Ultrastructural characterization of the early changes in intercellular junctions in response to cadmium (Cd2+) exposure in LLC-PK1 cells.

Previous studies have shown that Cd2+ can disrupt the Ca2+-dependent junctions between LLC-PK1 cells. The objective of the present studies was to further characterize the early junctional effects of Cd2+ in LLC-PK1 cells and to identify the initial site of injury. LLC-PK1 cells were grown on permeable membrane supports and were exposed to 10 microM Cd2+ from the basolateral compartment. The integrity of cell junctions was assessed by light and electron microscopy and by measuring the transepithelial electrical resistance. After as little as 15 min of Cd2+ exposure, there was an increase in the amount of light transmitted between the cells. The transepithelial resistance began to decline by 30 min and continued to fall until reaching zero after about 6 hr. Ultrastructural analysis showed that the initial disruption of cell-cell junctions coincided with a decrease in the density of intracellular plaques associated with the adhering junctions (zonulae adherens). This effect increased with time and paralleled an increase in the space between cells and a change in the shape of the cells from squamous to rounded. Sectioning the cells horizontally, in a plane parallel to the membrane support, allowed us to see large areas of zonulae adherens. Cd2+ caused the formation of gaps within the zonulae which increased with time of exposure. No significant changes in most occluding junctions (zonulae occludens) were seen until 6-8 hr after Cd2+ exposure. These results indicate that the adhering junctions and associated cytoplasmic components are primary sites of early Cd2+ injury in LLC-PK1 cells and they suggest that these effects may result from the interaction of Cd2+ with target sites associated with the basolateral cell surface.