Comparative effects of Cd2+ and Cd-metallothionein on cultured kidney tubule cells

The effects of Ca2(+) and Cd-metallothionein on two cultured cells with proximal tubule characteristics, mouse kidney cortical cells and pig kidney LLC-PK1 cells, have been compared. Cd2+ inhibits Na(+)-glucose cotransport in LLC-PK1 cells and in the process decreases the number of binding sites for [3H]phlorizin, a competitive inhibitor of glucose for the Na(+)-glucose cotransporter. During 24 hr incubation and over a range of concentrations in the two cell types, only Cd2+ inhibited Na(+)-glucose cotransport even when approximately equal concentrations of intracellular Cd resulted from these treatments. Indeed, at low concentrations of Cd-metallothionein in mouse cells, transporter activity was elevated. Extension of incubations to 72 hr in mouse cells led to increased Cd uptake and reduction in cell density with both sources of Cd but only a progressive decline in Na(+)-glucose cotransport activity with Cd2+. Zn-metallothionein was without effect under comparable conditions. Both forms of Cd were accumulated by these cells, with the large majority of the metal ion localizing in metallothionein as a Cd, Zn-protein in LLC-PK1 cells. Under equal exposure conditions, the net uptake of Cd from Cd-metallothionein in the two cell types. It is evident that the mechanisms of toxicity of Cd2+ and Cd-metallothionein as well as their modes of uptake differ in these two cell types.