The interaction of relaxin with the rat uterus. I. Effect on cyclic nucleotide levels and spontaneous contractile activity.

Three different preparations of relaxin elevated cAMP levels in isometrically suspended uterine strips obtained from estrogen-primed female rats in the presence of phosphodiesterase inhibitors. The effect was time and dose dependent, with maximal response at 20 min and 1 microgram/ml, respectively. No changes in cGMP levels were observed. No significant response was observed in ileum, vas deferens, epididymis, or testicular capsule. In the absence of phosphodiesterase inhibitors, relaxin markedly suppressed spontaneous contractile activity at doses of 0.36 and 0.96 microgram/ml but did not elevate cAMP in this dose range. While the beta-adrenergic antagonist propranolol suppressed both the relaxant action and cAMP stimulation promoted by isoproterenol, it did not interfere with the effect of relaxin on these same parameters. Relaxin is therefore capable of relaxing uterine muscle and elevating cAMP by mechanisms which probably do not require beta-adrenergic mediation. It is not clear at present whether the effects of relaxin on cAMP levels and contractile activity are causally related.