This study is the first report to elucidate the distribution and functional role of adenosine receptor subtypes in the bladder of male rats with bladder outlet obstruction (BOO).  Storage symptom, similar to overactive bladder (OAB), in elderly man with BOO related to benign prostate enlargement (BPE) is often resistant to treatments. Recent studies using female rats have reported that adenosine receptor modulation using A2A receptor antagonists or A2B receptor agonists including inosine can affect bladder function.  However, the adenosine receptor mechanism underlying BOO-induced bladder dysfunction is not well elucidated, Furthermore, previous BOO research mainly utilized female BOO animals.  Therefore, this study examined the expression of adenosine receptors and the effects of adenosine receptor modulation on bladder activity using the male BOO rat model.

In male Sprague-Dawley rats (8-weeks old), BOO was produced under isoflurane anesthesia.  After the lower abdominal incision, 4-0 silk suture was placed around the urethra, including a metal rod with an outer diameter of 1.2mm placed extraluminally at the urethrovesical junction level proximal to the urethral fenestration [1].  After tying the suture, the rod was removed. Sham operated animals were used as controls.   After baseline awake cystometrograms (CMG) were obtained with saline, we intravesically applied an adenosine A1 receptor agonist (CCPA, 4.1 μM) or an adenosine A2A antagonist (ZM241385, 15 μM), or inosine (1 mM) [2] to BOO (n=6, each agent) and sham rats (n=4, each agent).  Furthermore, mRNA levels of adenosine receptor subtypes such as A1 (ADORA1), A2A (ADORA2A), A2B (ADORA2B), and A3 (ADORA3) receptors in bladder mucosa and detrusor were analysed by RT-PCR. Totally, we used 38 BOO rats and 32 age-matched sham rats in this study.  Thirty of 38 BOO and 24 of 32 sham rats underwent CMG, and the remaining 8 BOO rats and 8 sham rats were used for molecular studies.

Bladder weight of BOO rats was significantly increased (0.35±0.041 g vs. 0.11±0.0048 g, respectively, p<0.0001) compared to sham rats.  In CMG, BOO rats were significantly increased in the amplitude during voiding (91±6.5 cmH2O vs. 40±2.2 cmH2O, respectively, p<0.0001), a number of non-voiding contractions (0.91±0.15 /min vs. 0.11±0.025 /min, respectively, p=0.00020), post-void residual (1200±230 µl vs. 20±8.4 µl, respectively, p=0.0004), bladder capacity (2000±240 µl vs. 860±32 µl, respectively, p=0.0006), and compliance (0.89±0.17 ml/H2O vs. 0.31±0.032 ml/H2O, respectively, p=0.0051) compared to sham rats (figure 1).  Voiding efficiency (VE) was decreased in BOO rats compared to sham rats (46±6.9 % vs. 98±0.89 %, respectively, p<0.0001).  In sham rats, intravesical administration of CCPA, ZM241385 or inosine did not have any effect on CMG parameters.  In BOO rats, CCPA and Inosine did not have any effect on CMG parameters; however, ZM241385 significantly reduced NVCs in BOO rats (0.50±0.079 /min vs. 0.90±0.15 /min, respectively, p=0.028) (figure 2).  mRNA expression levels of adenosine A2A receptors and A3 receptors in bladder mucosa were significantly increased in BOO rats compared to sham rats (p<0.0001, p=0.0145, respectively).  In contrast, mRNA expression levels of adenosine A2B receptors in bladder mucosa was significantly decreased in BOO rats compared to sham rats (p<0.0001).  There was no significant difference in adenosine receptor expression in detrusor between BOO and sham rats.

Referring to previous reports of female BOO rats [3], the results of bladder weight and CMG parameters in our male 4-week BOO rats were compatible to the early decompensated phase; i.e. bladder contractility and bladder capacity are preserved, but VE is significantly decreased.  The therapeutic effect of ZM241385 on bladder overactivity evident as reduced NVCs in our male BOO rats may be related to the increased expression of adenosine A2A subtype receptors in bladder mucosa.  In addition, the reduction of adenosine A2B mRNA levels may contribute to the negative effect of inosine, which reportedly suppresses bladder activity via adenosine A2B receptor stimulation [2].

The male BOO rat model is useful to understand the pathophysiology of early decompensated phase of bladder dysfunction related to male BOO.  Also, adenosine receptor subtypes such as A2A receptors could be a therapeutic target for male patients with OAB due to BPE-induced BOO.

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