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The effects of the heavy-metal ions Cd2+ and Zn2+ on the homoeostasis of intracellular free Ca2+ in E367 neuroblastoma cells were examined using 19F-NMR spectroscopy with the fluorinated chelator probe 1,2-bis-(2-amino-5-fluorophenoxy)ethane-N,N,N', N'-tetra-acetic acid (5F-BAPTA). First, the technique was used to quantify the uptake and intracellular free concentrations of the heavy metals after treatment of the cells with 20 microM CdCl2 or 100 microM ZnCl2. Secondly, metal-induced transients in intracellular free Ca2+ were recorded. Addition of 20 microM CdCl2, but not 100 microM ZnCl2, evoked a transient increase in Ca2+ from a resting level of 84 nM to approx. 190 nM within 15 min after addition of the metal. Zn2+ at 20 microM completely prevented the induction of a Ca2+ transient by Cd2+. Ca2+ was mobilized by Cd2+ from intracellular organelles, since depletion of these stores by thapsigargin abolished the effect of the toxic metal. Furthermore, 20 microM Cd2+ evoked a transient rise in cellular Ins(1,4,5)P3, reaching a maximum level within 5 min after addition of the metal. These results demonstrate that perturbation of the Ins(1,4,5)P3/Ca2+ messenger system is an early and discrete cellular effect of Cd2+.

The effect of Cd^2+, Pb^2+ and Hg^2+ on the Ca^2+-ATPase activity of sarcoplasmic reticulum from rabbit muscle was studied. The concentration of relevant free and complex species for the assay conditions have been computed. As a result, ATP hydrolysis was found to be inhibited with an IC(50) value of 950 nmol/l free Cd^2+ or 95 nmol/l free Pb^2+. Although calculation of the free Hg2+ was not possible, the comparison of the IC50 values for total metal ions show that Hg^2+ is the strongest inhibitor of enzyme activity. The inhibition by Cd^2+ seems to be independent of substrate concentration, whereas the inhibitory effect of Pb^2+ is lowered in the presence of higher MgATP concentrations. Our data illustrate that the three heavy metals are potent inhibitors of the Ca^2+ pump. Therefore low concentrations of these metal ions may disturb intracellular Ca^2+ homeostasis and act on Ca^2+-mediated cell functions.

The bioavailability of carcinogenic compounds plays an important role in the process of oncogenesis. Chromium in its hexavalent oxidation state is classified as carcinogen to humans. Therefore we studied the uptake of chromate(VI) into Chinese hamster V79 cells and nuclei isolated after the incubation of the whole cells with chromate. The chromium content of cells and nuclei was determined by atomic absorption spectroscopy. Chromate is taken up in a concentration dependent manner and accumulates to about 30 fold over the extracellular concentration of 0.1 mmol/l. Incubating whole cells with the same concentration results in an intranuclear concentration of up to 6 mmol/l after 3 h. The kinetic parameters (KM = 0.34 mmol/l, Vmax = 0.12 fmol per cell and minute) are in the same order of magnitude as previously published data. The consequences of the high intracellular and intranuclear concentrations are discussed in terms of the genotoxic effects.

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