OBJECTIVETo investigate the relationship between phenotype transformation and biomechanical properties of detrusor smooth muscle cell (DSMC) subjected to the cyclic mechanical stretch.

METHODSCultured rat DSMCS were grown on collagen-coated silicone membranes and subjected to continuous cycles of stretch-relaxation. All experiments were made on cells between passage 2 and 4. Each cycle consists of 5-second stretch and 5-second relaxation. The computer controlled vacuum induced 10% (I), 20% (II) and 30% (III) maximum elongation of the plate membrane at different designed pressures. We assessed DNA synthesis rate using tritiated thymidine incorporation assay. Using immunofluorescent assay and flow cytometer, we analysed the expression of SM-alpha-actin and proliferation of DSMC. The image analysis and micropipette aspiration systems were employed to investigate the single cell contraction and viscoelasticity. The elastic modulus K(1), K(2) and viscoelastic coefficient micro were determined using the three-element standard linear solid model, thus demonstrating the passive deformation ability of detrusor cells.

RESULTSAs the basic structural changes to mechanical stretch, DSMCs underwent phenotypic modulation from their normal contractile phenotype to a "synthetic" phenotype: the DSMCs became more proliferative and the actin less organized along the cell's long axis. The cell proliferation index (CPI) of control and stretched group (10%, 20%, 30% elongation) were 0.24, 0.43, 0.58 and 0.65 respectively. After mechanical stretch, the well-spread filaments changed their orientation. Contraction and viscoelasticity of single DSMC subjected to stretch both decreased significantly compared to control. The Vmax and. DeltaLmax of group III (30% elongation) saw significant decreases compared with unstretched control (P < 0.01). K(1) and K(2) decreased with the increasing of mechanical overload, however, there was no statistic difference between groups II and group III.

CONCLUSIONSStructure determines function. Conversely, dysfunction implies the structural transformation. Functional abnormalities of BOO have the structural basis: phenotype transformation of detrusor cells. Cyclic stretch and relaxation applied to DSMCs in vitro can be used to model the increases in urodynamic load experienced by the bladder detrusor muscle under the conditions of bladder outlet obstruction. Phenotypic transformation is the structural basis of functional changes of DSMC subjected to periodic overload mechanical stretch.