Effects of Silver Ion on the Calcium-Induced Calcium Release Channel in Isolated Sarcoplasmic Reticulum1

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Abstract
Ag+ Ca2+ release in isolated sarcoplasmic reticulum (SR) was studied by the stopped flow method monitoring chlortetracycline fluorescence change. After improving the experimental procedure, the initial rate of Ca2+ release could be determined more precisely than before. Micromolar concentrations of Ag+ specifically enhanced Ca2+ efflux from heavy fraction of SR vesicles (HSR). This specific effect was referred to as Ag+-induced calcium release. The Ag+ Ca2+ efflux was activated by caffeine and ATP, but was inhibited by Mg2+ and procaine. Further, Ag+ enhanced the Ca2+-induced Ca2+ release over the whole range of Ca2+ concentrations, similarly to ATP. Parallel to Ca2+ efflux, Mg2+ efflux, measured by the same method, was also activated by Ag+ Choline permeability determined by the light scattering method was also activated by Ag+ The results suggest that Ag+ binds to the activation site of the Ca+-induced Ca2+ release channel and opens the channel. The Ag+ binding site is different from the Ca2+ binding site but similar to the ATP binding site.