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Department of Physiology, Medical College of Virginia, Richmond.

1. Ventricular tachycardia generates complex changes in baroreceptor input to the central nervous system: arterial baroreceptors are unloaded while cardiopulmonary receptors are stimulated. In humans with heart diseases, muscle sympathetic nerve activity increases during ventricular tachycardia. This suggests that arterial baroreceptor-mediated sympathoexcitation overrides cardiopulmonary receptor-mediated sympathoinhibition. However, the relative roles of each reflex are difficult to determine in humans. 2. We measured efferent renal sympathetic neural responses to simulated ventricular tachycardia, to determine what pathophysiological mechanisms are invoked when inputs from different baroreceptive areas change in opposite directions. In alpha-chloralose anaesthetized, mechanically ventilated dogs, we recorded the electrocardiogram, mean left atrial and arterial pressures and multifibre efferent renal sympathetic nerve activity (RSNA) during 1 min of right ventricular pacing at 214 beats min-1. Pacing was repeated after either sinoaortic or vagal cardiopulmonary denervation and again after both sinoaortic and cardiopulmonary denervation. 3. With all afferent baroreceptor pathways intact, right ventricular pacing elicited transient sympathoinhibition (delta RSNA, -19 +/- 10%, mean +/- S.E.M.). After sinoaortic denervation (cardiopulmonary receptors intact), right ventricular pacing elicited abrupt and sustained sympathoinhibition (delta RSNA, -53 +/- 8%, P < 0.05 vs. intact). After vagal cardiopulmonary denervation (sinoaortic receptors intact), right ventricular pacing elicited abrupt and sustained sympathoexcitation (delta RSNA, + 56 +/- 19%, P < 0.05 vs. intact). After both sinoaortic and vagal cardiopulmonary denervation, right ventricular pacing elicited a gradual increase in sympathetic outflow (delta RSNA, + 16 +/- 6%, P < 0.05 vs. intact). 4. We conclude that interactions between vagal cardiopulmonary and arterial baroreflexes determine renal sympathetic outflow during simulated ventricular tachycardia. In healthy anaesthetized dogs, the balance of the two opposing reflexes is weighted towards vagal cardiopulmonary-mediated sympathoinhibition.

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