The rat external urethral sphincter an in vitro model to evaluate the activity of drugs on the smooth and striated components of the urinary bladder outlet.

Electrical field stimulation (EFS, 2.5 Hz, 60 V, 1 ms, trains of 5 sec every 5 min) of the rat external urethral sphincter (EUS) produced contractile responses characterized by a "slow" tonic contraction on which was superimposed a series of phasic "twitch-like" contractions. Both responses were abolished by tetrodotoxin (0.6 microM), and their amplitude was significantly lower in samples taken from denervated (15 days before) sphincters. The tonic contraction showed duration, voltage, and frequency dependency, whereas the twitches were markedly duration dependent. No correlation was found between the amplitude of the tonic and that of the twitch-like contractions. Phentolamine (3 microM) reduced by 95% the amplitude of the tonic contraction produced by maximal parameters, whereas it did not affect the twitches. On the other hand, hexamethonium (10 microM) was ineffective on both components. Dantrolene (10 microM) inhibited the twitch response, whereas it did not influence the tonic component. Preincubation with d-tubocurarine (0.2 mM) or succinylcholine (2 mM) significantly inhibited the amplitude of twitches produced by EFS (0.1 Hz, 60 V) up to a duration of 50 microseconds. Stimulus width higher than 50 microseconds, resulted in twitches that were resistant to neuromuscular blocking agents but sensitive to dantrolene (10 microM). Our results indicate that the rat external urethral sphincter is a reliable and easy "in vitro" model for studying the activity of drugs capable of interfering with the nerve-mediated activity of the striated and smooth muscle portion of the urinary bladder outlet.