This content is PDF only. Please click on the PDF icon to access.
Abstract
A theoretical model of a pump-leak system4 that takes into account the effect of concentration of both substrate and inhibitor on the pump in terms of Michaelis-Menten kinetics and the effect of electric, as well as chemical, forces on the leak has been shown to describe accurately the movement of potassium, rubidium, and cesium into and out of the lens. Values, calculated on the basis of the model for parameters governing the pump, support the conclusion that these cations are actively transported into the lens by a carrier-mediated system which has a single kind of active site and which preferentially transports potassium and rubidium with respect to cesium. The electric forces have a significant effect on transport of these ions, increasing influx by about 15 per cent and decreasing efflux by over 60 per cent. The computed values of the permeability coefficient for the leak are consistent with the hypothesis that permeability of the lens for the three ions is dependent on absorptive rather than frictional forces, i.e., on the anionic field strength of the membrane that limits diffusion. The values are incompatible with any model based on purely structural considerations of the lens membrane in which transport is assumed to occur by sieving through hydrated pores.