The effects of ryanodine on net Ca++ accumulation (Ca++ uptake) and on K+,Ca++-ATPase activities of cardiac sarcoplasmic reticulum vesicles were investigated. In the presence of 3 mM oxalate, ryanodine increased both the initial velocity of Ca++ uptake and the maximum amount of Ca++ accumulated 2- to 3-fold. Potassium ions potentiated the effects of ryanodine. In the presence of 100 mM KCl, 10-4 M ryanodine produced near maximal stimulation of uptake, whereas in the absence of KCl, a 10-fold higher concentration of ryanodine was required for appreciable stimulation. Choline chloride could not substitute for this effect of KCl. Ca++-dependent ATPase activities, whether measured with or without KCl, were not altered significantly by 10-4 M ryanodine when the concentration of Ca++ ions was not limiting and when oxalate was present. Thus, the stoichiometry of the net transport of Ca++ ions per molecule of ATP hydrolyzed was increased 3-fold by ryanodine to a value of approximately 1. Ryanodine stimulated net Ca++ accumulation at all Ca++ concentrations tested, but the effect was most marked at an ionized Ca++ concentration between 10-6 M and 10-5 M. The Mg++ concentration (1-6 mM), 3 mM ATP) had little effect on the stimulation of Ca++ uptake by 10-4 M ryanodine, and increasing the oxalate concentration to 6 mM did not alter the magnitude of stimulation of uptake by ryanodine. When oxalate was omitted from the incubation medium, K+,Ca++-ATPase activity was inhibited 50% by 10-4 M ryanodine, but this effect was completely reversed by addition of the Ca++-specific ionophore, A23187, which eliminated intravesicular accumulation of Ca++. The results suggest that ryanodine increases Ca++ uptake by cardiac sarcoplasmic reticulum vesicles by decreasing the rate of Ca++ efflux from the vesicles. No direct effect of the drug was observed on the turnover of the Ca++ pump or K+,Ca++-ATPase activity.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Pharmacology and Experimental Therapeutics|
|State||Published - Jan 1 1979|
ASJC Scopus subject areas
- Molecular Medicine