Functional role of an adrenergic sphincter in the female urethra of the guinea-pig.

By means of a microtransducer catheter, the urethral pressure profile was recorded in female guinea-pigs. The maximum urethral pressure averaged 24 mmHg (range 18-27 mmHg) and was found 4 to 7 mm from the vesico-urethral junction. Fluorescence microscopy of the urethra showed an irregular distribution of fluorescent nerve terminals in the most proximal part of the organ. In a distal direction, there was a continuous increase in the number of adrenergic nerves, with a maximum 4 to 6 mm from the internal meatus, corresponding to the maximum intraurethral pressure point. In this region, a dense plexus of nerves supplied thick muscle coats of smooth muscle having an outer circular and an inner longitudinal orientation. The findings suggest that a specific region of the female guinea-pig urethra may serve as an "internal sphincter".