Effects of activation on the elastic properties of intact soleus muscles with a deletion in titin.

Titin has long been known to contribute to muscle passive tension. Recently, it was also demonstrated that titin-based stiffness increases upon Ca2+ activation of wild-type mouse psoas myofibrils stretched beyond overlap of the thick and thin filaments. In addition, this increase in titin-based stiffness was impaired in single psoas myofibrils from mdm mice, characterized by a deletion in the N2A region of the Ttn gene. Here, we investigated the effects of activation on elastic properties of intact soleus muscles from wild-type and mdm mice to determine whether titin contributes to active muscle stiffness. Using load-clamp experiments, we compared the stress-strain relationships of elastic elements in active and passive muscles during unloading, and quantified the change in stiffness upon activation. Results from wild-type muscles show that upon activation, the elastic modulus increases, elastic elements develop force at 15% shorter lengths, and there was a 2.9-fold increase in the slope of the stress-strain relationship. These results are qualitatively and quantitatively similar to results from single wild-type psoas myofibrils. In contrast, mdm soleus showed no effect of activation on the slope or intercept of the stress-strain relationship, which is consistent with impaired titin activation observed in single mdm psoas myofibrils. Therefore, it is likely that titin plays a role in the increase of active muscle stiffness during rapid unloading. These results are consistent with the idea that, in addition to the thin filaments, titin is activated upon Ca2+ influx in skeletal muscle.