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1 Kinesiology, University of Waterloo, Waterloo, ON, Canada
2 Kinesiology, University of Western Ontario, London, ON, Canada
* To whom correspondence should be addressed. E-mail: hughson{at}uwaterloo.ca.
We tested the hypothesis that flow-mediated dilation (FMD) of the brachial artery would be impaired by acute increases in sympathetic nervous system activation (SNA) in models where similar peak shear stress stimulus was achieved by varying the duration of forearm muscle ischemia. Eleven healthy young men were studied under four different conditions, each with its own control: lower body suction (LBS), cold pressor test (CPT), mental arithmetic task (MAT) and activation of muscle chemoreflex (MCR). The duration of ischemia before observation of FMD by ultrasound imaging was 5-minutes for each of control, LBS and CPT, 3-minutes for MAT and 2-minutes for MCR. Peak shear rate was not different between control and any of the SNA conditions, although total shear in the first minute was reduced in MAT. MCR was the only condition in which brachial artery vasoconstriction was observed before forearm occlusion (4.38 mm SD 0.53 vs. control 4.60 mm SD 0.53, P<0.05) however diameter increased to the same absolute value as control so the percentage FMD was greater for MCR (9.85% SD 2.33 vs. control 5.29% SD 1.50). Blunting of the FMD response occurred only in the CPT model (1.51% SD 1.20). During SNA, the increase in plasma cortisol from baseline was significant only for MCR, the increase in plasma norepinephrine was significant for MCR, LBS and CPT, and the increase in epinephrine was significant only for MCR. These results showed that the four models employed to achieve increases in SNA had different effects on baseline brachial artery diameter and that blunted FMD is not a general response to increased SNA.
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