Effects of hydroxocobalamin and haemoglobin on no-mediated relaxations in the rat anococcygeus muscle.

1. The effects of hydroxocobalamin (Vitamin B12a) 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 mumol/L) significantly reduced relaxations induced by NO (0.1-3 mumol/L) and sodium nitroprusside (SNP; 0.01-0.3 mumol/L) but did not affect relaxations induced by glyceryl trinitrate (GTN; 0.01-1 mumol/L), S-nitrosocysteine (0.1-0.3 mumol/L) or stimulation of nitrergic nerves. A higher concentration of hydroxocobalamin (100 mumol/L) slightly reduced nitrergic nerve stimulation-induced relaxations. 3. Haemoglobin (10 mumol/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 mumol/L), hydroxocobalamin had only a weak and transient inhibitory effect. 5. Noradrenergic contractions induced by field stimulation were not affected by hydroxocobalamin (30 mumol/L), but were enhanced by haemoglobin (10 mumol/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.