Titin force enhancement following active stretch of skinned skeletal muscle fibres.

In actively stretched skeletal muscle sarcomeres, titin-based force is enhanced, increasing the stiffness of active sarcomeres. Titin force enhancement in sarcomeres is vastly reduced in mdm , a genetic mutation with a deletion in titin. Whether loss of titin force enhancement is associated with compensatory mechanisms at higher structural levels of organization, such as single fibres or entire muscles, is unclear. The aim of this study was to determine whether mechanical deficiencies in titin force enhancement are also observed at the fibre level, and whether mechanisms compensate for the loss of titin force enhancement. Single skinned fibres from control and mutant mice were stretched actively and passively beyond filament overlap to observe titin-based force. Mutant fibres generated lower contractile stress (force divided by cross-sectional area) than control fibres. Titin force enhancement was observed in control fibres stretched beyond filament overlap, but was overshadowed in mutant fibres by an abundance of collagen and high variability in mechanics. However, titin force enhancement could be measured in all control fibres and most mutant fibres following short stretches, accounting for ∼25% of the total stress following active stretch. Our results show that the partial loss of titin force enhancement in myofibrils is not preserved in all mutant fibres and this mutation likely affects fibres differentially within a muscle. An increase in collagen helps to reestablish total force at long sarcomere lengths with the loss in titin force enhancement in some mutant fibres, increasing the overall strength of mutant fibres.