The Relation of the Membrane Potential and Resistance of Nitella to Ion Fluxes

Abstract

The results of this study indicate membrane resistance of Nitella cannot be related to passive fluxes of ions as classical interpretation suggests. When a hyperpolarizing current of about 0. 6 µ amp cm-2 was passed through the membrane, with 42K present in the external solution, it was found that 50% of the current was carried by K+. The increase in K+ influx was 40 times the value predicted by the Goldman passive model for the observed changes in potential. The large transference number for K+ is not in agreement with Kitasato's conclusion that 95% of the cationic influx is due to H+. Membrane potential measurements and tracer studies of the major ions necessitate proposal of an active electrogenic cation efflux. Previous work suggests this is H+ extrusion. The depolarizing effects of increased external [H+] and DNP are explained as decreases in the rate of electrogenic H+ extrusion. Evidence against passive H+ fluxes is presented. Tracer studies also indicate a component of K+ transport which in most cases acts differently from a simple passive mechanism for K+ uptake. The depressing effects of increased external [H+] and DNP on the rate of active H+ extrusion and active K+ influx suggest the H+ pump can act partially as a H+/H+ or H+/K+ exchange pump. The simplest way of accounting for the large K+ transference number would be that the K+ current travels through the electrogenic mechanism.