Slowed Dynamics of Thin Filament Regulatory Units Reduces Ca²⁺-Sensitivity of Cardiac Biomechanical Function

Actomyosin kinetics in both skinned skeletal muscle fibers at maximum Ca²⁺-activation and unregulated in vitro motility 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 vitro motility 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 (pCa ) of s decreased markedly with elevated viscosity (η/η ≥ ~1.3). For comparison with unloaded motility assays, steady-state isometric force (F) and kinetics of isometric tension redevelopment (k ) were measured in single, permeabilized porcine cardiomyocytes when viscosity surrounding the myofilaments was altered. Maximum Ca²⁺-activated F changed little for sucrose ≤ 0.3 M (η/η ~1.4) or glucose ≤ 0.875 M (η/η ~1.66), but decreased at higher concentrations. Sucrose (0.3 M) or glucose (0.875 M) decreased pCa for F. k at saturating Ca²⁺ decreased steeply and monotonically with increased viscosity but there was little effect on k at 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.