EFFECTS OF HYDROXOCOBALAMIN AND HAEMOGLOBIN ON NO‐MEDIATED RELAXATIONS IN THE RAT ANOCOCCYGEUS MUSCLE

1. The effects of hydroxocobalamin (Vitamin B_12a) on relaxations produced by nitric oxide (NO), some NO‐donating compounds and nitrergic nerve stimulation in isolated preparations of the rat anococcygeus muscle were compared with the effects of haemoglobin. 2. Hydroxocobalamin (30 μmol/L) significantly reduced relaxations induced by NO (0.1–3 μmol/L) and sodium nitroprusside (SNP; 0.01–0.3 μmol/L) but did not affect relaxations induced by glyceryl trinitrate (GTN; 0.01–1 μmol/L), S‐nitrosocysteine (0.1–0.3 μmol/L) or stimulation of nitrergic nerves. A higher concentration of hydroxocobalamin (100 μmol/L) slightly reduced nitrergic nerve stimulation‐induced relaxations. 3. Haemoglobin (10 μmol/L) blocked relaxation induced by NO and reduced relaxations induced by SNP, GTN, S‐nitrosocysteine and nitrergic nerve stimulation. 4. When nitrergic nerve stimulation‐induced relaxations had been partially reduced by the NO synthase inhibitor l‐NAME (5–10 μmol/L), hydroxocobalamin had only a weak and transient inhibitory effect. 5. Noradrenergic contractions induced by field stimulation were not affected by hydroxocobalamin (30 μmol/L), but were enhanced by haemoglobin (10 μmol/L). 6. The results suggest that the transmitter released from nitrergic nerves in anococcygeus muscles resembles NO‐releasing compounds such as S‐nitrosocysteine and GTN but not SNP or free NO.