Calcium ionophores injure alveolar epithelial cells: Relation to phospholipase activity

Phospholipases and certain of their hydrolytic products are toxic to alveolar epithelial cells. Since many intracellular phospholipases are Ca²⁺ dependent, we postulated that elevating cytosolic Ca²⁺ with ionophores might cause epithelial injury via phospholipase activation. Isolated perfused hamster lungs exposed to an Ca²⁺ ionophore A23187 develop functional evidence of severe epithelial injury. Ultrastructural studies show widespread lysis of type I epithelial cells, with only minimal abnormalities in other lung cells, including the microvascular endothelium. Analysis of whole lung lipid extracts reveals a modest elevation in free arachidonic acid but no changes in other putative products of phospholipase activity. Parallel studies were performed in cultured cells of pulmonary origin. As measured by ⁵¹Cr release, A23187 causes substantial cytotoxicity in 3-day-old cultures of rat type II alveolar epithelial cells (RAEC) but not in cultured bovine pulmonary artery endothelial cells (BPAEC). RAEC prelabeled with [¹⁴C] stearic acid ([¹⁴C] SA) and [³H] arachidonic acid ([³H]AA) release radiolabeled free fatty acids (FFA) in response to A23187 in a dose- and time-dependent manner that parallels the cytotoxicity index. Analyses of putative phospholipase products in cells radiolabeled with [¹⁴C] SA and [³H]AA, with [¹⁴C] choline, or with [¹⁴C] ethanolamine suggest that liberation of radiolabeled FFA may be due to several phospholipases but with principal activity being exhibited by a phospholipase C having specificity toward phosphatidylcholine and phosphatidylethanolamine. Prelabeled BPAEC release only minimal quantities of FFA in response A23187 under the same conditions. These studies demonstrate that elevations of intracytoplasmic Ca²⁺ are capable of severely and selectively damaging alveolar epithelial cells and that the injury is associated with activation of intracellular phospholipases. These findings may have implications in regard to the pathogenesis of acute lung injury in humans.