Residual force enhancement is reduced in permeabilized fiber bundles from mdm muscles

Residual force enhancement (RFE) is the increase in steady-state force after active stretch relative to the force after isometric contraction at the same final length. The mdm mutation in mice, characterized by a small deletion in N2A titin, has been proposed to prevent N2A titin-actin interactions so that active mdm muscles are more compliant than WT. This decrease in active muscle stiffness should be associated with reduced RFE. We investigated RFE in permeabilized soleus (SOL) and extensor digitorum longus (EDL) fiber bundles from wild type and mdm mice. On each fiber bundle, we performed active and passive stretch from an average sarcomere length of 2.6 - 3.0 &[mu]m at a slow rate of 0.04 &[mu]m/s, as well as isometric contractions at the initial and final lengths. One-way ANOVA showed that SOL and EDL fiber bundles from mdm mice exhibited significantly lower RFE than WT (P < 0.0001). This result is consistent with previous observations in single myofibrils and intact mus...