Endplate currents in sucrose solution.

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Endplate currents were recorded from voltage-clamped frog muscle fibres bathed in an isotonic sucrose solution containing 2 mM K + . In this solution the major part of the current is carried by K + ions, and at negative potentials the membrane voltage–current amplitude relations of both miniature endplate currents and the single channel current estimated from noise analysis were linear, with smaller conductance than in normal Ringer solution. At positive potentials miniature endplate currents and currents induced by acetylcholine (ACh) showed a saturation, or sometimes even decline, with increasing potential. In contrast, the single channel current continued to increase linearly at these potentials. It is suggested that in sucrose solution the number of functional ACh receptors decreases as the endplate is depolarized to more positive potentials.