Characteristics of Current Fluctuations Originating from Activities of Inward-Rectifier K+ Channels in Guinea-Pig Heart Cells

Although outward current through inward-rectifier K+ channels has been observed in the whole-cell mode of the patch-clamp technique, no outward unitary current in single-channel studies has been recorded with the physiological ionic conditions. Hence, the relationship between single-channel activities and the inward rectification of the whole-cell current has been poorly understood. Therefore, characteristics of inward-rectifier K+ channels in guinea-pig ventricular myocytes were assessed by the noise analysis of the K+ current using the whole-cell patch clamp method. Partial blockade of the inward-rectifier K+ current by Ba2+ was used to obtain different levels of mean current and current fluctuation as needed for variance-to-mean analysis. The plot of variance of current fluctuation against mean currents was well fitted by theoretical parabolic curves, and the unitary conductance, the open probability, and the density of functional channels were deduced. The unitary conductance of the inward-rectifier K+ channel exhibited an inward-rectification, although the channel open probability and the density of functional channels were not much different at various holding potentials used. The unitary conductance was not changed when the intrapipette concentration of Mg2+ was reduced, but tended to be smaller when the pipette contained high Mg2+ concentration. Spermine also tended to reduce the outward unitary conductances, although the reduction was not statistically significant. These results suggest that the inward rectification in the whole-cell current was due to the inward-rectifying property of the unitary conductance of the K+ channels. Inward rectification of the unitary conductance may be caused by blocking of the channels by both Mg2+ and polyamines.