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The Relation of the Membrane Potential and Resistance of Nitella to Ion Fluxes
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1973-08-01
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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.