Summation of sinoaortic baroreflexes depends on size of input signals.

We studied sinoaortic baroreflex summation in seven anesthetized rabbits by paying special attention to the sigmoidal nature of the open-loop characteristic curve of the two reflexes. Carotid sinuses were isolated to control carotid sinus pressure (CSP). Aortic nerves were cut, and the left aortic nerve was electrically stimulated. Initially, CSP was set between 75 and 95 mmHg, and the parameters for aortic nerve stimulation (ANS) were chosen so that mean arterial pressure (MAP) was between 80 and 100 mmHg. We then increased CSP and ANS mildly so that MAP would fall by 10-20 mmHg in each case. Comparison of the sum of these decreases with the decrease caused by simultaneous increases in CSP and ANS showed no statistical difference, indicating a simply additive summation. With mild decreases in CSP and ANS, a similar additive summation was observed in the reflex increases in MAP. When we input slightly larger changes in CSP and ANS so that MAP fell by 20-30 mmHg for each stimulus, comparison of the sum of the separate effects with the combined effect on MAP indicated a mildly inhibitory summation. By analyzing the total peripheral resistance calculated from aortic flow data, we found this inhibitory summation occurred in the reflex controls of the total peripheral resistance. We conclude that the summation of the sinoaortic reflex controls of arterial pressure can be regarded as simply additive when small input signals are given in the physiological range of both reflexes, but the summation is mildly to moderately inhibitory when moderately larger input signals are given.