The urethra is capable of generating tone which is at least partly myogenic in nature. In the rabbit (Sergeant et al. 2000) and human urethra (Hollywood et al. 2002) we have characterised a number of currents that could play an important role in generating urethral tone. The purpose of the present study was to assess the importance of calcium influx and calcium stores 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 in accordance with Home Office guidelines. 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. Resting flow through the urethra under control conditions was 1.7 ± 0.14 ml min^-1 (mean ± S.E.M., n = 23) and was increased to 5.0 ± 0.2 ml min^-1 by electrical field stimulation (0.5 Hz, 0.3 ms pulse width, 50 V) consistent with a reduction in urethral tone.

To assess the contribution of calcium influx through L- and T-type channels to tone generation, the effects of both nifedipine and Ni²⁺ were examined on flow. In the presence of 10 µM nifedipine, resting flow was increased from 1.8 ± 0.2 to 2.5 ± 0.3 ml min^-1 (P < 0.01, paired t test, n = 7). When T-type calcium channels were blocked by 300 µM Ni²⁺ (in the presence of nifedipine) resting flow was further increased to 3.4 ± 0.5 ml min^-1 (P < 0.01, n = 6).

We next examined the effects of CPA and 2APB on flow to assess the contribution of calcium stores to myogenic tone. Inhibition of the calcium-ATPase with CPA (30 µM) significantly increased flow from 1.7 ± 0.5 to 5.1 ± 1.0 ml min^-1 (P < 0.01, n = 5). Prior to application of CPA, nerve stimulation (0.5 Hz, 1 min) increased flow to 5.2 ± 0.3 ml min^-1. However, in the presence of CPA nerve stimulation decreased urethral flow dramatically from 5.1 ± 1.0 to 0.3 ± 0.2 ml min^-1. Inhibition of calcium release from IP₃ dependent stores by 2APB (100 µM ) increased mean flow from 1.2 ± 0.2 to 3.9 ± 0.2 ml min^-1 (P < 0.001, n = 6) and nerve stimulation in the presence of 2APB further increased flow to 5.2 ± 0.2 ml min^-1.

These data support the idea that urethral tone in the rat is dependent on the influx of calcium through T and L channels and the release of calcium from intracellular stores.

This work was supported by Help the Aged, Wellcome Trust and Action Research.