Chorion-induced myometrial relaxation is mediated by large-conductance Ca2+-activated K+ channel opening in the guinea pig

Objective: We previously demonstrated that chorion releases a factor that inhibits both spontaneous and oxytocin-stimulated myometrial contractility. Here, we investigate the mechanism of action of this unidentified substance. Study Design: Myometrial strips from pregnant guinea pigs were mounted in an organ bath and contractility stimulated with oxytocin. Results: Guinea pig chorion produced a time-dependent decrease in oxytocin-induced myometrial contractility. The ability of the chorion to reduce contractility was unaltered by inhibiting chorionic synthesis of either nitric oxide (N [ω]-nitro-L-arginine), carbon monoxide (tin-protoporphyrin), prostaglandins (indomethacin), or the myometrial cyclic guanosine monophosphate pathway (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalime-1-one and Rp-8Br-cGMP). In contrast, iberiotoxin, an inhibitor of large conductance Ca2+-activated K+ channels reduced the quiescent effect of chorion by 40%; in contrast, inhibition of adenosine triphosphate–sensitive (glibenclamide) and voltage-gated K+ channels (4amynopyridine) had no effect. Conclusion: Chorion-induced relaxation of oxytocin-stimulated myometrial contraction is, in great part, the product of a paracrine substance that opens myometrial large conductance Ca2+-activated K+ channels. (Am J Obstet Gynecol 2003;188:84-91.)