Studies on the Crystalline Lens XIII. Kinetics of Potassium Transport

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Abstract

The rate of accumulation of ⁴²K in cultured rabbit lenses was measured and the coefficients for the parameters concerned with active transport into, and diffusion out of, the lens were evaluated using an analog computer to establish fits to a simple equation that described a pump-leak system. The turnover rate of potassium in lenses cultured under conditions essentially like those found in vivo was 4.5 to 5.0 per cent per hour, which corresponds to a flux of approximately 1.10 µmoles per lens per hour. The rate of accumulation of ⁴²K in cultured lenses decreased when concentration of nonlabeled potassium in the media was increased, suggesting that a carrier system responsible for active transport had become saturated and thus limited the capacity of the pump. A series of successive approximations, to compensate for the changed concentration of both labeled and nonlabeled potassium in the media during the culture experiments, was employed to establish the effect of concentration of potassium in the media on the velocity of the pump. The data from these experiments were found, to obey Michaelis-Menten kinetics: the apparent constant, K_m, and maximum velocity, V_max were 0.85 mmole per liter and 1.3 µmoles per lens per hour, respectively. With these fundamental constants which characterize the carrier system for active transport, a general pump-leak equation was devised. The equation was used to make separate calculations of the quantities of potassium that accumulated and that were actually pumped into lenses under conditions when the concentrationof labeled and nonlabeled ion remained constant.