Blockade by verapamil of cholinergic effects on atrial specialised tissue in the anaesthetised dog

The effects of verapamil were studied in anaesthetised dogs administered dextromoramide intrathecally to provide background vagal tone. Measurements were made of spontaneous heart rate, and, in paced hearts, of conduction times in atrial muscle, the atrioventricular node (A-V node) and His-Purkinje system by means of His bundle potential recording. The effective refractory period (ERP) of A-V node was measured by the extrastimulus method. In atropinised and vagotomised animals, verapamil reduced sinus rate and increased A-V nodal conduction time. In dogs high vagal tone after dextromoramide, however, verapamil increased sinus rate and reduced A-V nodal ERP. After dextromoramide alone, A-V block was observed at an atrial pacing rate of 150 beats X min-1, but after verapamil 1:1 A-V conduction was restored. The decrease in conduction velocity in the A-V node due to ACh was neither attenuated nor enhanced by verapamil.