Erythrocytes possess an intrinsic barrier to nitric oxide consumption

It has been reported that free hemoglobin (Hb) reacts with NO at an extreme ly high rate (K-Hb similar to 10(7) M-1 s(-1)) and that the red blood cell (RBC) membrane is highly permeable to NO. RBCs, however, react with NO 500- 1000 times slower. This reduction of NO reaction rate by RBCs has been attr ibuted to the extracellular diffusion limitation. To test whether additiona l Limitations are also important, we designed a competition test, which all ows the extracellular diffusion limitation to be distinguished from transme mbrane or intracellular resistance. This test exploited the competition bet ween free Hb and RBCs for NO generated in a homogenous phase by an NO donor . If the extracellular diffusion resistance is negligible, then the results would follow a kinetic model that assumes homogenous reaction without extr acellular diffusion limitation, In this case, the measured effective reacti on rate constant, K-RBC, would remain invariant of the hematocrit, extracel lular-free Hb concentration, and NO donor concentration. Results show that the K-RBC approaches a constant only when the hematocrit is greater than 10 % suggesting that at higher hematocrit, the extracellular diffusion resista nce is negligible. Under such a condition, the NO consumption by RBCs is st ill 500-1000 times slower than that by free Hb. This result suggests that i ntrinsic RBC factors, such as transmembrane diffusion limitation or intrace llular mechanisms, exist to reduce the NO consumption by RBCs.