836 is known that the functional changes in the obstructed rat bladder are stabilized slowly and gradually by 2 weeks af- ter the initiation of BOO. Around this period, those rats show variably different voiding functions, characterized by increased frequency, residual urine, and nonvoiding con- tractions (NVCs). After this, it is known that they enter the decompensated phase, which refers to a progressive loss of emptying detrusor function, resulting in increased re- sidual urine [7-9]. However, information remains frag- mentary on the initial functional changes during the acute obstruction. The aim of this study was to study bladder function, with special focus on initial functional changes, by objective re- port of decompensated bladder according to residual urine volume in awake, obstructed rats. To reduce the influence of confounding factors, we used a simultaneous recording of intravesical pressure (IVP) and intraabdominal pres- sure (IAP). MATERIALS AND METHODS Experimental animals. Female Sprague Dawley rats (200 to 250 g) obtained from Orient Bio Inc. (Seongnam, Korea) were used in this study. They were maintained under standard 12:12 hour light:dark cycles with free access to water and food. The method for creating urethral con- striction was identical to that previously described [10,11]. Sham-operated animals underwent similar surgery but no obstruction was created. After surgery, animals were cag- ed individually and animal weights were recorded at the time of utilization. All procedures for animal handling and treatment were performed in accordance with the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health and were approved by the Ethics Committee of the Inha University College of Medicine. Thirty rats were randomly subjected to sham operations (n=10) or partial BOO (n=20). Cystometric investigations were performed without anesthesia 1 week after BOO sur- gery in 10 rats in the BOO group (1 week BOO group) and 2 weeks after surgery in the sham group and in 10 rats in the BOO group (2 week BOO group). The catheters were implanted 3 days before the cystometries. Decompensated bladder was defined as a bladder with more than 20% of re- sidual urine compared to bladder capacity. On the basis of the cystometric criteria for the presence of detrusor overactivity (DO) during the filling phase, the rats were counted into subgroups showing DO (DO+) or not showing DO (DO-). DO in this study was defined as the oc- currence of NVCs without simultaneous IAP elevation. 1. Surgical method Rats were anesthetized with ketamine (Ketamine 50, Yuhan Corp., Seoul, Korea; 75 mg kg–1 intraperitoneally) and xylazine (RompunⓇ, Bayer Korea Ltd., Seoul, Korea; 15 mg kg–1 intraperitoneally). Through a lower midline in- cision, the bladder and the proximal urethra were freed from surrounding tissues. A 3/0 Novafil (monofilament pol- Korean J Urol 2011;52:835-841 Kang et al ybutester, Davis & Geck, Wayne, NJ, USA) ligature was placed around the urethra and tied in the presence of an intraluminally placed steel rod with a diameter of 0.9 mm. After tying the knot, the steel rod was removed and the bladder was repositioned. The abdominal wall was closed. 2. Intra-vesical and intra-abdominal pressure catheter im- plantations After an abdominal incision was made, a polyethylene catheter (PE-50; Becton Dickinson & Co., Franklin Lakes, NJ, USA) with a cuff was implanted into the dome of the bladder and held in place with a purse-string suture. To re- cord IAP, a catheter with an abdominal balloon (Latex, Dawoo Medical, Incheon, Korea) was placed superior to the bladder, as described previously [12]. The catheters were tunneled subcutaneously and anchored to the skin of the back with a silk ligature. The free end of the catheter was sealed. Animals were allowed to recover for at least 24 hour. 3. Cystometric investigations The conscious rats were placed in metabolic cages (Nalgene metabolic cage; Nalge Co., Rochester, NY, USA) without restraint. The indwelling bladder catheter was connected to a two-way valve that was connected to a pressure trans- ducer (Research Grade Blood Pressure Transducer; Harvard Apparatus, Holliston, MA, USA) as well as an in- fusion pump (PHD 22/2000 Programmable syringe pump; Harvard Apparatus), and the indwelling IAP catheter was connected to another pressure transducer. Micturition vol- ume was simultaneously recorded by using a fluid collector connected to a force displacement transducer (Research Isometric Transducer; Harvard Apparatus). Continuous bladder infusion was performed with room temperature, normal saline at a rate of 20 ml h–1 in all groups. Data were digitally stored and analyzed at a later time by using Acq Knowledge ver. 3.8.1 software (BIOPAC Systems Inc., Goleta, CA, USA) at a sampling rate of 100 Hz and an MP150 data acquisition system (BIOPAC Systems Inc.). The values from three reproducible micturition cycles were used for evaluation. IAP was defined as the recorded balloon pressure corrected by subtracting the lowest bal- loon pressure in each voiding cycle for zeroing. The de- trusor pressure (DP) was defined as the IVP minus the IAP. The NVCs during the filling phase were defined as incre- ments of IVP that exceeded 2 cmH2O from baseline without simultaneous changes in IAP and without fluid expulsion from the bladder. The following cystometric parameters were inves- tigated: 1) Cystometric pressure (by DP) and volume pa- rameters, including basal pressure (the lowest pressure during filling), threshold pressure (pressure immediately before bladder contraction), maximal pressure (highest pressure during micturition), micturition volume, residual volume (remaining urine after voiding), bladder capacity (micturition volume+residual volume), residual volume percent (residual volume/bladder capacity x100), and mic- turition interval (interval between micturition contrac-