamoxifen in both tissues (particularly in the vagina) stimulated the progesterone receptor very significantly. Progesterone blocked the number of specific binding sites of progesterone and the stimulatory effect provoked by tamoxifen.