Na+/Ca2+ countertransport in plasma membrane of rat pancreatic acinar cells

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Abstract
The presence of a coupled Na+/Ca2+ exchange system has been demonstrated in plasma membrane vesicles from rat pancreatic acinar cells. Na+/Ca2+ exchange was investigated by measuring45Ca2+ uptake and45Ca2+ efflux in the presence of sodium gradients and at different electrical potential differences across the membrane (=Δϕ) in the presence of sodium. Plasma membranes were prepared by a MgCl2 precipitation method and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the plasma membrane, (Na++K+)-ATPase was enriched by 23-fold. Markers for the endoplasmic reticulum, such as RNA and NADPH cytochromec reductase, as well as for mitochondria, the cytochromec oxidase, were reduced by twofold, threefold and 10-fold, respectively. For the Na+/Ca2+ countertransport system, the Ca2+ uptake after 1 min of incubation was half-maximal at 0.62 μmol/liter Ca2+ and at 20 mmol/liter Na+ concentration and maximal at 10 μmol/liter Ca2+ and 150 mmol/liter Na+ concentration, respecitively. When Na+ was replaced by Li+, maximal Ca2+ uptake was 75% as compared to that in the presence of Na+. Amiloride (10−3 mol/liter) at 200 mmol/liter Na+ did not inhibit Na+/Ca2+ countertransport, whereas at low Na+ concentration (25 mmol/liter) amiloride exhibited dose-dependent inhibition to be 62% at 10−2 mol/liter. CFCCP (10−5 mol/liter) did not influence Na+/Ca2+ countertransport. Monensin inhibited dose dependently; at a concentration of 5×10−6 mol/liter inhibition was 80%. A SCN− or K+ diffusion potential (=Δϕ), being positive at the vesicle inside, stimulated calcium uptake in the presence of sodium suggesting that Na+/Ca2+ countertransport operates electrogenically, i.e. with a stoichiometry higher than 2 Na+ for 1 Ca2+. In the absence of Na+, Δϕ did not promote Ca2+ uptake. We conclude that in addition to ATP-dependent Ca2+ outward transport as characterized previously (E. Bayerdörffer, L. Eckhardt, W. Haase & 1. Schulz, 1985,J. Membrane Biol. 84:45–60) the Na+/Ca2+ countertransport system, as characterized in this study, represents a second transport system for the extrusion of calcium from the cell. Furthermore, the high affinity for calcium suggests that this system might participate in the regulation of the cytosolic free Ca2+ level.

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