Calcium transport in the lens.

Evidence based on the following three observations suggests the existence of a calcium transport system in the mammalian lens: calcium levels in the lens are lower than that measured in the aqueous humor; calcium efflux is temperature-dependent and is reduced by inhibitors of Ca++ transport; and there exists a calcium-acivated, magnesium-dependent ATPase. In rat, bovine, dog, and rabbit lenses, the concentration of total calcium was found to be approximately 0.2 mM, at least an order of magnitude lower than that found in the aqueous humor. To determine the nature of the mechanism responsible for maintaining these low levels, calcium fluxes were measured. During the initial rapid phase of 45Ca efflux, the rate at 4 degrees C was reduced by 85% compared with that found at 37 degrees C. Efflux was not altered in the absence of external Na+. Calcium efflux was reduced, however, by lanthanum and propranolol, inhibitors of Ca/Mg ATPase. The presence of Ca/Mg ATPase was also demonstrated in the rat, bovine, and rabbit lens and was likewise inhibited by both lanthum and propranolol.