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Articles by B. M Palmer
Total Records ( 2 ) for B. M Palmer
  T Suzuki , B. M Palmer , J James , Y Wang , Z Chen , P VanBuren , D. W Maughan , J Robbins and M. M. LeWinter
 

Background— The left ventricles of both rabbits and humans express predominantly β-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 100% β-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 50% higher power production in TG40 versus NTG40 (P<0.001) and 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.

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.

  M. S Miller , P VanBuren , M. M LeWinter , S. H Lecker , D. E Selby , B. M Palmer , D. W Maughan , P. A Ades and M. J. Toth
 

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 Ca2+-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 Ca2+ 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.

 
 
 
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