ROLE OF INTRACELLULAR CALCIUM IN HYDROGEN PEROXIDE-INDUCED RENAL TUBULAR CELL INJURY

Both reactive oxygen metabolites and calcium have been implicated in ischemic and toxic renal tubular cell injury. However, the role of calcium in oxidant injury to renal tubular cells has not been previously examined. In the present study we examined the role of intracellular free Ca2+ ([Ca2+]i) in H2O2-mediated injury to LLC-PK1 cells, a renal tubular epithelial cell line. H2O2 induced a significant rise in [Ca2+]i within 1 min after exposure of cells to 5 mM H2O2, with a sustained rise in [Ca2+]i during the course of experiments, reaching a value of 1.3-mu-M at 60 min (n = 10). The rise in [Ca2+]i preceded sublethal cell injury as measured by [H-3]adenine release or irreversible cell injury as determined by trypan blue exclusion. Buffering [Ca2+]i with quin-2 (50-mu-M) and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAP-TA, 50-mu-M) was highly protective against the H2O2-induced cell injury. For example, at 120 min after exposure of cells to 5 mM H2O2, irreversible cell injury was reduced from 45 +/- 8 to 9 +/- 1% (n = 3) by quin-2. The acetoxymethyl ester of quin-2 (quin-2/AM) and BAPTA/AM did not interfere with the trypan blue exclusion assay or scavenge H2O2. Preventing mobilization of Ca2+ from intracellular storage sites using 8-(N,N-dimethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8, 10(-4) M) significantly reduced the rise in [Ca2+]i and thus prevented H2O2-mediated cytotoxicity to LLC-PK1 cells. In contrast, omission of Ca2+ from the medium or addition of verapamil, a voltage-gated Ca2+-channel blocker, to a cell suspension prepared in Ca2+-replete buffer has little effect on the H2O2-induced rise in [Ca2+]i or cytotoxicity. A Ca2+ ionophore, ionomycin, in a dose that caused a similar rise in [Ca2+]i produced cytotoxicity to LLC-PK1 cells, and quin-2 was also protective against ionomycin-induced cytotoxicity. Taken together, our data indicate an important role of [Ca2+]i in H2O2-mediated cytotoxicity to LLC-PK1 cells.