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

在分离的犬颌下腺细胞上，研究了肾上腺素能和胆碱能刺激粘蛋白释放和血小板活化因子(PAF)的产生以及钙离子的作用。Ach的分泌反应比肾上腺素能激动剂明显，并主要依赖于外源Ca~(2+)的存在，但对胞外Na~+的依赖性很小。离子霉素还能刺激细胞释放粘蛋白。毒扁豆碱胆碱能激动剂和离子霉素都不是大鼠颌下腺细胞中有效的β-肾上腺素能激动剂。EGTA与胞外Ca~(2+)~(2+)~(2+)结合可减少α-肾上腺素能受体介导的释放。β-肾上腺素能受体介导的反应因细胞大量暴露于EGTA而减弱，至少部分原因是由于需要钙离子…更多的是肾上腺素能刺激cAMP的形成。8-br-cAMP可刺激预先加载的细胞释放粘蛋白。离子霉素可消除8-br-cAMP诱导的粘蛋白释放。但不受细胞外钙离子螯合、TMB-8或负载BAPTA的细胞的抑制作用。这些结果提示，不仅肾上腺素能系统参与了犬颌下腺粘蛋白释放的调节，而且M胆碱能系统也可能参与了犬颌下腺粘蛋白释放的调节，并且可能除了cAMP介导的机制外，还可能有钙依赖的机制参与犬颌下腺的粘液分泌。PAF的这种产生是浓度和时间依赖的，并被抗胆碱能药物预处理所抑制。预先用ACh刺激细胞所积累的PAF随着阿托品的加入迅速消失。苯甲基磺酰氟在未受刺激的细胞中产生PAF的积聚，并大大增强ACh诱导的进一步积聚。ACh诱导的PAF生成和花生四烯酸(AA)释放随细胞外Ca~(2+)浓度的升高而增加，无Ca~(2+)时ACh不能刺激PAF的形成，但ACh仍能刺激[^&lt；14&gt；C]AA的释放。完整细胞中的钙离子载体离子霉素和洋地黄素通透性细胞中的钙离子载体(浓度大于或等于30 nM)均促进PAF的形成。Lyso-PAF：乙酰辅酶A乙酰转移酶活性在ACh或离子霉素孵育的细胞中迅速增加。这些结果表明，ACh诱导的PAF合成增加至少与重塑途径的刺激有关。ACh也能激活二硫苏糖醇不敏感的胆碱磷酸转移酶活性。然而，ACh对这两种酶的激活都是短暂的，尽管ACh刺激的PAF的形成是连续的。这可能表明，这些酶的激活以外的其他机制(S)在控制PAF的合成中起着重要作用。本研究进一步证明，犬颌下腺外分泌细胞能够以钙依赖的方式在刺激时增加PAF的周转率，并将PAF保留在部分与细胞膜相关的细胞内，部分释放于胞浆中。较少

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