Role of potassium in the phosphate efflux from mammalian nerve fibers.

Abstract

The efflux of phosphate was measured in rabbit vagus nerve loaded with radiophosphate. The efflux was found to depend on the K concentration of the bathing solutions; increasing the K from 5.6 up to 150mm produced a maximal lowering of 28%; K-free solution produced a transient increase whose peak was 86% above the normal efflux. In the presence of Na, the K-free effect could be repeated; in Na-free solution, it was found only for the first application of the K-free solution. The phosphate efflux was not altered when K was replaced by Rb; replacement with Cs showed that this ion only partially mimics the effect of K. The results suggest that the transient increase in phosphate efflux is due to release of label from a K-dependent saturable binding site, which is distinct from the main intracellular pool. The binding site appears to be labeled from the inside by the Na-dependent phosphate efflux previously described. It may correspond to the phosphorylation of membrane phospholipids. A mathematical model of this system is developed and curves simulated by an analog computer are compared to the experimental results. Measurements of the membrane potential and the internal inorganic phosphate showed that the effect of K on the phosphate efflux could not be explained by changes in the membrane potential or in the internal phosphate pool.