Relationship between Membrane Potential and Calcium Ion Fluxes in the Fragmented Sarcoplasmic Reticulum

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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 Cat+ionophore A23187.The addition of valinomycin or nigericin, and the replacement of Cl^-with gluconate^-affected potential formation.These facts suggest that Cat+uptake is accompanied by K⁺efflux and/or Cl^-influx for partial compensation of the change.