On the block of outward potassium current in rabbit Schwann cells by internal sodium ions

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Abstract
Currents through delayed rectifier-type K$^{+}$ channels in Schwann cells cultured from rabbit sciatic nerve were studied with patch-clamp techniques. When the internal and external solutions contained physiological concentrations of sodium, the amplitude of these outward currents declined as the cell was depolarized to potentials above about +40 mV, despite the increased driving force. This reduction in the amplitude of outward K$^{+}$ currents was observed in many cells before the subtraction of leakage currents; it was also observed for ensemble currents recorded in outside-out patches. It was therefore not the result of a leak-subtraction artefact nor of inadequate voltage-clamp control. Several lines of evidence also suggested that it was not the result of the extracellular accumulation of K$^{+}$. By contrast, when the Na$^{+}$ ion concentration of the internal solution was nominally zero, the reduction in the amplitude of outward K$^{+}$ currents at positive membrane potentials was not observed. The apparent amplitude of single-channel currents through two types of K$^{+}$ channel was reduced by 30 mM internal Na$^{+}$, apparently as the result of a rapid `flickery' block. The results suggest that channel block by internal Na$^{+}$ is largely responsible for the negative slope conductance seen in current-voltage plots of whole-cell K$^{+}$ currents at positive membrane potentials. In addition, our analysis of single-channel currents suggests that the current-voltage curve for a delayed rectifier channel in rabbit Schwann cells (in the absence of internal Na$^{+}$) is roughly linear with internal and external K$^{+}$ concentrations of 140 mM and 5.6 mM, respectively.