Effects of Cardiac Myosin Isoform Variation on Myofilament Function and Crossbridge Kinetics in Transgenic Rabbits
The left ventricles of both rabbits and humans express predominantly beta-myosin heavy chain (MHC). Transgenic (TG) rabbits expressing 40% alpha-MHC are protected against tachycardia-induced cardiomyopathy, but the normal amount of alpha-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. Papillary muscle strips from TG rabbits expressing 40% (TG40) and 15% alpha-MHC (TG15) and from nontransgenic (NTG) controls expressing approximately 100% beta-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 approximately 50% higher power production in TG40 versus NTG40 (P<0.001) and approximately 20% higher power in TG15 versus NTG15 (P<0.05). A characteristic of acto-myosin crossbridge kinetics, the "dip" frequency, was significantly higher in TG40 versus NTG40 (0.70+/-0.04 versus 0.39+/-0.09 Hz, P<0.01) but not in TG15 versus NTG15. The calculated crossbridge time-on was also significantly shorter in TG40 (102.3+/-14.2 ms) versus NTG40 (175.7+/-19.7 ms) but not in TG15 versus NTG15. The incorporation of 40% alpha-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 alpha-MHC in the failing heart, the 5% to 7% alpha-MHC content of the normal human heart has little if any functional significance.