It has been reported that β3-adrenoceptor (β3-AR) agonists can inhibit non-voiding contractions (NVCs) of the hypertrophied bladder in rats with bladder outlet obstruction (BOO) [1]. Moreover, bladder myogenic microcontractions may link to the single-unit mechanosensitive afferent activities (SAAs) of Aδ-fiber, and β3-AR agonists inhibit the afferent activities through the suppression of the microcontractions in normal rats [2]. In addition, SAAs of Aδ- and C-fibers were intermittently enhanced by propagation of bladder myogenic microcontractions in BOO rats [3]. In this study, we investigated the effects of CL316243, a β3-AR agonist, on the myogenic activities and on the SAAs of the bladder in BOO rats.

Female Sprague-Dawley rats weighing 190 to 210 gm were used. The proximal urethra was tightly ligated with a steel rod (1.1 mm in diameter), then the rod was removed. This ligation was kept before the experiments. Six weeks after the surgery, cystometry (CMG) and SAAs measurements were performed. Continuous CMG measurements were performed under an awake and free-moving condition. A PE-50 catheter with a cuff was implanted into the bladder a day before the measurements. Saline was instilled into the bladder at a rate of 20 mL/h. CMG recordings were repeated three times both before and after the intravenous (i.v.) administration of CL316243 (10 μg/kg). The following parameters were averaged and analyzed before and after CL316243-administration: voided volume (VV), residual volume (RV), intercontraction interval (ICI), basal pressure (BP), threshold pressure (TP), the maximum pressure (MP) during a micturition, and the number of NVCs. NVCs were defined as bladder contractions without micturition, of which amplitude was greater than 2 cmH2O.
SAAs measurements were performed under a urethane-anesthetized condition (1.0 g/kg, intraperitoneally). Bilateral L6 dorsal roots were transected via a laminectomy. The fine filaments were dissected from the left L6 dorsal roots and placed across a bipolar electrode for monitoring SAAs. Nerve fibers primarily originating from the bladder were identified by electrical stimulation of the left pelvic nerve and by bladder distension. Nerves with conduction velocities (CV) more than 2.5 m/second were designated as Aδ-fibers and those with CV less than 2.5 m/second as C-fibers. The bladder was emptied and saline instilled until the intravesical pressure reached 30 cmH2O. The bladder was kept under an isovolumetric condition, allowed to stabilize for 5 min, and then CL316243 (10 μg/kg, i.v.) was administered i.v. and recording was performed for 5 min. Mean bladder pressure, number of microcontractions and SAAs were analyzed before and after CL316243-administration. All data are expressed as mean ±SEM. Both results between before and after administration were analyzed using an unpaired Student’s t-test. P-values <0.05 were considered statistically significant.

In continuous CMG, 9 rats were used. After the administration of CL316243, the MP and the number of NVCs were decreased, while other parameters did not change (Table 1). In SAA measurements, 10 rats were used and totally 16 SAAs (Aδ-fibers: n=9, C-fibers: n=7) were isolated. CL316243 significantly decreased the mean intravesical pressure and the number of microcontractions. SAAs of Aδ-fibers, but not C-fibers, were also decreased after CL316243-administration (Fig. 1).

As harmonized with a previous study [1], CL316243 decreased NVCs. Also similarly in the SAAs measurements, bladder microcontractions were decreased after CL316243-administration. These present results suggest that CL316243 can inhibit bladder myogenic activities in BOO rats. In addition, the present SAAs measurements showed an inhibitory effect of CL316243 on the Aδ-fibers, but not on the C-fibers, suggesting that CL316243 can inhibit the SAAs of Aδ-fibers at least partly due to suppressing bladder myogenic activities in BOO rats like as in normal rats.

The present results suggest that β3-AR agonists inhibit the SAAs of Aδ-fiber at least partly due to suppressing bladder myogenic activities in female BOO rats. These results may give us a possible mechanism involved in the inhibitory action of β3-AR agonists on OAB associated with BOO.

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