The Effect of Intravesical Capsaicin on Detrusor Contractility－a Study of the Concentration and Duration in Rat Urinary Bladders

In order to clarify the influence of concentration and duration of intravesical capsaicin exposure on the micturition reflex and detrusor contractility, normal adult rats were used in in vivo cystometry, in vitro whole bladder study and in vitro muscle strip study. In in vivo cystometry, Capsaicin at the concentrations of 1, 10, 100 and 1000 µ M were infused into urethane-anesthetized rat bladders. Vehicle containing 10% Tween 80 and 80% normal saline was used for control studies. In the secoud study, vehicle and 1000 µ M capsaicin were infused into the bladders and were retained for 1, 2 and 4 hours. The bladder activities after each type of infusion were recorded and compared. Then the bladders were resected and suspended in the organ baths. In vitro whole bladder contractility was performed by recording of the intravesical pressure under electrical, bethanechol and KCl simulations. Some bladders were harvested after in vivo cystometry and isolated muscle strip contractility study were undertaken under the same stimulations. The results of this study showed that intravesical capsaicin administration could inhibit micturition reflex at the concentrations of at least 100 µ M. A higher concentration of capsaicin administered intravesically could activate concentration- dependent this concentration. A longer duration of intravesical exposure to 1000 µ M capsaicin did not show a better inhibitory effect on detrusor contractility than vehicle pretreated bladders. It is postulated that the neurokinin receptors in the urothelium can be activated by intravesical capsaicin administration. The tahykinins released from afferent nerve terminals can modulate the micturition reflex. However, only under a concentration of at least 100µ M capsaicin can inhibit the reflex through influencing the neurokinin receptors in the detrusor muscle or intramural ganglia. A longer duration of intravesical exposure up to 4 hours did not induce a larger amount of tachykinin releases, and hence had no effect on the detrusor contractility.