Effect of cadmium on active ion transport and cytotoxicity in cultured renal epithelial cells (A6)

Abstract

A cultured epithelial cell line from toad kidney (A6) was used to study the mechanism by which cadmium (Cd) affects transepithelial resistance (TER) and active transepithelial ion transport measured as short-circuit current (SCC) in vitro. The influence of Cd on cell integrity was investigated by measuring time-dependent TER under controlled conditions and the half-maximal inhibition concentration (IC 50) 24 hr after exposure to 1 m m CdCl 2. The data suggest that Cd deterioration of cell integrity is stronger when applied to the apical relative to the basolateral solution (IC 50 = 173.9 and 147.8 μ M, respectively). Also, the data demonstrate that addition of Cd to the basolateral solution results in a prompt and transient stimulation of the active ion transport from 2.6 ± 0.4 to 8.7 ± 1.1 μA/cm 2. Use of the sodium channel blocker amiloride indicate that Na transport is not involved in Cd-stimulated SCC. Substitution of Cl with SO 4 2− in the basolateral solution and use of the Cl channel inhibitors, diphenylamine-2-carboxylase (DPC) and niflumic acid indicate strongly that Cd increases Cl secretion in A6 epithelium. Thapsigargin (TG), an intracellular Ca-ATPase blocker, inhibits Cd-stimulated active ion transport indicating that Ca-activated Cl channels are probably involved. Therefore, we suggest that Cd by interaction with the basolateral membrane, become internalized and increase Ca intracellularly. In a dose- and time-dependent way an increase in Ca activates specific Cl channels leading to an increased SCC. Thereafter, the increase in Ca leads to disruption of tight junctions thereby decreasing TER. This may lead to deterioration of cell integrity and perhaps even cell death.