CHARACTERISTICS OF THE NANC POSTSTIMULUS (REBOUND) CONTRACTION OF THE URINARY-BLADDER NECK MUSCLE IN SHEEP

1 Strips of muscle from sheep bladder neck were set up for tension recording and subjected to electrical held stimulation (EFS) to stimulate their intramural nerves. 2 In the presence of atropine (1 mu M) and guanethidine (I mu M), the response to 1 Hz EFS was biphasic, characterized by a relaxation during the stimulus period, followed by a post-stimulus contraction. A similar biphasic response was also seen following bolus application of nitric oxide (NO). 3 In the absence of atropine and guanethidine, the relaxations were masked by contractions during stimulation; however, the post-stimulus contractions were unaffected. L-NAME (100 mu M) blocked the post-stimulus contractions and L-arginine (1 mM) restored them, suggesting that they were NO-mediated. 4 M&B 22948, a phosphodiesterase inhibitor, prolonged the relaxations and abolished the post-stimulus contractions. This suggests that rapid removal of cyclic GMP is required for post-stimulus contraction to occur. 5 When the number of pulses in the stimulus train was kept constant, the size of the post-stimulus contraction increased as the duration of the preceding period of stimulation increased. Maximal poststimulus contractions were obtained following stimulation for >40 s. 6 The L-channel antagonist, nifedipine (1 mu M) and verapamil(1 mu M), had little effect on the amplitude of the post-stimulus contractions. 7 In contrast, ryanodine (8 mu M) reduced the post-stimulus contractions by over 90%. Caffeine (20 mM) also abolished the post-stimulus contractions and cyclopiazonic acid (CPA, IO mu M) reduced them by 76%. However, in the presence of CPA a slower post-stimulus contraction developed. Nifedipine (1 mu M) reduced this by 40%. 8 In conclusion, these results support a role for NO in the post-stimulus contraction of the sheep bladder neck muscle. The post-stimulus contraction depends more on release of intracellular Ca2+, than Ca2+ influx through L-type channels.