Modification of the function of cardiac subcellular organelles by insulin

Although insulin is known to elicit a positive inotropic effect in cardiac-muscle preparations, little is known concerning the mechanism responsible for this action. Because various subcellular organelles such as mitochondria, sarcoplasmic reticulum (SR), sarcolemma, and myofibrils are intimately involved in determining the cardiac contractile function, the effects of insulin (0.1-1000 mU/ml) on some selected enzymatic activities associated with these organelles were investigated. Insulin significantly enhanced Ca2+ uptake and Ca2+-stimulated ATPase activities of SR preparations obtained by two different methods. Insulin had no effect on mitochondrial Ca2+ uptake and ATPase activities or myofibrillar ATPase activities. Sarcolemmal Na+, K+-ATPase activity was stimulated only in the presence of 1 U/ml insulin, whereas sarcolemmal Ca2+ pump activity was increased by all insulin concentrations employed. Sarcolemmal Ca2+-ATPase activity and ATP-independent Ca2+ binding were augmented in the presence of 1 U/ml insulin only. These subcellular effects of insulin, either alone or in concert, may partially explain the positive inotropic action of insulin.