Release of inorganic phosphate during activity in mammalian non‐myelinated nerve fibres

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Abstract
The efflux of labeled phosphate was measured in desheathed rabbit vagus nerve at rest and during activity. In solutions with 2 mM-phosphate and 1 mM-K the rate constant of the resting efflux was 2.7 .times. 10-3 min-1; stimulation caused an extra fractional loss of 2.8 .times. 10-6 impulse-1. Lowering the phosphate concentration decreased the resting and the stimulated efflux; with 0.2 mM-phosphate the corresponding values were 1.9 .times. 10-3 min-1 and 1.8 .times. 10-6 impulse-1, respectively. Increasing the K to 5.6 mM decreased resting and stimulated efflux. Lowering the temperature decreased the resting efflux with a Q10 of 2.9 and the stimulated efflux with a Q10 of 8.1. Chromatography of the effluent showed that at rest and during activity at least 96% of the radiophosphate was in the orthophosphate fraction. Replacing the Na of the solution by Li lowered the rate constant of the resting efflux to 0.8 .times. 10-3 min-1 and abolished the extra release during activity, without action potential reduction. The presence of ouabain did not affect the resting efflux, except at 100 .mu.M, when a transient reduction was found. The extra fractional loss was not affected with 0.001 .mu.M; with 0.01-0.5 .mu.M it was reduced without much change in the action potential, and abolished at higher concentrations. The extra release may result from an increase in internal Pi caused by increased breakdown of ATP during recovery. Comparison with the O2 consumption shows that about 1% of the Pi liberated at the inside of the axons escapes to the outside.