Our previous finding that increasing myocardial contractility caused reflex systemic hypotension, the left ventricular (LV) mechanoreceptor reflex, suggested that the classical Bezold reflex (systemic hypotension and bradycardia after intracoronary administration of veratrum alkaloids) may be initiated by these same LV mechanoreceptors. In our working LV preparation with the coronary and systemic circulations isolated and perfused separately, intracoronary injection of veratrum alkaloids, like that of catecholamines or ouabain, had a positive inotropic effect which produced the hypotensive response typical of the LV mechanoreceptor reflex. To test directly if veratridine's positive inotropic effect initiates the Bezold reflex, verapamil, which blocks the slow Ca2+ channels of myocardial cells but leaves intracardiac nerves unaffected, was injected by the intracoronary route to prevent the increased contractility from intracoronary injection of veratridine which also abolished the reflex hypotension, demonstrating conclusively that increasing myocardial contractility and thereby activating LV mechanoreceptors but not chemoreceptors initiates the Bezold reflex. Contrariwise, decreasing contractility or cardiac asystole by administration of tetrodotoxin, verapamil, or EDTA resulted in an increase in the systemic resistance, indicating that changes in the magnitude of the stimulus initiating the LV mechanoreceptor reflex (i.e., changes in myocardial contractility) lead to directionally opposite changes in peripheral resistance, as in the sino-aortic mechanoreflexes. Thus, it is concluded that the Bezold reflex is a special case of the LV mechanoreceptor reflex. The latter, by means of feedback mechanisms, functions normally by continuously matching the peripheral resistance to the LV contractile state so as to maintain the arterial pressure constant, thereby playing an important role in blood pressure regulation.
