Changes in the synaptic cleft potassium concentration produced by polarizing currents

Abstract

How much the cleft K + concentration ([K +] c) is changed by a step-like shift of an endplate membrane potential was estimated from the resulting acceleration of frequency of miniature endplate currents and the dependence of miniature endplate currents frequency on bath K + concentration ([K +] 0). The absolute change in [K +] c is generally constant and hence relatively more pronounced the lower the [K +] 0. It is estimated that in physiological solution ([K +] 0 = 2mM), [K +] c is at least tripled (~6 mM) if the muscle membrane is held at the reversal potential ( V r) for the endplate currents. The same step-like shift in muscle membrane potential leads to an almost instantaneous change of endplate current amplitudes or of their time constants of decay. This indicates that cleft K + changes have no effect on the voltage dependence of amplitudes and decay phases of endplate currents (and hence on V r also). The lack of postsynaptic effect of change in [K +] 0 from 0.1 to 10 mM and only small influence of [K +] 0 changes on nerve-evoked quantal release explain these findings.