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Endothelial cells (EC) contribute to the control of local vascular diameter by formation of an endothelium derived relaxant factor (EDRF) (1). Whether nitric oxide (NO) is identical with (EDRF) or might represent only one species of several EDRFs has not been decided as yet (2-5). Therefore, we have directly compared in cultured EC the kinetics of NO formation determined in a photometric assay with the vasodilatory effect of EDRF and NO in a bioassay. Basal release of NO was 16, 4 pmol/min/ml packed EC column. After stimulation with bradykinin (BK) and ATP onset of endothelial NO release and maximal response preceded the EDRF-mediated relaxation. Concentrations of NO formed by stimulated EC were quantitatively sufficient to fully explain the smooth muscle relaxation determined in the bioassay. Our data provide convincing evidence that under basal, BK and ATP-stimulated conditions 1. endothelial cells release nitric oxide as free radical, 2. nitric oxide is solely responsible for the vasodilatory properties of EDRF.

The response of endothelial energy metabolism to oxygen supply was studied in cultured coronary endothelial cells from the rat at defined PO2 levels between 0.1 and 100 Torr. In the presence of glucose (5 mM), endothelial respiration (4 nmol O2.min-1.mg protein-1) was independent of the exterior PO2 greater than 3 Torr; oxygen consumption was half maximal at 0.8 Torr. At 100 Torr, lactate production was 26 nmol.min-1.mg protein-1; the decrease of the PO2 to 0.1 Torr resulted in a 2.2-fold increase in lactate production. The contents of ATP, ADP, and AMP were 21, 4, and 2 nmol/mg protein, respectively; they remained constant for 2.5-h incubations at PO2 levels between 0.1 and 100 Torr. In the presence of palmitate (100 microM) plus glutamine (0.5 mM), oxygen consumption was 8 nmol.min-1.mg protein-1 at PO2 levels greater than 3 Torr, and the half-maximal rate was again observed at 0.8 Torr. Lactate production was negligible. At PO2 levels greater than 3 Torr, the cells remained well energized. Below 3 Torr, however, the adenine nucleotide contents rapidly declined. These results demonstrate that the oxygen demand of coronary endothelial cells is low compared with the beating myocardium. In the presence of glucose, aerobic glycolysis is pronounced and the Pasteur effect small. In severe hypoxia (PO2 less than 0.1 Torr) the energetic state remained stable. In the absence of glucose, the energetic state of coronary endothelial cells is sensitive to the exterior PO2 less than 3 Torr, declining concomitantly with the decrease in respiration.

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