[Effect of homologous n-alkanoic acids on the function of isolated skeletal muscles. V. Relations between twitch, tetanus and contracture].

Twitch, tetanus, and contractures induced by potassium ions (K+) or caffeine were investigated on isolated bundles of frog muscles. Using supramaximal stimuli the twitch amplitude amounts to about 50% of the tetanic tension (22 degrees C). Usually the contracture in 190 mM K+ is smaller than the tetanic tension, in 20 mM caffeine the contracture maximum corresponds to the tetanus amplitude. All of the mechanical answers are diminished if the fibres were bathed in solutions containing 2.5 to 10 mM Na-octanoate or 0.5 mM decanoic acid. Twitch and tetanic tensions decrease nearly in the same degree. A sequence of tetanic stimulations in a distance of 2 s induces a depression of the amplitudes. Fatty acids increase this depression and delay restoration between the series. Contractures in 190 mM K+ are diminished by a preceding bath in 10 mM Na-octanoate. Contractures with submaximum concentrations of K+ were decreased by lower concentrations of octanoate. The caffeine induced contractures are delayed by fatty acids, but the maximum tension is not reduced. It is supposed that the mobilisation of calcium ions is alterated by fatty acids. The decreasing effect of fatty acids on the mechanical activity, induced by different ways of the muscles' stimulation suggests that the main action may take place in the process of mechanical coupling, probably during the activation of calcium ions.