Melatonin inhibits spontaneous and oxytocin-induced contractions of rat myometrium in vitro.

OBJECTIVE: The aim of this study was to investigate the effects of melatonin on spontaneous and oxytocin-induced contractility of pregnant and non-pregnant rat myometrium in vitro.

DESIGN: Myometrial strips were removed from virgin or late pregnant (21 days gestation) Wistar rats following decapitation and placed in an organ bath containing Krebs' solution at 37 degrees C and pH 7.4, constantly bubbled with 95% oxygen-5% carbon dioxide and isometric contractions were recorded. Effects of cumulative concentrations of melatonin (0.1 to 10 microM) on spontaneous and oxytocin-induced contractions were studied. Possible involvement of Ca(2+)-activated K (+) channels in inhibitory actions of melatonin was investigated by using apamin (100 nM).

RESULTS: Melatonin inhibited spontaneous and oxytocin-induced contractions of myometrium from both virgin and late pregnant rats in a dose-dependent manner. After inhibition of oxytocin-induced contractions by melatonin, application of prostaglandin F (2alpha) (1 microM) but not high KCl (30 mM) containing solution initiated contractile activity. Inhibitory response induced by melatonin (13 microM) was not affected by apamin (100 nM).

CONCLUSIONS: Data from this study demonstrates that melatonin inhibits spontaneous and oxytocin-induced contractions of myometrium from pregnant and non-pregnant rats. Although the exact mechanism is not clear, melatonin-induced inhibition of myometrial contractions may results from its interactions with Ca(2+) channels.