Mediation by nitric oxide of neurogenic relaxation of the urinary bladder neck muscle in sheep.

1. Mechanical recordings were made in vitro from circularly oriented strips of the bladder neck muscle of sheep. In the absence of drugs, electrical field stimulation at frequencies of 0.2‐1 Hz evoked clear‐cut relaxations throughout 1 min stimulation periods, while higher stimulus frequencies (2‐8 Hz) evoked variable responses consisting of relaxation, contraction or a mixture of both. All of the responses were abolished by tetrodotoxin (10(‐6) M). 2. The contractions were reduced by guanethidine (10(‐6) M) and atropine (10(‐6) M), so that in the presence of these drugs clear‐cut relaxations were obtained at 0.2‐8 Hz stimulation, indicating that the relaxations were mediated by non‐adrenergic, non‐cholinergic (NANC) nerves. 3. The NANC relaxations were blocked by L‐NG nitro arginine methyl ester (L‐NAME), an inhibitor of nitric oxide synthesis. The antagonism by L‐NAME was reversed by L‐arginine. 4. Another feature of the NANC relaxation was ‘rebound contraction’ which occurred when the stimulus was switched off. The rebound contraction was also blocked by L‐NAME and restored by L‐arginine. 5. The relaxations and rebound contractions were unaffected by either alpha,beta‐methylene ATP (10(‐5) M) or 2‐methylthio ATP (10(‐5) M). 6. S‐Nitroso‐L‐cysteine, a substance which spontaneously releases NO at physiological pH, mimicked the relaxation and rebound contraction produced by nerve stimulation. 7. It is concluded that nerve‐evoked relaxation of the bladder neck is mediated by NO, or a closely related substance such as S‐nitroso‐L‐cysteine.