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

Abstract

Ag⁺-induced 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⁺-induced 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 Ca²⁺-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.