Attenuation of arterial baroreflex control of heart rate by left ventricular receptor stimulation in the conscious dog.

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Abstract
The purpose of this investigation was to determine whether left ventricular receptor stimulation attenuates the arterial baroreflex control of heart rate in the conscious dog and to determine the role of cardiac efferent sympathetic and parasympathetic pathways in any interaction observed. Mean arterial blood pressure-heart rate function curves, which characterized arterial baroreflex control of heart rate, were constructed before (control) and during an infusion of veratrine into the left circumflex coronary artery. Peak sensitivity, the maximum absolute slope along the mean arterial blood pressure-heart rate curve, and heart rate range (maximum minus minimum heart rate) were reduced during intracoronary infusion of veratrine. The mean arterial blood pressure-heart rate relationship also was shifted to a lower pressure during intracoronary infusion of veratrine. In order to study the role of cardiac efferents in this interaction, we constructed mean arterial blood-pressure-heart rate curves during cholinergic blockade, cholinergic blockade plus intracoronary infusion of veratrine, beta 1-adrenergic blockade, and beta 1-adrenergic blockade plus intracoronary infusion of veratrine. The addition of intracoronary infusion of veratrine during cholinergic blockade produced a shift of the mean arterial blood pressure-heart rate curve down the ordinate axis (heart rate) and to a lower pressure; however, peak sensitivity and heart rate range remained unchanged. The addition of intracoronary infusion of veratrine during beta 1-adrenergic blockade resulted in reductions in peak sensitivity and heart rate range. These data indicate that left ventricular receptor stimulation attenuates arterial baroreflex control of heart rate and that the reduction of sensitivity and heart rate range is mediated by parasympathetic motoneurons common to both reflex arcs. On the other hand, resetting to a lower operational set point may be mediated by cardiac sympathetic motoneurons common to both reflex arcs.