Synergistic effect of relaxin and progesterone on cyclic adenosine 3′,5′-monophosphate levels in the rat uterus

Cyclic adenosine 3′,5′-monophosphate is known to modulate smooth muscle contractility. Because both relaxin and progesterone have been demonstrated to affect myometrial cyclic adenosine monophosphate activity, we questioned whether the previously observed synergism of these two hormones in inhibiting uterine contractility is mediated via cyclic adenosine monophosphate. Immature rats were treated with estradiol benzoate (n = 7) or a combination of estradiol benzoate and progesterone (n = 7). Uterine horns were isolated, each horn was divided into two segments, and these horn segments were incubated in Ringer-Locke solution, either alone (control) or with 3-isobutyl-l-methylxanthine (MIX) 0.5 mM, MIX 0.5 mM + relaxin 10 ng/ml, or MIX 0.5 mM + relaxin 50 ng/ml. When compared with uterine segments incubated in MIX alone, treatment with MIX + relaxin 50 ng/ml significantly increased cyclic adenosine monophosphate levels in animals treated with estradiol benzoate alone or in combination with progesterone. Relaxin 10 ng/ml was sufficient to significantly elevate mean (± SEM) uterine cyclic adenosine monophosphate levels above that of control MIX-treated uteri in animals receiving both estradiol benzoate and progesterone (2.49 ± 0.39 pm/μg deoxyribonucleic acid [DNA] versus 1.08 ± 0.16 pm/μg DNA, p < 0.05) but not in animals receiving estradiol benzoate alone (2.08 ± 0.32 pm/μg DNA versus 1.28 ± 0.16 pm/μg DNA, NS). Compared with treatment with MIX only, MIX + relaxin 10 ng/ml and MIX + relaxin 50 ng/ml produced greater increases in uterine cyclic adenosine monophosphate in the steroid combination group than in the estradiol benzoate controls (144.8% and 233.7% versus 71.7% and 156.6%, respectively). These results suggest that the synergism of relaxin and progesterone in inhibiting uterine contractility may be mediated by intracellular cyclic adenosine monophosphate.