title: Calcium sensitivity, force frequency relationship and cardiac troponin I: critical role of PKA and PKC phosphorylation sites.
abstract: Transgenic models with pseudo phosphorylation mutants of troponin I, PKA sites at Ser 22 and 23 (cTnIDD(22,23) mice) or PKC sites at Ser 42 and 44 (cTnIAD(22,23)DD(42,44)) 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. cTnIDD(22,23) transgenic mice produce a force frequency relationship (FFR) equivalent to control NTG albeit at lower peak [Ca(2+)](i), while cTnIAD(22,23)DD(42,44) TG mice had a flat FFR with normal peak systolic [Ca(2+)](i), thus suggestive of diminished responsiveness to [Ca(2+)](i) at higher frequencies. Force-[Ca(2+)](i) hysteresis analysis revealed that cTnIDD(22,23) mice have a combined enhanced myofilament calcium peak response with an enhanced slope of force development and decline per unit of [Ca(2+)](i), whereas cTnIAD(22,23)DD(42,44) 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 Ca(2+) 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-[Ca(2+)](i) hysteresis loops. These findings may have important implications for understanding the specific contribution of cTnI to beta-adrenergic inotropy and lusitropy and to adverse contractile effects of PKC activation, which is relevant during heart failure development.
authors.affiliation: Department of Pediatrics/Division of Cardiology, Johns Hopkins University School of Medicine, Ross Bldg 1144/720 Rutland Avenue, Baltimore, MD 21205, USA.
authors.forename: Genaro A
authors.initials: GA
authors.lastname: Ramirez-Correa
authors.forename: Sonia
authors.initials: S
authors.lastname: Cortassa
authors.forename: Brian
authors.initials: B
authors.lastname: Stanley
authors.forename: Wei Dong
authors.initials: WD
authors.lastname: Gao
authors.forename: Anne M
authors.initials: AM
authors.lastname: Murphy
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003198:Computer Simulation; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D005260:Female; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D009206:Myocardium / Q000378:metabolism; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D019210:Troponin I / Q000378:metabolism*
chemical_list: D019210:Troponin I; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D002118:Calcium