Quantifying the influence of contraction history on ex vivo bladder mechanics using optogenetics

The micturition cycle involves passive expansion of the bladder as urine gradually accumulates, then active contraction during voiding. The high compliance of the bladder compared to other tissues is attributed to the interplay between smooth muscle cells and the extracellular matrix (ECM) within the detrusor muscle. Current ex vivo methods to characterize bladder mechanics entail passive loading and unloading, which do not include the active contraction of the detrusor. The bladder does not return to the original configuration during these tests, leading to strain softening. We hypothesized that inducing controlled ex vivo detrusor contraction during the unloading phase of mechanical testing would improve the physiological relevance of the data by minimizing the strain softening that occurs during passive-only mechanical testing. We combined optogenetically mediated smooth muscle contraction with a custom mechanical testing platform to analyze the mechanical changes in bladder tissue compared to standard mechanical testing methods. Using a murine model expressing channelrhodopsin-2 in smooth muscle cells, we performed cyclical mechanical tests on bladder rings, both with and without optogenetic stimulation. Passive-only tests resulted in increased tissue compliance and strain softening. In contrast, induction of concentric contraction during unloading preserved the original tissue configuration. This method will enable future studies to link changes in ECM and cellular organization with bladder mechanics, leading to improved constitutive descriptions that drive computational analyses of in vivo behavior. STATEMENT OF SIGNIFICANCE: Traditional passive mechanical testing methods do not fully capture the dynamic nature of bladder mechanics. Our results demonstrate that incorporating active contraction into testing protocols provides a more physiologically relevant representation of bladder function and mechanical behavior. In addition, our results highlight the importance of choosing the proper reference configuration to define stretch when analyzing mechanical data. This work improves physiological relevance of ex vivo bladder mechanical data and has the potential to enhance the understanding of the mechanics of other active smooth muscle cell-containing tissues.