The increase in non-cross-bridge forces after stretch of activated striated muscle is related to titin isoforms

Abstract

Skeletal muscles present a non-cross-bridge increase in sarcomere stiffness and tension on Ca(2+) activation, referred to as static stiffness and static tension, respectively. It has been hypothesized that this increase in tension is caused by Ca(2+)-dependent changes in the properties of titin molecules. To verify this hypothesis, we investigated the static tension in muscles containing different titin isoforms. Permeabilized myofibrils were isolated from the psoas, soleus, and heart ventricle from the rabbit, and tested in pCa 9.0 and pCa 4.5, before and after extraction of troponin C, thin filaments, and treatment with the actomyosin inhibitor blebbistatin. The myofibrils were tested with stretches of different amplitudes in sarcomere lengths varying between 1.93 and 3.37 μm for the psoas, 2.68 and 4.21 μm for the soleus, and 1.51 and 2.86 μm for the ventricle. Using gel electrophoresis, we confirmed that the three muscles tested have different titin isoforms. The static tension was present in psoas and soleus myofibrils, but not in ventricle myofibrils, and higher in psoas myofibrils than in soleus myofibrils. These results suggest that the increase in the static tension is directly associated with Ca(2+)-dependent change in titin properties and not associated with changes in titin-actin interactions.

Reply to “Letter to the editor: Comments on Cornachione et al. (2016): “The increase in non-cross-bridge forces after stretch of activated striated muscle is related to titin isoforms”

“…The isometric forces, the rates of force development, and the rates of relaxation produced by myofibrils from psoas, soleus, and ventricle myofibrils (Table 1) are all very similar to values reported in the literature; we actually cite several studies in our paper to prove this point [refs. 25, 45, 58, 59, 64, 65 in the paper (2)]. The values for the residual force enhancement and the static stiffness (Table 1) are also well within range published by others (e.g., refs.…”

“…REPLY: In a letter to the Editor of the American Journal of Physiology-Cell Physiology, Prof. Walter Herzog (6a) chose and edited two figures displaying raw data from our paper (2) to challenge the conclusions drawn from our study. Herzog disregards all the remaining data and results that base the conclusions of our paper, including the figure (Fig.…”

Titin‐based mechanosensing modulates muscle hypertrophy

“…However, these studies were performed in intact, contracting muscle. Consequently, the results might be confounded by the presence of calcium cycling: calcium binds to the PEVK spring‐domain of titin, changing its stiffness,15, 16, 17 and titin‐based stiffness modulates contractility20, 41, 42, 43, 44 which in turn might affect muscle growth. The importance of the current findings is that they show that the effects of titin‐based mechanosensing occur independent of contractile activity, and can be ascribed to titin's elastic properties.…”

Letter to the editor: “Titin-actin interaction: the report of its death was an exaggeration”

“…TO THE EDITOR: A recent paper by Cornachione et al (1) claims refutation of the hypothesis that an interaction between titin and actin may be responsible for Ca 2ϩ -activated tension or residual force enhancement in muscle. Examination of the evidential basis for this claim reveals that it is premature.…”

“…They used fluorescent phalloidin to visualize actin before and after gelsolin extraction [ Fig. 1 in Cornachione et al (1)]. Using these methods, the authors state, in the legend of Fig.…”