What Limits the Velocity of Fast-skeletal Muscle Contraction in Mammals?

• Published in: Journal of molecular biology 2006-01, Vol.355 (3), p.432-442
• Main Authors: Nyitrai, Miklós, Rossi, Rosetta, Adamek, Nancy, Pellegrino, Maria Antonietta, Bottinelli, Roberto, Geeves, Michael A.
• Format: Article
• Language: English
• Subjects: Adenosine Diphosphate - metabolism; Adenosine Triphosphate - metabolism; Animals; fast kinetics; flash photoplysis; In Vitro Techniques; Kinetics; Mice; Models, Biological; Muscle Contraction - physiology; Muscle Fibers, Fast-Twitch - physiology; Muscle, Skeletal - physiology; myosin isoforms; Myosin Subfragments - physiology; Myosin Type II - physiology; Osmolar Concentration; Protein Isoforms - physiology; Rabbits; Rats; subfragment 1; Swine; Temperature; temperature dependence
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/j.jmb.2005.10.063

In rat skeletal muscle the unloaded shortening velocity ( V o) is defined by the myosin isoform expressed in the muscle fibre. In 2001 we suggested that ADP release from actomyosin in solution (controlled by k −AD) was of the right size to limit V o. However, to compare mechanical and solution kinetic data required a series of corrections to compensate for the differences in experimental conditions (0.5 M KCl, 22 °C for kinetic assays of myosin, 200 mM ionic strength, 12 °C to measure V o). Here, a method was developed to prepare heavy meromyosin (HMM) from pure myosin isoforms isolated from single muscle fibres and to study k −AD (determined from the affinity of the acto-myosin complex for ADP, K AD) and the rate of ATP-induced acto-HMM dissociation (controlled by K 1 k +2) under the same experimental condition used to measure V o. In fast-muscle myosin isolated from a wide range of mammalian muscles, k −AD was found to be too fast to limit V o, whereas K 1 k +2 was of the right magnitude for ATP-induced dissociation of the cross-bridge to limit shortening velocity. The result was unexpected and prompted further experiments using the stopped-flow approach on myosin subfragment-1 (S1) and HMM obtained from bulk preparations of rabbit and rat muscle. These confirmed that the rate of cross-bridge dissociation by ATP limits the velocity of contraction for fast myosin II isoforms at 12 °C, while k −AD limits the velocity of slow myosin II isoforms. Extrapolating our data to 37 °C suggests that at physiological temperature the rate of ADP dissociation may limit V o for both isoforms.