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. © 1990.