In 30% of the patients with aldosterone-producing adrenal adenomas (APA), somatic mutations affecting the selectivity filter of the potassium channel KCNJ5 have been identified as a cause for APA formation and hyperaldosteronism. These mutations cause a loss of cation selectivity leading to Na⁺ influx. As a consequence, intracellular Ca²⁺ increases and stimulates aldosterone production. This study aimed at investigating the mechanisms that link pathological Na⁺ influx through mutated KCNJ5 (KCNJ5-mut) to increased intracellular Ca²⁺ levels.

As a cellular model, NCI-H295R cells originating from a human adrenal carcinoma were used. In patch clamp experiments, cells transfected with KCNJ5-mut were depolarized due to the pathological Na⁺ conductance. After removal of bath Na⁺, Na⁺ influx was abolished and cells hyperpolarized. In Fura-2 Ca²⁺ measurements, cells transfected with KCNJ5-mut (but not with KCNJ5-wildtype) showed a strong increase of Ca²⁺ when bath Na⁺ was removed. Due to the concomitant hyperpolarization, this Ca²⁺ increase could not be explained by activation of depolarization-activated Ca²⁺ channels. The Ca²⁺ increase after extracellular Na⁺ removal was blocked by the NCX-blocker KB-R7943 indicating that NCX was working in reversed mode (importing Ca²⁺ and extruding Na⁺). Due to high intracellular Na⁺ and the depolarized membrane, KCNJ5-mut transfected cells might indicate reversed NCX mode also at normal bath Na⁺.

These data suggest that mutations of KCNJ5 result in adenoma-promoting increases in cytosolic Ca²⁺ levels by two independent mechanisms: activation of depolarization-activated Ca²⁺ channels and Ca²⁺ influx through NCX working in pathological reversed mode.