Changes in left ventricular performance, stroke volume, and peripheral vascular resistance were studied in dogs after combined and separate stimulation of the aortic and carotid chemoreceptors. Selective stimulation of the aortic chemoreceptors produced an immediate increase in myocardial contractility as judged by the force developed by a strain gauge arch sewn into the left ventricular myocardium and by changes in the first derivative of the left ventricular pressure. Similar results were seen when heart rate changes were prevented by prior administration of atropine, when changes in outflow impedance were prevented by previous blockade of the alpha-adrenergic receptors, and when there was combined chemoreceptor stimulation. Betaadrenergic blockade prevented the increase in myocardial contractility observed after chemoreceptor stimulation. With selective carotid chemoreceptor stimulation there was no significant change in contractility. Aortic chemoreceptor stimulation increased the heart rate and peripheral vascular resistance, while stroke volume decreased; carotid chemoreceptor stimulation slowed the heart rate and increased the stroke volume, but not change peripheral vascular resistance.ADDITIONAL KEY WORDS ventricular performance chemoreflexes hemodynamics adrenergic receptor blockade sympathetic fibers anesthetized dogs • Recent reports on the effect of chemoreceptor stimulation on myocardial performance seemingly differ. Using anoxia as the stimulus to the carotid chemoreceptors, several investigators (1-3) described a negative inotropic effect. Using combined anoxic stimulation of the carotid and aortic chemoreceptors, other workers (4) described a positive inotropic effect. These opposing findings led us to reevaluate the effects of chemoreceptor stimulation on the performance of the left ventricle. Data indicate that the hemodynamic responses to stimulation of the aortic and carotid chemoreceptors differ (5, 6); therefore, we were careful to separate the effects arising from each of these receptors. We were also able to separate any changes in myocardial contractility induced by direct sympathetic stimulation from those induced by reflex peripheral vasoconstriction and resultant increase in outflow impedance (7, 8) by using drugs to produce separate and combined blockade of the alpha-and beta-adrenergic receptors. We also studied the immediate changes in stroke volume following chemoreceptor stimulation, since measurements of cardiac output in pre-
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