Background. The effectiveness of blockade of the inwardly rectifying K' current (IK1) in prevention of arrhythmias is unknown. We have examined the antiarrhythmic potential of a new selective IKI blocker, RP58866, in rat, rabbit, and primate (marmoset) isolated hearts in the settings of acute ischemia and reperfusion. Methods and Results. In concentration-response studies (n=12 per group), the drug reduced ischemiainduced ventricular fibrillation (VF) in rat from control incidence of 100 to

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... , 17% (p<0.05), and 0%7 (p<0.05) at 1, 3, and 10 gmol/L, respectively. RP58866 produced significant bradycardia at the 3-and 10-,gmol/L concentrations and significant QT interval widening at all three concentrations (p<0.05). When rat hearts (n=12 per group) were paced (5 Hz) via the left atrium to prevent bradycardia, the antiarrhythmic effects of 10-,umol/L RP58866 were unmodified (ischemia-induced VF incidence was reduced by drug from 83% in control hearts to 8%; p<0.05). Similarly, pacing did not prevent the drug's QT-widening activity at 90% repolarization (QT90 was 64±3 msec in control hearts versus 128±17 msec in the presence of 10 ,umol/L of drug after 10 minutes of ischemia; p<O.05). These values are similar to equivalent values in unpaced hearts (65±3 msec in control hearts versus 159±15 msec with 10 ,umol/L of drug; p<O.05). In separate groups of rat hearts (n=10 per group) subjected to 10 minutes of ischemia, reperfusion-induced VF incidence was reduced from 90%,o in control hearts to 10%o (p<O0.05), 0%o (p<O0.05), and 0%o (p<0.05) by 1-, 3-, and 10-,umol/L RP58866. To examine whether drug actions were speciesspecific, we performed further studies in rabbit and primate using the middle concentration of RP58866 (3 ,umol/L). Ischemia-induced VF incidence was too low in these species to assess the effects of the drug. However, RP58866 widened QT interval (p<0.05), slowed heart rate (p<0.05), and reduced the incidence of reperfusion-induced VF from 67% to 8% (p<0.05) in rabbit. Furthermore, in the more clinically relevant primate species (marmoset; n=9-12 per group), RP58866 (3 ,umol/L) abolished ischemiainduced VT (36% incidence in control hearts; p<0.05) and significantly reduced the incidence of ischemia-induced ventricular premature beats from 91% to 33% (p<0.05). The drug was also effective against reperfusion VF in primates (incidence reduced from 64% in control hearts to 11%;p<0.05). As in rat and rabbit, RP58866 significantly widened QT interval in primate and caused bradycardia before and during ischemia. RP58866 had no significant influence on coronary flow in any species. Finally, in further studies on rat, QT widening by RP58866 was found to persist relatively unmodified in nonischemic hearts perfused with solution containing K' elevated to 8 mmol/L to mimic the early ischemic milieu. Conclusions. RP58866, a selective IKI blocker, is a potent and efficacious new antiarrhythmic drug in ischemia and reperfusion in rat, rabbit, and primate. When tested in rat, pharmacological activity was undiminished by cardiac pacing or elevation of extracellular K'.