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We have studied the effect of Rutherium red on the sarcoplasmic reticulum Ca 2+ -ATPase. Ruthenium red does not modify the Ca 2+ pumping activity of the enzyme, despite its interaction with cationic binding sites on sarcoplasmic reticulum vesicles. Two pools of binding sites were distinguished. One pool (10 nmol/mg) is dependent upon the presence of micromolar Ca 2+ and may therefore represent the high-affinity Ca 2+ transport sites of the Ca 2+ -ATPase. However, Ruthenium red only slight, competes with Ca 2+ on these sites. The other pool (15–17 nmol/mg) is characterized as low-affinity cation binding sites of sarcoplasmic reticulum, distinct from the Mg 2+ site involved in the ATP binding to the Ca 2+ -ATPase. The interaction of Rutherium red with these low-affinity cation binding sites, which may be located either on the Ca 2+ -ATPase or on surrounding lipids, decrease tryptophan fluorescence level of the protein. As much as 25% of the tryptophan fluorescence of the Ca 2+ -ATPase is quenched by Ruthenium red (with a dissociation constant of 100 nM), tryptophan residues located near the bilayer being preferentially affected.

Using a rapid filtration method, we show that Ag+ is able to trigger Ca2+ release from sarcoplasmic reticulum vesicles at a rate as fast as that induced by Ca2+ itself. The Ag+ concentration dependence of the rate constant of Ca2+ release presents a bell shape, similar to that of Ca2+-induced Ca2+ release, with a maximum at 30 microM free Ag+. The rapid phase of Ca2+-release induced by Ag+ is activated by millimolar ATP and inhibited by 5 microM ruthenium red. Moreover, micromolar Ca2+ produces a shift of the Ag+ concentration dependence of the Ca2+ release rate. All these results suggest that Ag+ acts on the same sites as Ca2+ to regulate the release of Ca2+.