ABSTRACT
Tropomyosin (Tm) plays a central role in the regulation of muscle contraction and is present in three main isoforms in skeletal and cardiac muscles. In the present work we studied the functional role of α- and ßTm on force development by modifying the isoform composition of rabbit psoas skeletal muscle myofibrils and of regulated thin filaments for in vitro motility measurements. Skeletal myofibril regulatory proteins were extracted (78%) and replaced (98%) with Tm isoforms as homogenous ααTm or ßßTm dimers and the functional effects were measured. Maximal Ca(2+) activated force was the same in ααTm versus ßßTm myofibrils, but ßßTm myofibrils showed a marked slowing of relaxation and an impairment of regulation under resting conditions compared to ααTm and controls. ßßTm myofibrils also showed a significantly shorter slack sarcomere length and a marked increase in resting tension. Both these mechanical features were almost completely abolished by 10 mM 2,3-butanedione 2-monoxime, suggesting the presence of a significant degree of Ca(2+)-independent cross-bridge formation in ßßTm myofibrils. Finally, in motility assay experiments in the absence of Ca(2+) (pCa 9.0), complete regulation of thin filaments required greater ßßTm versus ααTm concentrations, while at full activation (pCa 5.0) no effect was observed on maximal thin filament motility speed. We infer from these observations that high contents of ßßTm in skeletal muscle result in partial Ca(2+)-independent activation of thin filaments at rest, and longer-lasting and less complete tension relaxation following Ca(2+) removal.
