Passive viscoelastic properties of costameres in EDL skeletal muscle in normal and dystrophin null mice

Costameres at the sarcolemmal skeletal myofibers transmit the lateral force generated by myofibrils from them to the extracellular matrix. We used an elastimeter method by which sucking pressure is applied through a micropipette to the surface membrane of single mice myofibers; of the Extensor digitorum longus to measure the viscoelasticity of the sarcolemma-costamere complex as a function of sarcomere length (SL). Constant suction pressure applied to the sarcolemma generated a sarcolemmal-costamere-myofibril bleb of variable height depending on the sucking pressure and SL. It took some time for the bleb to reach a stable height after applying the pressure. This time delay indicates that the sarcolemma-costamere-myofibril system acts as a viscoelastic system. We undertook the present experiments to measure the height stabilization time of the bleb at different sarcomere lengths from which we estimated the viscoelastic parameters of the system. The time course of the bleb formation was biphasic and reached a plateau between 3.5 to 1.3 and 4.9 to 3.5 min for normal and dystrophic mice respectively depending on SL of 3.0 to 5.6 mu m. Based on a Maxwell-Voigt system we found the viscoelastic parameters such as viscosity, friction coefficient and the costameres (k) and sarcolemma (k(1)) elasticity constants.