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

Force generation of fast-twitch and slow-twitch fibres exhibits large diffe rences 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 me asurements on segments of rehydrated freeze-dried fast-twitch rat leg muscl e fibres were executed in this study. Among these, the Ca2+- and Sr2+-force relationship and the unloaded shortening velocity were determined. The fib res were classified into subtypes according to their kinetics of stretch ac tivation (Galler et al. 1994). In all fibres, the maximal force under Sr2activation was about 0.9 of that under Ca2+ activation. The Ca2+- and Sr2+- force relationship exhibited a biphasic shape with a steeper part (Hill coe fficient, n(1)) below 50% and a flatter part (Hill coefficient. nz) above 5 0% of maximal force. The difference between the Ca2+ - and Si2+ -sensitivit y was independent of the fibre subtypes. The Hill coefficients were only pa rtially correlated with kinetic properties. The correlation was more pronou nced for the unloaded shortening velocity than for the kinetics of stretch activation. The data are consistent with the idea that the Ca2+ and Sr2+ se nsitivities 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.