Cadmium-induced Inhibition of ADH-stimulated Ion Transport in Cultured Kidney-derived Epithelial Cells (A6)

Abstract

An epithelial cell line (A6) derived from the distal tubule of toad kidney, was used to study the effect of cadmium (Cd²⁺) on the increase in active ion transport induced by antidiuretic hormone (ADH). Addition of Cd²⁺ (1mM) to the basolateral solution of A6 epithelia generated an immediate and transient increase in active ion transport, measured as short circuit current (SCC). This increase was not affected by prior addition of ADH. However, there was a distinct inhibition of ADH-induced stimulation of SCC in epithelia pre-treated with Cd²⁺. Since cAMP serves as an intracellular messenger for ADH by increasing the ion permeability of the apical membrane in A6 epithelial cells, the effects of Cd²⁺ on enzymes involved in cAMP metabolism were measured. The results showed that Cd²⁺ markedly inhibits cAMP production by inhibiting adenylate cyclase (which had been stimulated with forskolin, magnesium or a non-hydrolysed GTP-analog), indicating that Cd²⁺ inhibits the catalytic subunit of adenylate cyclase. Furthermore, degradation of cAMP by phosphodiesterase was not stimulated by Cd²⁺, also suggesting that the mechanism by which Cd²⁺ inhibits the ADH-induced ion transport could be through inhibition of adenylate cyclase. Taken together, these results indicate that, in addition to the well-known toxic effect on the proximal tubule, Cd²⁺ could also have an effect on the distal part of the kidney, where the important hormonal regulation of salt and water homeostasis takes place.