Dynamic vagal control of pacemaker activity in the mammalian sinoatrial node.

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Abstract
Dynamic heart rate control by parasympathetic nervous input involves feedback mechanisms and reflex bursting of efferent cardiac vagal fibers. Periodic vagal bursting induces phasic changes in sinoatrial cycle length and can entrain the pacemaker to beat at periods that may be identical to those of the vagal burst. We investigated the electrophysiological basis of these phenomena in isolated sinus node preparations (rabbit, cat, and sheep). In the presence of propranolol (3.9 X 10(-6)M), relatively brief (50-150 msec) trains of stimuli, applied onto the endocardial surface of the preparation, activated postganglionic vagal terminals and induced a brief hyperpolarization of sinoatrial pacemaker cells. This vagally mediated hyperpolarization could alter the pacemaker rhythm by an amount that depended on its duration and its position in the cycle, as well as on the duration of the free-running pacemaker period. When the free-running period was sufficiently long and the hyperpolarization was induced sufficiently early in the spontaneous cycle, a "paradoxical" acceleration of the pacemaker rhythm ensued. Phasic changes were plotted on phase-response curves, constructed by scanning systematically the sinoatrial pacemaker period with single or repetitive vagal trains. These phase-response curves enabled us to predict the entrainment characteristics and the levels of synchronization of the pacemaker to the vagal periodicity. The overall data explain the cellular mechanisms involved in the phasic effects of brief vagal discharges on sinoatrial periodicity, and provide conclusive evidence for the prediction that repetitive vagal input is capable of forcing the cardiac pacemaker to beat at rates that can be faster or slower than the intrinsic pacemaker rate. These data should improve our knowledge of the dynamic control of heart rate by neural reflexes and aid in our understanding of rhythm disturbances generated by the interaction of the cardiac pacemaker with vagal activity.