YM934, a novel K+ channel opener, activates ATP-sensitive K+ channels in cardiac myocytes

2-(3,4-Dihydro-2,2-dimethyl-6-nitro-2H-1,4-benzoxazin-4-yl)pyridin e N-oxide (YM934) is a newly synthesized benzoxazin. The effects of YM934 on ATP-sensitive K+ (KATP) channels in guinea pig cardiac ventricular myocytes and in an insulin-secreting cell line, HIT T15 beta-cells, were examined using the gigaohm-seal patch-clamp techniques. Under the whole-cell clamp condition, YM934 induced in ventricular myocytes a time-independent, glibenclamide-sensitive K+ current in a concentration-dependent fashion (EC50 = approximately 3 microM). On formation of inside-out patches in ATP-free solution, the KATP channel current abruptly appeared and then ran down. YM934 was applied to inside-out patches before, during and after channel "run-down." Because nucleoside diphosphates, such as uridine diphosphate (UDP), can induce channel openings after complete run-down, the effects of YM934 on the UDP-induced channel openings were also examined. Before run-down, YM934 enhanced KATP channel activity by decreasing the sensitivity of channels to intracellular ATP. YM934 also enhanced the partially run-down channel, even in the absence of ATP. After run-down, YM934 had no effect but could enhance the UDP-induced KATP channel openings. These effects of YM934 on cardiac KATP channels were similar to those of pinacidil and lemakalim. In HIT T15 beta-cells, 100 microM YM934 was ineffective in both cell-attached and inside-out patch configurations, suggesting the tissue-specific nature of the action of this novel K+ channel opener.