Interaction of Estrogens on the Vaginal Smear of Spayed Rats

Abstract

Employing the vaginal smear as an index of effect, combinations of various estrogenic substances were tested for interaction. Studies were concentrated at the approximate 50% response level. In a single experiment ED50 doses of the two estrogens in pure form were administered to two groups of spayed rats and a third group was injected with a mixture of one-half of each ed50; in one experiment three intermediate doses were employed. In all of the studies a given proportion of the ED50 of one compound was replaced by the same proportion of the ED50 of the other without mensurable change in biological response. These data are interpreted as indicating simple additive relationships among the compounds tested. Pairs studied were: estradiol-17ß with estriol; and estrone in combination with estradiol-17ß, estriol, methallenestril (Vallestril) and stilbestrol. The additive relationships described here are of particular interest in view of literature reports that indicate an interference between certain pairs of these compounds when uterine growth rather than vaginal cornification was used as an index of activity. This target organ differentiation suggests that various end points of estrogenic action differ in their responses to varying hormonal balances.