Active muscle stiffness is reduced during rapid unloading in muscles from TtnΔ112-158 mice with a large deletion to PEVK titin

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 TtnΔ112-158 mice characterized by a 75% deletion of PEVK titin and increased passive stiffness. We hypothesized that activated TtnΔ112-158 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 TtnΔ112-158 muscles increased with activation. However, elastic elements developed force at 7% longer lengths and exhibited 50% lower active stiffness in TtnΔ112-158 soleus muscles than in wild-type muscles. Thus, despite having a shorter, stiffer PEVK segment, during rapid unloading, TtnΔ112-158 soleus muscles exhibited reduced active stiffness compared with 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 TtnΔ112-158 muscles suggests that other mechanisms compensate for the increased PEVK stiffness.