The cardiodynamic and metabolic effects of glucagon.

The cardiac effects of a continuous infusion of glucagon at the rate of 10 mug/min were studied in the Starling heart-lung preparation, modified to measure coronary flow and myocardial oxygen consumption. A maximal increase in myocardial contractility, as reflected by maximal rate of rise of left ventricular pressure, dp/dt, of 31% was observed at a total dose of glucagon of 50 mug and was accompanied by an increase in heart rate and in myocardial oxygen consumption of 59% and 57%, respectively. At a total dose of glucagon of 100 mug, there was an additional and comparable increase only in heart rate and myocardial oxygen consumption of 11.2% and 6.4% respectively. Similarly, at a total dose of glucagon of 150 mug, only heart rate and myocardial oxygen consumption increased additionally by increments of 2.6% and 2.9% respectively. These effects occurred at constant aortic pressure and left ventricular volume. Further infusion of glucagon led to an additional increase only in myocardial oxygen consumption of 4.2%. When the increase in heart rate was largely prevented by prior treatment with veratramine, an increase in dp/dt, not significantly different from the maximal increase obtained with glucagon alone, was accompanied by much lower and closely comparable increases in heart rate and in myocardial oxygen consumption of 15% and 19%, respectively. Coronary flow increased more markedly when glucagon was administered alone and it paralleled the increase in myocardial oxygen consumption. It may be concluded from this study that, in the isolated dog heart preparation, glucagon increases contractility, heart rate and myocardial oxygen consumption and that the increase in myocardial oxygen consumption is related more closely to the increase in heart rate than to the increase in contractility, but a minor increment is referable to a calorigenic action. The increase in coronary flow is of a secondary nature, resulting from the increase in myocardial metabolic demands.