Characterization of Ca2+ release from the cardiac sarcoplasmic reticulum

The characteristics of Ca2+ release in relation to Ca2+ binding were studied in sarcoplasmic reticulum vesicles isolated from canine myocardium. The Ca2+ binding appeared to be dependent on ATP as a 4 fold increase in Ca2+ binding was observed upon the addition of ATP. In the presence of a suboptimal ATP concentration (20 mumol/l; without ATP regenerating system) a rapid release of Ca2+ started within 2 min. The rate of Ca2+ release was increased by increasing the concentration of Ca2+ in the preincubation medium when studied by diluting preloaded vesicles in medium free of Ca2+ and ATP; an apparent saturation was reached at 5 mmol/l Ca2+ but Ca2+ release again increased between 5 and 10 mmol/l Ca2+. High pH (8.0) enhanced the Ca2+ release process. When Ca2+ loaded vesicles were treated with various phospholipases and proteases, an enhanced Ca2+ release was observed in comparison to the control values. The release of Ca2+ was also increased by pharmacological agents like caffeine, ether and halothane. The Ca2+ release rate was stimulated by the p-chloromercurybenzoate treatment, which decreased ATP dependent Ca2+ binding and Ca2+-stimulated ATPase activities of the sarcoplasmic reticulum vesicles. The effect of temperature when evaluated by Arrhenius plots showed a higher energy of activation of Ca2+ release (66.15 kJ/mol) in comparison to that for Ca2+ binding (41.03 kJ/mol). These results indicate that, although Ca2+ release and Ca2+ binding activities of the cardiac sarcoplasmic reticulum appears to be related, Ca2+ release is probably a distinct process and is controlled differently. It seems that the Ca2+ release site in sarcoplasmic reticulum membranes is lipoprotein in nature.