Direct addition of insulin inhibits a high affinity Ca²⁺-ATPase in isolated adipocyte plasma membranes

Abstract

The mechanism by which insulin regulates cellular metabolism remains unknown although indirect evidence suggests that alterations in intracellular calcium are important. More specifically, it has been proposed that insulin triggers an increase in intracellular calcium which is responsible for the subsequent modification of metabolic activities^1,2. The cell maintains a large electrochemical gradient for ionised calcium between the cytoplasm (<10⁻⁶ M, as determined for muscle³ and nerve⁴) and the extracellular environment (>10⁻³ M). The plasma membrane may, therefore, be important in the regulation of calcium homeostasis, as a slight alteration in the processes maintaining this gradient could result in marked changes in cytoplasmic calcium. One such process is the active extrusion of calcium from the cell by a high affinity calcium-stimulated ATPase (Ca²⁺-ATPase). Such a mechanism has been well established in red cells⁵ and is postulated in nerve⁶, liver⁷ and muscle⁸. We have identified a high affinity Ca²⁺- ATPase in a plasma membrane-enriched subcellular fraction isolated from rat adipocytes⁹ which may provide the enzymatic basis for a calcium extrusion pump. We demonstrate here that the Ca²⁺-ATPase is specifically inhibited by the direct addition of physiological concentrations of insulin to the isolated plasma membranes. This effect suggests that direct regulation of calcium homeostasis may represent an important event in the mechanism of action of insulin.