Openings of cardiac KATP 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 increases in I(term) reflected activation of the ATP-sensitive K+ (KATP) channels, as glibenclamide (1 microM) 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 gliben-clamide. These findings are compatible with the notion that OFRs activate KATP channels by modulating ATP binding sites of the KATP channels, without affecting ADP binding or glibenclamide binding sites.