THE EFFECT OF GLUCAGON ON CALCIUM EXCHANGEABILITY, CORONARY BLOOD FLOW, MYOCARDIAL FUNCTION AND HIGH ENERGY PHOSPHATE STORES

¹ Baker Medical Research Institute, Melbourne, Victoria, Australia

The positive inotropic effect of glucagon on isometrically contracting dog papillary muscle preparations was found to be dose-dependent and antagonized by Mn⁺⁺. This positive inotropic effect of glucagon was accompanied by a significant increase in Ca⁺⁺ (as Ca⁴⁵) exchangeability in intact heart muscle, and this effect of glucagon was antagonized by Mn⁺⁺. Glucagon failed to cause any significant change in the Ca⁺⁺-accumulating activity of microsomal or mitochondrial fractions isolated from heart muscle, nor did it displace Ca⁺⁺ from these fractions. Left ventricular work function studies showed that glucagon increased left ventricular work per minute, coronary blood flow and myocardial oxygen consumption in such a way that the ratio between left ventricular work per minute and oxygen consumption was reduced. Adenosine triphosphate and creatine phosphate stores remained unchanged. These results have been interpreted to mean that glucagon increases the Ca⁺⁺ which is accumulated by heart muscle during excitation, and that this accumulation may result from the effect of glucagon on the adenyl cyclase enzyme.