Characterization of nitrergic neurotransmission during short- and long-term electrical stimulation of the rabbit anococcygeus muscle.

1. Isolated preparations of rabbit anococcygeus muscle were exposed to electrical field stimulation (EFS; 50V, 0.3 ms duration, 0.08-40 Hz) for periods of 1-60 s (short-term EFS) or 10 min-2 h (long-term EFS). 2. Both short- and long-term EFS caused a contractile response which was enhanced by the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG), showing that it is modulated by endogenous NO. 3. In preparations treated with scopolamine and guanethidine and in which a constrictor tone was induced by histamine, both short- and long-term EFS resulted in relaxation of the tissue. 4. Such relaxations were reversed by tetrodotoxin (TTX), omega-conotoxin, inhibitors of NO synthase and the NO scavenger, oxyhaemoglobin, indicating that they are neuronal in origin and nitrergic in nature. 5. The relaxations to long-term EFS persisted for the duration of the stimulation and were associated with sustained release of oxidation products of NO (NOx). The EFS-induced release of NOx was decreased by N-iminoethyl-L-ornithine (L-NIO), an inhibitor of NO synthase, and by TTX. 6. Inhibitors of NO synthase, in addition, increased the basal tone of the tissue and reduced the basal output of NOx. The basal output of NOx was also reduced by TTX. 7. Long-term EFS which induces approximately 50% of the maximum relaxation could be enhanced by addition of L-, but not D-, arginine to the perfusion medium. 8. These data show that there is a continuous basal release of NO from nitrergic nerve terminals which maintains a relaxant tone in the rabbit anococcygeus muscle. 9. In addition, NO is released during short- and long-term EFS which further relaxes the preparation and modulates sympathetic transmission. Activation of the L-argimne: NO pathway for periods up to2 h does not exhaust nitrergic transmission in any appreciable way.