Underactive bladder (UAB) has been described as a symptom complex suggestive of detrusor underactivity (DU) that is usually characterized by prolonged urination time with or without a sensation of incomplete bladder emptying, usually with hesitancy, reduced sensation on filling, and slow stream [1]. Diabetes mellitus (DM) is one of the causes of the UAB, and streptozotocin (STZ)-induced DM rats have been widely used as an animal model of UAB [2]. Diabetic bladders may undergo a transition from a compensated to a decompensated state [3], but this time point has not been fully investigated. The aim of this study is to assess the time-dependent changes of diabetic bladder and to investigate the chronic diabetic bladder induced by STZ.

Male Wistar rats (9-weeks old) were received intraperitoneal injection of 60 mg/kg of STZ. In vitro (muscle strip) and in vivo (cystometry: CMG) functional experiments were performed. In vitro muscle strip experiments using full-thickness of longitudinal strips taken from the bladder body were performed at 4, 8, 12,16 weeks after the induction of diabetes, in which smooth muscle contractile responses to high potassium chloride (high K+: KCl 62 mM), carbachol (CCh: 10-3-10-8 M), and electrical field stimulation (EFS; 2-64 Hz, 50V, 0.8m sec pulse duration, 5s train duration, 2min interval) were investigated. In vivo CMG was performed at 16 weeks after the induction of diabetes. Two days after a catheter-implantation through the bladder dome, single CMG measurements were performed with saline-instillation at a rate of 6 mL/hour under a conscious and restraint condition. At the end of each micturition, the residual volume was precisely measured by collecting naturally dripping of post-void residual through the bladder catheter for 10 minutes. After stable and reproducible recordings were obtained, 3 micturition cycles were averaged and the following parameters were analysed: voided volume, residual volume, bladder capacity, voiding efficiency, threshold pressure for inducing micturition, maximal voiding pressure, basal pressure, non-voiding contractions (NVCs).Non-voiding contractions (NVCs), defined as bladder contractions of which amplitude was more than 3 cmH2O observed during the filling phase, were analysed their amplitude (cmH2O) and frequency (times/min).

Diabetic rats showed significantly higher serum glucose level and bladder weight, and lower body weight compared with sham rats at all time points (4, 8, 12, and 16 weeks). In vitro muscle strip experiments, contractile responses to high K+ were not significantly different between diabetic and sham rats at all time points. On the other hand, contractile responses to CCh were significantly higher in diabetic rats compared with sham rats at all time points. In addition, diabetic rats showed a tendency of higher contractile responses to EFS at 4, 8, 12 weeks, but it reversed at 16 weeks (Figure 1A, 1B). In vivo CMG measurements, diabetic rats showed significant increases in voided volume, residual volume, bladder capacity, maximal voiding pressure, and the amplitude and frequency of NVCs compared with sham rats (Figure 1C, Table 1).

Contractile responses to high K+ did not change, whereas those to CCh showed increase at all time points in diabetic rats. These results suggested that hypersensitivity of the muscarinic receptors in the detrusor smooth muscle occurs at from early to late phases of STZ-induced diabetic bladder. On the other hand, in contractile responses to EFS, diabetic rats showed rather increased responses at 4, 8, and 12 weeks, and decreased responses at 16 weeks, suggesting that efferent nerve impairment of the bladder occurs at a late phase (16 weeks) induced by STZ. In this period of time (16 weeks), CMG measurements showed larger bladder capacity, voided volume, and residual volume, and higher maximal voiding pressure and increased NVCs, whereas voiding efficiency did not differ in comparison with sham rats. These results suggest that diabetic bladder dysfunction as a model of UAB (with or without detrusor overactivity) occurs at least more than 16 weeks after induction of DM. In addition, higher maximum voiding pressure on CMG measurements suggests that bladder contractile dysfunction is relatively limited, but rather urethral relaxation during voiding might be impaired at a late phase of DM induced by STZ.

The present results suggest that diabetic bladder dysfunction as a model of UAB occurs at least more than 16 weeks after DM-induction by STZ-injection. In addition, the present results may indicate urethral dysfunction rather than bladder dysfunction during voiding is possibly prominent in the chronic diabetic animal model induced by STZ.

Chapple, C. R.; Osman, N. I.; Birder, L.; van Koeveringe, G. A.; Oelke, M.; Nitti, V. W.; Drake, M. J.; Yamaguchi, O.; Abrams, P.; Smith, P. P., The underactive bladder: a new clinical concept? European urology 2015, 68 (3), 351-3.
Daneshgari, F.; Leiter, E. H.; Liu, G.; Reeder, J., Animal models of diabetic uropathy. The Journal of urology 2009, 182 (6 Suppl), S8-13.
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