Electrophysiological evidence for ammonium as a substitute for potassium in activating the sodium pump in a crayfish sensory neuron

Abstract

Electrophysiological studies were performed on slowly adapting cells of the crayfish (Astacus astacus) stretch receptor to examine some aspects of the operation of the sodium pump. Intracellular sodium activity [Formula: see text] and pH (pHi) were measured with liquid ion exchanger microelectrodes and the effects of [Formula: see text] were observed. In cells in which the sodium pump was inhibited by K+-free solution, [Formula: see text] induced a decrease of [Formula: see text] that can be explained only if Na+ extrusion is assumed. pHi measurements provide indirect evidence that [Formula: see text] was taken up at the same time as Na+ was extruded. Ouabain blocks the operation of the sodium pump in the presence of K+ and [Formula: see text]. This result suggests that the ammonium-mediated decrease in [Formula: see text] in K+-free solution was caused by activation of the sodium pump. The results obtained by electrophysiological methods in a living cell are qualitatively in good agreement when compared with biochemical investigations on assays of crustacean Na+–K+ ATPase.