Reduced preload increases Mechanical Control (strain-rate dependence) of Relaxation by modifying myosin kinetics

Abstract

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.

Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca2+ transients is modulated by preload

“…The preload dependence of Ca 2+ handling in cardiac muscle has been studied in isometrically contracting trabeculae ( 13 – 15 ) and auxotonically contracting myocytes ( 16 ), but it has not yet been examined in isolated cardiac muscle undergoing work-loop contractions. From a mechanistic standpoint, preload has a major role in modulating the ability of cardiac muscle to relax under afterloaded isotonic contractions in trabeculae ( 17 ) and myocytes ( 10 , 18 ), as well as on the mechano-Ca 2+ feedback as recently shown in silico ( 19 ). Hence, it is possible that preload modulates the afterload-dependent Ca 2+ handling observed in the literature.…”

Strain rate of stretch affects crossbridge detachment during relaxation of intact cardiac trabeculae

“…In contrast, we found that the relaxation-loading (stretch) was not only necessary, but also sufficient, to modify the relaxation rate without changes in afterload. While Mechanical Control of Relaxation was predicted to be related to crossbridge detachment, the mechanisms underlying this phenomenon remained speculative [ 3 , 4 ]. Modeling performed in these prior studies suggested that the bound crossbridge population was reduced more quickly by stretch, but the experimental protocols did not systematically control for strain rate and time-to-stretch for each trabecula.…”

“…After equilibration, isometric twitching was maintained except for a single twitch during each acquisition. For the experimental procedure, we first measured a normal twitch followed by a reference load-clamp twitch by controlling the developed force signal using a custom controller interfacing in SLControl data acquisition software [3][4][5]. The clamp was typically held at 50% of the developed, peak systolic force from the prior "normal" twitch, unless otherwise stated.…”

Mechanical Control of Relaxation using Intact Cardiac Trabeculae

“…The protocol described here utilizes trabeculae and not papillary muscles. The chordae of the papillary muscle induce a series elasticity that can inhibit changes to the MCR 9 . However, the exact placement of the attachments in the muscle is unlikely to impact the measures, because the trabecula length (and diameter) vary substantially.…”

“…Similarly, the data acquisition and analysis can also be applied to cardiomyocytes or other isolated muscle types 1 , 10 . The discussion includes commentary on possible alterations and adaptations to the method, along with limitations, such as caution against utilizing papillary muscles because of the mechanical properties of the chordae 9 .…”