The function of the smooth muscle of the urethra is to generate sufficient tone to prevent voiding of urine from the bladder and thus play an important role in continence (Bridgewater et al. 1993). This tone appears to be partly myogenic in nature and is associated with the spontaneous generation of spontaneous transient depolarisations (STDs) and large regularly occurring slow waves (Hashitani et al. 1996). These events are due to spontaneous release of calcium from intracellular stores which causes depolarisation by activating a calcium-activated chloride current. It was previously believed that STDs originate in the smooth muscle cells but we have recently demonstrated that they may originate in specialised non-contractile interstitial cells (Sergeant et al. 2000). The purpose of the present study was to assess the importance of calcium-activated chloride channels in the generation and modulation of myogenic tone in the rat urethra using a novel isolated urethra preparation. Female Sprague-Dawley rats were anaesthetised by CO2 inhalation and killed by cervical dislocation. The urethra was cannulated at the bladder neck and placed in a horizontal organ bath (perfused with oxygenated Krebs solution at 37 °C) and connected to a constant-pressure reservoir. Flow was estimated by measuring side-arm pressure near the inflow cannula. Mean flow through the urethra under control conditions was 0.93 ± 0.123 ml min-1 (± S.E.M., n = 14). This could be increased more than twofold by electrical field stimulation (1 Hz, 0.3 ms pulse width, 50 V). The flow increase in response to field stimulation was blocked by 1 µM TTX (2.4 ± 0.295 before to 0.07 ± 0.036 ml min-1 in the presence of TTX), suggesting that it was due to stimulation of the intrinsic inhibitory nerves. These nerves appeared to release nitric oxide since the response was reduced by 100 µM L-NOARG (3.1 ± 0.36 before to 1.0 ± 0.18 ml min-1 in the presence of NOARG). Addition of 1 mM A-9-C caused baseline flow to increase approximately threefold, almost to the peak level achieved during field stimulation such that further trains of stimulation had little extra effect. In four such experiments A-9-C increased baseline flow from 0.9 ± 0.03 to 3.1 ± 0.15 ml min-1 (P < 0.05, paired t test). We conclude that isolated rat urethra exhibits resting myogenic tone under the conditions of these experiments and that this is relaxed by the chloride channel blocker A-9C (1 mM) to a level comparable to that produced by stimulation of the inhibitory nerves.
- Bridgewater, M., MacNeill, H.F. & Brading, A.F. (1993). J. Urol. 48, 347-354.
Hashitani, H., Van Helden, D.F. & Suzuki, H. (1996). Brit. J. Pharmacol. 118, 1627-1632.
Sergeant, G.P., McCloskey, K.D., Hollywood, M.A., Thornbury, K.D. & McHale, N.G. (2000). J. Physiol. 526, 359-366. abstract