AJP - Heart and Circulatory Physiology, Vol 241, Issue 5 684-H690, Copyright © 1981 by American Physiological Society
Sensitivity differences of SA and AV node to vagal stimulation: attenuation of vagal effects at SA node
J. M. Loeb, D. P. Dalton and J. M. Moran
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.
