Arteriolar wall PO(2) and nitric oxide release during sympathetic vasoconstriction in the rat intestine

Endothelium-derived nitric oxide (NO) attenuates arteriolar constriction in the rat small intestine during periods of increased sympathetic nerve activity. This study was undertaken to test the hypothesis that a flow-dependent fall in arteriolar wall PO(2) serves as the stimulus for endothelial NO release under these conditions. Sympathetic nerve stimulation at 3-16 Hz induced frequency-dependent arteriolar constriction, with arteriolar wall O(2) tension (PO(2)) falling from 67 +/- 3 mmHg to as low as 41 +/- 6 mmHg. Arteriolar responses to nerve stimulation were enhanced after inhibition of NO synthase with N(G)-monomethyl-L-arginine (L-NMMA). Under a high-O(2) (20%) superfusate, the fall in wall PO(2) was significantly attenuated, arteriolar constrictions were increased by 57 +/- 9 to 66 +/- 12%, and these responses were no longer sensitive to L-NMMA. The high-O(2) superfusate had no effect on vascular smooth muscle responsiveness to NO (as judged by arteriolar responses to sodium nitroprusside) or on arteriolar wall oxidant activity (as determined by the reduction of tetranitroblue tetrazolium dye). These results indicate that a flow-dependent fall in arteriolar wall PO(2) may serve as a stimulus for the release of endothelium-derived NO during periods of increased sympathetic nerve activity.