Temperature dependence of the sodium-potassium permeability ratio of a molluscan neurone.

1. The temperature dependence of the membrane potential of a molluscan giant neurone was examined under conditions which block the electrogenic activity of the Na-K exchange pump.2. When the Na pump was blocked by ouabain or the removal of external K, the membrane potential depolarized as temperature was increased.3. This depolarization was prevented by the replacement of external Na with impermeant cations, but was greater when Na was replaced with Li.4. All observed effects of ouabain were attributable to inhibition of the Na pump. The depolarization in response to ouabain at warmer temperatures was completely reversible, and the rate of both onset and reversibility of the ouabain effect was dependent upon temperature.5. Using a modified form of the constant field equation, the internal K concentration and the Na-K permeability ratio, P(Na)/P(K), were calculated from the experimental data.6. P(Na)/P(K) was found to increase from 0.028 at 4 degrees C to 0.068 at 18 degrees C. It is suggested that this increase is due primarily to a change in P(Na).