In this study we measured urethral pressure changes in response to
efferent pudendal nerve stimulation in rats. All other neural pathways to
the urethra were transected, and the urethra was continuously perfused. We
found fast twitch-like contractions, superimposed on a slow relaxation.
The amplitude of the twitches was independent of the stimulation frequency
below 26 Hz, whereas the relaxation depended highly on this frequency. The
twitches were caused by striated urethral muscles, and the relaxation was
caused by smooth muscles. Both were mediated by acetylcholine. We
calculated the effective urethral relaxation as the absolute relaxation
multiplied by the time fraction between the twitches. Maximum effective
relaxation occurred at 8-10 Hz, exactly the frequency of spontaneous
oscillations during bladder voiding in rats. Although the oscillatory
sphincter contractions in rats during voiding may be needed in other
mechanisms for efficient voiding, our data suggest that they may be a side
effect of the actual purpose: urethral relaxation
