Effects of inorganic and organic mercury on intracellular calcium levels in rat t lymphocytes

Abstract

The importance of cytosolic free calcium level ([Ca²⁺]i) in lymphocyte activation prompted us to investigate changes in [Ca²⁺]i in T cells caused by mercury compounds, which have been shown to have immunomodulatory and immunotoxic properties. Using fura‐2 as fluorescent Ca²⁺ indicator, we found that both methyl‐mercury (MeHg; 0.02–2 μM) and inorganic mercury (HgCI₂; 0.01–1 μM) increased [Ca²⁺]i in lymphocytes from rat spleen in a concentration‐dependent manner. The effect of MeHg was rapid and the increase of Ca²⁺ level was sustained in time, while HgCI₂ caused a slow rise in [Ca²⁺]i. The effects of mercury compounds did not appear to be associated with alterations of membrane integrity, since there was no significant difference in the extent of MnCI₂ quench between control and mercury‐treated cells. However, HgCI₂ (1 μM) and MeHg (2 μM) appeared to cause membrane damage at longer incubation times (15 min). When cells were incubated in Ca²⁺‐free medium (in the presence of 1 mM EDTA) MeHg still increased [Ca²⁺]i, though to a lesser extent, while HgCI₂ had no effect. Heparin, an inhibitor of inositol 1,4,5,‐trisphosphate‐induced Ca²⁺ mobilization partially blocked this rise of [Ca²⁺]i, while carbonyl cyanide m‐chlorophenylhydraxone (CCCP), an inhibitor of mitochondrial function, had a lesser effect. When added together, heparin and CCCP almost completely block the response to MeHg. These results suggest that MeHg and HgCI₂ exert their effects of [Ca²⁺]i in different ways: MeHg‐induced increases in [Ca²⁺]i are due to influx from outside the cells as well as to mobilization from intracellular stores, possibly the endoplasmic reticulum, and, to a minor extent, the mitochondria; on the other hand, HgCI₂ causes only Ca²⁺ influx from the extracellular medium.