Contractile activation in slow and twitch muscle fibres of the frog

Abstract

VERTEBRATE slow and twitch skeletal muscle fibres differ qualitatively in many respects and constitute functionally distinct motor systems^1,2. Little is known, though, of the respective contractile activation properties. Since triad contacts between transverse (T) tubules and sarcoplasmic reticulum (SR), thought to be the pathway for activation of twitch fibres, are nearly identical in frog twitch and slow fibres³, similar processes would be expected to lead to Ca²⁺ release from the SR following T-tubule depolarisation in both muscle types. We describe here voltage-clamp studies of activating threshold contractions in slow and twitch fibres^4,5. Activation kinetics in these physiologically very different fibre types are remarkably similar. Thus, a given membrane potential change may result in an equivalent amount of Ca²⁺ delivered to both slow and twitch myofibrils. Once activated, each fibre type would shorten at its characteristic velocity⁶, probably determined by the rate of actomyosin cross-bridge turnover⁷.