Acute and Chronic Inhibition of NOS Causes a Switch in Vasodilator Mechanism from Nitric Oxide to Hydrogen Peroxide in the Human Microcirculation

Abstract

This study tests the hypothesis that inhibition of NO-synthase (NOS) increases mitochondria-derived (mt) reactive oxygen species (ROS), resulting in changes in the mediator of flow-induced dilation (FID). In healthy individuals nitric oxide (NO) is the primary vasodilator responsible for FID, while in microvessels (MV) of patients with coronary artery disease (CAD) mtROS are responsible. NO inhibits mtROS and reduces smooth muscle proliferation to contribute to prevention of atherosclerosis. Hydrogen peroxide (H2O2)promotes cell proliferation and is pro-atherosclerotic. MV (~200µm) obtained from surgical discarded adipose were dissected and cannulated on glass pipettes in a heated organ chamber, perfused with Krebs and aerated with 74% N2, 21% O2, 5% CO2. After equilibration, MV were constricted with endothelin-1 and responses to graded increases in flow were recorded. Papaverine (100µM) was used to assess endothelial independent relaxation. Neither 1h nor 16h incubation with L-NAME (100µM before and during experiment) affected the FID compared to Vehicle (V) (% Max dilation: V 76.1 ±5.1; L-NAME: 1h 84.3 ±3; 16h: 87.8±2.7 P< 0.05 ANOVA). In V treated MV FID was L-NAME inhibitable (10.1±8.3) but not effected by PEG-cat while with L-NAME pretreatment PEG-cat inhibited dilation (L-NAME: 1h +Peg-Cat 18.4±12.9 16h + Peg-Cat 12.6 ±1.6). Our results suggest that inhibition of NOS in MV from healthy individuals causes a primary shift in the mediator of dilation from NO to H2O2 , similar to subjects with CAD. Our data are consistent with the concept that that threshold levels of NO are necessary to suppress mtROS in human MV. Loss of NO bioavailability causes a compensatory increase in mtROS, which mediates FID.