Transgenic incorporation of skeletal TnT into cardiac myofilaments blunts PKC-mediated depression of force

Protein kinase C (PKC)-mediated phosphorylation of cardiac troponin I (cTnI ) and troponin T (cTnT) has been shown to diminish maximum activation of my ofilaments. The functional role of cTnI phosphorylation has been investigat ed. However, the impact of cTnT phosphorylation on myofilament force is not well studied. We tested the effect of endogenous PKC activation on steady- state tension development and Ca2+ sensitivity in skinned fiber bundles fro m transgenic (TG) mouse hearts expressing fast skeletal TnT (fsTnT), which naturally lacks the PKC sites present in cTnT. The 12-O-tetradecanoylphorbo l 13-acetate (TPA) treatment induced a 29% (46.1 +/- 2.5 vs. 33.4 +/- 2.6 m N/mm(2)) reduction in maximum tension in the nontransgenic (NTG) preparatio ns (n = 7) and was inhibited with chelerythrine. However, TPA did not induc e a change in the maximum tension in the TG preparations (n = 11). TPA indu ced a small but significant (P < 0.02) increase in Ca2+ sensitivity (untrea ted pCa(50) = 5.63 +/- 0.01 vs. treated pCa(50) = 5.72 +/- 0.01) only in TG preparations. In TG preparations, P-32 incorporation was not evident in Tn T and was also significantly diminished in cTnI, compared with NTG. Our dat a indicate that incorporation of fsTnT into the cardiac myofilament lattice blunts PKC-mediated depression of maximum tension. These data also suggest that cTnT may play an important role in amplifying the myofilament depress ion induced by PKC-mediated phosphorylation of cTnI.