Cromakalim- and RP 49356-induced 42K+ and 86Rb+ efflux in rat myometrium.

Previous studies had failed to observe cromakalim-induced 42K+ or 86Rb+ efflux from the myometrium of the pregnant rat in contrast to positive findings in other smooth muscles. In the current study, in myometrium from the non-pregnant rat, cromakalim (10 microM) and RP 49356 ([(+/-)-N-methyl-2-(3-pyridyl)-tetrahydrothiopyran-2-carbothioamid e-1-oxide); 10 microM) induced small increases in 42K+ or 86Rb+ efflux but much less than did oxytocin (20 nM) or KCl (20 mM). The cromakalim-induced increase in 42K+ efflux was enhanced 3.5-fold in the presence of KCl (20 mM) plus (+)-cis-diltiazem (3 microM), a property shared by RP 49356. Glibenclamide (10 microM) partially reduced the cromakalim-induced 42K+ efflux, in the presence of KCl and (+)-cis-diltiazem, but did not affect the KCl-induced 42K+ efflux. The data provides further support for the idea that cromakalim and RP 49356 are able to open potassium channels in rat myometrium. It would appear that their actions in this tissue are dependent on the extracellular K+ concentration and/or membrane potential.