title: Strongly binding myosin crossbridges regulate loaded shortening and power output in cardiac myocytes.
abstract: This study investigated the possible roles of strongly binding myosin crossbridges in determining loaded shortening and power output in cardiac myocytes. Single skinned cardiac myocytes were attached between a force transducer and position motor, and shortening velocities were measured over a range of loads during varying levels of Ca(2+) activation. Lowering the [Ca(2+)] slowed shortening velocities, decreased relative power output, and increased the curvature of length traces. We tested the hypothesis that Ca(2+) activation dependence of loaded shortening is determined primarily by strongly binding crossbridges or by [Ca(2+)] per se, which was done by measuring loaded shortening before and after addition of N-ethylmaleimide-conjugated myosin subfragment-1 (NEM-S1), a strongly binding myosin analogue that cooperatively enhances thin filament activation. At fixed [Ca(2+)], NEM-S1 reduced the curvature of length traces and sped loaded shortening velocities. Even when [Ca(2+)] was adjusted so that force was equal with and without NEM-S1, myocyte shortening was faster and exhibited less curvature with NEM-S1. In the presence of NEM-S1, peak relative power output was also significantly greater during activations either at the same [Ca(2+)] or when [Ca(2+)] was adjusted to achieve the same force. Consequently, NEM-S1 eliminated any Ca(2+) dependence of relative power output that is normally observed in cardiac myocytes. These results indicate that strongly binding crossbridges play a significant role in determining loaded shortening and power output and suggest that previously observed Ca(2+) dependence of power output is mediated by alterations in numbers of crossbridges bound to the thin filament.
authors.affiliation: Department of Physiology, University of Missouri School of Medicine, Columbia, MO, USA. mcdonaldks@missouri.edu
authors.forename: K S
authors.initials: KS
authors.lastname: McDonald
authors.forename: R L
authors.initials: RL
authors.lastname: Moss
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007551:Isotonic Contraction; D009200:Myocardial Contraction / Q000502:physiology*; D009206:Myocardium / Q000166:cytology; D015879:Myosin Subfragments / Q000737:chemistry / Q000494:pharmacology; D009218:Myosins / Q000737:chemistry / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013599:Systole; D013997:Time Factors
chemical_list: D015879:Myosin Subfragments; D009218:Myosins; D002118:Calcium