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 Ca 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 mice, characterized by a deletion in the N2A region of the gene. Here, we investigated the effects of activation on elastic properties of intact soleus muscles from wild-type and 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, 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 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 Ca influx in skeletal muscle.