Impact of β-myosin heavy chain isoform expression on cross-bridge cycling kinetics

Abstract

Myosin heavy chain (MHC) isoforms alpha and beta have intrinsically different ATP hydrolysis activities (ATPase) and therefore cross-bridge cycling rates in solution. There is considerable evidence of altered MHC expression in rodent cardiac disease models; however, the effect of incremental beta-MHC expression over a wide range on the rate of high-strain, isometric cross-bridge cycling is yet to be ascertained. We treated male rats with 6-propyl-2-thiouracil (PTU; 0.8 g/l in drinking water) for short intervals (6, 11, 16, and 21 days) to generate cardiac MHC patterns in transition from predominantly alpha-MHC to predominantly beta-MHC. Steady-state calcium-dependent tension development and tension-dependent ATP consumption (tension cost; proportional to cross-bridge cycling) were measured in chemically permeabilized (skinned) right ventricular muscles at 20 degrees C. To assess dynamic cross-bridge cycling kinetics, the rate of force redevelopment (ktr) was determined after rapid release-restretch of fully activated muscles. MHC isoform content in each experimental muscle was measured by SDS-PAGE and densitometry. alpha-MHC content decreased significantly and progressively with length of PTU treatment [68 +/- 5%, 58 +/- 4%, 37 +/- 4%, and 27 +/- 6% for 6, 11, 16, and 21 days, respectively; P < 0.001 (ANOVA)]. Tension cost decreased, linearly, with decreased alpha-MHC content [6.7 +/- 0.4, 5.6 +/- 0.5, 4.0 +/- 0.4, and 3.9 +/- 0.3 ATPase/tension for 6, 11, 16, and 21 days, respectively; P < 0.001 (ANOVA)]. Likewise, ktr was significantly and progressively depressed with length of PTU treatment [11.1 +/- 0.6, 9.1 +/- 0.5, 8.2 +/- 0.7, and 6.2 +/- 0.3 s(-1) for 6, 11, 16, and 21 days, respectively; P < 0.05 (ANOVA)] Thus cross-bridge cycling, under high strain, for alpha-MHC is three times higher than for beta-MHC. Furthermore, under isometric conditions, alpha-MHC and beta-MHC cross bridges hydrolyze ATP independently of one another.

Cardiac Myosin-binding Protein C and Troponin-I Phosphorylation Independently Modulate Myofilament Length-dependent Activation

“…Likewise, the impact of variable cTnI PKA phosphorylation was accounted for by the phospho-mimetic cTnI containing troponin exchange procedure, resulting in ϳ93% troponin replacement. Finally, the partial expression of ␤-myosin in the AAA group is unlikely to affect the results of the present study, as no impact on myofilament Ca 2ϩ has been observed upon the complete replacement of ␣-myosin with ␤-myosin in rat myocardium (36,37).…”

Cardiac contractile dysfunction in Lep/Lep obesity is accompanied by NADPH oxidase activation, oxidative modification of sarco(endo)plasmic reticulum Ca2+-ATPase and myosin heavy chain isozyme switch

“…An altered distribution of MHC isoforms has been shown in myocardial remodelling, cardiac hypertrophy and cardiomyopathy [17,[28][29][30][31], indicating that the regulation of cardiac contractile function is directly related to the relative amounts of MHC-α and MHC-β isozymes [25]. Even a small shift in the relative expression of these isoforms may significantly alter cardiomyocyte power output [30,31].…”

“…Although the predominant MHC isoform in healthy human hearts is β rather than α, overexpression of MHC-β in transgenic mice suggests that such an MHC-α to MHC-β switch is a maladaptive response to preserve energy [30]. The MHC isozyme switch from the fast type α isoform to the slow type β isoform (lower ratio of MHC-α to MHC-β) is believed to play a significant role in cardiac dysfunction in diabetes, cardiac hypertrophy and senescence [28,29,49,50]. This is supported by the fact that an increase in MHC-α and a decrease in MHC-β mRNA expression (increased ratio of MHC-α to MHC-β) is deemed beneficial in the improvement of left ventricular function in patients with idiopathic dilated cardiomyopathy [31].…”

“…5b,c). It has been reported that expression of MHC isozymes is associated with cardiac function, especially in myocardial remodelling and cardiomyopathies [17,[28][29][30][31]. We examined the pattern of MHC isozyme distribution in Lep/Lep heart.…”

Increased Cross-bridge Cycling Kinetics after Exchange of C-terminal Truncated Troponin I in Skinned Rat Cardiac Muscle

“…Exchange of Recombinant Cardiac cTn into Skinned Rat Trabeculae-Free running, unbranched right ventricular trabeculae were dissected from male Lewis Brown Norway-F1 rats (ϳ14 weeks of age; Harlan Laboratories) and, after extraction with Triton X-100, attached to aluminum T-clips (18,19). Exogenous Tn was exchanged for endogenous Tn by slight modification of previously described methods (18).…”