Calcium and pancreatic β-cell function. XI. Modification of ⁴⁵Ca fluxes by Na⁺ removal

Pancreatic islets were isolated from noninbred ob/ob mice and used for evaluating how removal of extracellular Na⁺ affects ⁴⁵Ca fluxes in the insulin-producing β cells. Isoosmotic replacement of Na⁺ with Li⁺ was less efficient in stimulating the incorporation of intracellular (La³⁺ nondisplaceable) ⁴⁵Ca than substitution with choline⁺ or sucrose. Ample support for the significance of Na⁺-Ca²⁺ exchange for the net efflux of Ca²⁺ was obtained with the observation that Na⁺ deprivation markedly inhibited the ⁴⁵Ca washout in a Ca²⁺-deficient medium. The inhibitory action of Na⁺ deficiency exceeded that obtained with 20 mM glucose. The ⁴⁵Ca incorporated after removal of Na⁺ differed from that taken in response to glucose in not being exchanged with the Ca²⁺ entering the β cells during glucose stimulation and in appearing also in the postmicrosomal supernatant obtained by differential centrifugation.