The electrical properties and responses to nerve stimulation of the proximal urethra of the male rabbit

Objective To identify the nature of neurotransmitters acting on the smooth muscle cells of the proximal urethra and the electrical activity underlying the mechanical responses to nerve stimulation. Materials and methods The electrical activity of longitudinal strips of proximal urethra obtained from male rabbits was recorded with microelectrode and double sucrose-gap techniques, Intramural nerves were stimulated with 200 mu s pulses. Drugs which selectively affected neurotransmission were added to the perfusing fluid. Results The mean resting potential of smooth muscle cells was -39 mV and they had infrequent spontaneous action potentials. The frequency was increased by noradrenaline without depolarization; ATP produced depolarization with increased action-potential frequency. Small (less than or equal to 16mV) spontaneous depolarizations and hyperpolarizations could be recorded from < 50% of cells; large (less than or equal to 40 mV) hyperpolarizations also occurred. Electrical stimulation envoked excitatory and inhibitory junction potentials (EJPs and IJPs). Most EJPs reached a maximum within 500 ms, decayed with a time-constant of 200-300 ms and were blocked by alpha,beta-methylene ATP. IJPs had a latency of > 1 s and showed much variation in amplitude and duration. IJPs could still be recorded in the presence of N-nitro-L-arginine methyl ester (L-NAME). In the double sucrose-gap, depolarization and associated contraction were reduced by alpha,beta-methylene ATP. Further reduction occurred with phentolamine and the remaining response was blocked by atropine. A large hyperpolarization could occur in response to a single stimulus but this decreased progressively with repeated stimulation. The hyperpolarization was partially blocked by L-NAME. Conclusions The proximal urethra receives excitatory innervation involving three neurotransmitters, i.e. ATP, acetylcholine and noradrenaline, and inhibition is associated with hyperpolarization which results at least in part from action of non-nitrergic nerves.