Control of salivary phospholipid content and composition

The mediation of phospholipid secretion in rat sublingual salivary gland cells maintained in the presence of [3H]choline was investigated. The secretion of [3H]choline-containing phospholipids was enhanced by beta-adrenergic agonist, isoproterenol, to a greater extent than the cholinergic agonist carbachol. A 2.9-fold increase in phospholipid secretion occurred with isoproterenol, while carbachol evoked only about 1.3-fold increase. In contrast to carbachol, the enhanced phospholipid secretion due to isoproterenol was accompanied by an increase in cAMP concentration. The secretion of phospholipids was also stimulated by dibutyryl-cAMP and the protein kinase C activator, phorbol myristate acetate, but not by 4 alpha-phorbol 12, 13-didecanoate which does not activate protein kinase C. Furthermore, the effects of dibutyryl-cAMP and phorbol myristate acetate were additive. The phospholipids secreted in response to isoproterenol exhibited a 52% decrease in lysophosphatidylcholine, while those secreted in response to carbachol showed a 23% lower content of phosphatidylcholine, and were enriched in lysophosphatidylcholine (2.8-fold) and sphingomyelin (1.4-fold). The results suggest that salivary phospholipid secretion remains mainly under beta-adrenergic control, while the phospholipid makeup is under cholinergic regulation.