Differential frequency-dependent reflex summation of the aortic baroreceptor afferent input

Reflex summation in the expression of left and right aortic baroreflex control of hemodynamic functions was investigated. In anesthetized Sprague-Dawley rats, mean arterial pressure (MAP), heart rate (HR), and mesenteric vascular resistance (MVR) were recorded following left, right, and bilateral stimulation of the aortic depressor nerve (ADN). Stimulation frequency was varied between low (1 Hz), moderate (5 Hz), and high (20 Hz). At 1 Hz, left and right ADN stimulation evoked similar depressor, bradycardic and MVR responses, whereas bilateral stimulation induced larger MAP, HR, and MVR reductions compared with stimulations of either side. The sum of the separate and combined stimulation effects on MAP, HR, and MVR was similar, indicating an additive summation. A similar additive summation was observed with HR responses at 5 and 20 Hz. Left-sided and bilateral stimulation produced greater depressor and MVR responses than right-sided stimulation, with responses of the bilateral stimulation mimicking those of the left side. The bilateral MAP or MVR response was smaller than the sum of the separate responses, suggesting an inhibitory summation. In conclusion, reflex summation of the left and right aortic baroreceptor afferent input is differentially expressed in relation to the frequency of the input signal. Summation of baroreflex control of HR is always additive and independent of stimulation frequency. Summation of baroreflex control of MAP is additive when the frequency input is small and inhibitory when the frequency input is moderate to high, with MAP changes mainly driven by parallel baroreflex-triggered changes in vascular resistance.