Comparison of the effects of hydroxocobalamin and oxyhaemoglobin on responses to NO, EDRF and the nitrergic transmitter

1. The effects of ranges of concentrations of oxyhaemoglobin (0.01-30 μM) and hydroxocobalamin (1-100 μM) were compared for their abilities to reduce relaxant responses to EDRF released by acetylcholine in endothelium-intact rat aortic rings, the nitergic transmitter in rat anococcygeus muscles, and NO in aqueous solution in both tissues (aortic rings were denuded of endothelium). 2. The concentrations of oxyhaemoglobin producing 50% reduction of responses to EDRF and NO in rat aorta correspond closely, the IC₅₀ values being 0.13 ± 0.02 μM and 0.11 ± 0.02 μM respectively. 3. Oxyhaemoglobin was equally effective in inhibiting responses to NO in anococcygeus muscles and in aortic rings with an IC₅₀ of 0.14 ± 0.05 μM. However, responses to the nitrergic transmitter were considerably less sensitive to inhibtion by oxyhaemoglobin, the IC₅₀ being 19.7 ± 5.1 μM. 4. The IC₅₀ values for hydroxocobalamin in inhibiting responses to EDRF and NO in aorta were 3.4 ± 0.2 μM and 8.4 ± 0.63 μM, respectively, but it was less effective against responses to NO in anococcygeus muscles the IC₅₀ being 46 ± 9.6 μM. However, even in the highest concentration used (100 μM), it did not reduce responses to the nitrergic transmitter. 5. The findings are compatible with the views that EDRF is NO, but suggest that the nitergic transmitter in the rat anococcygeus muscle does not behave like free NO.