ROK-induced cross-link formation stiffens passive muscle: reversible strain-induced stress softening in rabbit detrusor

1 Department of Mechanical Engineering, 2 Department of Biomedical Engineering and 3 Departments of Biochemistry and Pediatrics, Virginia Commonwealth University, School of Medicine, Richmond, Virginia Submitted 25 August 2004 ; accepted in final form 9 February 2005 ABSTRACT Passive mechanical properties of strips of rabbit detrusor smooth muscle were examined and found by cyclic loading in a calcium-free solution to display viscoelastic softening and strain-induced stress softening (strain softening). Strain softening, or the Mullins effect, is a loss of stiffness attributed to the breakage of cross-links, and appeared irreversible in detrusor even after the return of spontaneous rhythmic tone during 120 min of incubation in a calcium-containing solution. However, 3 min of KCl or carbachol (CCh)-induced contraction permitted rapid regeneration of the passive stiffness lost to strain softening, and 3 µM of the RhoA kinase (ROK) inhibitor Y-27632 prevented this regeneration. The degree of ROK-induced passive stiffness was inversely dependent on muscle length over a length range where peak CCh-induced force was length independent. Thus rabbit detrusor displayed variable passive stiffness both strain- and activation-history dependent. In conclusion, activation of ROK by KCl or CCh increased passive stiffness softened by muscle strain and thereby attributed to cross-links that remained stable during tissue incubation in a calcium-free solution. Degradation of this signaling system could potentially contribute to urinary incontinence. muscle mechanics; preconditioning; Y-27632; RhoA kinase; passive force Address for reprint requests and other correspondence: P. H. Ratz, Virginia Commonwealth Univ., School of Medicine, Depts. of Biochemistry and Pediatrics, 1101 E. Marshall St., PO Box 980614, Richmond, VA 23298-0614 (e-mail: phratz{at}vcu.edu )