Effects of Gonadotrophins on Secretion of Steroids by the Testis and Ovary

Abstract

This review considers 3 aspects of the effects of gonadotropins: whether they promote secretion of ovarian and testicular steroids; where they act in the biosynthetic sequence leading to the formation of the steroids; what is the mechanism of action for the gonadotropins. The mechanism of action of ACTH on the adrenal cortex is used as a comparison for the latter discussion. Only data dealing with chemical estimation of the steroids is presented. Data from in vivo and in vitro studies indicate that gonadotroplns promote an immediate increase in the secretion of steroid hormones by ovarian and testicular tissue. Increased biosynthesis of these compounds rather than increased release of stored tissue steroids appears to be a common feature of gonadotropic action on both glands. Evidence indicates distinct sites of action for the gonadotropins; some current data support the view that hydroxylation of cholesterol in the 20 alpha position is the rate-limiting step influenced by the gonadotropins. No investigation has revealed the ultimate site, i. e. whether on enzymes directly involved in steroid formation or on enzymes affecting the concentration of cofactors. Recent experiments indicate a similarity between the action of gonadotropins on the ovary and testis and the activity of ACTH in increasing the phosphorylase activity of the adrenal gland. The evidence that gonadotropic action is associated with increased NADPH production by way of the pentose phosphate pathway is not so clear. Available evidence indicates that the trophins and NADPH may act at different sites. The tropins could influence steroid biosynthesis before production of [DELTA]5-pregnenolone, and tropic-stimulated production of the nucleotide cofactor could affect biosynthetic steps between cholesterol and [DELTA]5-pregnenolone and also those subsequent with the formation of the latter steroid.