Cytotoxic effects of calcium on sodium-potassium transport in the mammalian lens

Abstract

Experiments were designed to investigate the effects of lens calcium on the Na/K pump since elevated calcium and sodium often accompanies lens opacification. An increase in lens calcium was accomplished by culturing lenses in calcium-rich media at 21 degrees C in the absence of poisons which might directly alter sodium transport. Calcium loaded lenses were subsequently incubated at 37 degrees C in a normal culture medium to assess membrane transport function. A five-fold increase in total lens calcium to 1.45mM, resulting in a 4% increase in membrane bound calcium, had little effect on the cation pump. However, an increase in lens calcium by a factor of 100 caused a 4-fold increase in membrane bound calcium, a 50% decrease in (Na/K)-ATPase activity, and a 40% decrease in 86Rb uptake after an incubation period of 20 hrs. Details of the mechanism of enzyme inhibition remain obscure, but the data suggest the inhibition is predominantly irreversible and does not involve an interaction between calcium and ATP.