Annulment of Cardiac Muscle Length-Dependent Force Activation in Transgenic Mice Bearing the HcTnT-I79N Mutation

Abstract

Missense mutation Ile79Asn in human cardiac troponin T (HcTnT-I79N) has been linked to phenotypic outcomes of hypertrophic cardiomyopathy (HCM) followed by adverse consequences including arrhythmias and sudden cardiac death. Little else is known about changes in cardiac muscle mechanics caused by the mutation. This study investigates cross-bridges kinetics of left ventricular papillary muscle bundles (Ca2+-sensitivity of isometric force, sinusoidal stiffness and rate of tension redevelopment (kTR)). Cardiac tissues were harvested from both non-transgenic wild-type (NTg) mice and transgenic mice bearing the HcTnT-I79N mutation. Sarcomere lengths (1.9, 2.1, and 2.3 μm) were set at pCa 8.0 using HeNe laser. Sinusoidal stiffness (0.2% PTP length oscillation) and kTR were obtained during Ca2+ activation when force was at least 20% of maximum. Increased Ca2+-sensitivity of isometric force was observed in NTg cardiac preparations upon an increase in SL (1.9 vs 2.1 μm, and 2.1 vs 2.3 μm). Interestingly, HcTnT-I79N exhibited an increased Ca2+-sensitivity at SL 1.9 vs 2.1 μm only. No further increase was observed at SL 2.1 vs 2.3 μm. This blunt effect indicates an abolishment of length-dependent activation at SL 2.1 vs 2.3 μm. Although HcTnT-I79N mutation exhibited a significant lower maximal kTR, neither NTg nor HcTnT-I79N showed significant changes in maximal kTR regarding length-dependent activation. After compressing the myofilament lattice spacing using 3% Dextran T500 (SL 2.1 μm), only NTg cardiac preparations displayed increased myofilament Ca2+ sensitivity. No changes in maximal kTR were observed for both NTg and HcTnT-I79N in the presence of dextran. In addition, no significant changes in passive force, maximal force and cooperativity of thin filament activation were observed between groups. Experiments were carried out at ∼21°C. Analysis of mathematical modeling of force and sinusoidal stiffness are in progress. NIH-HL128683.