Acinar structure and membrane regionalization as a prerequisite for exocrine secretion in the rat submandibular gland

The significance of glandular organization in exocrine secretion was examined by analysing the functional and morphological features of the dissociated rat submandibular gland with special reference to the acinar structure and luminal specialization. The digestion of the gland with collagenase (C preparation) produced relatively large cellular masses having well-preserved acinar structures. When EGTA and the proteolytic enzyme Dispase were added to the C preparation (CED preparation), the gland was dissociated into small cellular aggregates in which the acinar structure disintegrated. Upon stimulation with either isoproterenol or dibutyryl cyclic AMP, a large amount of peroxidase, one of the secretory products of the rat submandibular gland, was released from C-treated cells, while discharged peroxidase was greatly reduced after the CED preparation was used. Measurements of dye exclusion, oxygen consumption, protein synthetic activity and receptor binding, as well as ultrastructural features and the absence of inhibitory effects of EGTA and Dispase, suggested that the reduced secretory response of CED-treated cells was not attributable to cellular death, denaturation of receptors or the inhibitory effects of EGTA and Dispase. When the localization of dipeptidyl aminopeptidase IV was surveyed by both enzyme histochemistry and immuno-histochemistry, the luminal plasma membrane was the exclusive site for the reaction in C-treated cells as well as intact acini, whereas the entire cell surface was reaction-positive in CED-treated cells. In addition, the luminal microfilament system and tight junctions, as revealed by nitrobenz-oxadiazole-phallacidin staining and freeze-replica studies, respectively, were well-preserved in the C-treated cells, but considerably disorganized in the CED-treated cells. All these results strongly suggest that: (1) luminal specialization plays an important role in exocrine secretion; and (2) normal acinar arrangement provides the luminal specialization.