Importance of aortic baroreflex in regulation of sympathetic responses during hypotension. Evidence from direct sympathetic nerve recordings in humans.

Arterial baroreceptors in the carotid sinus and aortic arch regions reflexly regulate heart rate and peripheral vascular responses during changes in arterial pressure. The relative influence of these two arterial baroreflex pathways on the control of these autonomic responses is debatable. Recent studies in our laboratory demonstrate that the aortic baroreflex produces substantial and sustained inhibition of efferent sympathetic nerve activity to muscle (MSNA) during increases in arterial pressure. The regulation of MSNA by these two baroreflexes in humans during hypotension, and particularly the role of the aortic baroreflex, remains undefined. We therefore performed a new series of studies to assess the relative influence of the aortic and carotid baroreflexes on MSNA responses during sustained decreases in arterial pressure. In eight normal male subjects, aged 23 +/- 1 years (mean +/- SEM), we directly measured mean arterial pressure, heart rate, central venous pressure, and MSNA (microneurography) during hypotension (combined aortic and carotid baroreceptor deactivation) produced by intravenous infusion of sodium nitroprusside and during nitroprusside infusion with superimposed application of external neck suction. Neck suction was applied at levels sufficient to maintain transmural carotid sinus pressure above control levels (carotid baroreceptor activation) while the aortic baroreflexes remained deactivated. Central venous pressure was maintained constant with volume infusion. We also studied responses of these same subjects to direct carotid baroreceptor deactivation with the application of external neck pressure. During neck pressure alone, there was a reflex increase in mean arterial pressure; thus, during this portion of the protocol, we achieved carotid baroreceptor deactivation with some aortic baroreceptor activation. Nitroprusside infusion (combined aortic and carotid deactivation) decreased mean arterial pressure from 90.8 +/- 3.1 to 77.8 +/- 1.1 mm Hg (p less than 0.01) with concomitant increases in heart rate from 62.6 +/- 3.0 to 89.7 +/- 6.1 beats/min (p less than 0.001) and in MSNA from 273.8 +/- 43.0 to 950.6 +/- 133.5 units (p less than 0.001). During continued nitroprusside infusion with superimposed neck suction (aortic baroreceptor deactivation and carotid baroreceptor activation), mean arterial pressure decreased to 70.3 +/- 1.9 mm Hg (p less than 0.001 vs. control), heart rate decreased to 82.5 +/- 6.5 beats/min (p less than 0.01 vs. control or vs. nitroprusside alone), but MSNA remained markedly increased at 889.7 +/- 105.1 units (p less than 0.001 vs. control; p = NS vs. nitroprusside alone).(ABSTRACT TRUNCATED AT 400 WORDS)