ATP and nitrite represent independent mediators of RBC vasodilation. (A) Representative vessel tension traces (at 25 mmHg oxygen) showing that human and rat RBCs induce contraction or have no vasoactive effect on rat thoracic aorta in the absence or presence, respectively, of L-NMMA. (B) Representative vessel tension traces and (C) percentages of relaxation stimulated by the addition of human RBCs (0.3% HCT) or ATP (1 μM) to rabbit thoracic aorta at 15 mmHg oxygen tension alone or after preincubation with 8 U/mL apyrase, 100 μM reactive blue-2 (RB-2), or 100 μM L-NMMA. To control for the addition of oxygen and oxygen off-loading from RBCs, an equivalent volume of ice-cold PBS saturated with 95% oxygen was added that also resulted in relaxation in an L-NMMA–inhibitable manner. Values are mean ± SEM (n = 3-6). P values are shown. NS = not significant. (D) Human RBCs (0.3% HCT) and nitrite (2 μM) were added to rabbit thoracic aorta incubated with L-NMMA (100 μM) to inhibit ATP-dependent relaxation. Dilation was initiated in a manner similar to that observed with rat vessels (Figure 3) at an oxygen tension of approximately 27 mmHg. Data represent mean ± SEM (n = 4-5). *P < .04 relative to RBCs alone. #P < .01 relative to nitrite alone.