Early events in naphthalene-induced acute Clara cell toxicity - Comparisonof membrane permeability and ultrastructure

Naphthalene causes severe dose- and site-selective injury to mouse noncilia ted bronchiolar (Clara) epithelial cells. Toxicity is characterized by exfo liation of injured Clara cells into the airway lumen 24 h after exposure. T he purpose of this study was to define the temporal pattern of intracellula r changes immediately following naphthalene treatment, with the goal of ide ntifying critical early events involved in cytotoxicity. Mice were injected with naphthalene or carrier and were killed 1, 2, 3, and 6 h after treatme nt (PT). Loss of membrane integrity was assessed by ethidium homodimer-1 pe rmeability and confocal microscopy, Cell morphology and ultrastructure were evaluated using high-resolution light and election microscopy. Permeable c ells were found only in terminal bronchioles and increased in abundance wit h time PT. At 2 and 3 h PT, when most Clara cells had early signs of injury , few permeable cells were detected. Many Clara cells had apical membrane b lebs that contained abundant. swollen, smooth endoplasmic reticulum (SER) a nd few other organelles. By 6 h PT many Clara cells were membrane-permeable . However, many permeable Clara cells lacked apical blebs and SER was less abundant in these cells. Cytoplasmic blebbing may be a mechanism to protect the cell by isolating and removing damaged SER. We conclude that the early stages of injury include SER swelling and bleb formation which precede inc reases in cell membrane permeability after acute naphthalene injury to bron chiolar Clara cells in vivo.