Ca²⁺-, Sr²⁺-force relationships and kinetic properties of fast-twitch rat leg muscle fibre subtypes

Author: Galler

Source: Acta Physiologica, Volume 167, Number 2, October 1999 , pp. 131-141(11)

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Abstract:

Force generation of fast-twitch and slow-twitch fibres exhibits large differences in its sensitivity to Ca²⁺ and Sr²⁺ (e.g. Fink et al. 1986 ). Little is known about fast-twitch fibre subtypes. Thus, a variety of mechanical measurements on segments of rehydrated freeze-dried fast-twitch rat leg muscle fibres were executed in this study. Among these, the Ca²⁺- and Sr²⁺-force relationship and the unloaded shortening velocity were determined. The fibres were classified into subtypes according to their kinetics of stretch activation ( Galler et al. 1994 ). In all fibres, the maximal force under Sr²⁺ activation was about 0.9 of that under Ca²⁺ activation. The Ca²⁺- and Sr²⁺-force relationship exhibited a biphasic shape with a steeper part (Hill coefficient, n₁) below 50% and a flatter part (Hill coefficient, n₂) above 50% of maximal force. The difference between the Ca²⁺ - and Sr²⁺ -sensitivity was independent of the fibre subtypes. The Hill coefficients were only partially correlated with kinetic properties. The correlation was more pronounced for the unloaded shortening velocity than for the kinetics of stretch activation. The data are consistent with the idea that the Ca²⁺ and Sr²⁺ sensitivities of fast-twitch fibres are mainly determined by a single isoform of troponin C. Among several protein isoforms, the isoforms of the myosin light chains seem to be involved for determining the slope of the Ca²⁺- and Sr²⁺-force relationship of fast-twitch muscle fibres.

Keywords: muscle fibre types; myofibrillar Ca2+ sensitivity; myofibrillar protein isoforms; myofibrillar Sr2+ sensitivity; myosin isoforms; skeletal muscle

Document Type: Research article

DOI: http://dx.doi.org/10.1046/j.1365-201x.1999.00603.x

Affiliations: 1: Institute of Zoology, University of Salzburg, Austria

Publication date: 1999-10-01