Silica increases cytosolic calcium and causes cell injury in renal cell lines

The purpose of this study was to clarify the effect of silica-induced cytosolic free calcium mobilization and cell injury in immortalized cell lines from transgenic mice kidney harboring SV40 T-antigen gene. The proximal convoluted tubule (S1)- and the inner medullary collecting tubule (IMCT)-originated cell lines were used. Cytosolic free calcium concentration ([Ca2+]i) was measured employing Fura-2 fluorescence and cell injury was evaluated by a vital dye exclusion procedure. Silica increased [Ca2+]i in a concentration-dependent manner in S1 (60 micrograms/ml-600 micrograms/ml) and IMCT (6 micrograms/ml-600 micrograms/ml). Silica caused a biphasic increase in [Ca2+]i which was composed of an initial rapid rise and following sustained phase. Ca2+ removal from the medium resulted in abolishment of initial and sustained phase of silica (600 micrograms/ml)-induced [Ca2+]i in both cell lines. Silica-induced cell injury was increased in a dose-dependent manner. This silica-induced cell injury was attenuated by the pretreatment with EGTA (100 microM) and nifedipine (1 microM). Cellular ATP content ([ATP]i) by silica also decreased in a concentration-dependent manner. The relationship between [Ca2+]i and [ATP]i showed that [ATP]i depletion caused [Ca2+]i to rise. This study suggests that 1) the elevation of [Ca2+]i caused by silica is due mainly to influx through plasma membrane Ca2+ channel and non specific membrane damage (at high concentration) and 2) nephrotoxicity of silica shows site-specificity within the kidney.