The heavy metal ions Ag + and Hg 2+ trigger calcium release from cardiac sarcoplasmic reticulum

Abstract

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