The R403Q Mutation Alters Isometric and Energetic Properties in 2 Month Mice

Abstract

Introduction: Familial hypertrophic cardiomyopathy (FHC) is a primary disease of the sarcomere. The R403Q mutation resides at the actin-interaction site on myosin and leads to progressive hypertrophic cardiomyopathy that can result in sudden cardiac death or congestive heart failure. Previous studies examining the functional impact of the R403Q mutation give inconsistent results such as either reduced or enhanced actin-activated ATPase. We hypothesized that the R403Q mutation intrinsically increases the energetic cost of contraction.Methods: To do this, we assessed Ca2+-sensitivity of tension and cross-bridge kinetics in de-membranated cardiac trabeculae from wild-type (WT) and R403Q hearts at 2 months of age. Assuming a two-state model of cross-bridge binding and un-binding, we measured cross-bridge kinetics using 2 methods: (1) the rate of ATP hydrolysis (off-rate) by NADH oxidation that is enzymatically and stoichiometrically coupled to ATP consumption, and (2) the rate of force redevelopment (ktr; on-rate plus off-rate) by a rapid release-restretch protocol.Results: Male R403Q mice display a decrease in Ca2+ sensitivity (2.461μM ± 0.072) compared to WT males and females (2.189 μM ± 0.059 and 2.166 μM ±0.044, respectively) at SL 2.0μm. R403Q males display increased tension cost at SL 2.0μm when compared to WT counterparts (9.56 ± 0.68 and 6.82 ±1.07 pmol s-1 mm-1 mN-1, respectively). At maximal activation, the rate of force redevelopment (ktr) is not significantly different between R403Q and WT males (40.26±6.86 and 30.26±3.95 s-1).Conclusions: Although maximal ktr is not significantly different between WT and R403Q males, an increase in ATP hydrolysis (off rate) suggests an alteration in cross-bridge kinetics. Importantly, future studies will explore the relationship between submaximal activation and ktr. Along with no overt pathology at 2 months, the R403Q mutation alters the energetic cost of contraction and isometric properties.