Abstract
The urinary bladder is a distensible hollow muscular organ, which allows huge changes in size during absorption, storage and micturition. Pathological alterations of biomechanical properties can lead to bladder dysfunction and loss in quality of life. To understand and treat bladder diseases, the mechanisms of the healthy urinary bladder need to be determined. Thus, a series of studies focused on the detrusor muscle, a layer of urinary bladder made of smooth muscle fibers arranged in longitudinal and circumferential orientation. However, little is known about whether its active muscle properties differ depending on location and direction. This study aimed to investigate the porcine bladder for heterogeneous (six different locations) and anisotropic (longitudinal vs. circumferential) contractile properties including the force-length-(FLR) and force-velocity-relationship (FVR). Therefore, smooth muscle tissue strips with longitudinal and circumferential direction have been prepared from different bladder locations (apex dorsal, apex ventral, body dorsal, body ventral, trigone dorsal, trigone ventral). FLR and FVR have been determined by a series of isometric and isotonic contractions. Additionally, histological analyses were conducted to determine smooth muscle content and fiber orientation. Mechanical and histological examinations were carried out on 94 and 36 samples, respectively. The results showed that maximum active stress (pact) of the bladder strips was higher in the longitudinal compared to the circumferential direction. This is in line with our histological investigation showing a higher smooth muscle content in the bladder strips in the longitudinal direction. However, normalization of maximum strip force by the cross-sectional area (CSA) of smooth muscle fibers yielded similar smooth muscle maximum stresses (165.4 ± 29.6 kPa), independent of strip direction. Active muscle properties (FLR, FVR) showed no locational differences. The trigone exhibited higher passive stress (ppass) than the body. Moreover, the bladder exhibited greater ppass in the longitudinal than circumferential direction which might be attributed to its microstructure (more longitudinal arrangement of muscle fibers). This study provides a valuable dataset for the development of constitutive computational models of the healthy urinary bladder. These models are relevant from a medical standpoint, as they contribute to the basic understanding of the function of the bladder in health and disease.
Highlights
The urinary bladder is a hollow organ, which absorbs, stores and releases urine, and is thereby exposed to enormous deformation
The shape of the active isometric force-length relationship is similar for all groups investigated
The passive force-length dependency of the bladder tissue strip was characterized by an exponential increase of force with muscle length
Summary
The urinary bladder is a hollow organ, which absorbs, stores and releases urine, and is thereby exposed to enormous deformation. The detrusor muscle is of special physiological and medical interest, as it provides contraction during micturition It generates high inner pressure by a coordinated, fast reduction of bladder volume in order to successfully propel urine out of the body. The required muscle forces mainly depend on biomechanical properties such as the force-length- (FLR) and the force-velocity-relationship (FVR) Characterization of these biomechanical properties is essential to better understanding bladder functioning. The development of a computational model requires the determination of characteristic passive and active biomechanical and physiological properties (FLR and FVR). For such investigations, whole bladder experiments are insufficient and in vitro tests on isolated tissue strips are required. The examination of pig bladder strips is of special importance due to the structural and mechanical similarities shared with the human bladder (Teufl et al, 1997; Dahms et al, 1998)
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