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Nerve mediated relaxation in human detrusor muscle.

James et al. [1] describe relaxation of precontracted detrusor muscle in response to electrical field stimulation. The magnitude of these relaxations ranges from approximately -8.0 mg to -85 mg (calculated from their Figure 2). We would question the physiological relevance of such small relaxations. There is no doubt that relaxation occurs in urethral smooth muscle in animals [2-5]. The magnitude of such relaxation obtained in different species ranges from -3 mN to -6 mN, which is up to 30 times greater than described by James et al. [1]. Persson et al. [2] was unable to obtain relaxation in rat isolated detrusor muscle precontracted by carbachol or potassium in response to electrical field stimulation. This was also true in pig detrusor muscle [6], although minor relaxation was observed when the preparation was almost maximally contracted. Because of the difficulties in evoking nerve induced detrusor relaxation they questioned the physiological relevance of these findings. Klarskov [7] could only find minor and variable equivocal nerve mediated relaxation in human detrusor, but the evidence was not convincing. We investigated nerve mediated relaxation in human bladder biopsies obtained from four patients. Two samples were from normal bladders and two from patients undergoing prostatic resection. Muscle strips were mounted between parallel electrodes and maintained in a 50 ml organ bath at 370 C aerated with 95% oxygen and 5% carbon dioxide. The samples were precontracted with different concentrations of KCl 10-124 mm. Electrical field stimulation was applied in the presence of 10 FM atropine using 1.5.15.20.25.30 pulses per second. The parameters used were 10 volts with pulse width 0.5 ms which produced contractions that were abolished by 1.3 x 10-6 M TTX. We could not produce relaxation in our detrusor samples using these parameters despite wide experience with organ bath preparations [8].