Faster bladder filling in rats reduces detrusor overactivity but worsens (i.e., decreases) bladder compliance

The impact of bladder filling rate on cystometric outcomes remains unclear. Clinically, faster bladder filling is believed to increase the likelihood of observing detrusor overactivity (DO) in those with bladder dysfunction, although evidence of this is lacking. We executed this study to clarify how changes in bladder filling rate impact cystometric parameters. Urethane-anesthetized female CD rats (<i>n</i> = 19) underwent bladder filling at five different fill rates, a baseline rate scaled to have a filling phase of ∼7 min (in line with our previous work) and scaled rates of 1/3×, 2×, 4×, and 8× that speed. Contrary to expectations, filling at faster rates decreased the likelihood of observing detrusor overactivity, with 4× and 8× filling rates demonstrating less detrusor overactivity than the baseline (1×) rate (<i>P</i> = 0.0091 for 4× and <i>P</i> = 0.019 for 8×). However, faster filling rates did decrease bladder compliance. Filling at 4× and 8× demonstrated decreased bladder compliance compared to 1× (<i>P</i> = 0.032 for 4× and <i>P</i> < 0.0001 for 8×). Finally, increasing the filling rate led to increases in bladder capacity at 4× (<i>P</i> = 0.034) and 8× (<i>P</i> = 0.0066) relative to 1×. These results suggest that, contrary to expectations, faster filling may not be more effective at eliciting detrusor overactivity (i.e., not a better diagnostic approach). As reductions in detrusor overactivity and increases in bladder capacity are critical parameters for evaluating preclinical therapeutics, faster filling may impair the ability to demonstrate further improvements.<b>NEW & NOTEWORTHY</b> Little is known about the effects of different bladder filling rates on cystometric results. Various sources have suggested that faster filling is "provocative" to the bladder. However, in this study we varied bladder filling rates in anesthetized rats and observed less detrusor overactivity with faster filling, not more. We explain this discrepancy as a miscommunication about what being provocative means, where faster filling leads to worse bladder compliance (as we observed), not more detrusor overactivity.