Myocardial fiber architecture is a physiologically important regulator of ejection fraction, strain, and pressure development. Apparent ultrasonic backscatter has been shown to be a useful method for recreating the myocardial fiber architecture in human-sized sheep hearts, due to the dependence of its amplitude on the relative orientation of a myofiber to the angle of ultrasonic insonification. Thus, the anisotropy of the backscatter signal is linked to, and provides information about, the fiber orientation. In this study, we sought to determine if apparent backscatter could be used to measure myofiber orientation in rodent hearts. Fixed adult rat hearts were imaged intact, and both a transmural cylindrical core and transmural wedge of the LV free wall were imaged. Cylindrical core samples confirmed that backscatter anisotropy could be measured in rat hearts. Ultrasound and histological analysis of transmural myocardial wedge samples confirmed that the apparent backscatter could be reproducibly mapped to fiber orientation (angle of the fiber relative to the direction of insonification). These data provided a quantitative relationship between the apparent backscatter and fiber angle that was applied to whole heart images. Myocardial fiber architecture was successfully measured in rat hearts. Quantifying myocardial fiber architecture using apparent backscatter provides a number of advantages, including its scalable use from rodents to man, its rapid low-cost acquisition, and minimal contraindications. The method outlined in this study provides a method for investigators to begin detailed assessments of how the myocardial fiber architecture changes in pre-clinical disease models, which can be immediately translated into the clinic.

Rapid myocardial relaxation is essential in maintaining cardiac output, and impaired relaxation is an early indicator of diastolic dysfunction. While the biochemical modifiers of relaxation are well known to include calcium handling, thin filament activation, and myosin kinetics, biophysical and biomechanical modifiers can also alter relaxation. We have previously shown that the relaxation rate is increased by an increasing strain rate, not a reduction in afterload. The slope of the relaxation rate to strain rate relationship defines Mechanical Control of Relaxation (MCR). To investigate MCR further, we performed in vitro experiments and computational modeling of preload-adjustment using intact rat cardiac trabeculae. Trabeculae studies are often performed using isometric (fixed-end) muscles at optimal length (Lo, length producing maximal developed force). We determined that reducing muscle length from Lo increased MCR by 20%, meaning that reducing preload could substantially increase the sensitivity of the relaxation rate to the strain rate. We subsequently used computational modeling to predict mechanisms that might underlie this preload-dependence. Computational modeling was not able to fully replicate experimental data, but suggested that thin-filament properties are not sufficient to explain preload-dependence of MCR because the model required the thin-filament to become more activated at reduced preloads. The models suggested that myosin kinetics may underlie the increase in MCR at reduced preload, an effect that can be enhanced by force-dependence. Relaxation can be modified and enhanced by reduced preload. Computational modeling implicates myosin-based targets for treatment of diastolic dysfunction, but further model refinements are needed to fully replicate experimental data.

Background: Heart failure and especially heart failure with preserved ejection fraction are significant burdens on health. Intact cardiac trabeculae may reveal functional changes that cannot be seen in other ex vivo (skinned, single molecule) preparations. For example, using intact trabeculae, we have shown that relaxation is mechanically controlled by the lengthening strain rate at end systole, not afterload. Objective: We sought to evaluate the effect of myosin activator Omecamtiv Mercarbil (OM) and inhibitor Mavacamten (Mav) on physiologic function in intact trabeculae. Methods/Results: Afterload-clamp protocols were applied to intact cardiac trabeculae from Sprague Dawley rats to simulate physiologic work-loops and evaluate mechanical control of relaxation. Both OM and Mav reduced stroke work (force x length) by >50% and power (force x velocity) by ~50% at doses reducing developed force by 50%. These were mediated by dose-dependent reductions in both force and shortening length. We have recently reported preliminary results that OM improves contraction-relaxation coupling and makes the relaxation rate more sensitive to strain rate in a dose-dependent manner. Mav does not lead to significant changes in contraction-relaxation coupling at any dose; Mav alters the sensitivity of relaxation rate to strain rate only at doses that reduce developed force by 50%. Summary/Perspective: Intact rat cardiac trabeculae reveal function mimicking physiology. OM and Mav show remarkable similarities in force, work, and power, which may be due to the high expression of alpha-myosin heavy chain. OM treatment not only modified contractility but increased sensitivity of the relaxation rate to strain rate in a dose dependent manner, which may explain why diastolic dysfunction is not more prevalent in clinical studies. Mav appears to only modify the attachment of crossbridges as expected.

DCM phenotype are lacking. However, we recently introduced a metric capable of predicting the type and severity of myocardial remodeling in progressive cardiomyopathies (Davis et al. Cell, 2016), which hinges on relating force-time integrals of computationally derived twitches of diseased cardiomyocytes to wild-type. Negative values of this metric (the 'tension index') correlate with eccentric hypertrophy associated with DCM, and the magnitude correlates with the severity of phenotype. Here, we test the hypothesis that an experimental analogue of the tension index can be generated from twitches of intact cardiac trabeculae from genetically engineered murine models of DCM, and that it will predict the degree of myocardial remodeling. Intact trabeculae from a DCM model-a calcium-desensitizing tropomyosin mutation (D230N)-have significantly decreased twitch force compared to wild-type trabeculae (1652 versus 3153 kPa, respectively). The corresponding tension index for D230N trabeculae is -6.62x10 3 (normalized force,ms), predicting significant eccentric hypertrophy. Indeed, our predictions are verified by echocardiographic measurements of ventricular diameters in vivo. In D230N hearts, the left-ventricular systolic and diastolic diameters are significantly greater than wild-type (3.1 and 4.3mm, versus 2.4 and 3.8mm, respectively). Furthermore, intact trabeculae from a double-transgenic (DTG) murine model, (D230N plus the calcium-sensitizing troponin mutation, L48Q), produce twitch forces similar to wild-type (3253 kPa). The tension index for DTG trabeculae is -1.48x10 3 (normalized force,ms), predicting less remodeling than D230N alone. Consistently, systolic and diastolic diameters of DTG hearts are not different from wild-type (2.5 and 3.9mm, respectively). Our work demonstrates the ability of a trabecula-based tension index to predict organ-level morphology in DCM hearts and to use these predictions to develop interventions that prevent pathological remodeling. 1306-PosMolecular Dynamics Studies of Myosin Structure with 2-Deoxy-ADP The mitochondrial inner membrane (IM) has a dynamic and complex structure that plays a significant role in mitochondrial function and energy metabolism.