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Spectral analysis of the systolic arterial pressure (SAP) and R-R interval changes to define the influence of captopril infusion on autonomic circulation regulation in conscious dogs shows that captopril suppresses low-frequency SAP (LFSAP) and blocks the cardiac sympathetic excitatory processes. Captopril suppression of LFSAP sympathetic vasomotor regulation marker during excitation experiments with dynamic treadmill exercise, transient coronary artery occlusion and bilateral carotid artery occlusion confirm the effect of captopril on excitatory responses.

Importance of ventilation in modulating interaction between sympathetic drive and cardiovascular variability. Am J Physiol Heart Circ Physiol 280: H722-H729, 2001.--Chemoreflex stimulation elicits both hyperventilation and sympathetic activation, each of which may have different influences on oscillatory characteristics of cardiovascular variability. We examined the influence of hyperventilation on the interactions between changes in R-R interval (RR) and muscle sympathetic nerve activity (MSNA) and changes in neurocirculatory variability, in 14 healthy subjects. We performed spectral analysis of RR and MSNA variability during each of the following interventions: 1) controlled breathing, 2) maximal end-expiratory apnea, 3) isocapnic voluntary hyperventilation, and 4) hypercapnia-induced hyperventilation. MSNA increased from 100% during controlled breathing to 170 [+ or -] 25% during apnea (P = 0.02). RR was unchanged, but normalized low-frequency (LF) variability of both RR and MSNA increased markedly (P [is less than] 0.001). During isocapnic hyperventilation, minute ventilation increased to 20.2 [+ or -] 1.4 l/min (P [is less than] 0.0001). During hypercapnic hyperventilation, minute ventilation also increased (to 19.7 [+ or -] 1.7 l/min) as did end-tidal [CO.sub.2] (both P [is less than] 0.0001). MSNA remained unchanged during isocapnic hyperventilation (104 [+ or -] 7%) but increased to 241 [+ or -] 49% during hypercapnic hyperventilation (P [is less than] 0.01). RR decreased during both isocapnic and hypercapnic hyperventilation (P [is less than] 0.05). However, normalized LF variability of RR and of MSNA decreased (P [is less than] 0.05) during both isocapnic and hypercapnic hyperventilation, despite the tachycardia and heightened sympathetic nerve traffic. In conclusion, marked respiratory oscillations in autonomic drive induced by hyperventilation may induce dissociation between RR, MSNA, and neurocirculatory variability, perhaps by suppressing central genesis and/or inhibiting transmission of LF cardiovascular rhythms. autonomic nervous system; spectral analysis; hyperventilation; apnea; chemoreflexes

Several letters to the editor are presented in response to the article "Point:Counterpoint: Respiratory sinus arrhythmia is due to central mechanism vs. respiratory sinus arrhythmia is due to the baroreflex mechanism."