Comparison of forearm blood flow responses to incremental handgrip and cycle ergometer exercise: relative contribution of nitric oxide

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Abstract
The contribution of endothelium‐derived nitric oxide (NO) to exercise hyperaemia remains controversial. Disparate findings may, in part, be explained by different shear stress stimuli as a result of different types of exercise. We have directly compared forearm blood flow (FBF) responses to incremental handgrip and cycle ergometer exercise in 14 subjects (age ±s.e.m.) using a novel software system which calculates conduit artery blood flow continuously across the cardiac cycle by synchronising automated edge‐detection and wall tracking of high resolution B‐mode arterial ultrasound images and Doppler waveform envelope analysis. Monomethyl arginine (l‐NMMA) was infused during repeat bouts of each incremental exercise test to assess the contribution of NO to hyperaemic responses. During handgrip, mean FBF increased with workload (P < 0.01) whereas FBF decreased at lower cycle workloads (P < 0.05), before increasing at 120 W (P < 0.001). Differences in these patterns of mean FBF response to different exercise modalities were due to the influence of retrograde diastolic flow during cycling, which had a relatively larger impact on mean flows at lower workloads. Retrograde diastolic flow was negligible during handgrip. Although mean FBF was lower in response to cycling than handgrip exercise, the impact of l–NMMA was significant during the cycle modality only (P < 0.05), possibly reflecting the importance of an oscillatory antegrade/retrograde flow pattern on shear stress‐mediated release of NO from the endothelium. In conclusion, different types of exercise present different haemodynamic stimuli to the endothelium, which may result in differential effects of shear stress on the vasculature.