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

Abstract

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 (Circulation 1989;79:83-92)

Recent Insights into Carotid Baroreflex Function in Humans Using the Variable Pressure Neck Chamber

“…pharmacological and non-pharmacological techniques that alter systemic blood pressure to estimate aortic baroreflex function (Ferguson et al 1985;Sanders et al 1988Sanders et al , 1989Fadel et al 2001bFadel et al , 2003Smith et al 2001). Basically, the approach in these experiments is to increase or decrease systemic blood pressure in combination with the application of NP or NS, respectively, to maintain carotid sinus pressure and negate the influence of the carotid baroreceptors.…”

“…The use of NP and NS in this way has elucidated the importance of the CBR to the integrated reflex control of the circulation when both the carotid and aortic baroreflexes are engaged. Studies that have estimated the contribution of the CBR to overall baroreflex control have indicated a role of the CBR ranging from 30 % to 50 % (Ferguson et al 1985;Sanders et al 1988Sanders et al , 1989Fadel et al 2001bFadel et al , 2003Smith et al 2001). More importantly, it has been shown that both baroreceptor populations operate over the same range of arterial pressures .…”

Naloxone potentiates cardiopulmonary baroreflex sympathetic control in normal humans.

“…This technique has been validated and extensively described in studies from our laboratory and elsewhere. [19][20][21][22][23][24][25][26][27] In brief, recordings were obtained by percutaneous insertion of tungsten microelectrodes into the peroneal nerve. The electrodes were connected to a preamplifier, and the nerve signal was fed through a band-pass filter and routed through an amplitude discriminator to a storage oscilloscope and loudspeaker.…”

Arterial baroreflex buffering of sympathetic activation during exercise-induced elevations in arterial pressure.

“…Eight healthy male volunteers, ages [23][24][25][26][27][28][29][30][31][32][33] yr, participated in this study after providing informed written consent. All subjects were normotensive (supine blood pressures < 140/90 mmHg), were taking no medications, and had no evidence of cardiopulmonary disease by history and physical examination at the time of the study.…”