A mechanical model for adjustable passive stiffness in rabbit detrusor

Departments of 1 Mechanical Engineering, 2 Biomedical Engineering, 3 Surgery, and 4 Biochemistry and Pediatrics, Virginia Commonwealth University, Richmond, Virginia Submitted 4 April 2006 ; accepted in final form 7 June 2006 Strips of rabbit detrusor smooth muscle (DSM) exhibit adjustable passive stiffness characterized by strain softening: a loss of stiffness on stretch to a new length distinct from viscoelastic behavior. At the molecular level, strain softening appears to be caused by cross-link breakage and is essentially irreversible when DSM is maintained under passive conditions (i.e., when cross bridges are not cycling to produce active force). However, on DSM activation, strain softening is reversible and likely due to cross-link reformation. Thus DSM displays adjustable passive stiffness that is dependent on the history of both muscle strain and activation. The present study provides empirical data showing that, in DSM, 1 ) passive isometric force relaxation includes a very slow component requiring hours to approach steady state, 2 ) the level of passive force maintained at steady state is less if the tissue has previously been strain softened, and 3 ) tissues subjected to a quick-release protocol exhibit a biphasic response consisting of passive force redevelopment followed by force relaxation. To explain these and previously identified characteristics, a mechanical model for adjustable passive stiffness is proposed based on the addition of a novel cross-linking element to a hybrid Kelvin/Voigt viscoelastic model. smooth muscle mechanics; preconditioning; strain softening; passive force; muscle model Address for reprint requests and other correspondence: J. E. Speich, Virginia Commonwealth Univ., Mechanical Engineering, 601 West Main St., P. O. Box 843015, Richmond, VA 23284–3015 (e-mail: jespeich{at}vcu.edu )