Differences in myofilament calcium sensitivity in rat psoas fibers reconstituted with troponin T isoforms containing the α- and β-exons

The carboxy terminus of fast skeletal muscle troponin T (fsTnT) is highly conserved. However, mutually exclusive splicing of exons 16 and 17 in the fsTnT gene results in the expression of either the α- or β-fsTnT isoform. The α-isoform is expressed only in adult fast skeletal muscle, whereas the β-isoform is expressed in varying quantities throughout muscle development. Reconstitution of detergent-skinned adult rat psoas muscle fibers with rat fast skeletal troponin complexes containing either fsTnT isoform demonstrated that reconstitution with α-fsTnT resulted in greater myofilament Ca2+ sensitivity than reconstitution with β-fsTnT, without changes to Ca2+-activated maximal tension, ATPase activity or tension cost. The observed isoform-specific differences in myofilament Ca2+ sensitivity may be due to changes in the transition of the thin-filament regulatory unit from the off to the on state, possibly due to altered interactions of the C-terminus of fsTnT with troponins I and/or C.