Relative roles of vagal and sympathetic effector mechanisms in the baroreflex control of myocardial contractility in conscious rabbits

The relative roles of vagal and sympathetic effector mechanisms in the baroreflex control of myocardial contractility have been assessed in the conscious normotensive and hypertensive rabbit. Graded increases in mean arterial pressure (MAP) were produced by inflation of a balloon occluder around the abdominal aorta. Stimulus response curves relating the change in MAP to the induced change in peak rate of change of left ventricular pressure (peak LV dP/dt) were produced when heart rate was allowed to change and when it was held constant by atrial pacing. These curves were repeated after sympathetic blockade with propranolol, vagal blockade with methylscopolamine and combined blockade with the two drugs together. Increase in MAP produced a reflex fall in peak LV dP/dt which was due to two components. There was a reflex negative inotropic effect which was independent of heart rate, occurring in animals in whom heart rate was held constant by atrial pacing, and there was also a reduction in peak LV dP/dt which was caused by the reflex bradycardia when the heart rate was allowed to change. Both sympathetic and vagal efferents contributed to the reflex fall in peak LV dP/dt seen after elevation of MAP, the sympathetic being primarily responsible for the direct negative inotropic effect and the vagus for the bradycardia and hence the secondary effects on peak LV dP/dt. The slope of the stimulus response curves relating the fall in peak LV dP/dt to the increase in MAP was similar in intact normotensive and hypertensive rabbits, both with and without atrial pacing. This indicates that the sensitivity of the baroreceptor-myocardial contractility reflex was not impaired in the hypertensive animals, 6 weeks after renal wrapping, even though reflex control of heart rate is blunted at this time. Furthermore, the relative contribution of the vagus and the sympathetic to the control of contractility was similar in normotensive and hypertensive animals when heart rate was allowed to change. On the other hand, when the heart rate was held constant with atrial pacing, vagal blockade with methyl scopolamine revealed a contribution of the vagus to the reflex negative inotropic effect in hypertensive rabbits that was not evident in normotensive animals.