Doublet Potentiation During In Vitro Cyclical Movement of Mdm Mouse Muscles

Abstract

It has been observed that the addition of a stimulus (doublet) to a train of stimuli increases the force output of muscles, and that the effect of a doublet on force diminishes as muscle length increases. In natural movements such as running, muscles are activated at various lengths during cyclical motion. Thus, the increase in force due to the addition of a doublet during cyclical movements likely depends on the length at which a muscle is stimulated during the cycle (i.e. phase of activation). Recent work has suggested that the elastic protein, titin, may play a role in doublet potentiation. Using the muscular dystrophy with myositis (mdm) mouse, which is characterized by a deletion in the N2A region of titin, we investigated the role of titin in doublet potentiation during work loop experiments. We measured force output of soleus muscles from wildtype and mdm mice subjected to cyclical changes in length and stimulated at various phases. The length and force data were used to construct work loops, which were integrated to determine the work output of each muscle. Results show that the addition of a doublet increased muscle work at lower stimulation frequencies (p < 0.0001), and that at these same frequencies the relative increase in work was greater in wildtype compared to mdm muscles (p = 0.0003). In addition, both genotypes showed the largest increase in work when stimulated at the shortest lengths, and the smallest increase when stimulated at the longest lengths (p < 0.0001). These interactions between activation and length suggest that elastic elements play a role in doublet potentiation. Our work is consistent with the winding filament hypothesis, which suggests that titin contributes to active muscle force.Support or Funding InformationThis work was funded by the Anderson Endowment at Denison University, an APS UGSRF award to D.E.E., and NSF 1456868.