Reduced perivascular PO2 increases nitric oxide release from endothelial cells

Many studies have suggested that endothelial cells can act as "oxygen sensors" to large reductions in oxygen availability by increasing nitric oxide (NO) production. This study determined whether small reductions in oxygen availability enhanced NO production from in vivo intestinal arterioles, venules, and parenchymal cells. In vivo measurements of perivascular NO concentration (NO]) were made with NO-sensitive microelectrodes during normoxic and reduced oxygen availability. During normoxia, intestinal first-order arteriolar NO] was 397 +/- 26 nM (n = 5), paired venular NO] was 298 +/- 34 nM (n = 5), and parenchymal cell NO] was 138 +/- 36 nM (n = 3). During reduced oxygen availability, arteriolar and venular NO] significantly increased to 695 +/- 79 nM (n = 5) and 534 +/- 66 nM (n = 5), respectively, whereas parenchymal NO] remained unchanged at 144 +/- 34 nM (n = 4). During reduced oxygenation, arteriolar and venular diameters increased by 15 +/- 3% and 14 +/- 5%, respectively: NG-nitro-L-arginine methyl ester strongly suppressed the dilation to lower periarteriolar Po2. Micropipette injection of a CO2 embolus into arterioles significantly attenuated arteriolar dilation and suppressed NO release in response to reduced oxygen availability. These results indicated that in rat intestine, reduced oxygen availability increased both arteriolar and venular NO and that the main site of NO release under these conditions was from endothelial cells.