The occupancy of high-affinity ryanodine-binding sites of isolated heavy sarcoplasmic reticulum vesicles occurring in concentrated salt solutions affects ATP-dependent calcium accumulation and caffeine-induced calcium release. The initial suppression of calcium uptake is followed by a marked uptake activation resulting in a reduction of the final calcium level in the medium. Simultaneously, caffeine-induced calcium release is blocked. The dependence of inhibition of calcium uptake and caffeine-induced calcium release observed in assay media containing physiological concentrations of magnesium and ATP on the concentration of ryanodine corresponds to the drug's effectiveness in living muscles.
