Effects of extracellular sodium concentration on null potential, conductance and open time of endplate channels.

(i) Effects of extracellular sodium concentration, [Na]o, on endplate channel characteristics were investigated in voltage-clamped, glycerol-treated toad sartorius fibres. (ii) The relation between [Na]o (and [K]o) and acetylcholine null potential could be reasonably well fitted by the Goldman-Hodgkin-Katz type of equation, except when [Na]o was higher than normal. Anions had no significant effect on the null potential. (iii) Endplate channel open time (phi), whether measured from miniature endplate currents or from current fluctuations induced by iontophoresis of acetylcholine, varied inversely with [Na]o. The relation between phi-1(= alpha) and [Na]0 could be fitted by alpha = alpha max [Na]o/(Km + [Na]o) with a Km of 92 mM. (iv) Endplate conductance, measured at the peak of endplate currents or at the peak of spontaneous miniature endplate currents, increased nonlinearly with [Na]o. (v) Single channel conductance, gamma, also increased nonlinearly with [Na]o. Experimental observations at -90 mV could be fitted by the relation gamma = gamma max [Na]o/(Km + [Na]o), giving values for gamma max and Km of 47 pS and 146 mM respectively. Correcting channel conductance for the contribution from potassium ions gave values of gamma max and Km of 78 pS and 423 mM respectively. (vi) The results are consistent with the hypothesis that binding sites for Na ions can modulate both channel lifetime and conductance and that these sites become saturated at higher sodium concentrations.