Mechanism of carbachol-stimulated diacylglycerol formation in rat parotid acinar cells

P2Y purinoceptors mediate ATP-induced changes in intracellular calcium and amylase release in acinar cells of mouse parotid glands

Adenosine 5'-triphosphate (ATP) can act as an extracellular signal that regulates various cellular functions. The present study aimed to determine which purinoceptors play a role in ATP-induced changes in intracellular Ca(2+) ([Ca(2+)]i) and amylase secretion in mouse parotid glands. ATP induced a steep increase in [Ca(2+)]i in acinar cells. The removal of extracellular Ca(2+) or the use of Ca(2+) channel blockers slightly inhibited this increase. Inhibition of PLCγ by U73122 and of IP3 by xestospongin C did not completely block this increase. The purinoceptor antagonists suramin and reactive blue-2 strongly inhibited the ATP-induced changes in [Ca(2+)]i. 2-MeSATP induced a strong increase in [Ca(2+)]i, while Bz-ATP induced a small [Ca(2+)]i increase, and UTP and α,β-MeATP had no effect. The potency order of ATP analogs (2-MeSATP > ATP >> UTP) suggested that P2Y1 and P2Y12 play a significant role in the cellular response to ATP. RT-PCR revealed that P2X2,4,7 and P2Y1,2,10,12,14 were expressed in acinar cells. Ca(2+)-dependent exocytotic secretion of amylase was detected in parotid glands. These findings indicated that ATP activates P2Y receptors more than P2X receptors at low concentrations. Thus, P2Y receptors were found to be the main receptors involved in Ca(2+)-related cell homeostasis and amylase secretion in mouse parotid glands.

Impaired microvascular response to cholinergic stimuli in primary Sjögren's syndrome

OBJECTIVE

Signs of a parasympathetic dysfunction have been revealed in primary Sjögren's syndrome (SS). Its role in the pathogenesis and the clinical picture of the disease is not clear. To investigate the responsiveness of SS patients to a cholinergic agonist, a model was used involving examination of the cutaneous microcirculation. The microvascular response to the administration of carbachol was measured, a muscarinic cholinergic agonist.

METHODS

Twenty two SS patients and 12 controls were examined. Carbachol and 0.9% saline solution were administered intracutaneously into the forearm skin at two distinct places. Skin blood flow (SBF) in the injected areas was measured continuously before and for 10 minutes after the injections by means of a laser Doppler perfusion monitor. The increase in SBF in response to carbachol (dSBF), reflecting vasodilatation, was calculated by a formula including the baseline and the maximum SBF values after the injections of carbachol and saline solution.

RESULTS

The vasodilatation was significantly lower in SS patients than in the controls (mean dSBF: 2.1 (range: 1.0-4.5) versus 3.3 (range: 1.7-7.6), p=0.02). With non-responder patients defined as those in whom a smaller response was observed than in any of the controls, 11 of the 22 SS patients proved to be non-responders to carbachol. Comparisons of demographic, clinical and laboratory characteristics and HLA class II genotypes between responder and non-responder SS patients did not show any significant differences.

CONCLUSIONS

A diminished or absent response to carbachol indicates a cholinergic dysfunction in SS patients. A disturbance in the neurotransmission at a receptorial or postreceptorial level is hypothesised. Unresponsiveness to cholinergic stimuli may contribute to exocrine insufficiency.

Asymmetric signal transduction in polarized ileal Na(+)-absorbing cells: carbachol activates brush-border but not basolateral-membrane PIP2-PLC and translocates PLC-gamma 1 only to the brush border

In ileal Na+ absorptive cells, carbachol inhibits NaCl absorption and its component brush-border Na+/H+ exchanger, acting via basolateral membrane (BLM) receptors. This carbachol effect involves brush-border but not BLM protein kinase C. In the present work we describe another asymmetric aspect of signal transduction in these epithelial cells, this time involving phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phospholipase C (PLC). Thirty seconds and 1 min after carbachol treatment, brush-border PIP2-specific PLC activity increased, returning to control levels by 2.5 min. Involvement of brush-border tyrosine kinase(s) in this effect was suggested by inhibition of the carbachol effect on NaCl absorption by the tyrosine kinase inhibitor genistein, added to the mucosal but not the serosal surface. Luminal genistein pretreatment also prevented the carbachol-induced increase in brush-border PLC activity. In contrast, carbachol exposure did not change the BLM PIP2-specific PLC activity. Western analysis and immunoprecipitation demonstrated that PLC-gamma 1 is present in the brush border and that carbachol increases the PLC-gamma 1 amount in the brush border. Both the brush border and BLM contain PLC-beta 3 and a small amount of PLC-delta 1 but no PLC-beta 1, whereas BLM lacks detectable PLC-gamma 1. No change in PLC-beta 3 or PLC-delta 1 amount in the brush border occurred with carbachol exposure. No change in tyrosine phosphorylation of brush-border PLC-gamma 1 occurred with carbachol treatment. The Ca2+ ionophore A23187 did not alter PIP2-specific PLC activity in either the brush border or the BLM. These studies demonstrate that carbachol but not Ca2+ ionophore effects on brush-border NaCl absorption are associated with increases in brush-border but not BLM PIP2-specific PLC activity and in the amount of brush-border PLC-gamma 1, and involve tyrosine phosphorylation. This asymmetric aspect of epithelial signal transduction, together with the previous demonstration of localization of high-sensitivity IP3 stores to the apical membrane area in intestinal epithelial cells, shows that different aspects of signal transduction occur at the apical and basolateral membranes in epithelial and requires studies in both domains to define mechanisms of intracellular signalling.

Carbachol, substance P, and phorbol ester promote the tyrosine phosphorylation of protein kinase C delta in salivary gland epithelial cells

The initiation of saliva formation by parotid acinar cells, which comprise the majority of cells in this salivary gland, is initiated by the release of neurotransmitters (acetylcholine, substance P) from parasympathetic nerves. In response to substance P and the muscarinic agonist carbachol, two ligands that activate phospholipase C-linked receptors, which stimulate fluid secretion, PKC delta was phosphorylated on tyrosine residues. The maximal agonist-dependent tyrosine phosphorylation occurred within seconds of the addition of either agonist and then returned rapidly to a smaller increased level. Phorbol ester also caused a rapid increase in tyrosine phosphorylation, which reached a maximal level 5 min after the addition of phorbol 12-myristate 13-acetate. The increase in tyrosine phosphorylation of PKC delta was blocked by tyrosine kinase inhibitors genistein and staurosporine. Ionophore-mediated elevation of [Ca2+]i or activation of the beta-adrenergic receptor, epidermal growth factor receptor, or insulin receptor did not promote the tyrosine phosphorylation of PKC delta. These results indicate that tyrosine phosphorylation plays a role in early signal transduction events promoted by the activation of muscarinic and substance P receptors and suggests that the tyrosine phosphorylation of PKC delta has a role in the activation of fluid secretion by neurotransmitters binding to phospholipase C-linked receptors.

Diacylglycerol elevations in control platelets are unaccompanied by pleckstrin phosphorylation. Implications for the role of diacylglycerol in platelet activation

Several laboratories have reported that diacylglycerol levels in human platelets (approximately 100 pmol/10(9) platelets) increased severalfold in response to 0.5-1 U/ml thrombin. We report here fluctuations in diacylglycerol mass in control platelets, the magnitude of which were 60-90% of that measured in platelets treated with 0.2-0.5 U/ml of thrombin. These control platelets were not activated by such criteria as absence of aggregation, secretion, phosphatidic acid production and phosphorylation of the protein kinase C substrate, pleckstrin. Thrombin treatment evoked all of the above responses. Analysis of the diacylglycerol molecular species by reverse-phase HPLC of the dimethylated, phosphorylated derivatives showed that all of the molecular species that were present in control platelets were also present in thrombin-treated platelets. Most of the species appeared to fluctuate at random in control platelets with the exception of 1-stearoyl-2-arachidonoyl-sn-glycerol which was more or less stable and increased severalfold over control values only upon thrombin treatment. Furthermore, only this species accumulated as [32P]phosphorylated PtdOH in thrombin-treated platelets prelabelled with [32P]Pi. Our findings show that, in platelets, elevation of diacylglycerol molecular species other than the 1-stearoyl-2-arachidonoyl species occurs, but these changes are not necessarily linked to activation of protein kinase C as measured by pleckstrin phosphorylation which was observed only upon elevation of 1-stearoyl-2-arachidonoyl-sn-glycerol.

Carbachol-induced phosphatidylcholine hydrolysis and choline efflux in rat submandibular gland involves phospholipase D activation and is modulated by protein kinase C and calcium

The role of calcium and protein kinase C activation in carbachol-induced choline efflux from submandibular glands was investigated. The participation of phospholipase D in this signal transduction pathway was demonstrated by the formation of [14C]phosphatidylethanol in [14C]lysophosphatidylcholine-labelled submandibular gland cells treated with carbachol or noradrenaline in the presence of ethanol. Chelation of the intracellular calcium with BAPTA/AM reduced the carbachol stimulated outflow of [3H]choline. The calcium ionophore A23187 in a high concentration (10 microM) increased the basal [3H]choline outflow, but decreased the carbachol-induced outflow. Removal of the extracellular calcium enhanced the carbachol-stimulated outflow, which returned to control when calcium was re-added to the medium. Activation of protein kinase C by phorbol-12,13-dibutyrate (100 nM) or 1-oleyl-2-acetyl-sn-glycerol (20 microM) was without effect per se, but enhanced the carbachol-mediated outflow of [3H]choline. Phorbol-12,13-dibutyrate in combination with 1 microM A23187 induced a small efflux of [3H]choline. A 2 h treatment with phorbol-12,13-dibutyrate (1 microM), causing down-regulation of protein kinase C, significantly decreased the carbachol-stimulated [3H]choline outflow. In conclusion, elevation of intracellular calcium levels and protein kinase C activation are of importance for the carbachol-stimulated outflow of [3H]choline. Inflow of calcium, if anything, reduces the carbachol-stimulated outflow of [3H]choline. Since phosphatidylcholine breakdown generates diacylglycerol and this could lead to activation of protein kinase C, activation of this signal transduction pathway may be important for the protein content of the saliva and for the known trophic effects of parasympathetic innervation.

Breakdown of membrane choline-phospholipids induced by endogenous and exogenous muscarinic agonist is potentiated by VIP in rat submandibular gland

The outflow of tritium from rat submandibular gland fragments, pre-labelled with [3H]choline, following electrical or pharmacological stimulation was studied. Electrical stimulation of the tissue increased the outflow of tritium in a frequency dependent manner. Atropine treatment decreased the electrically-induced release, indicating that the outflow did not reflect acetylcholine from nerve endings, but was largely brought about by postsynaptic receptors. In agreement with this hypothesis, treatment with noradrenaline or carbachol induced a dose dependent increase in tritium outflow from the gland fragments which could be blocked by prazosin or atropine, respectively. Moreover, analysis of the tissue-associated tritium revealed an incorporation primarily in the lipid fraction of the tissue (almost 80%), of which about 90% was in phosphatidylcholine, indicating that this was the source of the tritium outflow. Pre-incubation with vasoactive intestinal polypeptide (VIP), which coexists with acetylcholine in the parasympathetic neurons innervating the submandibular gland, increased the carbachol-induced tritium overflow significantly. The effect of VIP could be imitated by the adenylyl cyclase stimulator forskolin, which increased the carbachol-stimulated tritium efflux in a dose dependent manner. Taken together, our results suggests that muscarinic- and alpha 1-receptor agonists may activate a phospholipase coupled to phosphatidylcholine hydrolysis in the rat submandibular gland. Endogenous acetylcholine released from parasympathetic nerve endings appear to activate this mechanism. Furthermore, VIP treatment, and the concomitant cAMP-accumulation, potentiates the acetylcholine induced phosphatidylcholine hydrolysis, demonstrating a new type of interaction between the classical transmitter acetylcholine and the co-stored neuropeptide VIP.

G protein beta gamma-subunits inhibit purified adenylate cyclase independent of the activation by Ca2+ and calmodulin

Adenylate cyclase purified by affinity chromatography was activated about 2.5-fold in a Ca(2+)- and calmodulin-dependent fashion. G protein beta gamma-subunits, an inhibitor in the receptor-mediated inhibition of adenylate cyclase, inhibited the purified cyclase by more than 80%. The extent of beta gamma-induced inhibition was not affected by the activation with Ca2+ and calmodulin. Moreover, the prior addition of the beta gamma-subunits to the cyclase did not prevent the subsequent activation of the enzyme by Ca2+ and calmodulin. We conclude that the beta gamma-subunits inhibit adenylate cyclase activity in a calmodulin-independent mode.

SR 46559A: a novel and potent muscarinic compound with no cholinergic syndrome

The cholinergic activities of SR 46559A, 3-[N-(2 diethyl-amino-2-methylpropyl)-6-phenyl-5-propyl] pyridazinamine sesquifumarate, have been investigated in vitro and in vivo, in rodents. Using rat brain cortical membranes, SR 46559A was a competitive ligand (Ki = 112 nM) at muscarinic M1 receptors, its affinity for muscarinic M2 (cardiac) and M3 (glandular) receptors being 6-7 times lower. SR 46559A did not interact with brain nicotinic receptors and high affinity choline uptake sites nor did it inhibit brain acetylcholinesterase activity. In contrast to reference muscarinic agonists, SR 46559A (1 mM) did not inhibit the forskolin-induced activation of cAMP synthesis nor did it stimulate phosphoinositides breakdown in various brain preparations. However, this compound enhanced (+67% at 1 mM) diacylglycerol formation in rat striatal miniprisms, an effect fully reversed by atropine. As shown with reference agonists, SR 46559A inhibited (IC50 = 10 microM) the K(+)-evoked release of [3H]GABA from rat striatal slices and reduced at 0.5 and 1 microM, the population spike amplitude of the CA1 pyramidal cells induced by stimulation of the Schaffer's collateral commissural pathway in rat hippocampal slices. In mice, SR 46559A at a near lethal dose (200 mg/kg PO) did not induce the typical cholinergic syndrome nor did it modify at 30 mg/kg PO the oxotremorine-induced hypothermia. Like muscarinic agonists, SR 46559A (1 mg/kg PO) potentiated haloperidol-induced catalepsy in rats and inhibited (ED50 = 0.12 mg/kg PO) rotations induced in mice by intrastriatal injection of pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ ionophore and phorbol ester stimulate diacylglycerol formation and phosphatidylcholine hydrolysis in rat parotid acinar cells

We investigated the effects of A23187 and phorbol 12,13-dibutyrate (PDBu) on sn-1,2-diacylglycerol (DAG) accumulation and phosphatidylcholine (PC) hydrolysis in rat parotid acinar cells. Both A23187 and PDBu, in concentration ranges of 0.001-0.1 microM, stimulated DAG accumulation and PC hydrolysis in a time- and concentration-dependent manner. Treatment with A23187 and PDBu stimulated the release of [3H]choline and [3H]phosphocholine into the medium, indicating [3H]PC hydrolysis is due to the activation of phospholipases C and D; however, [3H]phosphatidylethanolamine hydrolysis was not indicated. These releases were unaffected by the addition of glucose 6-phosphate, a phosphatase inhibitor. Staurosporine, a protein kinase C inhibitor, significantly inhibited the DAG accumulation and the PC hydrolysis stimulated by these agents. Combinations of A23187 and PDBu potentiated the stimulatory effect which each of these agents alone had on DAG accumulation and PC hydrolysis. This mode of action was additive but not synergistic. These results suggest that DAG accumulation induced by A23187 and PDBu is related to the PC hydrolysis mediated via the activation of phospholipases C and D, and that it is not related to phosphatidylethanolamine hydrolysis.