title: Slowed Dynamics of Thin Filament Regulatory Units Reduces Ca2+-Sensitivity of Cardiac Biomechanical Function.
abstract: Actomyosin kinetics in both skinned skeletal muscle fibers at maximum Ca2+-activation and unregulatedin vitromotility assays are modulated by solvent microviscosity in a manner consistent with a diffusion limited process. Viscosity might also influence cardiac thin filament Ca2+-regulatory protein dynamics.In vitromotility assays were conducted using thin filaments reconstituted with recombinant human cardiac troponin and tropomyosin; solvent microviscosity was varied by addition of sucrose or glucose. At saturating Ca2+, filament sliding speed (s) was inversely proportional to viscosity. Ca2+-sensitivity (pCa50) ofsdecreased markedly with elevated viscosity (η/η0≥ ~1.3). For comparison with unloaded motility assays, steady-state isometric force (F) and kinetics of isometric tension redevelopment (kTR) were measured in single, permeabilized porcine cardiomyocytes when viscosity surrounding the myofilaments was altered. Maximum Ca2+-activatedFchanged little for sucrose ≤ 0.3 M (η/η0~1.4) or glucose ≤ 0.875 M (η/η0~1.66), but decreased at higher concentrations. Sucrose (0.3 M) or glucose (0.875 M) decreasedpCa50forF.kTRat saturating Ca2+decreased steeply and monotonically with increased viscosity but there was little effect onkTRat sub-maximum Ca2+. Modeling indicates that increased solutes affect dynamics of cardiac muscle Ca2+-regulatory proteins to a much greater extent than actomyosin cross-bridge cycling.
authors.affiliation: Department of Biological Science, The Florida State University, Tallahassee, FL, 32306, USA ; Department of Physics, The Florida State University, Tallahassee, FL, 32306, USA.
authors.forename: Campion K P
authors.initials: CK
authors.lastname: Loong
authors.forename: Aya K
authors.initials: AK
authors.lastname: Takeda
authors.forename: Myriam A
authors.initials: MA
authors.lastname: Badr
authors.forename: Jordan S
authors.initials: JS
authors.lastname: Rogers
authors.forename: P Bryant
authors.initials: PB
authors.lastname: Chase
keywords: actin; disaccharide sucrose; in vitro motility assay; kinetics of isometric tension redevelopment; microviscosity; monosaccharide glucose; myosin; skinned myocyte; tropomyosin; troponin