Relationship between membrane potential and calcium ion fluxes in the fragmented sarcoplasmic reticulum

The relation between membrane potential and Ca2+ fluxes in the skeletal fragmented sarcoplasmic reticulum (FSR) was investigated using cyanine dye (diS-C3-(5) as a potential probe. Change of membrane potential and Ca2+ release from FSR were induced by changing the ionic composition of the medium. When the medium was changed from K-gluconate to Tris-gluconate, a large fluorescence intensity change was observed, while no Ca2+ release occurred. On the other hand, when the medium was changed from K-gluconate to KCl, Ca2+ release occurred notwithstanding a relatively small fluorescence intensity change. This KCl-induced Ca2+ release from FSR was inhibited by the addition of sucrose. Also, the presence of both permeable anion and cation was required for induction of Ca2+ release. These results suggest that there is no correlation between "depolarization" and Ca2+ release. Ca2+ uptake by FSR created the positive-inside potential which was reversed by the application of Ca2+ ionophore A23187. The addition of valinomycin or nigericin, and the replacement of Cl- with gluconate-affected potential formation. These facts suggest that Ca2+ uptake is accompanied by K+ efflux and/or Cl- influx for partial compensation of the change.