Myosin heavy chain isoform composition and stretch activation kinetics in single fibres of Xenopus laevis iliofibularis muscle

Abstract

Skeletal muscle is composed of specialized fibre types that enable it to fulfil complex and variable functional needs. Muscle fibres of Xenopus laevis, a frog formerly classified as a toad, were the first to be typed based on a combination of physiological, morphological, histochemical and biochemical characteristics. Currently the most widely accepted criterion for muscle fibre typing is the myosin heavy chain (MHC) isoform composition because it is assumed that variations of this protein are the most important contributors to functional diversity. Yet this criterion has not been used for classification of Xenopus fibres due to the lack of an effective protocol for MHC isoform analysis. In the present study we aimed to resolve and visualize electrophoretically the MHC isoforms expressed in the iliofibularis muscle of Xenopus laevis, to define their functional identity and to classify the fibres based on their MHC isoform composition. Using a SDS-PAGE protocol that proved successful with mammalian muscle MHC isoforms, we were able to detect five MHC isoforms in Xenopus iliofibularis muscle. The kinetics of stretch-induced force transients (stretch activation) produced by a fibre was strongly correlated with its MHC isoform content indicating that the five MHC isoforms confer different kinetics characteristics. Hybrid fibre types containing two MHC isoforms exhibited stretch activation kinetics parameters that were intermediate between those of the corresponding pure fibre types. These results clearly show that the MHC isoforms expressed in Xenopus muscle are functionally different thereby validating the idea that MHC isoform composition is the most reliable criterion for vertebrate skeletal muscle fibre type classification. Thus, our results lay the foundation for the unequivocal classification of the muscle fibres in the Xenopus iliofibularis muscle and for gaining further insights into skeletal muscle fibre diversity.

An Embryonic Myosin Isoform Enables Stretch Activation and Cyclical Power in Drosophila Jump Muscle

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“…This principle was originally reported in seminal studies demonstrating a direct correlation between MHC isoform content and the physiological properties of muscles (Reiser et al, 1988a(Reiser et al, ,b, 1985. Subsequent studies have confirmed that hybrid fibers exhibit intermediate shortening velocities, and in hybrid fibers with different proportions of MHC isoforms, the precise shortening velocity is proportional to the relative amount of these isoforms (Andruchov et al, 2004;Andruchova et al, 2006;Larsson and Moss, 1993). It is now generally understood that a 'continuum' of fiber types exists (Fig.…”

Explosive Jumping: Extreme Morphological and Physiological Specializations of Australian Rocket Frogs (Litoria nasuta)

“…However, previous studies also indicate that the intrinsic contractile properties measured in isometric and force velocity studies are not always good indicators of the likely power output that can be produced under work loop conditions Caizzo and Baldwin 1997). The relatively high optimal cycle frequency for power output achieved in L. nasuta plantaris longus muscle also suggests that this muscle is of a relatively fast fiber type, probably containing more muscle fibers with type I myosin heavy chain and/or other fast muscle protein isoforms (such as myosin light chains) than other frog muscles (Lutz et al 1998;Andruchova et al 2006).…”