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

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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 .**GRAPHIC**. and pH (pHi) were measured with liquid ion exchanger microelectrodes and the effects of .**GRAPHIC**. were observed. In cells in which the sodium pump was inhibited by K+-free solution, .**GRAPHIC**. induced a decrease of .**GRAPHIC**. that can be explained only if Na+ extrusion is assumed. pHi measurements provide indirect evidence that .**GRAPHIC**. 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 .**GRAPHIC**. This result suggests that the ammonium-mediated decrease in .**GRAPHIC**. 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.