Ruthenium red: differential effects on excitation and excitation-contraction coupling in frog skeletal muscle

The effects of purified ruthenium red (RR6+) on action potentials, resting potentials, twitches, tetani and potassium contractures in fibre bundles dissected from frog leg muscles were compared to the effects of supranormal concentrations of Ca2+. The effects of RR6+ include a shift of action potential threshold and reduction in rates of depolarization, repolarization and conduction velocity. Resting potentials and action potential overshoots are unaffected by RR6+ in concentrations of 10 microM or less. Higher RR6+ concentrations cause membrane depolarization. Low concentrations of RR6+ cause twitch potentiation but do not affect tetanus tension. Unlike Ca2+, RR6+ has no effect on the mechanical activation curve. However, crude preparations of RR6+ contain an unidentified contaminant which does shift the mechanical activation curve to the right, i.e. it increases mechanical threshold. RR6+ itself appears to be largely excluded from the t-system.