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Abstract
In many tissues, the level of cytoplasmic calcium mediates cell function. Since cytoplasmic calcium is often maintained at a low level by active calcium extrusion, we examined whether calcium-stimulated ATPase is present in the rabbit ciliary epithelium. A technique was developed to measure calcium-stimulated ATPase in a partially enriched plasma membrane preparation. The enhancement of Na,K-ATPase activity was used to indicate the enrichment of plasma membrane. Marked stimulation of ATPase activity by calcium was observed over a range of calcium concentrations (10(-8) to 10(-3) M). The calcium concentration necessary to elicit half-maximal ATPase activity was 10(-6) M, which is similar to that reported for other membrane preparations. Calcium-stimulated ATPase activity was significantly inhibited in the presence of low concentrations of sodium orthovanadate. The inhibitory influence of vanadate was examined over a range of vanadate concentrations (10(-8) to 10(-3) M). The vanadate concentration needed to produce half-maximal inhibition of calcium-stimulated ATPase was 2 X 10(-6) M. These studies show that calcium-stimulated ATPase inhibition can occur, in vitro, at very low levels of vanadate; it is possible that this might contribute to the chain of events which results in the lowering of aqueous humor secretion reported in vanadate-treated rabbits.