title: Myosin binding-induced cooperative activation of the thin filament in cardiac myocytes and skeletal muscle fibers.
abstract: Myosin binding-induced activation of the thin filament was examined in isolated cardiac myocytes and single slow and fast skeletal muscle fibers. The number of cross-bridge attachments was increased by stepwise lowering of the [MgATP] in the Ca(2+)-free solution bathing the preparations. The extent of thin filament activation was determined by monitoring steadystate isometric tension at each MgATP concentration. As pMgATP (where pMgATP is -log [MgATP]) was increased from 3.0 to 8.0, isometric tension increased to a peak value in the pMgATP range of 5.0-5.4. The steepness of the tension-pMgATP relationship, between the region of the curve where tension was zero and the peak tension, is hypothesized to be due to myosin-induced cooperative activation of the thin filament. Results showed that the steepness of the tension-pMgATP relationship was markedly greater in cardiac as compared with either slow or fast skeletal muscle fibers. The steeper slope in cardiac myocytes provides evidence of greater myosin binding-induced cooperative activation of the thin filament in cardiac as compared with skeletal muscle, at least under these experimental conditions of nominal free Ca2+. Cooperative activation is also evident in the tension-pCa relation, and is dependent upon thin filament molecular interactions, which require the presence of troponin C. Thus, it was determined whether myosin-based cooperative activation of the thin filament also requires troponin C. Partial extraction of troponin C reduced the steepness of the tension-pMgATP relationship, with the effect being significantly greater in cardiac than in skeletal muscle. After partial extraction of troponin C, muscle type differences in the steepness of the tension-pMgATP relationship were no longer apparent, and reconstitution with purified troponin C restored the muscle lineage differences. These results suggest that, in the absence of Ca2+, myosin-mediated activation of the thin filament is greater in cardiac than in skeletal muscle, and this apparent cooperativity requires the presence of troponin C on thin filament regulatory strands.
authors.affiliation: Department of Physiology, University of Michigan School of Medicine, Ann Arbor 48109, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Metzger
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism*; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D014336:Troponin / Q000378:metabolism; D019209:Troponin C
chemical_list: D014336:Troponin; D019209:Troponin C; D000255:Adenosine Triphosphate; D009218:Myosins; D002118:Calcium