Calcium sensitivity, force frequency relationship and cardiac troponin I: Critical role of PKA and PKC phosphorylation sites

Transgenic models with pseudo phosphorylation mutants of troponin I, PKA sites at Ser 22 and 23 (cTnIDD22,23 mice) or PKC sites at Ser 42 and 44 (cTnIAD22,23DD42,44) displayed differential force-frequency relationships and afterload relaxation delay in vivo. We hypothesized that cTnI PKA and PKC phosphomimics impact cardiac muscle rate-related developed twitch force and relaxation kinetics in opposite directions. cTnIDD22,23 transgenic mice produce a force frequency relationship (FFR) equivalent to control NTG albeit at lower peak [Ca2+]i, while cTnIAD22,23DD42,44 TG mice had a flat FFR with normal peak systolic [Ca2+]i, thus suggestive of diminished responsiveness to [Ca2+]i at higher frequencies. Force-[Ca2+]i hysteresis analysis revealed that cTnIDD22,23 mice have a combined enhanced myofilament calcium peak response with an enhanced slope of force development and decline per unit of [Ca2+]i, whereas cTnIAD22,23DD42,44 transgenic mice showed the opposite. The computational ECME model predicts that the TG lines may be distinct from each other due to different rate constants for association/dissociation of Ca2+ at the regulatory site of cTnC. Our data indicate that cTnI phosphorylation at PKA sites plays a critical role in the FFR by increasing relative myofilament responsiveness, and results in a distinctive transition between activation and relaxation, as displayed by force-[Ca2+]i hysteresis loops. These findings may have important implications for understanding the specific contribution of cTnI to β-adrenergic inotropy and lusitropy and to adverse contractile effects of PKC activation, which is relevant during heart failure development.