Supersensitivity to methacholine in rat urethra following hypertrophy or disuse.

It has been possible to sensitize the rat urethra without cutting its nerves. This was achieved by hypertrophy or disuse. Hypertrophy of the urethra (2-4-fold weight increase) was caused by the presence of an intraluminal paraffin bolus. Disuse was caused by diverting the flow of urine from the lower urinary tract. When examined in vitro after an experimental period of 1 to 4 weeks the EC50 value of the parasympathomimetic drug methacholine was in both cases half of that of controls. The common cause of the development of supersensitivity in the two types of experiments is thought to be decreases in the local concentration of transmitter at the muscle cells. The present findings favour the idea of a parasympathetic motor control of the rat urethra.