Rapid unloading experiments from Ttn112-158 muscles

Evidence suggests that the giant muscle protein, titin, functions as a tunable spring in active muscle. However, the mechanisms for increasing titin stiffness with activation are not well understood. Previous studies have suggested that during muscle activation, titin binds to actin which engages the PEVK region of titin thereby increasing titin stiffness. In this study, we investigated the role of PEVK titin in active muscle stiffness during rapid unloading. We measured elastic recoil of active and passive soleus muscles from Ttn112-158 mice characterized by a 75% deletion of PEVK titin and increased passive stiffness. We hypothesized that activated Ttn112-158 muscles are stiffer than wild-type muscles as a result of the increased stiffness of PEVK titin. Using a servomotor force lever, we compared the stress-strain relationships of elastic elements in active and passive muscles during rapid unloading and quantified the change in stiffness upon activation. The results show that the elastic modulus of Ttn112-158 muscles increased with activation. However, elastic elements developed force at 7% longer lengths and exhibited 50% lower active stiffness in Ttn112-158 soleus muscles than wild-type muscles. Thus, despite having a shorter, stiffer PEVK segment, during rapid unloading, Ttn112-158 soleus muscles exhibited reduced active stiffness compared to wild-type soleus muscles. These results are consistent with the idea that PEVK titin contributes to active muscle stiffness, however, the reduction in active stiffness of Ttn112-158 muscles suggests that other mechanisms compensate for the increased PEVK stiffness.