Importance of cardiac pacing in the analysis of vagal and sympathetic contributions to baroreflex control of cardiac performance in the conscious rabbit.

The baroreflex control of myocardial contractility has been assessed in the conscious normotensive and hypertensive rabbit. Stimulus response curves relating graded changes in mean arterial pressure (MAP) to induced changes in peak rate of change of left ventricular pressure (peak LV dP/dt) were compared during constant heart rate controlled by atrial pacing and when heart rate was uncontrolled. The experiments were repeated after beta-block, cholinergic block and during combined block in both A rise in MAP produced a 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 and a reduction in peak LV dP/dt due to reflex bradycardia. The sympathetic nerves were primarily responsible for the direct negative inotropic effect and the vagus for the indirect effect, secondary to the bradycardia. The slope of the stimulus response curves relating the baroreflex fall in peak LV dP/dt to rises in MAP were similar in normotensive and hypertensive animals, in contrast to the sensitivity of the baroreflex heart rate response which is impaired in animals with chronic hypertension.