The heavy metal ions Ag⁺ and Hg²⁺ trigger calcium release from cardiac sarcoplasmic reticulum

Heavy metal ions have been shown to induce Ca²⁺ release from skeletal sarcoplasmic reticulum (SR) by binding to free sulfhydryl groups on a Ca²⁺ channel protein and are now examined in cardiac SR. Ag⁺ and Hg²⁺ (at 10-25 μm) induced Ca²⁺ release from isolated canine cardiac SR vesicles whereas Ni²⁺, Cd²⁺, and Cu²⁺ had no effect at up to 200 μm. Ag²⁺-induced Ca²⁺ release was measured in the presence of modulators of SR Ca²⁺ release was compared to Ca²⁺-induced Ca²⁺ release and was found to have the following characteristics, (i) Ag⁺-induced Ca²⁺ release was dependent on free [Mg²⁺], such that rates of efflux from actively loaded SR vesicles increased by 40% in 0.2 to 1.0 mm Mg²⁺ and decreased by 50% from 1.0 to 10.0 mm Mg²⁺. (ii) Ruthenium red (2-20 μm) and tetracaine (0.2-1.0 mm), known inhibitors of SR Ca²⁺ release, inhibited Ag⁺-induced Ca²⁺ release, (iii) Adenine nucleotides such as cAMP (0.25-2.0 mm) enhanced Ca²⁺-induced Ca²⁺ release, and stimulated Ag⁺-induced Ca²⁺ release. (iv) Low Ag⁺ to SR protein ratios (5-50 nmol Ag⁺/mg protein) stimulated Ca²⁺-dependent ATPase activity in Triton X-100-uncoupled SR vesicles. (v) At higher ratios of Ag⁺ to SR proteins (50-250 nmol Ag⁺/mg protein), the rate of Ca²⁺ efflux declined and Ca²⁺-dependent ATPase activity decreased gradually, up to a maximum of 50% inhibition, (vi) Ag⁺ stimulated Ca²⁺ efflux from passively loaded SR vesicles (i.e., in the absence of ATP and functional Ca²⁺ pumps), indicating a site of action distinct from the SR Ca²⁺ pump. Thus, at low Ag²⁺ to SR protein ratios, Ag²⁺ is very selective for the Ca²⁺ release channel. At higher ratios, this selectivity declines as Ag⁺ also inhibits the activity of Ca²⁺,Mg²⁺-ATPase pumps. Ag⁺ most likely binds to one or more sulfhydryl sites "on" or "adjacent" to the physiological Ca²⁺ release channel in cardiac SR to induce Ca²⁺ release.