Effects of Cardiac Myosin Isoform Variation on Myofilament Function and Crossbridge Kinetics in Transgenic Rabbits

Abstract

Background-The left ventricles of both rabbits and humans express predominantly ␤-myosin heavy chain (MHC).Transgenic (TG) rabbits expressing 40% ␣-MHC are protected against tachycardia-induced cardiomyopathy, but the normal amount of ␣-MHC expressed in humans is only 5% to 7% and its functional importance is questionable. This study was undertaken to identify a myofilament-based mechanism underlying tachycardia-induced cardiomyopathy protection and to extrapolate the impact of MHC isoform variation on myofilament function in human hearts. Methods and Results-Papillary muscle strips from TG rabbits expressing 40% (TG40) and 15% ␣-MHC (TG15) and from nontransgenic (NTG) controls expressing Ϸ100% ␤-MHC (NTG40 and NTG15) were demembranated and calcium activated. Myofilament tension and calcium sensitivity were similar in TGs and respective NTGs. Force-clamp measurements revealed Ϸ50% higher power production in TG40 versus NTG40 (PϽ0.001) and Ϸ20% higher power in TG15 versus NTG15 (PϽ0.05). A characteristic of acto-myosin crossbridge kinetics, the "dip" frequency, was significantly higher in TG40 versus NTG40 (0.70Ϯ0.04 versus 0.39Ϯ0.09 Hz, PϽ0.01) but not in TG15 versus NTG15.The calculated crossbridge time-on was also significantly shorter in TG40 (102.3Ϯ14.2 ms) versus NTG40 (175.7Ϯ19.7 ms) but not in TG15 versus NTG15. Conclusions-The incorporation of 40% ␣-MHC leads to greater myofilament power production and more rapid crossbridge cycling, which facilitate ejection and relengthening during short cycle intervals, and thus protect against tachycardia-induced cardiomyopathy. Our results suggest, however, that, even when compared with the virtual absence of ␣-MHC in the failing heart, the 5% to 7% ␣-MHC content of the normal human heart has little if any functional significance. (Circ Heart Fail. 2009;2:334-341.)

The Rates of Ca2+ Dissociation and Cross-bridge Detachment from Ventricular Myofibrils as Reported by a Fluorescent Cardiac Troponin C

“…This result may not be surprising considering that previous studies have shown that myosin S1 and singleheaded myosin have different properties than even heavy meromyosin (55). In agreement with our results, the apparent rates of ADP dissociation measured from the rabbit, rat, and human myofibrils were similar to the calculated rates of ADP dissociation measured by the laser trap assay (rabbit and rat) (41,44) and from the sinusoidal length perturbation analysis of crossbridge detachment (human, rabbit, and rat) (52)(53)(54).…”

“…Previously, the rates of ADP dissociation from different cardiac myosins were inferred from the study of unregulated actomyosin in solution (20), laser trap experiments (44), and sinusoidal analysis on skinned cardiac muscle (52)(53)(54). The apparent rate of ADP dissociation measured by our fluorescent TnC is nearly one order of magnitude slower than that measured from actomyosin in solution for the different fast and slow myosins (20).…”

Ca 2+ ‐independent positive molecular inotropy for failing rabbit and human cardiac muscle by α‐myosin motor gene transfer

“…Increasing ␣-MyHC by just 12% is sufficient to increase single cardiac myocyte power output by ϳ50% (43). In transgenic rabbits that express 15% ␣-MyHC, ventricular papillary muscles reportedly generate 20% greater peak power output than nontransgenic samples expressing 100% ␤-MyHC (49). Finally, mathematical modeling predicts that just 10% ␣-MyHC (90% ␤-MyHC) can enhance myocardial twitch kinetics and amplitude independent of the calcium transient (15).…”

Updating the Physiology and Pathophysiology of Cardiac Myosin-Binding Protein-C

“…3,4 The same directional shift occurs in humans; because human myocardium normally consists of ≈90% β-MyHC, its magnitude is small and functional consequences seem modest. 4,5 Ventricular expression of the atrial isoform of the myosin essential light chain is elevated in DCM and would be expected to improve contractile performance. 6 Titin, the giant protein that accounts for cardiomyocyte passive stiffness, undergoes a shift to its more compliant N2BA isoform in human DCM.…”