Effects of the estrous cycle on the sensitivity of rat uterus to oxytocin and desamino-oxytocin.

Abstract

The sensitivity of the isolated rat uterus to oxytocin and the USP Standard has been found to undergo a cyclic change according to the estrous cycle. The sensitivity of the tissue is lowest at metestrus, and from there it increases progressively during the successive stages of the estrous cycle, reaching the peak at estrus. When desaminooxytocin, a synthetic analogue of oxytocin, is tested on the rat uterus, a similar cyclic change is observed. However, the increase in sensitivity as the uterus approaches estrus is greater to oxytocin and the USP Standard than to desamino-oxytocin. This difference in sensitivity markedly affects the oxytocic potency values obtained for desamino-oxytocin when it is assayed against either oxytocin or the USP Standard on uteri at different stages of estrus. The calculated values for units per mg of oxytocic activity of desaminooxytocin on the basis of the USP Standard are: 771 ±36 at diestrus, 564 +17 at proestrus, 551 ±17 at estrus and 899 ±24 at metestrus.