Comparison of Methods for Eliciting the Baroreceptor‐heart Rate Reflex in Conscious Rabbits

SUMMARY 1. Two methods were used for altering blood pressure in conscious rabbits by up to ± 30 mmHg from the resting level in order to characterize the barorecep‐tor‐heart rate reflex. These were to inflate descending aortic or inferior vena caval cuffs, or to give brief intravenous infusions of phenylephrine or glycerol trinitrate. The relation of change in blood pressure to change in heart interval was examined, both during the initial ‘ramp’ changes of these variables and when they had reached a ‘steady‐state'. 2. Both methods allowed the construction of'steady‐state’ sigmoid stimulus‐response curves whose parameters were reproducible within animals, and which were attended by a relatively small variance between animals. The inflatable‐cuff method gave a higher average value for maximum gain than the vasoactive drug method (10.0 v 5‐4 ms/mmHg) and a narrower pressure range between the threshold and saturation points of the response, but values for the other parameters were similar. Corresponding parameters obtained by the two methods correlated closely. 3. After denervating all arterial baroreceptors except one carotid sinus, ‘steady‐state’ maximum gain by cuff‐inflation was reduced to 3.1 ms/mmHg, and to 2‐6 ms/mmHg by the vasoactive drug method. The heart interval range between upper and lower plateau levels was reduced, but the pressure range between threshold and saturation points was widened, with both methods. 4. During the initial ‘ramp’ changes of blood pressure the sensitivity of the reflex was described by the slope of the linear regression of heart interval on ‘mean blood pressure. The slopes obtained by aortic cuff inflation, and by infusion of either vasoactive drug, correlated positively with ‘steady‐state’ maximum gain. However, the reproducibility of the ‘ramp’ method was inferior to that of the ‘steady‐state’ method with respect to reflex sensitivity, and other parameters of the stimulus‐response relation cannot be estimated. 5. Complete baroreceptor denervation virtually eliminated heart rate changes over the range of blood pressures usually employed. However, when blood pressure was increased by more than 40 mmHg a profound reflex bradycardia and hypopnoea were then evoked.