Identification of troponin subunits which regulate the length dependent change in the Ca sensitivity in cardiac muscles

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Cardiac muscle contraction is significantly altered by a change in initial muscle length before contraction. This is considered to be due to the crossbridge-dependent change in the affinity of troponin-C (Tn-C) for Ca ion. The hypothesis was examined to measure the relation between Ca ion concentration and tension (pCa-tension relation) at different muscle lengths in intact and skinned preparations of mammalian cardiac muscles. In intact preparations, the papillary muscles treated with ryanodine were repetitively stimulated and tetanus was induced for measuring pCa-tension relation at different muscle lengths in the presence or absence of isoproterenol (ISO). ISO increased the length-dependent shift of pCa-tension relation. Thin trabeculae were treated with Triton X and stored in 50% glycerol containing relaxing solution. pCa-tension relations at different muscle lengths were measured before and after phosphorylation by A-kinase. After phosphorylation, length-dependent shift of pCa-tension relation was lessened. These results suggest that Tn-I and/or C-protein which can be phosphorylated by A-kinase might be involved in the signal transduction from crossbridges to Tn-C.The different effects of A-kinase dependent phosphorylation in intact and skinned preparation should be further investigated.