Activation of Ca²⁺ release in isolated sarcoplasmic reticulum

The relationship between Ca²⁺ release from sarcoplasmic reticulum, induced by elevated pH, tetraphenylboron (TPB^-) or chemical modification, and the change in the surface charge of the membranes as measured by the fluorescence intensity of anilinonaphthalene sulfonate (ANS) is examined. The stimulated Ca²⁺ release is inhibited by dicyclohexylcarbodiimide and external Ca²⁺. TPB^-, but not tetraphenylarsonium (TPA⁺), causes a decrease in ANS^- fluorescence, with 50% decrease occurring at about 5 μm TPB^-. The decrease in ANS^- fluorescence as well as the inhibition of Ca²⁺ accumulation induced by TPB^- are prevented by TPA⁺. A linear relationship between the decrease in membrane surface potential and the extent of the Ca²⁺ released by TPB^- is obtained. Similar levels of [³H]TPB^- bound to sarcoplasmic reticulum membranes were obtained regardless of whether or not the vesicles have taken up Ca²⁺. The inhibition of Ca²⁺ accumulation and the [³H]TPB^- incorporation into the membranes were correlated. Ca²⁺ release from sarcoplasmic reticulum, by pH elevation, chemical modification or by addition of NaSCN (0.2 to 0.5 m) or the Ca²⁺ ionophore ionomycin, is also accompanied by a decrease in ANS^- fluorescence intensity. However, chemical modification and elevated pH affects the surface potential much less than SCN^- or TPB^- do. These results suggest that the enhancement of Ca²⁺ release by these treatments is not due to a general effect on the membrane surface potential, but rather through the modification of a specific protein. They also suggest that membrane surface charges might play an important role in the control mechanism of Ca²⁺ release.