Glucagon. Its enhancement of cardiac performance in the cat and dog and persistence of its inotropic action despite beta-receptor blockade with propranolol.

The action of glucagon on cardiac performance was studied -in 21 isolated cat papillary muscle preparations, in 13 spontaneously beating cat atria, in 15 intact dog hearts, and in 4 isolated perfused dog hindlimbs. In each papillary muscle preparation, addition of glucagon produced marked increases in maximal developed tension, averaging 36 ± 4.2% (SEM) (P<0.01) , and shifted the force-velocity curve upwards and to the right, indicating that contractility was augmented. Glucagon always increased the rate of the spontaneously beating atrium, the rise averaging 28.8 ± 5.5 contractions/min (P<0.01). In the dog, myocardial performance was markedly augmented by the administration of glucagon, 50 /xg/kg iv, as indicated by an average increase of 72.2 ± 18.4% (P<0.01) in the left ventricular peak dP/dt and of 58.9 ± 12.8% (P<0.01) in force recorded by a strain gauge arch, despite an average decrease of 3.8 ± 1.2 cm H2O (P<0.02) in left ventricular end-diastolic pressure. Heart rate rose an average of 38.7 ± 10.9 beats/min (P<0.02) . Small but significant decreases in peripheral vascular resistance were produced. Single intravenous injections produced effects lasting 15 to 20 minutes. Propranolol did not prevent die inotropic responses in either the cat or dog preparations but markedly decreased the chronotropic effects. ADDITIONAL