Mercury-induced Ca²⁺ increase and cytotoxicity in renal tubular cells

The effect of mercury (Hg²⁺), a known nephrotoxicant, on intracellular free Ca²⁺ levels ([Ca²⁺]_i) in Madin Darby canine kidney (MDCK) cells was explored. [Ca²⁺] _i was measured by using the Ca²⁺-sensitive dye fura-2. Hg²⁺ increased [Ca²⁺]_i in a concentration-dependent manner with an EC₅₀ of 6 μM. The Ca ²⁺ signal comprised a gradual increase. Removal of extracellular Ca²⁺ decreased the Hg²⁺-induced [Ca²⁺] _i increase by 27%, suggesting that the Ca²⁺ signal was due to both extracellular Ca²⁺ influx and store Ca²⁺ release. In Ca²⁺-free medium, the Hg²⁺-induced [Ca ²⁺]_i increase was nearly abolished by pretreatment with 1 μM thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor), and conversely, pretreatment with Hg²⁺ abolished thapsigargin- induced Ca²⁺ increase. Hg²⁺-induced Ca²⁺ release was not altered by inhibition of phospholipase C but was potentiated by activation of protein kinase C. Overnight treatment with 1 μM Hg ²⁺ did not alter cell proliferation rate and mitochondrial activity, but 10 μM Hg²⁺ killed all cells. Collectively, this study shows that Hg²⁺ induced protein kinase C-regulated [Ca²⁺] _i increases in renal tubular cells via releasing store Ca ²⁺ from the endoplasmic reticulum in a manner independent of phospholipase C activity. Hg²⁺ also caused cytotoxicity at higher concentrations.