Interaction of baroreflex bradycardia and inotropy in the rabbit

In urethane-anaesthetized, closed-chest rabbits negative inotropic reflex responses from the left ventricle were elicited by aortic nerve (AoN) and carotid sinus nerve (CSN) stimulation in the paced and spontaneously beating heart and characterized as percent decrease of the contractility index (d log p/dt)max. The negative inotropic reflex action upon the left ventricle is, at freely changing heart rate, masked by the concomitant reflex bradycardia, in dependence on the prestimulation spontaneous heart rate. Therefore, inotropic reflex responses to AoN and CSN stimulation, as well as before and after vagotomy are comparable only on the condition of paced heart. In hearts with a prestimulation rate below 245 beats/min the negative reflex inotropy at freely changing rate is higher than under pacing, and at rates above 245 beats/min vice versa. It is assumed that at lower rates the nervous inotropy is augmented by negative rate-dependent inotropy (Bowditch staircase), at higher rates counteracted by a positive staircase, caused by reflex bradycardia. This nonuniform interaction can be explained by the strength-interval relation of the left ventricle which has an optimum at 245 ms (= 245 beats/min). The inotropic effect differences between stimulations at freely changing and paced heart rate are correlated to the effective heart intervals; the regression lines both for AoN and for CSN stimulation show equal intercepts with the abscissa at 245 ms. This is further evidence that at high heart rate the negative nervous inotropy counteracts, and at low rate cooperate with the rate-dependent inotropy.