Studies on the crystalline lens. XXV. An analysis of the dependence of the components of the potential on sodium-potassium fluxes based on the pump-leak model

Abstract

A formulation which shows the equality of influx and efflux of sodium and potassium ions in the rabbit lens at steady state was derived on the basis of the pump-leak model of ion transport and used to derive an equation which expresses the electrogenic and diffusional components of the potential difference in terms of active and passive transport coefficients and the cation concentration gradients existing in lenses maintained in culture. The various components of the potential are calculated from experimentally determined values for the coefficients of active and passive transport for Na and K and the concentration gradients found experimentally in cultured lenses. The value for the electrogenic component is −12 mV, the diffusional component is −32 mV and the total potential is −44 mV. The latter value agrees closely with that obtained experimentally for lenses cultured to measure the coefficients of active and passive transport, −42 mV. The agreement between the potential difference obtained experimentally and that calculated theoretically provides furthersupport for the adequacy of the pump-leak model. However, it appeared paradoxical that cultured lenses showing a potential difference significantly below that found in situ, namely 60–70 mV, still maintained cation concentration gradients at essentially the level found in vivo, since the gradients themselves are thought to be potentially dependent. In an effort to resolve the apparent discrepancy, the rate of active transport of 86Rb, the equivalent of K, was studied under conditions in which the potential in the culture period involved was maintained at −60 mV and found to decrease by about 10%. When the electrogenic component was recalculated using the reduced value of the coefficient of active transport, K p , the electrogenic component was found to be approximately −20 mV. The higher transport rate for potassium associated with the lower potential is consistent with the operation of a feedback mechanism in which the rate of active transport increases as the potential falls, thus keeping the influx constant which results in maintaining normal concentration gradients.