Ultrastructural histochemical studies of secretory processes in rat submandibular granular tubules during intermittent sympathetic nerve stimulation

Secretory changes in the cells of granular tubules in rat submandibular gla nds have been studied sequentially during electrical stimulation of their s ympathetic nerves. Results were assessed in a series of biopsied lobes from the same gland, taken at different times during the sympathetic stimulatio n. Changes were not synchronous between adjacent cells and it appeared that the time for the onset of secretory events differed between cells but, onc e set in action, a chain of similar events occurred. Nevertheless, some cel ls appeared to remain refractory throughout. Initially, some alignment of g ranules to the adjacent plasma membrane occurred and occasional evidence fo r classical exocytosis was seen. However, from early on microvesicles appea red in more luminally located granule membranes and were associated with gr anule fusions, that became common and led to the formation of large irregul ar aggregates. Most of the secretion of granule contents appeared to be thr ough openings of aggregates into lumina. With granule fusions the intra-mem brane microvesicles became internalised and tended to increase in size with time; they were commonly expelled with the contents of the aggregates. Fra gments of cytoplasm also became incorporated in aggregate formation. Cytopl asm, often containing glycogen, also formed luminal blebs over some granula r tubule cells and appeared to pass into the secretion by an apocrine proce ss. At the end of stimulation multivesicular bodies were seen in associatio n with redundant aggregates.