ATP-SENSITIVE K- CHANNEL INHIBITOR IN VASCULAR( CHANNEL OPENER ACTS AS A POTENT CL)

We describe the activation of a K+ current and inhibition of a Cl- cur rent by a cyanoguanidine activator of ATP-sensitive K+ channels (K-ATP ) in the smooth muscle cell line A10. The efficacy of U83757, an analo gue of pinacidil, as an activator of K-ATP was confirmed in single cha nnel experiments on isolated ventricular myocytes. The effects of U837 57 were examined in the clonal smooth muscle cell line A10 using volta ge-sensitive dyes and digital fluorescent imaging techniques. Exposure of A10 cells to U83757 (10 nM to 1 mu M) produced a rapid membrane hy perpolarization as monitored by the membrane potential-sensitive dye b is-oxonol ([diBAC(4)(3)], 5 mu M). The U83757-induced hyperpolarizatio n was antagonized by glyburide and tetrapropylammonium (TPrA) but not by tetraethlylammonium (TEA) or charybdotoxin (ChTX). The molecular ba sis of the observed hyperpolarization was studied in whole-cell, volta ge-clamp experiments. Exposure of voltage-clamped cells to U83757 (300 nM to 300 mu M) produced a hyperpolarizing shift in the zero current potential; however, the hyperpolarizing shift in reversal potential wa s associated with either an increase or decrease in membrane conductan ce. In solutions where E(K) = -82 mV and E(Cl) = 0 mV, the reversal po tential of the U83757-sensitive current was approximately -70 mV in th ose experiments where an increase in membrane conductance was observed . In experiments in which a decrease in conductance was observed, the reversal potential of the U83757-sensitive current was approximately 0 mV, suggesting that U83757 might be acting as a Cl- channel blocker a s well as a K+ channel opener. In experiments in which Cl- current act ivation was specifically brought about by cellular swelling and perfor med in solutions where Cl- was the major permeant ion, U83757 (300 nM to 300 mu M) produced a dose-dependent current inhibition. Taken toget her these results (i) demonstrate the presence of a K+-selective curre nt which is sensitive to K-ATP channel openers in A10 cells and (ii) i ndicate that the hyperpolarizing effects of K+ channel openers in vasc ular smooth muscle may be due to both the inhibition of Cl- currents a s well as the activation of a K+-selective current.