Measurements have been made of K influx in squid giant axons under internal solute control by dialysis. With [ATP]i = 1 µM, [Na]i = 0, K influx was 6 ± 0.6 pmole/cm2 sec; an increase to [ATP]i = 4 mM gave an influx of 8 ± 0.5 pmole/cm2 sec, while [ATP]i 4, [Na]i 80 gave a K influx of 19 ± 0.7 pmole/cm2 sec (all measurements at ∼16°C). Strophanthidin (10 µM) in seawater quantitatively abolished the ATP-dependent increase in K influx. The concentration dependence of ATP-dependent K influx on [ATP]i, [Na]i, and [K]o was measured; an [ATP]i of 30 µM gave a K influx about half that at physiological concentrations (2–3 mM). About 7 mM [Na]i yielded half the K influx found at 80 mM [Na]i. The ATP-dependent K influx responded linearly to [K]o from 1–20 mM and was independent of whether Na, Li, or choline was the principal cation of seawater. Substances tested as possible energy sources for the K pump were acetyl phosphate, phosphoarginine, PEP, and d-ATP. None was effective except d-ATP and this substance gave 70% of the maximal flux only when phosphoarginine or PEP was also present.

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