Obligatory role of hyperaemia and shear stress in microvascular adaptation to repeated heating in humans

Abstract

The endothelium, a single layer of cells lining the entire circulatory system, plays a key role in maintaining vascular health. Endothelial dysfunction independently predicts cardiovascular events and improvement in endothelial function is associated with decreased vascular risk. Previous studies have suggested that exercise training improves endothelial function in macrovessels, a benefit mediated via repeated episodic increases in shear stress. However, less is known of the effects of shear stress modulation in microvessels. In the present study we examined the hypothesis that repeated skin heating improves cutaneous microvascular vasodilator function via a shear stress-dependent mechanism. We recruited 10 recreationally active males who underwent bilateral forearm immersion in warm water (42 • C), 3 times per week for 30 min. During these immersion sessions, shear stress was manipulated in one arm by inflating a pneumatic cuff to 100 mmHg, whilst the other arm remained uncuffed. Vasodilatation to local heating, a NO-dependent response assessed using laser Doppler, improved across the 8 week intervention period in the uncuffed arm (cutaneous vascular conductance week 0 vs. week 4 at 41• C: 1.37 ± 0.45 vs. 2.0 ± 0.91 units, P = 0.04; 42 • C: 2.06 ± 0.45 vs. 2.68 ± 0.83 units; P = 0.04), whereas no significant changes were evident in the cuffed arm. We conclude that increased blood flow, and the likely attendant increase in shear stress, is a key physiological stimulus for enhancing microvascular vasodilator function in humans.

Repeated increases in blood flow, independent of exercise, enhance conduit artery vasodilator function in humans

“…It appears that the brachial artery became somewhat hyperresponsive following heat training, as shear AUC decreased in the presence of maintained FMD and increased ischemic handgrip responses. These findings contrast somewhat with the impact of exercise training on shearmediated dilator responses, and the explanation is not entirely clear, but it may be that heat training has less impact on resistance vessel remodeling than exercise training, even if it does modulate cutaneous microvascular function (8).…”

Repeated ischaemic preconditioning: a novel therapeutic intervention and potential underlying mechanisms

“…Previous work has demonstrated that repeated increases in shear represent an important stimulus for vascular adaptations in function and structure (Green et al 2004;Tinken et al 2010). The perception that vascular adaptations are mediated by shear-stress-dependent mechanisms is well supported by both in vivo animal models and human experiments (Hambrecht et al 2003;McAllister et al 2005;Green et al 2010). In addition, repeated exposure to short periods of hypoxia (or ischaemia) may also contribute to cardiac and vascular adaptations (Laughlin et al 2008;Newcomer et al 2011).…”

Impact of 2‐Weeks Continuous Increase in Retrograde Shear Stress on Brachial Artery Vasomotor Function in Young and Older Men

“…Several studies have explored the impact of repeated exposure to elevations in antegrade shear stress via training, heating, or in vitro manipulation of antegrade shear 7, 13, 14, 15, 16, 17. These studies consistently report a dose‐dependent increase in arterial caliber and improvement in endothelial function, possibly through upregulation of endothelial nitric oxide synthase 1, 18.…”