I Im mm mu un no oc cy yt to oc ch he em mi ic ca al l e ev vi id de en nc ce e f fo or r e ex xt tr ra a--c ce el ll lu ul la ar r i in ni it ti ia at ti io on n o of f e el la as st ta as se e--i in nd du uc ce ed d b br ro on nc ch hi ia al l s se ec cr re et to or ry y c ce el ll l m me et ta ap pl la as si ia a i in n h ha am ms st te er rs s ABSTRACT: The bronchus is the only region of the hamster conducting airways to develop secretory cell metaplasia after an intratracheal instillation of human neutrophil elastase (HNE). We tested the hypothesis that this pathological change occurs because of cellular uptake of the enzyme that is specific to this region. HNE, dissolved in saline, was instilled into the trachea of hamsters, that were sacrificed 5, 15, 30 or 60 min later for immunocytochemical localization of the enzyme. Saline-treated animals served as controls.By light microscopy, HNE was evident only in the lumen and upon the epithelial surface in all airways, at all time points. Saline control tissues were negative. Electron microscopic immunogold staining revealed HNE within luminal macrophages and associated with mucus and, to a limited extent, upon the apical cell surface both in trachea and bronchus. A small amount of HNE staining occurred in the intercellular space and lamina propria of bronchi. Cytoplasmic gold particles were sparse both in treated and control animals.We conclude that instilled neutrophil elastase is excluded from the epithelial cytoplasm regardless of region. We thus reject the hypothesis of airway cellular uptake of HNE and suggest that stimulation of bronchial secretory cells to accumulate mucin granules is initiated at the cell surface, possibly by unmasking or altering region-specific receptors involved in signal transduction pathways governing mucin granule synthesis.