Pharmacological studies on the action of relaxin upon KCl-contracted rat uterus

The present experiments were designed to elucidate the pharmacological mechanism of the relaxing action of the ovarian hormone, relaxin, upon KCl-contracted rat uteri in vitro. Rat uterine segments were made to contract by superfusion with a physiological solution, in which 15% of the normal NaCl content had been replaced by an equimolar amount of KCl. The effect of relaxin, in reversing this contraction, was achieved in part by a direct action upon the tissue and in part (about 45% at the concentration of relaxin used) by the liberation of endogenous catecholamines which relax the uterus via beta-adrenergic activity. The effect of relaxin was reduced by propranolol, and it was not blocked by indomethacin, cimetidine or diphenhydramine. The effect of the ovarian peptide on uterine segments was potentiated in the presence of theophylline and indomethacin in the superfusing fluid. This effect of theophylline was shown to be insensitive to beta-adrenergic blockade. The results suggest that relaxin relaxes the KCl-contracted rat uterus in part by a direct inhibitory action upon the uterine muscle and in part by facilitating the release of intramural catecholamines. It is also suggested that relaxin may act via the production of cyclic AMP.