Radiologic study of the canine urethra.

The structure and function of the canine urethra were studied during retrograde and voiding urethrography in 9 male and 8 female dogs. The lumen of the prostatic portion of the urethra was variable in diameter. During retrograde urethrography, the lumen was narrow with streaks of contrast medium outlining mucosal folds, but during voiding, it was dilated and a spindle-shaped seminal hillock was detected. Retrograde urethrography revealed other regions of the urethra of males and females had a smooth radiographic outline. During voiding urethrography, characteristic identical urethral contractions were observed in the caudal portion of the pelvic urethra (male) and in the caudal half of the urethra (female). Contractions were progressive, occurred where striated muscle was present in the urethral wall, and resulted in an intermittent flow of urine from the urethra. In the male, contractions had a rate of 2.02 +/- 0.23 contraction/s and 1.65 +/- 0.53 in the females. Contractions may have a role in the controlled voiding of small volumes of urine necessary for the repeated marking of territory, in the passage of the ejaculate along the male urethra during copulation, and in the clearance of any residual urine from the urethra at the completion of micturition.