Sympatholytic effect on striated urethral sphincter. A peripheral or central nervous system effect?

Effects of intravenous administration of sympatholytic drugs on urethral pressure and electromyographic activity from urethral and anal sphincter, were investigated in five normal women. Clonidine (Catapresan), which easily passes the blood brain barrier, resulted in a pronounced decrease in urethral pressure and electromyographic activity from both sphincters. Phentolamine (Regitine), which does not pass the blood brain barrier, also lowered urethral pressure but had no effect on electromyographic activity from the sphincters. Neither drug depressed reflex induced increase in urethral pressure and sphincter activity during bladder filling or during voluntary contraction of the pelvic floor muscles. Phenoxybenzamine administration to one normal female had the same effect as clonidine. It is suggested that part of the clinical effect on voiding disorders seen after treatment with sympatholytic drugs passing the blood brain barrier (e.g. phenoxybenzamine) might be due to an effect on the central part of the striated urethral sphincter innervation.