Contractile properties of frog twitch fibres after D600 paralysis.

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 microM D600 at low temperature (around 5 degrees 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 Q gamma 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 degrees 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.