Uptake and release of radiophosphate were measured in desheathed rabbit vagus nerve. 2. During incubation in Locke containing 0.2 mM-[32P]phosphate a slow labelling of water soluble compounds of the nerve was found; the labelling of the non water soluble compounds was much smaller. During washing with inactive Locke, the label was almost entirely released from the water soluble compounds; the radioactivity of these compounds was therefore used as the basis for the calculation of the efflux rate constants. 3. The efflux of radiophosphate increased with increasing phosphate concentration of the washing fluid. 4. A similar effect of external phosphate on the efflux of radiophosphate was seen when the phosphate concentration was suddenly changed. The rate constants were in 0.2 mM-phosphate 1.29 X 10(-3) min-1, in 0.2 mM 1.95 X 10(-3) min-1, and in 2 mM 3.21 X 10(-3) min-1 at 37 degrees C. After changing the external solution the efflux reached a new level with a time constant of about 9 min. 5. Addition of arsenate also increased the efflux of radiophosphate; on a molar basis the effect of arsenate was slightly smaller than the effect of phosphate. 6. Addition of malate or malonate did not affect the efflux of radiophosphate. 7. When the Na of the Locke was replaced by Tris, the efflux of radiophosphate was lowered by 76%, the new level was reached with a time constant of 7.7 min. In Tris-Locke changes in external phosphate did not affect the phosphate efflux. 8. A lowering of the phosphate efflux by 52% was found in Li-Locke; the efflux was then no longer affected by the external phosphate concentration. 9. The effect of external phosphate on the efflux and in the radiophosphate was also measured at intermediate Na concentrations. At different Na concentrations the ratio between the Na dependent effluxes in 2 and 0.2 mM-phosphate was approximately equal to the ratio between the Na dependent influxes in 2 and 0.2 mM-phosphate. 10. The efflux of 22Na had a rate constant of 0.050 min-1 in Locke and 0.046 min-1 in Tris-Locke. 11. It is concluded that part of the efflux of phosphate is mediated by a Na-dependent transport system; the system appears to be able to exchange phosphate between the inside and the outside and to mediate net movements of phosphate in both directions.
