Direct Activation of the ATP-sensitive Potassium Channel by Oxygen Free Radicals in Guinea-pig Ventricular Cells:its Potentiation by MgADP

The effects of hydrogen peroxide (H 2O 2) on the activity of adenosine 5′-triphosphate-sensitive potassium (K + ATP) channels in ventricular cells isolated from guinea-pig hearts were investigated in inside-out membrane patches using the patch-clamp technique. H 2O 2at concentrations of 5 m mbut not 1 m mincreased K + ATPchannel activity in the presence of 0.3 m mATP. The presence of 10 μ mADP together with 0.3 m mATP led to activation of the K + ATPchannel by 1 m mH 2O 2. This potentiation of the H 2O 2-induced activation of the K + ATPchannel by ADP depended on the presence of Mg-ATP. Channel activation was due to an increase in the open-state probability and was not associated with a change in the single-channel conductance or the mean open and closed times during burst-like openings. The relationship between channel activity and ATP concentration could be fitted by the Hill equation with a Hill coefficient of 2 and the half maximal inhibition at 85 μ mATP in the presence of 10 μ mADP. The curve was shifted to higher ATP concentrations in a non-parallel manner by 1 m mH 2O 2. Analysis of open-state probability for membrane patches containing several channels indicated that H 2O 2activated the individual channels to a differing extent. It is concluded that H 2O 2activates the K + ATPchannel directly by decreasing the sensitivity to ATP. This effect was potentiated by the presence of ADP.