EVENTS IN THE LUMINAL MEMBRANE DURING THE EARLY STAGE OF SECRETION IN RAT PAROTID ACINAR CELLS: AN IMMUNOCYTOCHEMICAL STUDY

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Movements of the luminal plasma membrane during the early stage of secretion in rat parotid acinar cells were investigated by an immunocytochemical technique using dipeptidyl peptidase (DPP) IV as a marker. In acinar cells stimulated by a single intraperitoneal injection of isoproterenol, numerous pits and tubular invaginations rapidly appeared in the luminal plasmamembrane preceding exocytosis. At a subsequent stage, membranes of these invaginations came into contact and fused with those of secretory granules, and the contents of granules were discharged into the lumen through pits and tubular invaginations. After the termination of discharge, the membrane of emptied granules acquires the property of the luminal plasma membrane with regard to DPP IV immunoreactivity. At later stages of secretion, membranes of discharged granules fused successively with those of neighbouring undischarged granules, resulting in an enlargement of the lumen. The results indicate that the luminal plasma membrane of the acinar cell plays a significant role in the process of secretion in the parotid gland.