Hypothesis / aims of study
Suprasacral spinal cord injury (SCI) can result in neurogenic detrusor overactivity (NDO). Antimuscarinic agents have long been used to treat NDO, but these agents are fraught with systemic and neurological side-effects. β3-adrenoceptor agonists (BARA) have been FDA approved for the treatment of overactive bladder since 2012; mechanistically, they are thought to work predominantly by direct smooth muscle relaxation and they maintain a superior side effect profile when compared to antimuscarinics. We previously demonstrated significant positive effects with a rat-specific BARA, CL-316,243 (CL), in chronic suprasacral SCI rats. We present the results of an opportunistic post-hoc analysis comparing the cystometric effects of CL to those of atropine under the same chronic SCI conditions in an experiment originally designed to pharmacologically assess myogenic vs neurogenic contributions to NDO.
Study design, materials and methods
The source for the current analysis was a multidrug study with experimental arms developed to interrogate myogenic vs neurogenic pathophysiological changes associated with NDO after SCI. An Institutional Animal Care and Use Committee approved the study and the experiment conformed to all institutional guidelines. For the parent study, 4 week post SCI T9-10 transected female Sprague-Dawley rats received external urethral sphincter EMG electrodes, femoral vein catheters for drug administration, bilateral ureteral diversion catheters, and a transvesical catheter under isoflurane anesthesia. Conscious continuous cystometry was performed for ≥60 min while animals were mounted in Ballman cages. The infusion was stopped, bladder emptied, and vehicle (normal saline) was administered prior to resuming bladder infusion. Following 30min, the infusion was again stopped, the bladder emptied, and i.v. drugs were delivered prior to subsequent 30min infusion cycles. In this post-hoc analysis, we compared specific treatments of two arms of the parent study, one which received atropine and one that received CL. In these arms, 6 rats received 0.4mg/kg atropine and 8 rats received 10mg/kg verapamil; following the same methods, the verapamil-treated group received 100ug/kg CL prior to the next fill cycle. We measured total bladder capacity (TBC) and filling compliance, non-voiding contraction (NVC) counts, frequency, and average amplitude, voiding duration and efficiency, and EUS phasic firing frequency. Data were analyzed using non-parametric 1- or 2-Way RM-ANOVA, or linear regression.
Results
Because verapamil had no observable or measurable effect on any parameter and, due to its ~2min half-life, we directly compared the effects of atropine to those of CL. While both atropine and CL significantly increased TBC (47%, P=0.0016 and 61%, P=0 0.0018, respectively) and decreased NVC average amplitude (-20% and -21%, P=0.0119), atropine increased NVC counts and frequency (167%, P=0.0001 and 31%, P<0.0001, respectively) and CL decreased NVC count and frequency (-57%, P=0.0034 and -22%, P=0.0009). Only atropine significantly decreased voiding efficiency (-68%, P=0.0060) and voiding duration (-42%, P=0.0009), and voiding duration and efficiency were positively associated (P=0.0007, R2=0.70) for these animals. Compliance and EUS phasic firing frequency were unaffected by any drug treatment.
Interpretation of results
While TBC was increased and NVC amplitude decreased by both drugs, atropine reduced voiding efficiency and increased NVC count and frequency. In contrast, CL preserved voiding efficiency and reduced NVC count and frequency. The strong relationship between voiding duration and efficiency in the atropine group suggests yet another mechanism by which antimuscarinics may reduce voiding efficiency. Despite longer filling time, the CL treated rats had reduced NVC count and frequency, supporting a myogenic contribution to theses contractions. Limitations include those inherent to post-hoc analysis, unknown or carry-over effects of verapamil, and generalizations to clinical applications from our specific SCI rodent model.