title: Cardiac transgenic and gene transfer strategies converge to support an important role for troponin I in regulating relaxation in cardiac myocytes.
abstract: Elucidating the relative roles of cardiac troponin I (cTnI) and phospholamban (PLN) in beta-adrenergic-mediated hastening of cardiac relaxation has been challenging and controversial. To test the hypothesis that beta-adrenergic phosphorylation of cTnI has a prominent role in accelerating cardiac myocyte relaxation performance we used transgenic (Tg) mice bearing near complete replacement of native cTnI with a beta-adrenergic phospho-mimetic of cTnI whereby tandem serine codons 23/24 were converted to aspartic acids (cTnI S23/24D). Adult cardiac myocytes were isolated and contractility determined at physiological temperature under unloaded and loaded conditions using micro-carbon fibers. At baseline, cTnI S23/24D myocytes had significantly faster relaxation times relative to controls, and isoproterenol stimulation (Iso) had only a small effect to further speed relaxation in cTnI S23/24D myocytes (delta Iso: 7.2 ms) relative to the maximum Iso effect (31.2 ms) in control. The Ca(2+) transient decay rate was similarly accelerated by Iso in Tg and nontransgenic (Ntg) myocytes. Gene transfer of cTnI S23/24D to myocytes in primary culture showed comparable findings. Gene transfer of cTnI with both serines 23/24 converted to alanines (cTnI S23/24A), or gene transfer of slow skeletal TnI, both of which lack PKA phosphorylation sites, significantly blunted Iso-mediated enhanced relaxation compared with controls. Gene transfer of wild-type cTnI had no effect on relaxation. These findings support a key role of cTnI in myocyte relaxation and highlight a direct contribution of the myofilaments in modulating the dynamics of myocardial performance.
authors.affiliation: Department of Molecular and Integrative Physiology, 7730 Medical Science II, University of Michigan, Ann Arbor, MI 48109-0622, USA.
authors.forename: So-ichiro
authors.initials: S
authors.lastname: Yasuda
authors.forename: Pierre
authors.initials: P
authors.lastname: Coutu
authors.forename: Sakthivel
authors.initials: S
authors.lastname: Sadayappan
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Robbins
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Metzger
mesh_terms: D000256:Adenoviridae / Q000235:genetics; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000378:metabolism; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D018014:Gene Transfer Techniques; D007537:Isometric Contraction / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D051379:Mice; D008822:Mice, Transgenic; D018716:Molecular Mimicry / Q000235:genetics; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019210:Troponin I / Q000235:genetics* / Q000378:metabolism*; D000099211:Phospholamban
chemical_list: D002118:Calcium; D002135:Calcium-Binding Proteins; D019210:Troponin I; D000099211:Phospholamban