Does brachial artery flow-mediated vasodilation provide a bioassay for NO?

Is Flow-Mediated Dilation Nitric Oxide Mediated?

Flow-mediated dilation (FMD) is a noninvasive index of endothelial function and vascular health in humans. Studies examining the role of nitric oxide (NO) are not conclusive. In this article, we quantified the contribution of NO in FMD of conduit arteries and explored the effect of the protocol (ie, distal cuff, ≈5-minute ischemia) and method of analysis (ie, automated and continuous edge detection) on the NO dependency of this test. A systematic review and 3-stage meta-analysis of published crossover studies that measured FMD under local infusion of saline or the NO synthase blocker N G monomethyl-L-arginine (L-NMMA) was undertaken. Twenty studies met the inclusion criteria for stage 1 (374 individual comparisons). The meta-analyzed outcome was the difference in FMD between infusion of saline (ie, FMD saline ) and NO synthase blocker (ie, FMD L-NMMA ). Overall, FMD saline was 8.2% (95% confidence interval [CI], 6.8%–9.6%) compared with FMD L-NMMA of 3.7% (95% CI, 3.1%–4.3%; P <0.001). Stage 2 analysis focused on studies that used the most commonly adopted approach in healthy volunteers (ie, distal cuff placement, ≈5-minute occlusion), which similarly revealed a significant NO contribution to FMD (FMD saline , 6.5% [95% CI, 5.7%–7.3%]; FMD L-NMMA , 0.9% [95% CI, 0.5%–1.3%]; P <0.001). Stage 3 meta-analyzed the studies that adopted the commonly adopted approach and automated, continuous method of analysis, which also revealed a significant contribution of NO to the FMD (FMD saline , 6.9% [95% CI, 6.0%–7.8%]; FMD L-NMMA , 2.4% [95% CI, 1.1%–3.7%]; P <0.001). This comprehensive analysis demonstrates that FMD of conduit arteries in humans is, at least in part, mediated by NO.

Passive leg movement-induced hyperaemia with a spinal cord lesion: evidence of preserved vascular function

A spinal cord injury (SCI) clearly results in greater cardiovascular risk; however, accompanying changes in peripheral vascular structure below the lesion mean that the real impact of a SCI on vascular function is unclear.

AIM

Therefore, utilizing passive leg movement-induced (PLM) hyperaemia, an index of nitric oxide (NO)-dependent vascular function and the central hemodynamic response to this intervention, we studied eight individuals with a SCI and eight age-matched controls (CTRL).

METHODS

Specifically, we assessed heart rate (HR), stroke volume (SV), cardiac output (CO), mean arterial pressure (MAP), leg blood flow (LBF) and thigh composition.

RESULTS

In CTRL, passive movement transiently decreased MAP and increased HR and CO from baseline by 2.5 ± 1 mmHg, 7 ± 2 bpm and 0.5 ± 0.1 L min(-1) respectively. In SCI, HR and CO responses were unidentifiable. LBF increased to a greater extent in CTRL (515 ± 41 ∆mL min(-1)) compared with SCI, (126 ± 25 ∆mL min(-1)) (P < 0.05). There was a strong relationship between ∆LBF and thigh muscle volume (r = 0.95). After normalizing ∆LBF for this strong relationship (∆LBF/muscle volume), there was evidence of preserved vascular function in SCI (CTRL: 120 ± 9; SCI 104 ± 11 mL min(-1) L(-1)). A comparison of ∆LBF in the passively moved and stationary leg, to partition the contribution of the blood flow response, implied that 35% of the hyperaemia resulted from cardioacceleration in the CTRL, whereas all the hyperaemia appeared peripheral in origin in the SCI.

CONCLUSION

Thus, utilizing PLM-induced hyperaemia as marker of vascular function, it is evident that peripheral vascular impairment is not an obligatory accompaniment to a SCI.

Contribution of nitric oxide to brachial artery vasodilation during progressive handgrip exercise in the elderly

The reduction in nitric oxide (NO)-mediated vascular function with age has largely been determined by flow-mediated dilation (FMD). However, in light of recent uncertainty surrounding the NO dependency of FMD and the recognition that brachial artery (BA) vasodilation during handgrip exercise is predominantly NO-mediated in the young, we sought to determine the contribution of NO to BA vasodilation in the elderly using the handgrip paradigm. BA vasodilation during progressive dynamic (1 Hz) handgrip exercise performed at 3, 6, 9, and 12 kg was assessed with and without NO synthase (NOS) inhibition [intra-arterial N(G)-monomethyl-l-arginine (l-NMMA)] in seven healthy older subjects (69 ± 2 yr). Handgrip exercise in the control condition evoked significant BA vasodilation at 6 (4.7 ± 1.4%), 9 (6.5 ± 2.2%), and 12 kg (9.5 ± 2.7%). NOS inhibition attenuated BA vasodilation, as the first measurable increase in BA diameter did not occur until 9 kg (4.0 ± 1.8%), and the change in BA diameter at 12 kg was reduced by ∼30% (5.1 ± 2.2%), with unaltered shear rate (

CONTROL

407 ± 57, l-NMMA: 427 ± 67 s(-1)). Although shifted downward, the slope of the relationship between BA diameter and shear rate during handgrip exercise was unchanged (

CONTROL

0.0013 ± 0.0004, l-NMMA: 0.0011 ± 0.007, P = 0.6) as a consequence of NOS inhibition. Thus, progressive handgrip exercise in the elderly evokes a robust BA vasodilation, the magnitude of which was only minimally attenuated following NOS inhibition. This modest contribution of NO to BA vasodilation in the elderly supports the use of the handgrip exercise paradigm to assess NO-dependent vasodilation across the life span.