Positive inotropic effect of insulin on rabbit auricle in vitro

Water soluble pig insulin (4 x 10(-8) to 4 x 10(-7) g/ml) produced a marked and long-lasting increase in the contractile force of the rabbit auricle in vitro. Once the maximum effect for a given insulin concentration had been reached, addition of more insulin did not produce any further increase in inotropic effect. Insulin was without effect in reserpinized animals. Inhibition of cardiac beta-receptors by propranolol suppressed the positive inotropic effect of insulin. These findings support the hypothesis that insulin releases catecholamines from the myocardium.