A Role for Titin During Doublet Potentiation of Fast and Slow Twitch Muscles

Abstract

Doublet potentiation is the increase in muscle force as a result of the addition of a single pulse, or doublet, at the beginning of a low‐frequency stimulus train. The force output is higher in amplitude, generated at a faster rate, persists throughout activation. However, current muscle models fail to accurately predict muscle force during doublet potentiation. Recent work suggests that the elastic protein, titin, may play a role. In this study, we investigated the role of titin in doublet potentiation by using the muscular dystrophy with myositis (mdm) mouse, which is characterized by a deletion in the N2A region of the titin gene. Previous research on the winding filament hypothesis suggests that the N2A region of titin binds to the thin filaments upon activation, which increases titin stiffness and total muscle force. We hypothesized that the absence of N2A‐thin filament binding in mdm muscles reduces doublet potentiation. Using a servomotor force lever, we measured doublet potentiation in the soleus and extensor digitorum longus (EDL) muscles of wildtype and mdm mice at different muscle lengths. Potentiation was 20% lower in mdm than in wildtype soleus at all lengths (p = 0.04). In contrast to soleus, there was no difference in potentiation between wildtype and mdm EDL muscles (p = 0.7). In addition, potentiation was greater at shorter lengths than on the descending limb of the force‐length relationship in both soleus and EDL wildtype muscles. Mdm muscles did not show length dependence. Results from soleus muscles are consistent with the hypothesis that titin contributes to the increase in muscle force during doublet potentiation and that the lack of titin‐actin binding in mdm soleus muscles reduces doublet potentiation of muscle force. Differences in myosin isoforms, calcium flux, or titin isoforms, alone or in combination, may contribute to the observed differences between soleus and EDL in doublet potentiation.Support or Funding InformationThis work was funded by the Anderson Endowment at Denison University and NSF IOS‐1456868.