1. The activity of the Na+ pump in an Na+-rich yeast was compared with that in an Na+-rich frog sartorius muscle, and found to be very similar to it over the first hour if both were immersed in fluid containing 104mm-Na+ plus 10mm-K+. 2. The efflux of labelled Na+ from an Na+-rich yeast into an Na+-free medium was investigated. In this Na+-free medium, Li+ or choline replaced the Na+, and the efflux–content curves obtained with either of these ions were very similar. The curves were sigmoid, reaching or approaching a saturation at the higher internal Na+ concentrations. 3. The curves obtained with yeast resembled those similarly obtained with frog sartorius muscle by Keynes & Swan (1959), Mullins & Frumento (1963), Harris (1965) and Keynes (1965). The slope of the plot of the logarithm of the Na+ efflux against the logarithm of the Na+ concentration in the cells reached its highest value at an internal Na+ concentration of 15m-equiv./kg. (27m-equiv./l. of cell water). 4. The effect of external K+ concentration on the efflux–content relationship was examined. An increased K+ concentration was found to increase the Na+ efflux by raising the saturation value, which is similar to observations made by Harris (1965) with frog muscle. 5. The effect of increasing the external carbon dioxide concentration was investigated. No effect on the slope of the plot of the logarithm of the Na+ efflux against the logarithm of the Na+ content was noticed even when the yeast suspension was equilibrated with 100% carbon dioxide. There was, however, a decrease in the amount of Na+ efflux on equilibrating the solution with carbon dioxide.
The relation between sodium ion content and efflux of labelled sodium ions from yeast
