Dependence of cross‐bridge kinetics on myosin light chain isoforms in rabbit and rat skeletal muscle fibres

Abstract

Cross-bridge kinetics underlying stretch-induced force transients was studied in fibres with different myosin light chain (MLC) isoforms from skeletal muscles of rabbit and rat. The force transients were induced by stepwise stretches (< 0.3% of fibre length) applied on maximally Ca 2+ -activated skinned fibres. Fast fibre types IIB, IID (or IIX) and IIA and the slow fibre type I containing the myosin heavy chain isoforms MHC-IIb, MHC-IId (or MHC-IIx), MHC-IIa and MHC-I, respectively, were investigated. The MLC isoform content varied within fibre types. Fast fibre types contained the fast regulatory MLC isoform MLC2f and different proportions of the fast alkali MLC isoforms MLC1f and MLC3f. Type I fibres contained the slow regulatory MLC isoform MLC2s and the slow alkali MLC isoform MLC1s. Slow MLC isoforms were also present in several type IIA fibres. The kinetics of force transients differed by a factor of about 30 between fibre types (order from fastest to slowest kinetics: IIB > IID > IIA I). The kinetics of the force transients was not dependent on the relative content of MLC1f and MLC3f. Type IIA fibres containing fast and slow MLC isoforms were about 1.2 times slower than type IIA fibres containing only fast MLC isoforms. We conclude that while the cross-bridge kinetics is mainly determined by the MHC isoforms present, it is affected by fast and slow MLC isoforms but not by the relative content of MLC1f and MLC3f. Thus, the physiological role of fast and slow MLC isoforms in type IIA fibres is a fine-tuning of the cross-bridge kinetics.

Influence of fast and slow alkali myosin light chain isoforms on the kinetics of stretch-induced force transients of fast-twitch type IIA fibres of rat

“…It is an open question if the N-terminal extension contributes to the slowing effect of MLC1s on myosin head kinetics. The fact that the MLC1f/MLC3f ratio does not influence the myosin head kinetics [5] excludes this possibility at first glance. However, studies on heart preparations showed that the myosin head kinetics is decelerated by the interaction of the N-terminal extension of the ventricular alkali MLC isoform MLC1v with the actin filament [35,37].…”

“…The presence of MLC1s implicates a reduction of either MLC1f or MLC3f because of substitution. Therefore, in principle, the observed differences in kinetics could be due to the varying MLC1f/ MLC3f ratio, but this was excluded in a former study showing that a varying MLC1f/MLC3f ratio does not influence myosin head kinetics [5]. Therefore, it can be concluded that MLC1s causes a slowing of the myosin head kinetics in type IIA fibres.…”

“…After the mechanical measurements, the muscle preparations were removed from the apparatus for biochemical analysis of MHC and MLC isoforms, as described in a previous study [5]. Shortly, the muscle fragments were dissolved in sodium dodecyl sulphate (SDS) lysis buffer (62 mM Tris-HCl, pH 6.8, 10% (v/v) glycerol, 5% (w/v) SDS, 5% (v/v) 2-mercaptoethanol).…”

“…They strongly correlate with the MHC isoforms [3-5, 16, 18, 19], but not with the alkali MLC isoforms MLC1f and MLC3f [5]. Other MLC isoforms were hardly considered in correlation studies.…”

Human genome comparison of paretic and nonparetic vastus lateralis muscle in patients with hemiparetic stroke

“…Gene expression changes for other muscle contractile proteins, as well. Myosin regulatory light chains regulate the kinetics of force transmission of the specific MHC isoforms by modulating the binding of fast and slow myosin light chain (MLC) isoforms [27][28]. In the current study, the MRLC3 and myosin light polypeptide 6B (MYL6B) genes were significantly down-regulated in the paretic leg after chronic stroke.…”