EXOCYTOSIS FROM RAT SUBMANDIBULAR GRANULAR TUBULES DURING CYCLOCYTIDINE STIMULATION SHOWS UNUSUAL FEATURES, INCLUDING CHANGES IN THE GRANULE MEMBRANE

Sequential secretory changes in granular tubule cells caused by the se cretagogue cyclocytidine (75 mg/kg i.p.) were studied at the ultrastru ctural level, in perfusion (n = 5 animals) and immersion (n = 8 animal s) fixed rat submandibular glands, using the periodic acid-thiocarbohy drazide-silver proteinate technique (PA-TCH-SP). The onset of secretio n varied from 45 to 75 minutes after administering the cyclocytidine. During the initial stages of overt secretion, structural changes occur red irregularly in a progressive fashion with: (1) an increase in gran ule membrane staining with PA-TCH-SP and a parallel alignment of the s ecretory granules with the adjacent apical plasma membrane, which deve loped a honeycomblike appearance; (2) docking of these secretory granu les to the apical plasma membrane; (3) early secretion of some secreto ry granules in a semiclassical exocytotic fashion (but this was rarely witnessed). During stages (1) and (2), the cytochemical characteristi cs of the membrane of the secretory granules, as well as of the plasma membrane, suggest a priming process is occurring. After these initial preparatory phases, further structural changes occurred in the granul e membranes with a gradually progressive formation of microvesicles an d granule fusions; secretion continued in an explosive manner with pro teinaceous material being transferred to lumina in at least three diff erent ways: (1) by typical exocytosis (but it was infrequent); (2) fro m granules fused intracellularly into aggregates (compound exocytosis) ; and (3) some apocrine-type of secretion through bleb formation. The formation of these intracellular aggregations was associated with the microvesicles in the granule membranes and some aggregates became very large. Secretion of their contents into lumina occurred through elong ated membrane channels. The material secreted included microvesicular forms that had become interiorised in the granular aggregates, and any cytoplasm that may also have been entrapped. (C) 1996 Wiley-Liss, Inc .