Evidence for an electrogenic ion pump in Nitella translucens. I. The effects of pH, K +, Na +, light and temperature on the membrane potential and resistance

Abstract

1. 1. The effects of external pH and K + in the light and dark were investigated to establish conditions in which the membrane potential of Nitella translucens was more negative than the diffusion potential for the major ions. 2. 2. At high external K + concentrations there is an apparent increase in K + permeability and the membrane potential becomes equal to the calculated K + equilibrium potential. It is then possible to calculate the K + equilibrium potential for individual cells in solutions having low K + concentrations. 3. 3. In a solution containing 0.5 mM K + at pH 6, the membrane potential in the dark is close to the K + equilibrium potential. At a light intensity of 1.0 mW · cm −2 the membrane potential is hyperpolarized by 50 mV and there is a decrease in the membrane resistance from 153 kΩ · cm 2 to 17 kΩ · cm 2. 4. 4. In the dark, a decrease in the temperature depolarizes the membrane by 0.97 mV · °C − and there is no significant effect on the membrane resistance. In the light, the temperature coefficient for the membrane potential is 2.5 mV · °C −1 and the membrane resistance is approximately doubled by a 10 °C decrease in temperature. 5. 5. In the light, the current required to reduce the potential across thee plasmalemma to the K + equilibrium potential was equivalent to a flux of about 20 pmoles · cm −2 · s −1. This was tentatively identified with the flux through the electrogenic H + pump postulated by H. Kitasato ( J. Gen. Physiol., 52 (1968) 60). 6. 6. It is suggested that the results cannot be accounted for by the diffusion of the major ions or by an electrogenic pump in combination with a large passive H + flux. An alternative analysis in terms of a voltage-dependent, light-stimulated electrogenic pump (S.I. Rapoport, Biophys. J. 10 (1970) 246) is presented.