Inhibition of secretion from isolated rat alveolar epithelial type II cells by the cell permeant calpain inhibitor II (N-acetyl-leucyl-leucyl-methioninal).

Although several signal transduction pathways, including activation of specific protein kinases have been proposed and studied for the secretory processes of lung surfactant from alveolar epithelial type II cells, the role of proteolytic processing by calpains (calcium-activated neutral proteases) in secretion has not been investigated. Therefore, we examined the effect of cell permeable calpain inhibitor I (N-acetyl-leucyl-leucyl-norleucinal) and II (N-acetyl-leucyl-leucyl-methioninal) on secretion to test the hypothesis that calpains participate in the secretory processes of alveolar epithelial type II cells. Calpain inhibitor I preferentially inhibits micro (mu)-calpain while inhibitor II inhibits milli (m)-calpain. Isolated type II cells were prelabelled with [3H]-choline for 18-24 h. To measure secretion, [3H]-labelled disaturated phosphatidylcholine (DSPC) released in the medium was monitored. Basal secretion of DSPC was maximally (87%) depressed by the presence of 10 microM inhibitor II. Secretagogue-stimulated secretion was also modulated by inhibitor II treatment. Stimulation with calcium ionophore A23187 enhanced secretion 3-fold. However, cells pre-exposed to inhibitor II displayed a 90% reduction of calcium-stimulated secretion. Terbutaline (10 microM) and ATP (1 mM) each increased secretion 2- and 4-fold, respectively. However, the inhibitor-treated cells, exposed to the same stimuli, attained only 53 or 62% of these increases. Calpain inhibitor I, on the other hand, inhibited neither basal nor stimulated secretion. The results suggest that m-calpain, the major isozyme of lung calpain requiring mM calcium for activity in vitro, is involved in the secretory pathways of alveolar epithelial type II cells.