Login to Access Video or Poster Abstract: MP94-10
Sources of Funding: None

Introduction

: Biological soft tissues are viscoelastic materials because they display time-independent pseudo-elasticity and time-dependent viscosity. Upon an imposed ramp increase then decrease in strain, the resultant increase and decrease in stress, termed loading and unloading respectively, produce nonlinear stress-strain curves, and a reversible reduction in the stress-strain work area that identifies the viscous component. However, there is evidence that bladder tissue may also display plastic behavior, defined as an increase in strain that is unrecoverable unless work is done by the material. In the present study, an electronic lever was used to induce controlled changes in stress and strain to determine whether the material properties of rabbit detrusor smooth muscle (rDSM) represent a viscoelastic or viscoelastic-plastic material.

Methods

Strips of DSM free from underlying mucosa were removed from rabbit bladders. Each tissue was placed in an organ bath connected to an electronic lever and length-adjuster and subjected to a length-tension protocol to identify the length (Lref) that produced the strongest active force induced by KCl. Each ring was subsequently set to 80% Lref and subjected to sequential ramp loading and unloading cycles, stress-strain and stiffness-stress analyses, and a step-loading, load-clamp, step-unloading (creep) protocol.

Results

Ramp loading-unloading cycles revealed that rDSM displayed reversible viscoelasticity. The viscous component was responsible for establishing a high stiffness at low stresses that increased only modestly with increasing stress compared to the large increase produced when the viscosity was absent and only pseudo-elasticity governed tissue behavior. The creep protocol revealed that rDSM underwent extensive softening correlating with plastic deformation and creep that was reversible upon activation of muscle contraction. Softening reversal was prevented by inhibitors of actomyosin crossbridge cycling.

Conclusions

Together, the data support a model of DSM as exhibiting not only viscoelasticity, but also plasticity, the degree of which is controlled by the degree of motor protein activation. This model explains the mechanism of instrinsic bladder compliance as “slipping� crossbridges, predicts that wall tension is dependent not only on vesicle pressure and radius but also on actomyosin crossbridge activity, and identifies a novel molecular target for compliance regulation both physiologically and therapeutically.

Funding

None