Surface binding and uptake of cadmium (Cd2+) by LLC-PK1 cells on permeable membrane supports

Recent studies have shown that Cd²⁺ has relatively specific damaging effects on cell-cell junctions in the renal epithelial cell line, LLC-PK₁. The objective of the present studies was to examine the surface binding and uptake of Cd²⁺ by LLC-PK₁ cells in relation to the disruption of cell-cell junctions. LLC-PK₁ cells on Falcon Cell Culture Inserts were exposed to CdCl₂ containing trace amounts of¹⁰⁹Cd²⁺ from either the apical or the basolateral compartments, and the accumulation of¹⁰⁹Cd²⁺ was monitored for up to 8 h. The integrity of cell-cell junctions was assessed by monitoring the transepithelial electrical resistance. The results showed that the cells accumulated 3–4 times more Cd²⁺ from the basolateral compartment than from the apical compartment. The accumulation of Cd²⁺ from the basolateral compartment occurred in two phases: a rapid, exponential phase that occurred in 1–2 h and coincided with a decrease in transepithelial resistance, and a slower, linear phase that continued for 6–8 h. The Cd²⁺ that accumulated during the rapid phase was easily removed by washing the cells in EGTA, indicating that most of it was bound to sites on the cell surface. By contrast, most of the Cd²⁺ that accumulated during the slower phase could not be removed by EGTA, indicating that it had been taken up by the cells. Additional studies showed that the rapid phase of Cd²⁺ accumulation was enhanced when Ca²⁺ was present at low concentrations (0.1 mM), and was greatly reduced when Ca²⁺ was present at high concentrations (10 mM). These results suggest that Cd²⁺ damages the junctions between LLC-PK₁ cells by interacting with Ca²⁺-sensitive sites on the basolateral cell surface.