REFLEX VASCULAR-RESPONSES TO CHANGES IN LEFT-VENTRICULAR PRESSURES, HEART-RATE AND INOTROPIC STATE IN DOGS

Dogs were anaesthetized with chloralose, artificially ventilated and the chests widely opened. Left ventricular mechanoreceptors, including those in or near the coronary arteries, were stimulated by changing the pressure in the aortic root. The pressures distending the left atrium and the aortic and carotid baroreceptors were controlled. Reflex vascular responses were assessed from changes in perfusion pressures to a hind limb and to the rest of the systemic circulation, which were perfused independently at constant flows. Physiological increases in peak left ventricular and coronary arterial pressures resulted in vasodilatation in both regions. These responses were not influenced by changes in the heart rate. Stimulation of the left cardiac sympathetic nerves resulted in increases in peak ventricular pressure and in the maximal rate of change of pressure (dP/dt(max)). This also resulted in increases in perfusion pressures (vasoconstriction) at all levels of peak ventricular pressure although there was little effect on the responses to changes in ventricular pressure. Sympathetic stimulation had little effect on the relationship between perfusion pressures and aortic root pressure. Increases in ventricular filling also resulted in vasoconstriction at all levels of peak ventricular pressure. Increases in filling, however, did not affect the relationship between either perfusion pressure and aortic root pressure. Conversely, decreases in left ventricular filling, by bypassing some of the left atrial blood, resulted in vasodilatation at all levels of peak ventricular pressures but had no effect on the perfusion pressures at any aortic root pressure. The combination of sympathetic stimulation with decreased ventricular filling resulted in little effect on perfusion pressures or on their responses to changes in either aortic root or ventricular systolic pressures. We conclude that the vascular responses to stimulation of left ventricular mechanoreceptors are not enhanced by sympathetic stimulation, decreases in ventricular filling or the combination of the two. The apparent effects of each of these interventions alone on the relationships between perfusion pressures and ventricular, but not aortic root, pressure, could be explained if the receptors responsible were sensitive more to changes in aortic root and coronary arterial pressures than to pressure changes in the ventricle itself.