Impact of tropomyosin isoform composition on fast skeletal muscle thin filament regulation and force development

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 alpha- and beta Tm 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 alpha alpha Tm or beta beta Tm dimers and the functional effects were measured. Maximal Ca2+ activated force was the same in alpha alpha Tm versus beta beta Tm myofibrils, but beta beta Tm myofibrils showed a marked slowing of relaxation and an impairment of regulation under resting conditions compared to alpha alpha Tm and controls. beta beta Tm 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 Ca2+-independent cross-bridge formation in beta beta Tm myofibrils. Finally, in motility assay experiments in the absence of Ca2+ (pCa 9.0), complete regulation of thin filaments required greater beta beta Tm versus alpha alpha Tm 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 beta beta Tm in skeletal muscle result in partial Ca2+-independent activation of thin filaments at rest, and longer-lasting and less complete tension relaxation following Ca2+ removal.