Pinacidil activates the ATP-sensitive K⁺ channel in inside-out and cell-attached patch membranes of guinea-pig ventricular myocytes

Abstract

Patch-clamp techniques were used to study the effects of pinacidil on the adenosine-5′-triphosphate (ATP)-sensitive K⁺ channel current in guinea-pig ventricular myocytes. In inside-out patches, the ATP-sensitive K⁺ channel current could be recorded at an internal ATP concentration of 0.5 mM or less and almost complete inhibition was achieved by raising the concentration to 2 mM. Application of pinacidil (10–30 μM) in the presence of 2 mM ATP restored the current, whereas 5 mM ATP antagonized the effect of pinacidil. The conductance of the channel at symmetrical K⁺ concentrations of 140 mM was 75 pS with a slight inward rectification at voltages positive to +40 mV. There was no significant change in the conductance after application of pinacidil. In 0.5 mM ATP, at −80 mV, both the distributions of the open time and the life-time of bursts could be fitted by a single exponential. An increase in ATP concentration decreased the mean life-time of bursts, whereas pinacidil increased it with little increase in the mean open time. Closed time distributions of the channel were fitted by at least two exponentials, with a fast and a slow time constant. An increase in ATP concentration markedly increased the slow time constant associated with a decrease in the number of bursts, whereas the effect of pinacidil was opposite to that of increased ATP. These results indicate that pinacidil increases the open-state probability of the ATP-sensitive K⁺ channel. In cell-attached patches, application of pinacidil (100–200 μM) to the extracellular solution reversibly induced the channel activity, which showed similar properties to those of the ATP-sensitive K⁺ channel recorded in cell-free patches.