Abstract

Although insulin is known to elicit a positive inotropic effect in cardiac muscle preparations, very little is known concerning the mechanism of this action. In view of the crucial role played by the sarcoplasmic reticular (SR) calcium transport in cardiac contractile events, the effects of insulin on the pig heart SR were investigated. Insulin activated the SR Ca++-stimulated adenosine triphosphatase (ATPase) in a concentration-dependent manner (0.1 mU to 1 U/ml); maximal activation (125%) was seen at 0.1 to 1 U/ml of insulin. Kinetic studies revealed that the insulin-induced activation was due to an increase in the apparent Vmax of Ca++-stimulated ATPase without any alteration in the Km. Insulin was found to bind with SR membranes in a specific manner and this binding was rapid, saturable and displacable. The dose-related increase in the activation of Ca++-stimulated ATPase was related linearly (r = 0.98) to binding of insulin with SR membranes; 50% activation of Ca++-stimulated ATPase was found to occur at 13.5 fmol of insulin binding per mg of SR protein. When insulin was allowed to dissociate by a 100-fold dilution of the insulin-receptor complex, the activity of SR Ca++-stimulated ATPase also declined gradually. Furthermore, proteolytic digestion on the membrane with trypsin (3 micrograms/mg of protein) decreased both insulin binding as well as the increase in Ca++-stimulated ATPase activity by about 50%.(ABSTRACT TRUNCATED AT 250 WORDS)