Contractile properties of frog twitch fibres after D600 paralysis

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Abstract
Twitch and contracture tensions were measured in single intact fibres of semitendinosus muscles with a sensitive miniature transducer. After a fibre was paralysed by a conditioning depolarization in the presence of 30 (μm D600 at low temperature (around 5° C), no twitch tension could be detected. When a paralysed fibre was warmed, its ability to give potassium-contractures recovered almost completely but its twitch tension revived to a variable extent, ranging from fully to partially or not at all. This variable recovery of twitch tension appeared to correlate very well with the variable repriming of Qγ observed previously in revived fibres. The optimal temperature at which twitch tension could be revived readily lay within a narrow window roughly between 8 to 16° C, within which the rate and extent of revival of twitch tension were temperature-dependent. Removal of D600 from the bathing solution after conditioning depolarization facilitated the revival of twitch tension but was neither a necessary nor a sufficient condition for revival. A reduction of potassium concentration in the high K solution or an abbreviation of the duration of conditioning depolarization could bring a fibre to a partially paralysed state (or equivocally a partially revived state) without going through complete paralysis. The paralysing actions of submaximal condition depolarizations were additive. The partially revived state was unstable and affected by repetitive stimulations in a use-dependent manner. The effect of a 0.1 Hz train of action potentials on twitch tension was generally biphasic, with a small initial suppression followed by an enhancement. It is speculated that this use-dependent enhancement could be due to a competition between D600 molecules and intracellular Ca2+ ions.