Meeting Abstract
39.7 Sunday, Jan. 5 09:30 Residual force enhancement in the extensor digitorum longus muscle: The effect of titin length in activated muscle FUQUA, R.D.*; PACE, C.M.; JENSEN, D.; MONROY, J.A.; NISHIKAWA, K.C.; Northern Arizona University; Northern Arizona University; Northern Arizona University; Northern Arizona University rene.fuqua@nau.edu
When active muscles are stretched, tension increases and then settles to a steady state that is greater than the isometric force at the stretched length. The mechanism underlying this behavior, termed residual force enhancement (RFE), remains unknown. Previous studies have suggested a role for the elastic protein, titin, in generating force enhancement. In this study, we investigated titin’s role in contributing to force enhancement in the extensor digitorum longus muscle (EDL). As a fast contracting muscle, the EDL expresses a relatively short titin isoform resulting in higher titin-based stiffness compared to the soleus muscle. We hypothesized that RFE will be greater along the length tension curve in the EDL compared to the soleus as a result of its shorter titin isoform. To determine how force enhancement changes along the length tension curve, we measured RFE in EDL and soleus muscles from wild-type mice. An isometric contraction, an active 5% Lo stretch contraction, and a passive 5% Lo stretch were performed at 95% Lo, Lo, 105% Lo, 110% Lo, 115% Lo and 120% Lo. Along the length-tension curve, both muscles showed an increase in total force enhancement, passive force enhancement, and the force enhancement due to activation alone. The EDL showed greater passive force enhancement and a greater effect of activation than the soleus. This suggests that the shorter, stiffer titin in the EDL not only increases the passive force but also, that titin stiffness increases upon activation. Our data support the hypothesis that titin is activated by Ca2+-influx which causes titin stiffness to increase and demonstrates that the length of titin influences active force enhancement. This work was supported by NSF IOS-1025806.
