The influence of calcium on the rabbit lens sodium pump

Purpose. Experiments were conducted to examine how elevation of calcium in the lens impairs the lens sodium pump. Methods. Rabbit lenses were incubated in the presence or absence of calcium ionophore A23187. ⁸⁶Rb uptake by the intact lens was measured as an index of sodium pump activity. Na,K-ATPase (ouabain-sensitive adenosine triphosphate [ATP] hydrolysis) activity was determined in membrane material isolated from the lens. Lens ion content and ATP content also were determined. Results. Rabbit lenses gained calcium after exposure to calcium ionophore A23187 or ionomycin. Ionophore-treated lenses also gained sodium and lost potassium. A diminished rate of ⁸⁶Rb uptake observed in ionophore-treated lenses suggests that elevation of lens calcium leads to sodium pump inhibition. In contrast, the rate of ⁸⁶Rb efflux was not altered by A23187, indicating that elevated lens calcium causes little change in passive cation permeability. Membranes isolated from A23187- treated lenses were found to have normal Na,K-ATPase activity. However, calcium had a small direct inhibitory effect upon the Na,K-ATPase activity measured in freshly prepared lens membranes isolated from control (nonionophore-treated) lenses. Using a luciferase assay, A23187-treated lenses were found to have a normal ATP content. Conclusions. Calcium may impair the ability of the lens Na,K-ATPase to pump ions in the intact lens, but appears to leave the ATP-hydrolyzing capability of the isolated enzyme unchanged.