Effects of vigorous isometric muscle contraction on titin stiffness-related contractile properties in rat fast-twitch muscles

This study was conducted to examine the effects of an acute bout of vigorous isometric contractions on titin stiffness-related contractile properties in rat fast-twitch skeletal muscles. Intact gastrocnemius muscles were electrically stimulated in situ until the force was reduced to similar to 50% of the initial force. Immediately after cessation of the stimulation, the superficial regions of the muscles were dissected and subjected to biochemical and skinned fiber analyses. The stimulation resulted in a decrease in the titin-based passive force. The amounts of fragmented titin were unchanged by the stimulation. Protein kinase C alpha-treatment increased the passive force in stimulated fibers to resting levels. The stimulation had no effect on the maximum Ca2+-activated force (max Ca2+ force) at a sarcomere length (SL) of 2.4 mu m and decreased myofibrillar (my)-Ca2+ sensitivity at 2.6-mu m SL. Stretching the SL to 3.0 mu m led to the augmentation of the max Ca2+ force and my-Ca2+ sensitivity in both rested and stimulated fibers. For the max Ca2+ force, the extent of the increase was smaller in stimulated than in rested fibers, whereas for myCa(2+) sensitivity, it was higher in stimulated than in rested fibers. These results suggest that vigorous isometric contractions decrease the titin-based passive force, possibly because of a reduction in phosphorylation by protein kinase C alpha, and that the decreased titin stiffness may contribute, at least in part, to muscle fatigue.

Automated summary

This study demonstrates that acute vigorous isometric contractions decrease titin-based passive force, possibly due to reduced phosphorylation by protein kinase C alpha, and this decreased titin stiffness may contribute, at least in part, to muscle fatigue.