Active transport and passive fluxes of K, Na, and Li in mammalian non-myelinated nerve fibres.

Abstract

1. The ionic content of nerve fibres in the rabbit's cervical vagus was measured by flame photometry. Immediately after dissection the intracellular K was 98.0±4.0 and the Na 42.0±2.5 mM/l. Incubation for 5 hr in Locke produced little change in these values. 2. In K-free Locke the fibres lost K and gained Na. The loss of K had a rate constant of 0.0142 min−1, corresponding to an efflux of 0.313 p-mole/cm2·sec. Lowering the pH from 7 to 6 did not affect the efflux of K. 3. The K and Na contents were restored to the initial values within 30 min after addition of 5.6 mM K to the K-free solution. The restoration was inhibited by ouabain. 4. The rate of increase in intracellular K produced by different extracellular concentrations of K indicated a half-maximal activation of the Na-K-pump with 3.8 mM K. 5. Lowering the pH from 7 to 6 increased the half-maximal activation constant to 13.3 mM K, suggesting inhibition of the pump by H+. 6. A study of the effect of intracellular Na suggests that the simultaneous presence of 3–4 Na ions is necessary for activating the internal site of the pump. Half-maximal activation was found with an intracellular Na content of 29.5 mM. 7. Li ions showed a slight activating effect at the external site; their activating effect on the inside was at least 43–44 times smaller than the effect of Na.