Effect of tannic acid on excitation-contraction coupling in skeletal muscle

Toe muscles of the frog were exposed to the action of solutions containing 60-300 micrometer tannic acid (TA). The effects of K-induced contractures of the muscle and on the resting potential of the fibres were studied. After an initial transient potentiation, K contractures are progressively slowed and depressed, the effect being irreversible. After their abolition, caffeine is still able to induce a mechanical response with a reduced amplititude and a pronounced slowing in both tension development and relaxation after withdrawal of the drug. The blocking effects on excitation-contraction coupling are independent of the presence or absence of Ca from the bath. During the initial potentiation the muscle acquires the ability to respond by a contracture to EDTA solutions. TA induces a moderate membrane depolarization with a more rapid fall in resting potential during the first tens of minutes. EDTA contracture appears to be initiated at the level of T tubules, being not associated to a rapid membrane depolarization. These results suggest that TA effects on excitation-contraction coupling are primarily due to a surface mechanism involving probably the signal transmission at the level of T system as well. These effects may be the result of its capacity to crosslink protein macromolecules, interfering with conformational changes underlying initiation and propagation of excitation from membrane to the interior of muscle fibre.