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.
The authors declare no competing or financial interests.
J.A.M.: data collection, analysis and interpretation, paper preparation and editing; K.L.P.: data collection; C.M.P.: mouse genotyping; T.U.: data analysis and programming; K.C.N.: conceptual development, funding, interpretation and editing.
This work was supported by National Science Foundation grants IOS-1025806, IOS-1456868; and an award from the W. M. Keck Foundation.
- Received March 9, 2016.
- Accepted December 12, 2016.
- © 2017. Published by The Company of Biologists Ltd