Histochemical study of urethral striated musculature in the dog.

The striated musculature of the dog urethra was studied histochemically. Two main groups of muscle fibers could be identified: 1 with slow twitch oxidative fibers, which are fatigue resistant (type 1), and 1 with fast twitch fibers (type 2). The fast twitch fibers were subdivided into glycolytic (fatiguable) and oxidative glycolytic (fatigue resistant) fibers: the latter constitute 20 per cent of all fast twitch fibers. Type 1 constitutes of 35 per cent of the whole musculature and its proportion tends to decrease toward the distal end of the external urethral sphincter. From these observations we infer that type 1 fibers are likely responsible for continence at rest and that type 2 fibers are recruited in stress conditions, for example, during coughing or sneezing. Additional studies are needed to confirm this conjecture. The clinical implications of these studies for the control of incontinence, urinary retention and dyssynergic urethral sphincter are presented.