Adjustable Preload and Dynamic Compliance in the Human Detrusor

Hypothesis / aims of study Acute changes in detrusor tension may be important in both overactive bladder syndrome (OAB) and detrusor underactivity (DU). Total detrusor tension can be analyzed according to its component parts: the active tension (Ta) generated by detrusor smooth muscle (DSM) contraction and the preload (Tp). Tp can be further sub-divided into passive tension (Tpas) derived from nonregulatable structural components (i.e. collagen and elastin) and an adjustable preload (Tap) that has been identified in rabbit DSM (rDSM) [1] and other smooth muscle types. Alterations of Tap are observed through the process of reversible strain softening [1]. Our published data using rDSM supports the hypothesis that Tap is adjusted by breakage and reformation of slowly cycling actomyosin crossbridges, permitting reduced muscle stiffness (increases compliance) at increased muscle lengths [2]. Thus, we propose that Tap is a mechanism by which bladder compliance is adjusted acutely during filling (dynamic compliance). In biologic tissues, strain softening is defined as progressive loss of stiffness with repeat stretches due to cross-link breakage. Unlike the irreversible strain softening that occurs when a latex balloon is stretched and released prior to its initial inflation, Tap can be actively restored after contraction at short muscle lengths [1]. Recent pre−clinical studies have established that repeated passive fills of the bladder can acutely increase compliance. Moreover, acute compliance changes can be reversed by active contraction, a process we term "dynamic compliance.”