Mechanisms underlying skeletal muscle weakness in human heart failure: alterations in single fiber myosin protein content and function

Abstract

Background: Patients with chronic heart failure (HF) frequently experience skeletal muscle weakness that limits physical function. The mechanisms underlying muscle weakness, however, have not been clearly defined.

Methods and results: This study examined the hypothesis that HF promotes a loss of myosin protein from single skeletal muscle fibers, which in turn reduces contractile performance. Ten patients with chronic HF and 10 controls were studied. Muscle atrophy was not evident in patients, and groups displayed similar physical activity levels, suggesting that observed differences reflect the effects of HF and not muscle atrophy or disuse. In single muscle fibers, patients with HF showed reduced myosin heavy chain protein content (P<0.05) that manifested as a reduction in functional myosin-actin cross-bridges (P<0.05). No evidence was found for a generalized loss of myofilament protein, suggesting a selective loss of myosin. Accordingly, single muscle fiber maximal Ca(2+)-activated tension was reduced in myosin heavy chain I fibers in patients (P<0.05). However, tension was maintained in myosin heavy chain IIA fibers in patients because a greater proportion of available myosin heads were bound to actin during Ca(2+) activation (P<0.01).

Conclusions: Collectively, our results show that HF alters the quantity and functionality of the myosin molecule in skeletal muscle, leading to reduced tension in myosin heavy chain I fibers. Loss of single fiber myosin protein content represents a potential molecular mechanism underlying muscle weakness and exercise limitation in patients with HF.

Figure 1

Tissue homogenate MHC protein content (A; arbitrary densitometry units (AU) per μg of protein) and relative isoform distribution (B; % of total) in controls (C; n=10) and heart failure patients (HF; n=9) with representative sections of gels. Bar graphs represent mean ± SE. **, P<0.01.

Figure 2

Single fiber MHC (n=10/group) protein content from controls (C) and heart failure patients (HF) and representative gel images of MHC bands. Data are shown including all fibers measured and only those fibers that contained a measurable MHC band (density > background). The number of fibers is indicated at the base of each bar. Bar graphs represent mean ± SE. *, P<0.05; **, P<0.01.

Figure 3

Skeletal muscle fiber ultrastructural data in controls (n=8) and heart failure patients (n=7). Representative cross-sectional (8,000X; A and B; scale bar=1 μm and 60,000X; G and H; bar=100 nm) and longitudinal (5,000X; D and E; scale bar=1 μm) images are presented. Bar graphs in panels C,F and I represent mean ± SE.

Figure 4

Single skeletal muscle fiber Ca²⁺-activated (pCa 4.5) tension (A) and dynamic stiffness data (B, C, D) in controls (n=5) and heart failure patients (n=9). The number of fibers is indicated at the base of each bar. Bar graphs represent mean ± SE. *, P<0.05; **, P<0.01.

Figure 5

MHC I protein breakdown fragments (A,C) and ubiquitinated MHC (B,D) in controls (C; n=4) and heart failure patients (HF; n=4) and E3 ubiquitin ligase expression (E,F; n=6 controls and 4 heart failure). Representative western blots are shown for MHC I breakdown fragments (A) and ubiquitinated MHC (B), including a Simple Blue-stained gel to indicate total MHC protein content. Bar graphs represent mean ± SE.