Steroid effects on relaxin secretion in the rat

These studies were performed to determine if estrogen and/or androgens are involved in the regulation of relaxin secretion in the pregnant rat. Animals were hysterectomized at Day 15 of pregnancy and either untreated or given a) a subcutaneous Silastic capsule containing crystalline testosterone, b) a subcutaneous Silastic capsule containing crystalline dihydrotestosterone, or c) daily subcutaneous injections of 100 micrograms estradiol. A group of sham-operated animals served as controls. Serum samples taken at Days 14, 16, and 18 of pregnancy were assayed for relaxin and progesterone. Treatment with either estradiol or testosterone attenuated the decline in serum relaxin and progesterone concentrations observed following hysterectomy, whereas dihydrotestosterone (a nonaromatizable androgen) had no effect upon either relaxin or progesterone levels. These results suggest that estradiol and testosterone, previously implicated in the control of luteal progesterone production, also stimulate luteal relaxin secretion. The mechanism by which testosterone stimulates relaxin secretion is probably via its aromatization to estradiol within the corpus luteum.