Cardiac myosin isoforms exhibit differential rates of MgADP release and MgATP binding detected by myocardial viscoelasticity

We measured myosin crossbridge detachment rate and the rates of MgADP release and MgATP binding in mouse and rat myocardial strips bearing one of the two cardiac myosin heavy chain (MyHC) isoforms. Mice and rats were fed an iodine-deficient, propylthiouracil diet resulting in similar to 100% expression of beta-MyHC in the ventricles. Ventricles of control animals expressed similar to 100% alpha-MyHC Chemically-skinned myocardial strips prepared from papillary muscle were subjected to sinusoidal length perturbation analysis at maximum calcium activation pCa 4.8 and 17 degrees C. Frequency characteristics of myocardial viscoelasticity were used to calculate crossbridge detachment rate over 0.01 to 5 mM [MgATP]. The rate of MgADP release, equivalent to the asymptotic value of crossbridge detachment rate at high MgATP, was highest in mouse alpha-MyHC (111.4 +/- 62 s(-1)) followed by rat alpha-MyHC (65.0 +/- 7.3 s(-1)), mouse beta-MyHC (243 +/- 1.8 s(-1)) and rat beta-MyHC (15.5 +/- 0.8 s(-1)). The rate of MgATP binding was highest in mouse alpha-MyHC (325 +/- 32 mM(-1) s(-1)) then mouse beta-MyHC (152 +/- 23 mM(-1) s(-1)), rat alpha-MyHC (108 +/- 10 mM(-1) s(-1)) and rat beta-MyHC (55 +/- 6 mM(-1) s(-1)). Because the events of MgADP release and MgATP binding occur in a post power-stroke state of the myosin crossbridge, we infer that MgATP release and MgATP binding must be regulated by isoform- and species-specific structural differences located outside the nucleotide binding pocket, which is identical in sequence for these four myosins. We postulate that differences in the stiffness profile of the entire myosin molecule, including the thick filament and the myosin-actin interface, are primarily responsible for determining the strain on the nucleotide binding pocket and the subsequent differences in the rates of nucleotide release and binding observed among the four myosins examined here. (c) 2012 Elsevier Ltd. All rights reserved.