Relationship between Extracellular Na⁺ and the Total Ionized Ca²⁺ Content of Rat Pancreatic Islets

Abstract

It is assumed that sodium ions, Na⁺, are involved in a Na-Ca exchange process at the β-cell membrane and in the mobilization of calcium ions, Ca²⁺, from intracellular stores^4,5. Na-Ca exchange, which exchanges extracellular Na⁺ for intracellular Ca²⁺, has been observed in several cell types and has been characterized in detail in cardiac cells and membrane vesicles⁶. Na-Ca exchange depends on the electrochemical gradient of Na⁺ (and of Ca²⁺) across the cell membrane. This process is sensitive to changes in the membrane potential and the extracellular Na⁺ concentration, [Na⁺]_o ⁶. If this process exists in β-cells, depolarization of the membrane will inhibit the exchange and enhance the intracellular Ca²⁺ concentration, [Ca²⁺]_i, in the β-cells.