Background and purpose: Overactive bladder is a complex and widely prevalent condition, but little is known about its physiopathology. We have carried out morphological, biochemical and functional assays to investigate the effects of long-term nitric oxide (NO) deficiency on muscarinic receptor and β-adrenoceptor modulation leading to overactivity of rat detrusor muscle. Experimental approach: Male Wistar rats received N ω-nitro-L- arginine methyl ester (L-NAME) in drinking water for 7-30 days. Functional responses to muscarinic and β-adrenoceptor agonists were measured in detrusor smooth muscle (DSM) strips in Krebs-Henseleit solution. Measurements of [ 3H]inositol phosphate, NO synthase (NOS) activity, [ 3H]quinuclidinyl benzilate ([ 3H]QNB) binding and bladder morphology were also performed. Key results: Long-term L-NAME treatment significantly increased carbachol-induced DSM contractile responses after 15 and 30 days; relaxing responses to the β 3-adrenoceptor agonist BRL 37-344 were significantly reduced at 30 days. Constitutive NOS activity in bladder was reduced by 86% after 7 days and maintained up to 30 days of L-NAME treatment. Carbachol increased sixfold the [ 3H]inositol phosphate in bladder tissue from rats treated with L-NAME. [ 3H]QNB was bound with an apparent K D twofold higher in bladder membranes after L-NAME treatment compared with that in control. No morphological alterations in DSM were found. Conclusions and implications: Long-term NO deficiency increased rat DSM contractile responses to a muscarinic agonist, accompanied by significantly enhanced K D values for muscarinic receptors and [ 3H] inositol phosphate accumulation in bladder. 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