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 obse rved in the whole-cell mode of the patch-clamp technique, no outward unitar y current in single-channel studies has been recorded with the physiologica l ionic conditions. Hence, the relationship between single-channel activiti es and the inward rectification of the whole-cell current has been poorly u nderstood. Therefore, characteristics of inward-rectifier K+ channels in gu inea-pig ventricular myocytes were assessed by the noise analysis of the K current using the whole-cell patch clamp method. Partial blockade of the i nward-rectifier K+ current by Ba2+ was used to obtain different levels of m ean current and current fluctuation as needed for variance-to-mean analysis . The plot of variance of current fluctuation against mean currents was wel l 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 fun ctional channels were not much different at various holding potentials used . The unitary conductance was not changed when the intrapipette concentrati on of Mg2+ was reduced, but tended to be smaller when the pipette contained high Mg2+ concentration. Spermine also tended to reduce the outward unitar y conductances, although the reduction was not statistically significant. T hese results suggest that the inward rectification in the whole-cell curren t was due to the inward-rectifying property of the unitary conductance of t he K+ channels. Inward rectification of the unitary conductance may be caus ed by blocking of the channels by both Mg2+ and polyamines.