Glucagon-Enhanced Ventricular Automaticity in Dogs

Ventricular automaticity was studied in dogs with intact atrioventricular conduction systems and in dogs with surgically interrupted conduction systems before and after glucagon, 50 µ.g/kg. Results of these studies are in agreement with previous studies. When the influence of rate-dependent suppression of ventricular automaticity was nullified by comparing automaticity during periods of equal heart rates, highly significant increases in automaticity were observed in both preparations after glucagon administration. One index of automaticity, the number of ventricular beats occurring in a 30-second period following onset of vagal stimulation, increased from 8.7 ± 2.6 to 14.7 ± 3.1 after glucagon administration, an increase of 69%. The number of ventricular beats in a 30-second period following cessation of ventricular pacing increased from 8.6 ± 2.5 to 14.9 ± 2.1 after glucagon administration in dogs with heart block, an increase of 73%. Automaticity increases were also noted using other indexes. Beta-receptor blockade with propranolol failed to abolish automaticity changes effected by glucagon. This study demonstrated increases in ventricular automaticity produced by glucagon that heretofore had been overlooked, because a rate-dependent decrease in ventricular automaticity secondary to the positive chronotropic effect of this hormone masked the increases in automaticity.