Static Stiffness in Slow and Fast Mouse Muscle Fibers Expressing Different Titin Isoforms

Abstract

We showed previously (Bagni et al., 2002) that most of the increase of muscle fiber stiffness during the early phases of a tetanic contraction is due to a non-crossbridge sarcomere component whose stiffness (called static stiffness) increases after stimulation with a time course very similar to the internal Ca2+ concentration. This led us to speculate that Ca2+ concentration, in addition to promote crossbridge formation, could also leads to a stiffening of a sarcomere structure, identified with the titin filament, either directly or through a titin-actin interaction leading to the observed sarcomere stiffness increase. According to this hypothesis, it is expected that static stiffness has different properties in muscles expressing titin with different mechanical properties. Therefore we compared the static stiffness values in soleus and EDL adult mouse muscles, which express titin isoforms with long and short PEVK segment, respectively. Considering that Ca2+ binding to E-rich motifs in the PEVK segment increases its bending rigidity, the higher proportion of these motifs in EDL compared to soleus is expected to lead to a greater static stiffness in EDL. Our results showed that in agreement with the titin hypothesis, the static stiffness measured in single fibers at 25°C was more than five times greater in EDL than in soleus and about two times greater than previously reported on FDB muscle. The static stiffness time course in EDL was about the same as in FDB but slightly faster than in soleus, and it became much faster at 35°C in both EDL and soleus similarly to tension time course. These results are in agreement with the idea that static stiffness depends on the increment of titin stiffness due to the interaction between Ca2+ and E-rich motifs in PEVK segment.