Role of potassium in the phosphate efflux from mammalian nerve fibers

Abstract

Summary. 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 150 mM 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 effiux 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 Nadependent 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.We have shown previously that a large proportion of the transmembrane fluxes of inorganic phosphate in nonmyelinated nerve fibers is mediated by a saturable Na-dependent mechanism (Armor et al., 1976;Ferrero et al., 1978). This mechanism is different from the phosphate transport in other tissues, e.g., erythrocytes (Rothstein, Cabantchick & Knauf, 1976), mitochondria (Banerjee et al., 1977) or Escherichia coli (Rosenberg, Gerdes & Harold, 1979).In the course of further investigations on the phosphate efflux, a transient release of phosphate was observed when the potassium of the incubation solution was withdrawn. Conversely, when the potassium was increased, the efflux of phosphate was diminished. In the present study, we have analyzed these phenomena in more detail. Our results suggest that the phosphate released in the absence of K originates from a specific K-dependent phosphate binding site. Materials and MethodsDesheathed rabbit vagus nerves were mounted in a polyethylene tube which was perfused with 32p-phosphate Locke for 150 rain at 37 ~ The preparation then was washed with inactive Locke solution, and the effluent was collected and counted. At the end of the experiment the preparation was homogenized in 0.1 g triethanolamine buffer (pH 8). The homogenate was mixed with chloroform, centrifuged at 3,000 xg for 15 min and the activity of the water-soluble fraction was then counted. These counts were used for the calculation of the efflux rate constant. The composition of the Locke was (raM): 154 NaC1, 5.6 KC1, 0.9 CaC12, 0.5 MgCI2, 5 glucose, 0.2 Na2HPO4-NaH2PO4, 1 Tris. Potassium isethionate (Eastman-Kodak) was added to prepare K-rich Locke. In Na-free solutions, sodium wa...

Involvement of intracellular calcium in the phosphate efflux from mammalian nonmyelinated nerve fibers

“…In addition to the sodium and phosphate dependence of the phosphate efflux, we have found a transient release of phosphate when the potassium of the extracellular medium is reduced as well as a decrease of phosphate efflux in high potassium [15]. Phosphate efflux is also modified by changes in extracellular calcium concentration [16], and as briefly reported, there is a stimulation of phosphate efflux on application of the calcium ionophore A23187 [13].…”

“…Removal of extracellular sodium increases the membrane potential of rabbit vagus nerve by about 10 mV [15]. Qualitatively, this hyperpolarization could be at the origin of the observed increase in the efflux of the negatively charged phosphate ion in Na-free solution.…”