Nitric oxide is not obligatory for radial artery flow-mediated dilation following release of 5 or 10 min distal occlusion
This study investigated the nitric oxide (NO) dependence of radial artery (RA) flow-mediated dilation (FMD) in response to three different reactive hyperemia (RH) shear stimulus profiles. Ten healthy males underwent the following three RH trials: 1) 5 min occlusion (5 trial), 2) 10 min occlusion (10 trial), and 3) 10 min occlusion with cuff reinflation at 30 s (10–30 trial). Trials were performed during saline infusion and repeated during NG-monomethyl-l-arginine (l-NMMA) infusion in the brachial artery. RA blood flow velocity was measured with Doppler ultrasound, and B-mode RA images were analyzed using automated edge detection software. Shear rate estimation of shear stress was calculated as the blood flow velocity/vessel diameter. l-NMMA decreased baseline vascular conductance by 35%. l-NMMA infusion did not affect the peak shear rate stimulus (P = 0.681) or the area under the curve (AUC) of shear rate to peak FMD (P = 0.088). The AUC was significantly larger in the 10 trial vs. the 10–30 or 5 trial (P < 0.001). Although percent FMD (%change in diameter) in the 10 trial was larger than that in the 5 trial (P = 0.035), there was no significant difference in %FMD between the saline and l-NMMA conditions in any trial: 5 trial, 5.62 ± 1.48 vs. 5.63 ± 1.27%; 10 trial, 9.07 ± 1.16 vs. 11.22 ± 2.21%; 10–30 trial, 6.52 ± 1.43 vs. 7.98 ± 1.51% for saline and l-NMMA, respectively (P = 0.158). We conclude the following: 1) RH following 10 min of occlusion results in an enhanced stimulus and %FMD compared with 5 min of occlusion. 2) When the occlusion cuff is reinflated 30 s postrelease of a 10 min occlusion, it does not result in an enhanced %FMD compared with that which results from RH following 5 min of occlusion. 3) The lack of effect of l-NMMA on FMD suggests that NO may not be obligatory for radial artery FMD in response to either 5 or 10 min of occlusion in healthy volunteers.