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

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Abstract
The relation between membrane potential and Ca2+ fluxes in the skeletal fragmented sarcoplasmic reticulum (FSR) of the bullfrog 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. 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. The presence of both permeable anion and cation was required for induction of Ca2+ release. There apparently 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 [calcimycin]. The addition of valinomycin or nigericin and the replacement of Cl- with gluconate- affected potential formation. Ca2+ uptake apparently is accompanied by K+ efflux and/or Cl- influx for partial compensation of the change.