The Super-Relaxed State and Length Dependent Activation in Porcine Myocardium

Abstract

Rationale: Myofilament length-dependent activation (LDA) is the key underlying mechanism of cardiac heterometric autoregulation, commonly referred as the Frank-Starling Law of the heart. Although alterations in LDA are common in cardiomyopathic states, the precise structural and biochemical mechanisms underlying LDA remain unknown. Objective: Here, we examine the role of structural changes in the thick filament during diastole, in particular changes in the availability of myosin heads, in determining both calcium sensitivity and maximum contractile force during systole in permeabilized porcine cardiac fibers. Methods and Results: Permeabilized porcine fibers from ventricular myocardium were studied under relaxing conditions at short and long sarcomere length using muscle mechanics, biochemical measurements, and X-ray diffraction. Upon stretch, the porcine myocardium showed the increased calcium sensitivity and maximum calcium-activated force characteristic of LDA. Stretch increased diastolic ATP turnover, recruiting reserve myosin heads from the super-relaxed state at longer sarcomere length. Structurally, X-ray diffraction studies in the relaxed-muscle confirmed a departure from the helical ordering of the thick filament upon stretch which occurred concomitantly with a displacement of myosin heads towards actin, facilitating cross-bridge formation upon systolic activation. Mavacamten, a selective myosin-motor inhibitor known to weaken the transition to actin-bound power-generating states and to enrich the ordered super-relaxed state myosin population, reversed the structural effects of stretch on the thick filament, blunting the mechanical consequences of stretch; mavacamten did not, however, prevent other structural changes associated with LDA in the sarcomere, such as decreased lattice spacing or troponin-displacement. Conclusions: Our findings strongly indicate that in ventricular muscle, LDA and its systolic consequences are dependent on the population of myosin heads competent to form cross bridges and involves the recruitment of myosin heads from the reserve super-relaxed state pool during diastole.