Na⁺ Ca²⁺ exchange in plasma membrane vesicles from a glucose-responsive insulinoma

Plasma membrane vesicles from a glucose-responsive insulinoma exhibited properties consistent with the presence of a membrane Na⁺ Ca²⁺ exchange. The exchange was rapid, reversible, and was dependent on the external Ca²⁺ concentration (K_m = 4.1 ± 1.1 μM). External Na⁺ inhibited the uptake in a dose-dependent manner (IC₅₀ = 15 mM). Dissipation of the Na⁺ gradient by 10 μM monensin decreased Na⁺ Ca²⁺ exchange from 0.74 ± 0.17 nmoles/mg protein/s to 0.11 ± 0.05 nmoles/mg protein/s. Exchange was not influenced by veratridine, tetrodotoxin and ouabain, or by modifiers of cAMP. No effect was seen using the calcium channel blockers, nitrendipine or nifedipine. Glucose had no direct effect on Na⁺ Ca²⁺ exchange, while glyceraldehyde, glyceraldehyde-3-phosphate and dihydroxyacetone inhibited the exchange. Na⁺ induced efflux of calcium was seen in Ca²⁺ loaded vesicles and was half maximal at [Na⁺] of 11.1 ± 0.75 mM. Ca²⁺ efflux was dependent on [Na⁺], with a Hill coefficient of 2.7 ± 0.07 indicating that activation of Ca²⁺ release involves a minimum of three sites. The electrogenicity of this exchange was demonstrated using the lipophilic cation tetraphenylphosphonium ([³H]-TPP), a membrane potential sensitive probe. [³H]-TPP uptake increased transiently during Na⁺ Ca²⁺ exchange indicating that the exchange generated a membrane potential. These results show that Na⁺ Ca²⁺ exchange operates in the beta cell and may be an important regulator of intracellular free Ca²⁺ concentrations.