The Mucosecretion in the Submandibular Gland and the Role of Platelet-Activating Factor.

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Adrenergic and cholinergic stimulation of mucin release and platelet-activating factor (PAF) production and the role of Ca^<2+> have been investigated in the isolated dog submandibular gland cells.Stimulation of muscarinic cholinergic, alpha-adrenergic and beta-adrenergic receptors elicited mucin release from dispersed dog submandibular gland cells. The secretory response to Ach was much pronounced than those to adrenergic agonists, and largely dependent on the presence of extrace llul) Ca^<2+>, but the dependency on the extracellular Na^+ was slight. Ionomycin also stimulated mucin release from the cells. Neither muscarinic cholinergic agonists nor ionomycin were effective mucosecretagogues as beta-adrenergic agonists in rat submandibular gland cells. alpha-adrenoceptor-mediated release was decreased by chelating extracellular Ca^<2+> with EGTA. beta-adrenoceptor-mediated response was diminished by extensive exposure of cells to EGTA, due at least in part to the requirement of Ca^<2+> … More for -adrenocetor stimulation of cAMP formation. 8-br-cAMP stimulated ^<45>Ca^<2+> release from the cells preloaded with ^<45>Ca^<2+>.8-br-cAMP-induced mucin release was eliminated in ionomycin-pretreated cells. But not inhibited by chelating extracellular Ca^<2+> and by the treatment of the cells with TMB-8 or in the cells loaded with BAPTA. These results suggest that not only adrenergic system but also muscarinic cholinergic system may participate in the regulation of mucin release in dog submandibular gland, and also provide the possibility that, in addition to cAMP mediated mechanism, Ca^<2+>-dependent mechanisms may be involved in the mucosecretion in dog submandibular acini.Isolated dog submandibular gland cells synthesize platelet-activating factor (PAF) when stimulated with ACh. This production of PAF was concentration- and time-dependent, and was inhibited by pretreatments with anticholinergic agents. PAF accumulated in cells through prior stimulation with ACh vanished rapidly with the addition of atropine. Phenylmethylsulfonylfluoride produced an accumulation of PAF in non-stimulated cells and greatly potentiated further ACh-induced accumulation. PAF production and [^<14>C] arachidonic acid (AA) liberation induced by ACh were increased by higher concentrations of extracellular Ca^<2+>, and ACh failed to stimulate PAF formation in the absence of Ca^<2+>, though ACh still stimulated the liberation of [^<14>C] AA without Ca^<2+>. Both the calcium ionophor ionomycin in intact cells and Ca^<2+> (at concentrations <greater than or equal> 30 nM) in digitonin-permeabilized cells facilitated PAF formation. Lyso-PAF : acetyl-CoA acetyltransferase activity rapidly increased in cells incubated with ACh or ionomycin. These results suggest, at least, that the stimulation of a remodeling pathway is involved in the increased PAF synthesis induced by ACh. Dithiothreitol-insensitive cholinephosphotransferase activity was also activated by ACh. However, the activation of both enzymes by ACh was transient, in spite of the fact that ACh-stimulated PAF formation was continuous. This may suggest that additional mechanism (s) other than the activation of these enzymes play an important role in controlling PAF synthesis. The present study provides further evidence that exocrine submandibular gland cells of dogs have the capacity to increase PAF turnover upon stimulation in a Ca^<2+>-dependent manner and retain PAF within the cells partly associated with the membrane and partly released in the cytosol. Less