HgCl2-induced alteration of actin filaments in cultured primary rat proximal tubule epithelial cells labelled with fluorescein phalloidin.

When proximal tubule epithelial cells are exposed to HgCl2, cytoplasmic blebs are formed. These represent an early, potentially reversible response to injury. These blebs are accompanied by reorganization of cytoskeletal proteins, and presumably by alternations in cytoskeletal-plasma membrane interactions. Ca(2+)-activated proteinases, such as calpain, are known to affect cytoskeletal proteins and to be involved in diverse cellular processes. However, the role of calpains in cytotoxicity due to HgCl2 is unknown. To determine the relationship between F-actin, calpain, and HgCl2 toxicity, cells were stained with fluorescein phalloidin before and after treatment with HgCl2. Cells were grown on coverslips and exposed to HgCl2 (10 or 25 microM) in the presence or absence of the calpain inhibitor, leupeptin. Untreated cells were flat, polygonal, and contained many fluorescent-stained cables of actin filaments. Generally, cells exposed to HgCl2 became pleomorphic and contracted as the blebs formed. These cells showed fewer actin cables and fluorescence was seen mostly as either compact areas of dense stain or as peripheral rings. In many cells, actin cables and filaments were completely absent. Disappearance of F-actin was initially seen by 2 min after exposure to HgCl2. Thus, disruption of the actin cytoskeleton and blebbing were found to be early events in HgCl2 toxicity. When leupeptin was used with HgCl2 treatment, the actin staining appeared similar to that of untreated cells.