Effect of Graded Sympathetic Activation on Regional Cerebral Vascular Conductance

Abstract

The role of sympathetic nervous system in cerebral blood flow regulation remains poorly characterized. Previous studies have shown heterogeneity in regional cerebral circulation in response to various stimuli such as exercise, hypoxia, hypocapnia, etc. Recently, Ogoh et al. demonstrated that a moderate level (−50 Torr) of lower body negative pressure (LBNP) caused a significant reduction in internal carotid artery (ICA) blood flow, whereas vertebral artery (VA) blood flow remained unchanged, albeit trending towards a lower blood flow. Although a moderate increase in sympathetic nerve activity (SNA) occurs at −50 LBNP, it has been shown that SNA increases progressively with higher levels of LBNP. Whether VA blood flow is protected at higher levels of LBNP remains unknown. In this study, we tested the hypothesis that greater reflex‐mediated sympathetic activation via a higher level of LBNP will reduce conductance and blood flow to both the ICA and VA. In 9 young, healthy men, heart rate (ECG), arterial blood pressure (brachial sphygmomanometer), VA, ICA, and brachial artery blood flow (duplex Doppler ultrasound), middle cerebral artery (MCA) blood velocity (transcranial Doppler) and end‐tidal CO2 (ETCO2; capnograph) were measured at rest, and during 3–5 min of low, moderate and high LBNP (−30, −50, and −70 Torr, respectively). Conductance was calculated as blood flow of the respective artery/mean arterial pressure. Brachial artery blood flow and conductance were significantly reduced during all LBNP trials, indicating reflex‐mediated sympathetic vasoconstriction. Neither ETCO2 (Δ −1.9 ± 1.3 mmHg relative to baseline, p=0.18) nor any of the cerebral blood vessel measures were affected during −30 Torr LBNP. At −50 Torr LBNP, a reduction in ETCO2 (Δ −5.3 ± 1.6 mmHg relative to baseline, p=0.01) was accompanied by a reduction in ICA conductance (Δ −17.0 ± 8.4% baseline, p=0.057) whereas VA conductance remained unchanged (Δ −8.6 ± 5.6% baseline, p=0.24). Interestingly, with a similar reduction in ETCO2 at −70 Torr LBNP (Δ −4.3 ± 1.3 mmHg relative to baseline, p=0.04), there was a significant reduction in both ICA (Δ −13.1 ± 3.1% baseline, p=0.017) and VA conductance (Δ −20.4 ± 5.3% baseline, p=0.019). Thus, VA conductance was unaffected during low and moderate levels of LBNP however, high levels of LBNP induced a significant reduction in VA conductance and blood flow and this reduction appeared to be independent of the decrease in ETCO2. Collectively, these preliminary findings suggest that large increases in reflex‐mediated sympathetic activation via high levels of LBNP causes significant reductions in both ICA and VA conductance and blood flow.Support or Funding Information1 RO1 HL 127071This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.