Endothelial cell regulation of nitric oxide production during hypoxia in coronary microvessels and epicardial arteries

Nitric oxide (NO) synthesized by endothelial cell nitric oxide synthase (eN OS) elicits vasodilation of resistance-sized coronary microvessels. Since c oronary blood flow increases during hypoxia, we tested the hypotheses that: (1) hypoxia results in increased blood flow through increased NO productio n mediated by the upregulation of both eNOS mRNA and protein and (2) the re gulation of NO production in response to hypoxia differs in microvascular e ndothelial cells and nonresistance, epicardial endothelial cells. Monocultu res of vascular endothelium from resistance (similar to 100 mu) and nonresi stance epicardial arteries were established and characterized. Nitric oxide was quantitated using a chemiluminescence method. Hypoxia (pO(2) = 10 mmHg ) significantly increased NO production in both cell lines, with less NO pr oduced in microvascular endothelium. Western blots demonstrated that hypoxi a caused a time-dependent increase in eNOS protein in both lines, with an a verage 2.5-fold increase in nonresistance, epicardial endothelial cells com pared to an average 1.7-fold increase in protein from microvascular endothe lium. Total mRNA recovery increased 2.4 +/- 0.6-fold within 30 min of hypox ia in nonresistance, epicardial endothelial cells with no increase in micro vascular endothelial cells. Although hypoxia increased NO production in bot h populations of endothelial cells, the increase in NO production and eNOS protein in microvascular endothelium was less compared to nonresistance, ep icardial endothelial cells. Furthermore, there was no significant upregulat ion of total mRNA for eNOS in microvascular endothelium. The data indicate that increased NO production in microvascular endothelium during hypoxia ma y be through translational or posttranslational modifications of the enzyme , whereas transcriptional upregulation may account for the increased NO pro duction in nonresistance, epicardial endothelial cells. Oxygen-sensitive re sponse mechanisms that modulate NO production may be different in endotheli um from different coronary artery vascular beds. (C) 2000 Wiley-Liss, Inc.