Opposing effects of shear-mediated dilation and myogenic constriction on artery diameter in response to handgrip exercise in humans.

While the impact of changes in blood flow and shear stress on artery function are well documented, the acute effects of increases in arterial pressure are less well described in humans. The aim of this study was to assess the effect of 30 min of elevated blood pressure, in the absence of changes in shear stress or sympathetic nervous system (SNS) activation, on conduit artery diameter. Ten healthy male subjects undertook three sessions of 30 min unilateral handgrip exercise at 5, 10, and 15% of maximal voluntary contractile (MVC) strength. Brachial artery shear rate and blood flow profiles were measured simultaneously during exercise in the active and contralateral resting arms. Bilateral brachial artery diameter was simultaneously assessed before and immediately postexercise. In a second experiment, six subjects repeated the 15% MVC condition while continuous vascular measurements were collected during muscle sympathetic nerve activity (MSNA) assessment using peroneal microneurography. We found that unilateral handgrip exercise at 5, 10, and 15% MVC strength induced stepwise elevations in blood pressure (P < 0.01, Δmean arterial pressure: 7.06 ± 2.44, 8.50 ± 2.80, and 18.35 ± 3.52 mmHg, P < 0.01). Whereas stepwise increases were evident in shear rate in the exercising arm (P < 0.001), no changes were apparent in the nonexercising limb (P = 0.42). Brachial artery diameter increased in the exercising arm (P = 0.02), but significantly decreased in the nonexercising arm (P = 0.03). At 15% MVC, changes in diameter were significantly different between arms (interaction effect: P = 0.01), whereas this level of exertion produced no significant changes in MSNA. We conclude that acute increases in transmural pressure, independent of shear rate and changes in SNS activation, reduce arterial caliber in normotensive humans in vivo. These changes in diameter were mitigated by exercise-induced elevations in shear rate in the active limb.