Abstract 2734: Role of Cardiac Myosin Heavy Chain Isoform Variation in Cross-bridge Kinetics: Results from Transgenic Rabbits
Background: The small amount of α-myosin heavy chain (MHC) normally present in non-failing human myocardium (5–7%) is downregulated in failing myocardium to nearly undetectable levels. The physiological relevance of this small shift is uncertain, but some studies employing manipulation of thyroid state in rodents to vary MHC isoform ratio suggest that it has a significant, negative functional consequence. In order to determine the significance of small MHC isoform shifts in a preparation that is closer to human myocardium and does not require thyroid manipulation, we examined the effects of α-MHC content on myofilament function by using transgenic (TG) rabbits containing varying levels of α-MHC superimposed on a normal α-MHC background, as is present in both rabbits and humans.
Methods: Papillary muscle strips were obtained from two types of TG rabbits with different α-MHC contents [5 TG rabbits with 15 % α-/85% β-MHC (TG15) and 4 TG rabbits with 40% α-/60% β-MHC (TG40)] and age-matched NTG controls containing 100% β-MHC (4 NTG15 and 4 NTG 40, respectively). Dissected strips were chemically demembranated with detergent, attached to a length motor and force gauge, and stretched to sarcomere length 2.2 μm. Myofilaments were calcium activated and small-amplitude sinusoidal length perturbation analysis was performed at 17°C over a wide range of frequencies to delineate cross-bridge kinetics.
Result: Maximum isometric tension was similar in TG and NTG groups (17.6 ± 2.0 in TG15 vs 18.9 ± 2.8 in NTG15 and 26.6 ± 3.1 in TG40 vs 24.3 ± 3.7 in NTG40). ``Dip” frequency, the frequency of minimal dynamic stiffness determined during sinusoidal analysis that correlates with the rate of actomyosin cross-bridge kinetics, was higher in TG40s compared with NTG40s (0.70 ± 0.04 vs 0.39 ± 0.09 Hz, P<0.01). In contrast, there was no difference between TG15s and NTG15s in dip frequency (0.71 ± 0.09 vs 0.67 ± 0.10 Hz).
Conclusion: An increased proportion of α-MHC to ~40% on a α-MHC background results in more rapid rates of cross-bridge cycling. Decreasing α-MHC content to 15% leads to undetectable differences in cross-bridge kinetics. These findings suggest that the much smaller change in α-MHC that occurs in failing human myocardium is not functionally significant.