Studies on the crystalline lens: XIX. Quantitative aspects of active and passive transport of sodium

A theoretical model of a pump-leak system that assumes sodium enters the lens by passive diffusion and leaves primarily by active transport against both an electric and chemical gradient has been used to evaluate rate constants for parameters controlling movement of this cation into and out of cultured rabbit lenses. The system is regarded as consisting of three compartments, the medium, capsule and region behind an ion restricting barrier, the fibers. Because of transient changes in the rate of passive diffusion of 22Na in cultured lenses it was not possible to fit data showing the rate of accumulation and net rate of efflux with a single pair of rate constants, unless lenses were precultured for a period of at least 3 hr prior to making the determinations. Under these circumstances, two sets of rate constants ( K p = 0·35 hr −1, K d = 0·013 hr −1 and K p = 0·58 hr −1, K d = 0·010 hr −1) were found that would produce fits to the experimental data showing fluxes in both directions. These were dependent upon the technique employed in cutting the zonules when removing lenses from the eye. The latter pair of values, which predicts the steady state ratio concentration of sodium lens/medium observed in vivo, is thought to correspond to those prevailing in the living eye. Ouabain (10 −5 m) markedly inhibits active transport ( K p to 0·18 hr −1) and increases passive diffusion ( K d to 0·020 hr −1).