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Investigating the role of TRPV1 in bladder function using SB-705498 Transient receptor potential vanilloid 1 (TRPV1) is a ligand gated cation channel, predominately expressed on unmyelinated C-fibres. In vivo activation of TRPV1 reduces bladder capacity (Zhang et al., 2003) and TRPV1 knockout mice have an increased bladder capacity compared with their wild type littermates (Birder et al., 2002). In addition, TRPV1 antagonists have been shown to increase bladder capacity during cystometry in animal models containing an inflammatory component (Yura et al., 2004), although, a TRPV1 antagonist had little effect on conscious cystometry in normal mice (Nakamura et al., 2003). The aim of this study was to investigate the effect of the TRPV1 antagonist SB-705498 (Rami et al., 2006) on rat bladder function in vitro and in vivo. All experiments were carried out in compliance with UK legislation. Bladders were removed from euthanased, female, CD (200-250g) rats and longitudinal strips dissected from the bladder dome. Isometric tension recordings were performed from bladder strips bathed in Krebs’ solution (37ºC, 95% O2/ 5% CO2) containing naproxen 10 µM. A capsaicin (0.1 nM – 30 µM) cumulative concentration response curve (CCRC) was carried out in the absence and presence of 5 antagonist concentrations (50 nM – 1 µM) following a 1 hour incubation period. A pKB of -7.15 (95% confidence interval of -6.9, -7.41) n=4-13, was calculated utilising the Angus method of analysis ( Lew & Angus, 1995). Antagonism of TRPV1 in terminally anaesthetised, female, CD rats was confirmed via blockade of the capsaicin-induced pulmonary chemoreflex (4 µg/kg i.v.). SB-705498 was infused i.v. over 2 hours, achieving a final concentration of 400nM. A pulmonary chemoreflex was induced every 30 min. Statistical significance was established with an unpaired Student’s t-test (p<0.05). SB-705498 significantly inhibited the capsaicin induced pulmonary chemoreflex at free plasma concentrations of 187nM (n=6). In vivo efficacy of SB-705498 at the bladder was investigated in terminally anaesthetised, female, CD rats. The bladder was emptied and filled with saline (5.4 ml/hr), until a void was triggered. Bladder capacity was determined as percentage change in bladder capacity from the mean baseline measurement prior to compound/vehicle infusion. After 90 minutes the intravesical saline infusion was replaced by capsaicin (30 µM) and a further micturition cycle performed. Effect of capsaicin was expressed as percentage change from the previous saline micturition cycle. Statistical significance was established with an unpaired Student’s t-test (p<0.05). At a free plasma concentration of 298 nM, SB-705498 inhibited capsaicin-induced bladder contractions compared to vehicle, (57.1% ± 5.2 vs 31.9% ± 7.8 respectively). However this did not translate to an increase in functional bladder capacity during anaesthetised cystometry with saline at 30 min (135% ± 22 vs. 96% ± 18), 60 min (163% ± 27 vs. 95% ± 17) and 90 min (146% ± 22 vs. 118% ± 15) following infusion start. The lack of a TRPV1 antagonist effect on bladder capacity in vivo, despite demonstrable antagonism of the pulmonary chemoreflex, suggests a low importance of bladder C-fibres in normal bladder function in response to filling.
Birder, L et al (2002). Nat Neurosci. 5, 856-860 |
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