Openings of cardiac K-ATP channel by oxygen free radicals produced by xanthine oxidase reaction

We examined the effects of oxygen free radicals (OFRs) on action potentials and membrane currents of guinea pig ventricular myocytes. OFRs produced biphasic changes in the action potential duration, initial lengthening (30 s after exposure to OFRs) and subsequent shortening (within 5 min). In voltage-clamp experiments, OFRs suppressed the L-type calcium current, the delayed rectifier K+ current, and the inward rectifier K+ current. In addition, OFRs increased the time-independent outward current (I-term) at potentials greater than -30 mV. The increase in I-term reflected activation of the ATP-sensitive K+ (K-ATP) channels, as glibenclamide (1 mu M) blocked this current. In inside-out patches, OFRs significantly increased the open probability of the channel at a relatively narrow range of ATP concentrations (0.2-2 mM), and this effect was enhanced in the presence of ADP (0.1 mM) and abolished in the presence of either free radical scavengers or glibenclamide. These findings are compatible with the notion that OFRs activate K-ATP channels by modulating ATP binding sites of the K-ATP channels, without affecting ADP binding or glibenclamide binding sites.