Interstitial cystitis is a syndrome characterized by detrusor overactivity and chronic inflammation of the bladder. The mechanisms responsible for the altered smooth muscle contractility remain poorly understood. The aim of the study was to investigate the role of intracellular signalling pathways in carbachol-induced detrusor contraction in a rat model of interstitial cystitis. Cyclophosphamide (150 mg/kg, dissolved in saline) was injected to rats (Sprague-Dawley, female, 200-250 g) intraperitoneally once a day on days 1, 4 and 7 to induce interstitial cystitis. Control groups were injected with saline (0.9% NaCl). Detrusor smooth muscle strips were mounted in 1-ml organ baths containing HEPES-buffered modified Krebs' solution and permeabilized with 40 M -escin for 30 min. Carbachol-induced contractions were significantly increased from 21.2 +/- 1.6% (saline-treated) to 44 +/- 4.4% in cyclophosphamide-treated group. The Rho kinase inhibitor Y-27632 (8.8 +/- 2%) and the protein kinase C inhibitor GF-109203X (11.7 +/- 2.8%) inhibited the increased contractile response (44 +/- 4.4%) in rats with cystitis. The increased carbachol-induced contraction (44 +/- 4.4%) was also significantly inhibited by the sarcoplasmic reticulum ryanodine channel blocker ryanodine (25.8 +/- 3.2%) and the sarcoplasmic reticulum IP3 receptor blocker heparin (17.2 +/- 2.2%) in cystitis. RhoA protein levels in the bladder of cyclophosphamide-treated rats were significantly increased while pan-protein kinase C (, and isoforms) protein expression was unaltered between experimental groups. Carbachol-induced calcium sensitization at constant and clamped calcium (pCa 6) was also increased in cystitis (from 15.8 +/- 2.2% to 24.7 +/- 2.8%). This increased response (24.7 +/- 2.8%) was significantly inhibited by both Y-27632 (7.9 +/- 0.7%) and GF-109203X (4.4 +/- 1.5%). We conclude that interstitial cystitis is characterized by an enhanced carbachol contractile response as well as by calcium sensitization of the detrusor smooth muscle. Activation of Rho kinase and protein kinase C pathways may be the molecular culprits responsible for the augmented muscarinic response observed in cystitis.