BackgroundThe purpose of this study was to determine the effects of systemic infusions of nitroglycerin and sodium nitroprusside on flow distribution and wall shear stress in the microcirculation.MethodsWith university approval, the cremaster muscle of 28 anesthetized (70 mg/kg pentobarbital given intraperitoneally) hamsters (Harlan Sprague Dawley: Syrian; weight, 121 ± 11 g [mean ± SD]) was observed usingin vivofluorescence microscopy. Arteriolar diameter, erythrocyte flux, and velocity were measured for a feed arteriole and its sequential branches. Observations were made during control (mean arterial pressure, 88 ± 4 mmHg) and after 30 min of intravenous delivery of sodium nitroprusside or nitroglycerin, titrated to decrease mean arterial pressure by 20 mmHg.ResultsSodium nitroprusside significantly dilated select upstream portions of the network (23 ± 2.6 to 29 ± 2.6 &mgr;m); no arterioles were dilated with nitroglycerin. Erythrocyte flux into the feed (i.e., inflow into the arteriolar network) and into the sequential branches (i.e., distribution within the network) were evaluated. With nitroglycerin, inflow decreased significantly from 1,560 ± 335 to 855 ± 171 cells/s, and flux into the branches decreased evenly. With sodium nitroprusside, inflow increased significantly to 2,600 ± 918 cells/s, yet cells were “stolen” from upstream branches (a decrease from 425 ± 67 to 309 ± 87 cells/s in the first branch). Excess flow passed into a downstream “thorough‐fare channel,” significantly increasing flux from 347 ± 111 to 761 ± 246 cells/s. Wall shear stress decreased uniformly with nitroglycerin infusion, with a decrease in the feed from 8.8 ± 2.5 to 6 ± 1.7 dyn/cm2. With sodium nitroprusside, variable changes occurred that were location specific within the network. For instance, at the inflow point to the network, wall shear stress changed from 8.3 ± 2.5 to 4.2 ± 3.3 dyn/cm2.ConclusionsNitroglycerin infusion promoted homogeneity of flow. Sodium nitroprusside significantly increased the heterogeneity of flow within this arteriolar network; an anatomic location for steal induced by sodium nitroprusside is identified.