Acetylcholine-like effects of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine ('platelet-activating factor') and its analogues in exocrine secretory glands

Therapy for acute pancreatitis with platelet-activating factor receptor antagonists

Acute pancreatitis (AP) causes release of platelet-activating factor (PAF), which induces systemic effects that contribute to circulatory disturbances and multiple organ failure. PAF is a cell surface secretion of bioactive lipid, which could produce physiological and pathological effects by binding to its cell surface receptor called platelet-activating factor receptor (PAF-R). Studies showed that PAF participates in the occurrence and development of AP and administration of platelet-activating factor receptor antagonists (PAF-RAs) could significantly reduce local and systemic events after AP. PAF has also been implicated as a key mediator in the progression of severe AP, which can lead to complications and unacceptably high mortality rates. Several classes of PAF-RA show PAF-RAs significant local and systemic effects on reducing inflammatory changes. As a preventive treatment, PAF-RA could block a series of PAF-mediated inflammatory injury and thus improve the prognosis of AP. This review introduces the important role of PAF-RA in the treatment of AP.

PAF, a putative mediator of oral inflammation

PAF, or platelet-activating factor, is a family of structurally related phospholipids (1-O-alkyl/acyl/alkenyl-2-acetyl-sn-glycero-3-phosphocholine) which possesses a wide spectrum of potent pro-inflammatory actions. These phospholipids are synthesized by a diverse array of cells, including neutrophilic polymorphonuclear leukocytes (PMN), platelets, mast cells, monocytes/macrophages, vascular endothelial cells, and lymphocytes. PAF targets these and other cells via specific, G-protein-coupled receptors to initiate intracrine, autocrine, paracrine, and juxtacrine cell activation. Of importance, these unique acetylated phospholipids are frequently synthesized in concert with pro-inflammatory lipid mediators derived from arachidonic acid. Since PAF synergizes with these and other mediators to amplify the inflammatory response, it seems likely that PAF plays an integral, perhaps pivotal, role in acute and chronic inflammatory processes. PAF is present in the mixed saliva of dentate, but not edentulous, human subjects. The levels of PAF in mixed saliva or in gingival crevicular fluid and tissues are significantly increased during oral inflammatory conditions such as periodontitis and mucositis. Interestingly, the levels of salivary PAF correlate with the extent/severity of these oral diseases. These observations suggest that PAF may participate in pathophysiologic events during the course of oral inflammation. The availability of specific PAF receptor antagonists and human recombinant PAF-acetylhydrolase (PAF-AH), a plasma enzyme which rapidly destroys PAF, should provide clinical tools for the investigation of the role of PAF in these and other inflammatory disorders; and perhaps, ultimately, some of these reagents may prove to be therapeutically useful in the treatment and management of these conditions.

Inhibition of Na+,K(+)-ATPase by platelet-activating factor in dog submandibular glands

Physiological stimulation of dog submandibular gland has been shown to generate platelet-activating factor (PAF). However, PAF is not released from cells in the tissue. To assess its intracellular activity, the effect of PAF on Na+,K(+)-ATPase was examined in dog submandibular gland cells. PAF inhibited Na+,K(+)-ATPase in membrane preparations, and the inhibitory effect was dependent on the protein concentration in the enzyme preparation. The inhibitory effect of a low concentration of PAF was antagonized by a PAF-receptor antagonist, BN 50,739, but at high concentrations, PAF was not antagonized. Kinetic analysis of PAF inhibition of Na+,K(+)-ATPase suggests that the inhibition of Na+,K(+)-ATPase by PAF is not due to competition by PAF at K(+)- or Na(+)-binding sites on the enzyme, but by complex inhibitory mechanisms. These results suggest that PAF may interact with specific and nonspecific site of action resulting in the inhibition of Na+,K(+)-ATPase. Ouabain increased mucin release from dog submandibular gland cells. Because Na+,K(+)-ATPase and ion exchange pathways are important in the secretory responses of acinar cells, PAF may regulate intracellularly the secretory function of acinar cells by modulating Na+,K(+)-ATPase and ionic homeostasis.

Stimulation of platelet-activating factor synthesis by neurotransmitters in salivary glands

Platelet-activating factor (PAF), a phospholipid mediator exhibiting potent biological activities, has been shown to stimulate amylase release from the pancreas and salivary glands. The capacity of salivary glands for PAF biosynthesis in response to stimulation has also been demonstrated. To elucidate the role of PAF in salivary glands, we studied the regulation of platelet-activating factor synthesis by the autonomic nervous system in canine salivary glands. Acetylcholine and ionomycin stimulated PAF production in dispersed cells from parotid, submandibular, and sublingual glands of dogs. Norepinephrine and phenylephrine, but not isoproterenol, also stimulated PAF production in submandibular gland cells. Norepinephrine-induced PAF production was blocked by phentolamine but not by propranolol. Acetylcholine and norepinephrine increased both the PAF production and liberation of [14C]arachidonic acid from cells pre-labeled with [14C]arachidonic acid in the presence of Ca2+ in the medium. These stimulants increased [14C]arachidonic acid liberation without the accompanying production of PAF in Ca(2+)-deprived medium. No activators or inhibitors of protein kinase C produced or affected acetylcholine-induced PAF production. Lyso-PAF:acetyl-CoA acetyltransferase was activated in the cells treated with acetylcholine, norepinephrine, isoproterenol, and 8Br-cyclic AMP. Deprivation of Ca2+ in the medium markedly reduced acetylcholine-induced activation of the transferase, but little affected norepinephrine-, isoproterenol-, and 8Br-cyclic AMP-induced activation. Dithiothreitol-insensitive cholinephosphotransferase activity was also increased by acetylcholine, norepinephrine, isoproterenol, and 8Br-cyclic AMP, and the deprivation of Ca2+ in the medium further increased the activation of the enzyme activity by these agents. These results suggest that PAF synthesis in canine salivary glands is under the control of muscarinic cholinergic and alpha-adrenergic systems via Ca(2+)-dependent remodeling pathways, and that the independent activation of either phospholipase A2 or acetyltransferase is insufficient for PAF production in submandibular gland cells, i.e., the concurrent activation of these enzymes is required.

Role of endogenous platelet-activating factor in the regulation of pancreatic blood flow during caerulein stimulation

The role of endogenous platelet-activating factor (PAF) in the control of pancreatic blood flow during caerulein stimulation was investigated. Pancreatic blood flow in anesthetized rats was measured continuously by laser Doppler flowmetry for 2 h during the intravenous infusion of caerulein (0.25 micrograms/kg per h). Pancreatic blood flow showed a gradual, consistent, and significant increase, reaching 114.2 +/- 2.3% of the basal value after 120 min. Changes in pancreatic blood flow induced by caerulein were completely inhibited by a cholecystokinin (CCK) antagonist (loxiglumide, 5 mg/ kg per h, i.v.) and by a specific PAF antagonist (CV-6209, 1 mg/kg, i.v.-bolus). Systemic blood pressure remained stable in all groups. These results suggest an important role of endogenously yielded PAF in regulating pancreatic blood flow during caerulein stimulation to the pancreas.

Platelet-activating factor mediated potentiation of stimulation-evoked catecholamine release and the rise in intracellular free Ca2+ concentration in adrenal chromaffin cells

The effects of platelet-activating factor (PAF) on catecholamine (CA) release and intracellular free Ca2+ concentration ([Ca2+]i) were studied in cultured bovine adrenal chromaffin cells. PAF (1 nM-1 micron) alone had no effect on [Ca2+]i and basal CA release, but potentiated the [Ca2+]i rise and CA release evoked by acetylcholine (ACh) and by elevated extracellular K+. PAF did not potentiate the responses to caffeine in Ca(2+)-deficient medium or to Bay K 8644. In chromaffin cells pretreated with either BN 50739, tetrodotoxin and amiloride or in Na(+)-deficient medium, PAF failed to potentiate the stimulation-evoked [Ca2+]i rise and CA release. In contrast, neomycin, U 73122, 5-(N-ethyl-N-isopropyl)amiloride or pertussis toxin were ineffective in blocking the potentiating effect of PAF. In a membrane fraction prepared from fresh bovine adrenal medulla, ligand-binding studies using [3H]WEB 2086 identified a PAF-displaceable binding site. These results are consistent with a model in which PAF modulates CA release by activating plasma membrane receptors that can enhance the [Ca2+]i rise via an Na(+)-dependent mechanism.

Analogs of alkyllysophospholipids: chemistry, effects on the molecular level and their consequences for normal and malignant cells

In the search for new approaches to cancer therapy, the first alkyllysophospholipid (ALP) analogs were designed and studied about two decades ago, either as potential immunomodulators or as antimetabolites of phospholipid metabolism. In the meantime, it has been demonstrated that they really act in this way. However, their special importance is based on the fact that, in addition, they interfere with key events of signal transduction, such as hormone (or cytokine)-receptor binding or processing, protein kinase C or phospholipase C function and phosphatidylinositol and calcium metabolism. There are no strict structural requirements for their activity. Differences in the cellular uptake or the state of cellular differentiation seem to be mainly responsible for higher or lower sensitivities of cells towards ALP analogs. Consequences of the molecular effects mentioned on the cellular level are cytostasis, induction of differentiation (while in contrast the effects of known inducers of differentiation such as 12-O-tetradecanoylphorbol-13-acetate are inhibited, probably as a consequence of protein kinase C inhibition) and loss of invasive properties. Already in sublytic concentrations, alterations in the membrane structure were observed, and lysis may begin at concentrations not much higher than those causing the other effects described. Few ALP analogs have already entered clinical studies or are in clinical use. ALP analogs are the only antineoplastic agents that do not act directly on the formation and function of the cellular replication machinery. Therefore, their effects are independent of the proliferative state of the target cells. Because of their interference with cellular regulatory events, including those failing in cancer cells, ALP analogs, beyond their clinical importance, are interesting model compounds for the development of new, more selective drugs for cancer therapy.

Modulation by drugs acting on the autonomic nervous system of platelet-activating factor in the stomach of rats

Platelet-activating factor (PAF), an ether linked choline glycerophospholipid, is a potent initiator of diverse physiological and pathological processes. We have reported that gastric endogenous PAF levels were reduced and the contents of each of its molecular species changed during water-immersion stress in rats (Sugatani J et al., FASEB J 3: 65-70, 1989 and Sugatani J et al., Lipids 26: 1347-1353, 1991). In this study, we determined the effects of autonomic drugs on the level of gastric PAF, its molecular heterogeneity and formation of gastric erosions in unstressed rats and those subjected to water-immersion stress. Atropine, an anticholinergic drug, suppressed both the stress-induced changes and development of gastric lesions. 6-Hydroxydopamine-induced sympathectomy induced a small decrease in the gastric PAF levels and the addition of stress further decreased the PAF levels and development of gastric lesions. Carbamylcholine induced a transient decrease in the gastric PAF level of normal rats, which was not associated with gastric erosion formation. In contrast, the endogenous gastroprotective factor dopamine evoked transient dose- and time-dependent increases in the gastric PAF levels. These observations indicate that cholinergic muscarinic-receptor activation in rats led to decreases in gastric PAF levels and a prolonged and marked decrease in its level was associated with the development of gastric lesions, and that dopamine increases gastric PAF levels. Gastric endogenous PAF levels are closely associated with the autonomic nervous system and should be considered further in investigations of gastric function.

Influence of a platelet-activating factor antagonist on severe pancreatitis in two experimental models

This study investigates the role of platelet-activating factor (PAF) in experimental pancreatitis. The concentration of PAF quantified in ascites of bile-induced pancreatitis by radioimmunoassay (RIA) ranged from 3.67 +/- 0.39 pmol/mL 2 h to 0.954 +/- 0.39 pmol/mL 10 h after injection of taurocholate. Administration of a potent PAF antagonist, WEB-2170, prior to injection of taurocholate prolonged mean survival time in rats receiving i.v. camostate and albumin (46.4 h, n = 15, vs controls 38.3 h, n = 13). However, the survival rate after 72 h was not improved. The histologically estimated severity of pancreatitis and pancreatic enzymes in blood, tissue, or ascites was not affected. WEB-2170 had no effect on survival when injected simultaneously with taurocholate into the pancreatic duct or given i.v. after induction of pancreatitis (1, 0.1, or 0.01 mg/kg WEB-2170 vs controls). Subcutaneous injection of 10 mg/kg WEB-2170 also did not improve survival in pancreatitis induced by choline-deficient, ethionine-supplemented diet in mice. It is concluded that administration of a PAF antagonist after the onset of severe experimental pancreatitis does not influence its outcome, although activation of PAF may play a role in the pathogenesis of pancreatitis.

Role of platelet activating factor in pathogenesis of acute pancreatitis in rats

The importance of platelet activating factor in acute pancreatitis was examined by determining the tissue content of endogenous platelet activating factor and the protective effects of TCV-309, a highly selective platelet activating factor blocker, against caerulein induced pancreatitis in rats. Infusion of caerulein (10 micrograms/kg/h) for five hours resulted in about 70% increase in pancreatic weight, 22% rise in protein content, 50% reduction in tissue blood flow, nine fold increase in tissue level of platelet activating factor and 165% rise in plasma amylase as well as histological evidence of acute pancreatitis. Such infusion of caerulein in chronic pancreatic fistula rats caused a marked increase in protein output from basal secretion of 10 mg/30 minutes to 40 mg/30 minutes in the first hour of infusion followed by a decline in protein output to 15-20 mg/30 minutes in the following hours of the experiment. Exogenous platelet activating factor (50 micrograms/kg) injected ip produced similar alterations in weight, protein content, blood flow, and histology of the pancreas but the increment in serum amylase was significantly smaller and pancreatic secretion was reduced below the basal level. TCV-309 (50 micrograms/kg) given ip before caerulein or platelet activating factor administration significantly reduced the biochemical and morphological alterations caused by caerulein and abolished those induced by exogenous platelet activating factor. These results indicate that platelet activating factor plays an important role in the pathogenesis of acute pancreatitis probably by reducing the blood flow and increasing vascular permeability in the pancreas.

Calcium-dependent biosynthesis of platelet-activating factor by submandibular gland cells

Isolated dog submandibular gland cells synthesize platelet-activating factor (PAF) when stimulated with acetylcholine (ACh). This production of PAF was concentration- and time-dependent, and was inhibited by pretreatment with anticholinergic agents. PAF that had accumulated in cells through prior stimulation with ACh vanished rapidly on addition of atropine. Phenylmethanesulphonyl fluoride produced an accumulation of PAF in non-stimulated cells and greatly potentiated further ACh-induced accumulation. PAF production and [14C] arachidonic acid (AA) liberation induced by ACh were increased by higher concentrations of extracellular Ca2+, and ACh failed to stimulate PAF formation in the absence of Ca2+, although ACh still stimulated the liberation of [14C]AA without Ca2+. Both the Ca2+ ionophore ionomycin in intact cells and Ca2+ (at concentrations greater than or equal to 300 nM) in digitonin-permeabilized cells facilitated PAF formation. 1-O-Alkyl-2-lyso-sn-glycero-3-phosphocholine (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 remodelling 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 the 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 into the cytosol.

Interaction between PAF and drugs that stimulate formation of inositol phosphates

1. We examined the effect of platelet-activating factor (PAF) on alpha 1-adrenergic and muscarinic cholinergic receptors in the membranes of rat parotid and the functional responses to norepinephrine (NE) and carbamylcholine (CCh) in exocrine secretion. 2. Pretreatment of parotid slices with 1 microM PAF caused a significant decrease the density of [3H]prazosin and [3H]quinuclidinyl benzilate binding sites without a change in the affinity. 3. In functional studies, pretreatment of parotid slices with 1 microM PAF markedly reduced the increase in intracellular 3H-labeling of inositol phosphates induced by NE and CCh. However, the EC50 values for NE and CCh stimulation of [3H]inositol phosphates in 1 microM PAF treated slices were significantly higher when compared to those of controls. 4. In addition, NE- and CCh-induced K+ release from parotid slices were also inhibited by PAF (1 microM) treatment. CV3988, a PAF antagonist, protected against these PAF-induced changes. 5. These results suggests that PAF-induced downregulation of alpha 1-adrenergic and muscarinic cholinergic receptors is mediated through specific PAF receptors in rat parotid glands, and that PAF could be a mediator which alters alpha 1-adrenergic and muscarinic cholinergic responses.

The effect of platelet activating factor antagonist (BN 52021) on cerulein-induced acute pancreatitis with reference to oxygen radicals

Acute edematous pancreatitis was induced in Wistar male rats by iv infusion of cerulein (CR) in the dose of 5.10(-6)g.kg-1.h-1 during 3 or 6 h. The effect of BN 52021--platelet activating factor (PAF) receptor antagonist, against this model of disease was examined. BN 52021 was applied iv as a bolus injection in the dose of 5.10(-3)g.kg-1 at 0 time. Treatment with this agent significantly ameliorates cerulein-induced acute pancreatitis in rats. The effect of BN 52021 was expressed by significant reduction of pancreas edema, diminution of hyperamylasemia, lack of superoxide dismutase activity depletion, and inhibition of lipid peroxidation in pancreatic tissue. These changes were accompanied by significant reduction of acinar cells vacuolization and remarkable inhibition of infiltration with inflammatory cells in the interacinar space. We suppose that beneficial effect of BN 52021 against cerulein-induced acute pancreatitis in rats depends on the prevention of inflammatory cells activation and subsequent generation of oxygen radicals within pancreatic tissue.

Interaction of platelet-activating factor with endothelial and vascular smooth muscle cells in coculture

Platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine [PAF]) is a vasoactive ether lipid produced by activated blood cells. To examine the molecular traffic and sites of metabolism of PAF released in the vascular wall, we used a coculture system in which endothelial cells are grown on micropore filters suspended over confluent cultures of vascular smooth muscle cells. The endothelial cells took up PAF 5-7 times more readily from the apical than from the basolateral surface, converting it to 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (2-acyl-PAF) and other minor metabolites. Intact endothelial monolayers effectively shielded the underlying smooth muscle cells from PAF present in the apical fluid; after a 30-min incubation with [3H]-PAF, only 1% of the radioactivity was transferred to the interstitial fluid. By contrast, PAF readily entered the interstitial fluid when the endothelial monolayers were injured by exposure to xanthine and xanthine oxidase. PAF did not significantly increase the permeability of endothelial monolayers to albumin. Smooth muscle cells took up and metabolized interstitial PAF more quickly and more completely than did endothelial cells; 65% was converted to 2-acyl-PAF in 15 min by the smooth muscle cells. PAF enhanced the proliferative effect of PDGF on smooth muscle cells, as assessed by [3H]-thymidine incorporation. These findings suggest that endothelial cells form a barrier to PAF released at the luminal surface, but PAF released in the vascular intima interacts primarily with smooth muscle cells, possibly stimulating proliferation in these cells.

Biosynthesis of platelet-activating factor in glandular gastric mucosa. Evidence for the involvement of the 'de novo' pathway and modulation by fatty acids

The biosynthesis of platelet-activating factor (PAF), a phospholipid autocoid with potent ulcerogenic properties that is produced in secretory exocrine glands by physiological secretagogues, was assessed in microsomal preparations of glandular gastric mucosa. For this purpose, 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF):acetyl-CoA acetyltransferase (EC 2.3.1.67); the enzymes of the 'de novo' pathway: 1-O-alkyl-2-lyso-sn-glycero-3-phosphate (alkyl-lyso-GP):acetyl-CoA acetyltransferase and 1-O-alkyl-2-acetyl-sn-glycerol (alkylacetyl-G):CDP-choline cholinephosphotransferase (EC 2.7.8.16); and some enzymes involved in the catabolism of PAF and lyso-PAF were assayed. Only the enzymes of the 'de novo' pathway and small amounts of PAF acetylhydrolase, phospholipase A2 and a lysophospholipase D acting on either lipids could be detected in the gastric preparations, whereas lyso-PAF:acetyl-CoA acetyltransferase activity was undetectable. The specific activity of alkyl-lyso-GP:acetyl-CoA acetyltransferase in the gastric mucosa was about one-tenth of that found in spleen microsomes and its apparent Km for acetyl-CoA was 454 microM compared with 277 microM in spleen microsomes. Glandular mucosa homogenates contained preformed PAF at a concentration of 2.7 +/- 0.7 ng equivalents of PAF (hexadecyl)/mg of protein. When gastric microsomes were incubated with micromolar concentrations of fatty acids (arachidonic, palmitic and oleic) prior to the assay of dithiothreitol (DTT)-insensitive cholinephosphotransferase, a dose-dependent reduction in the formation of PAF was observed, arachidonic acid being the most potent inhibitor, followed by linoleic acid (only tested on spleen microsomes) and oleic acid. By contrast, 1,2-diolein and phosphatidylcholine (dipalmitoyl) showed no or little effect. These results indicate that glandular gastric mucosa can produce PAF through the 'de novo' pathway, and that fatty acids, especially unsaturated, can reduce that synthesis by modulating the expression of DTT-insensitive cholinephosphotransferase.

Characterization of 3H-labelled platelet activating factor receptor complex solubilized from rabbit platelet membranes

Rabbit platelet membranes, preincubated with 3H-labeled platelet activating factor ([3H]PAF), were solubilized with 2% digitonin. Sedimentation of the detergent extract in a sucrose density gradient revealed a major labeled component with a sedimentation coefficient (s20,omega) of 10.5 S, which was substantially diminished when an excess of unlabeled PAF or L-652,731, (trans-2,5-bis(3,4,5-trimethoxyphenyl)tetrahydrofuran), (PAF antagonist) was present in the preincubation mixture, suggesting that the 10.5 S component is a specific receptor-bound [3H]PAF complex. Gel filtration of the [3H]PAF-receptor complex on Sephacryl S-300 revealed a single radiolabeled fraction with an apparent Stokes' radius of 4.9 nm. The apparent molecular weight and the frictional ratio of the agonist-receptor complex were computed to be 220,000 and 1.13, respectively. Dissociation of [3H]PAF from the radioligand-receptor complex was facilitated by Na+ and Li+, whereas K+ and Cs+ were ineffective. The guanine nucleotide, GTP, was also found to promote the dissociation in a manner that is additive with the effect of Na+, suggestive of the coupling of a guanine nucleotide binding protein to the solubilized PAF-receptor complex.

Phosphorylation of the ribosomal protein S6 during agonist-induced exocytosis in exocrine glands is catalyzed by calcium-phospholipid-dependent protein kinase (protein kinase C). Experiments with guinea pig parotid glands

The ribosomal protein S6 in exocrine cells is phosphorylated during stimulation of exocytosis by cAMP-dependent or calcium-dependent agonists. Under both conditions the same tryptic S6 phosphopeptides (termed A, B, and C) were found [Padel, Kruppa, Jahn & Söling (1983) FEBS Lett. 159, 112-118]. Studies have now been made of the phosphorylation pattern of protein S6 from purified guinea pig parotid ribosomes following in vitro phosphorylation with calmodulin-dependent, phospholipid-dependent, and cAMP-dependent protein kinases. Only the phospholipid-dependent enzyme led to the phosphorylation of peptides A, B, and C, while the cAMP-dependent enzyme phosphorylated only peptides A and C, and the calmodulin-dependent enzyme did not phosphorylate any of the phosphopeptides found in S6 from unstimulated or stimulated intact cells. Guinea pig parotid microsomes contain substantial phospholipid-dependent protein kinase activity. Stimulation of intact parotid glands with tetradecanoylphorbol acetate led to a significant phosphorylation of S6 and a similar tryptic S6 phosphopeptide pattern as seen with carbamoylcholine. It is concluded that activation of phospholipid-dependent protein kinase is responsible for the phosphorylation of protein S6 during stimulation with calcium-dependent and cAMP-dependent secretagogues.

Regulation of protein kinases in exocrine secretory cells during agonist-induced exocytosis

Stimulation of exocytosis in exocrine glands is associated with an increased phosphorylation of several particulate proteins. Irrespective of the type of secretagogue (cAMP-dependent agonists, calcium-dependent agonists, calcium ionophores, phorbol esters) exocytosis is always accompanied by an enhanced phosphorylation of the ribosomal protein S6. It is shown by an analysis of the phosphopeptide pattern of the in vivo and the in vitro phosphorylated S6 protein that the protein kinase responsible for phosphorylation of the S6 protein during enhanced exocytosis is protein kinase C. This is so irrespective of whether the agonist uses cAMP or calcium as second messenger. Experiments with isolated guinea pig parotid gland lobules reveal that not only the acetylcholine analog carbamoylcholine, but also the beta-agonist isoproterenol lead within seconds to an increased formation of diacylglycerol. As diacylglycerol increases the affinity of protein kinase C for calcium this finding would explain why the phosphorylation pattern of the S6 protein reflects activation of protein kinase C also under conditions where (as in the case of stimulation with beta-agonists) cAMP is the primary second messenger. It would further explain why the changes of the phosphorylation of individual histones observed during agonist-induced exocytosis in the parotid gland are quite similar for isoproterenol on one hand and carbamoylcholine on the other. A 22 K protein which becomes phosphorylated only when cAMP serves as second messenger is located in the membrane of the endoplasmic reticulum. A possible relationship of this protein with the calcium transport ATPase of the endoplasmic reticulum is under investigation.