Sensitivity differences of SA and AV node to vagal stimulation: attenuation of vagal effects at SA node

The present study sought to characterize vagally induced desensitization at the sinoatrial (SA) node of the intact dog and to determine whether these findings could be extended to atrioventricular (AV) Nodal conduction. Autonomically decentralized anesthetized dogs were instrumented to record electrocardiograms, arterial pressure, and electrograms from the SA node, right atrium, right ventricle, and His bundle. During right- or left-vagal stimulation for 60 s, cycle length (CL) increased initially, followed by a gradual recovery toward control during stimulation; atrio-His (AH) interval remained prolonged throughout the stimulation. Atrial drive increased AH interval during stimulation, but AH prolongation was maintained throughout the stimulation period. To rule out acetylcholine (ACh) depletion, ACh was administered before and during vagal stimulation. Although control CL and AH interval were lengthened by ACh, no significant CL increases occurred when ACh was given during stimulation, but the AH interval became longer. Propranolol increased control CL and AH interval but did not alter the relationship between vagal stimulation and CL or AH interval. Thus the negative chronotropic effect of ACh at the SA node is diminished upon prolonged vagal stimulation, whereas the negative dromotropic effect upon AV nodal conduction remains unaltered. These results suggest differences in muscarinic receptor behavior at the SA and AV nodes.