Permissive effect of nitric oxide in arachidonic acid induced dilation in isolated rat arterioles

Objective: Prostaglandins and nitric oxide play an important role in the regulation of arteriolar tone. L-Arginine analogues inhibit nitric oxide formation, but may also inhibit arachidonic-acid induced dilation. Nitric oxide was found to stimulate cyclooxygenase activity in cultured endothelial cells. Therefore, we hypothesized that the non-specific inhibition of prostaglandin-related dilation by L-arginine analogues is a consequence of the absence of nitric oxide. Methods: To test this hypothesis, arteriolar segments from rat cremaster muscle were studied in a pressure myograph at 75 mmHg. Segments developed spontaneous tone, the diameter reduced from 179 +/- 3 to 98 +/- 3 mu m (n = 41). In this condition, responses to exogenous arachidonic acid (1 mu M) were recorded and compared with responses after addition of L-NNA, and addition of either SNAP, nitroprusside or 8-Br-cGMP in the presence of L-NNA. Results: Inhibition of basal nitric oxide production with L-NNA (0.1 mM) reduced arachidonic acid-induced dilation (from 52 +/- 9 to 31 +/- 6 mu m). In the presence of L-NNA, responses to arachidonic acid were augmented when exogenous nitric oxide was also present (SNAP, 31 +/- 6 mu m vs. 75 +/- 5 mu m; nitroprusside, 31 +/- 8 mu m vs. 42 +/- 7 mu m). Responses were not augmented with the second messenger of nitric oxide-mediated dilation 8-Br-cGMP (37 +/- 9 mu m vs. 32 +/- 9 mu m). Conclusions: These results indicate that nitric oxide directly increases arachidonic acid-induced dilation. Thus, the non-specific effect of L-arginine analogues can be explained by a permissive effect of nitric oxide on endothelial arachidonic acid metabolism. (C) 1998 Elsevier Science B.V. All rights reserved.