Functional interdependence of discrete vagal projections to SA and AV nodes

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Abstract
Dynamic modulation of chronotropic and dromotropic function was evaluated in anesthetized dogs before and after selective parasympathectomy of the atrioventricular (AV) node. Vagal and cardiac sympathetic efferent nerves were decentralized, and the distal cut ends of the cervical vagi were stimulated at frequencies from 1 to 25 Hz with step-wise voltage changes. To investigate parasympathetic modulation of AV conduction, stimulations were performed with and without atrial pacing. After determination of chronotropic and dromotropic responses, in the intact state, selective AV node parasympathectomy was performed as previously described [Am. J. Physiol. 248 (Heart Circ. Physiol. 17): H61-H68, 1985]. Stimulation protocols were then repeated. Control results in intact animals reveal a parallel and finely balanced vagal regulation of chronotropic and dromotropic responses over a wide range of frequency stimulations. Conversely, selective parasympathectomy of the AV node interrupts extrinsic vagal modulation of AV conduction without affecting parasympathetic control of chronotropic function. The dynamic balance of chronotropic and dromotropic function, modulated by extrinsic vagal input, reflects the parallel activation of functionally and anatomically distinct parasympathetic projections to the sinoatrial (SA) and AV nodal tissue. Thus present experiments allow a more detailed examination of vagal innervation of the SA and AV nodes, the importance of their balance through neuroregulation, and progress toward understanding the pathophysiology of disturbances to this balance.