HgCl2 is known to be a renal toxin, but its mechanisms of toxicity are not well understood. The cell line LLC-PK1 was used as a model for renal proximal tubule cells, and the effects of different concentrations of HgCl2 were studied. Apoptosis in response to 35 microM HgCl2 was confirmed by observation of morphological features characteristic of apoptotic cells as well as cleavage of chromosomal DNA into fragments of multiples of 200 base pairs. Ten percent of LLC-PK1 cells in a monolayer underwent apoptosis. These cells detached from the culture flask before apoptosis. Measurement of transepithelial resistance (TER) was used as a functional assay of junctional complex integrity in a novel approach to characterize preapoptotic events in this cell line. Monolayers of LLC-PK1 cells that contained apoptotic cells showed a transient decrease in TER followed by a recovery of TER to the initial levels. The decrease in TER was accompanied by a loss of hemicysts within the monolayer. These data indicate a temporary loss of junctional complexes within the monolayer during apoptosis. One hundred micromolar HgCl2 caused all cells to become necrotic within 3 hr. HgCl2 (10 microM) caused some changes in cell morphology, but no cell death.