Osmotic Compression Influences Cross-Bridge Detachment Rate in Transgenic Hypertrophic Cardiomyopathy Variant Hctnt-I79N and Non-Transgenic Mouse Cardiac Muscle

Abstract

Missense variant Ile79Asn in human cardiac troponin T (HcTnT-I79N) has been linked to adverse phenotypic outcomes of hypertrophic cardiomyopathy (HCM) including arrhythmia and sudden cardiac death. This study investigates cross-bridge kinetics of left ventricular papillary muscle bundles (Ca2+-sensitivity of isometric force, sinusoidal stiffness and rate of tension redevelopment (kTR)) in a (1) length-dependent activation manner and (2) under osmotic compression. Cardiac tissues were harvested from both non-transgenic wild-type (NTg) mice and transgenic mice bearing the HcTnT-I79N variant. Sarcomere lengths (1.9, 2.1 or 2.3 μm) were set at pCa 8.0 using HeNe laser diffraction. 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 and maximum force were 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 increased Ca2+-sensitivity and maximum force at SL 1.9 vs 2.1 μm, while only maximum force increased upon further stretch (SL 2.1 vs 2.3 μm). This blunted effect at SL 2.1 vs 2.3 μm cannot be explained by differences in thin filament length in HcTnT-I79N vs NTg. Although HcTnT-I79N exhibited significantly lower maximum kTR, neither NTg nor HcTnT-I79N showed significant changes in maximum kTR over the entire range of SL (1.9-2.3 μm). After osmotically compressing the myofilament lattice using 3% Dextran T500 (SL 2.1 μm), (1) only NTg cardiac preparations displayed increased myofilament Ca2+-sensitivity; (2) kTR increased at most activation levels for both NTg and HcTnTI79N; (3) no changes in maximum kTR were observed; (4) both NTG and HcTnT-I79N displayed increased cross-bridge detachment rates (g); (5) HcTnT-I79N showed slower g when compared to NTG with and without Dextran. NIH-HL128683.