Inhibitory effects of arachidonic acid on muscarinic current response in single pancreatic acinar cells of rats

Effect of glycine on the cyclooxygenase pathway of the kidney arachidonic acid metabolism in a rat model of metabolic syndrome

The kidneys are organs that can be severely impaired by metabolic syndrome (MS). This is characterized by the association of various pathologies such as hypertension, dyslipidemia, and type-2 diabetes. Glycine, a nonessential amino acid, is known to possess various protective effects in the kidney, such as a decrease in the deterioration of renal function and a reduction of the damage caused by hypoxia. In a rat model of MS, the effect of glycine on the cyclooxygenase (COX) pathway of arachidonic acid (AA) metabolism was studied in isolated perfused kidney. MS was induced in Wistar rats by feeding them a 30% sucrose solution for 16 weeks. The addition of 1% glycine to their drinking water containing 30% sucrose, for 8 weeks, reduced high blood pressure, triglyceride levels, insulin concentration, homeostatis model assessment (HOMA) index, albuminuria, AA concentration in kidney homogenate, renal perfusion pressure, prostaglandin levels, PLA2 expression, and COX isoform expression, compared with MS rats that did not receive the glycine supplement. Glycine receptor expression decreased significantly with MS, but glycine treatment increased it. The results suggest that in the MS model, 1% glycine treatment protects the kidney from damage provoked by the high sucrose consumption, by acting as an anti-inflammatory on the COX pathway of AA metabolism in kidney.

Cholecystokinin-evoked Ca(2+) waves in isolated mouse pancreatic acinar cells are modulated by activation of cytosolic phospholipase A(2), phospholipase D, and protein kinase C

We employed confocal laser-scanning microscopy to monitor cholecystokinin (CCK)-evoked Ca(2+) signals in fluo-3-loaded mouse pancreatic acinar cells. CCK-8-induced Ca(2+) signals start at the luminal cell pole and subsequently spread toward the basolateral membrane. Ca(2+) waves elicited by stimulation of high-affinity CCK receptors (h.a.CCK-R) with 20 pM CCK-8 spread with a slower rate than those induced by activation of low-affinity CCK receptors (l.a. CCK-R) with 10 nM CCK-8. However, the magnitude of the initial Ca(2+) release was the same at both CCK-8 concentrations, suggesting that the secondary Ca(2+) release from intracellular stores is modulated by activation of different intracellular pathways in response to low and high CCK-8 concentrations. Our experiments suggest that the propagation of Ca(2+) waves is modulated by protein kinase C (PKC) and arachidonic acid (AA). The data indicate that h.a. CCK-R are linked to phospholipase C (PLC) and phospholipase A(2) (PLA(2)) cascades, whereas l.a.CCK-R are coupled to PLC and phospholipase D (PLD) cascades. The products of PLA(2) and PLD activation, AA and diacylglycerol (DAG), cause inhibition of Ca(2+) wave propagation by yet unknown mechanisms.

Inhibition of antagonist and agonist binding to the human brain muscarinic receptor by arachidonic acid

Arachidonic acid (AA) inhibits the binding of [3H]quinclidinyl benzilate ([3H]QNB) to the human brain muscarinic cholinergic receptor (mAChR). AA inhibits at lower concentrations in the absence of glutathione (I50 = 15 microM) than in the presence of glutathione (I50 = 42 microM). Inhibition of mAChR binding shows specificity for AA and is reduced with loss of one or more double bonds or with either a decrease or increase in the length of the fatty acid chain. Metabolism of AA by the lipoxygenase, epoxygenase, or fatty acid cyclooxygenase pathways is not required for the inhibitory activity of AA on mAChR binding. Inhibition of [3H]QNB binding by AA is reversible. While decreasing Bmax, AA increased the apparent KD for [3H]QNB and for the more polar antagonist [3H]NMS. In addition, AA inhibits binding of the agonist [3H]oxotremorine-M (I50 = 60 microM) and is the first mediator of mAChR action to be shown to reversibly inhibit mAChR binding. The feedback inhibition of the mAChR by AA may serve a homeostatic function similar to the reuptake and hydrolysis of acetylcholine following cholinergic nerve transmission.

Ins(1,3,4,5)P4 is effective in mobilizing Ca2+ in mouse exocrine pancreatic acinar cells if phospholipase A2 is inhibited

In enzymically isolated mouse pancreatic acinar cells, under conditions of whole-cell patch-clamp current recording, the effect of phospholipase C-coupled agonists can be mimicked by internal perfusion of the intracellular second messenger Ins(1,4,5)P3 (10 microM) or its analogue Ins(2,4,5)P3 (10 microM). The inositol trisphosphates mimic receptor activation by releasing Ca2+ from intracellular stores and by promoting Ca2+ influx across the surface membrane. This Ca(2+)-mobilizing role of inositol polyphosphates seems to be confined to the inositol trisphosphates because internal perfusion of Ins(1,3,4,5)P4 (10 microM) is not associated with any Ca(2+)-dependent current activation. In this study we investigate the effects of 4-bromophenacyl bromide (4BPB), a putative inhibitor of phospholipase A2 and arachadonic acid production, on inositol polyphosphate-induced Ca2+ signalling. At 10 microM, 4BPB has no effect on unstimulated Ca(2+)-dependent membrane currents. However, if 4BPB is applied to cells internally perfused with 10 microM Ins(1,4,5)P3 or Ins(2,4,5)P3 then the current responses are rapidly potentiated. In cells internally perfused with 10 microM Ins(1,3,4,5)P4, which has itself no effect on membrane currents, application of 4BPB resulted in the activation of Ca(2+)-dependent currents, seen either as repetitive spikes of current or as sustained current activations. The application of arachidonic acid blocks the current responses evoked by the inositol trisphosphates and by Ins(1,3,4,5)P4/4BPB. These results suggest that in enzymically isolated pancreatic acinar cells phospholipase A2 activity is exerting an inhibitory effect on inositol polyphosphate-mediated Ca2+ mobilization. 4BPB removes this inhibition and potentiates the responses to internally perfused inositol trisphosphates and, importantly, makes 10 microM Ins(1,3,4,5)P4 as effective as 10 microM Ins(1,4,5)P3 in mobilizing intracellular Ca2+ and in promoting Ca2+ influx.

Selective activation of exocytosis by low concentrations of ACh in rat pancreatic acinar cells

1. We have monitored changes in membrane capacitance (delta C) and conductance (delta G) induced by muscarinic acetylcholine stimulation in single rat pancreatic acinar cells. 2. Acetylcholine (ACh, 500 nM) induced simultaneous increases of delta C and delta G. In contrast, a low concentration (50 nM) of ACh exclusively induced delta C increases without delta G. These responses were abolished by the internal perfusion of heparin. This indicates that inositol 1,4,5-trisphosphate-mediated internal Ca2+ mobilization either simultaneously activates exocytosis and ion channels or exclusively initiates exocytosis. In comparison, a low concentration of A23187 selectively activated ion channels but a high concentration activated exocytosis and ion channels simultaneously. 3. These selective response patterns of delta C and delta G depend on the choice of agonist and the internal EGTA concentration. From this, we postulated two explanations for the selective action of muscarinic ACh stimulation on exocytosis. First, an area of high [Ca2+]i, spatially close to secretory granules, activates exocytosis. Second, an as yet unknown signalling factor sensitizes the Ca2+ affinity of the exocytotic apparatus.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Photodynamic drug action on isolated rat pancreatic acini. Mobilization of arachidonic acid and prostaglandin production

Chloro-aluminium phthalocyanine sulphonate (SALPC) when photon-activated generates singlet oxygen, elicits amylase release and causes plasma membrane permeabilization of pancreatic acinar cells (Matthews and Cui, Biochem Pharmacol 39: 1444-1457, 1990). Amylase release precedes membrane permeabilization suggesting that the initial release of amylase may be due to direct stimulation by singlet oxygen of secretagogue receptors or their coupled guanine nucleotide binding proteins (G-proteins) and effector systems including phospholipase A2 (PLA2). The aim of the experiments reported here was to establish the extent to which PLA2 activation, arachidonic acid mobilization, and prostaglandin production are involved in the photon-induced action of SALPC on dispersed, perifused acini isolated from the rat pancreas. The mobilization of arachidonic acid by a major secretory stimulant of pancreatic exocrine cells, cholecystokinin octapeptide, was also assessed: it produced a time- and concentration-dependent (10(-10)-10(-6) M) stimulation of arachidonic acid output from acini prelabelled with [1-14C]arachidonic acid. In contrast, the kinetics of arachidonic acid mobilization with photon-activated SALPC 1 microM, 4500 or 18,400 lux light intensity (lambda > 570 mm), was biphasic, an intensity-dependent stimulation being preceded by a more immediate initial inhibition of output. Light activation of SALPC and singlet oxygen generation may evoke the stimulatory phase of arachidonic acid release by an action on G-proteins, or by PLA2 activated directly, or via calcium influx, because NaF 20 mM, mellitin 2 mg/mL and the calcium ionophore A23187 1 microM caused a 2.9-, 33- and 5-fold increase, respectively, in arachidonic acid output. However, not only was the arachidonate stimulation delayed in response to SALPC but in other experiments designed to gain more insight into the turnover of arachidonic acid and its metabolites, the photodynamic release of amylase preceded maximum prostaglandin E2 (PGE2) output and amylase release was completely unaffected when PGE2 production was blocked by the cyclo-oxygenase inhibitor, indomethacin 10 microM. It is therefore likely that the rapid initial photodynamic release of amylase from pancreatic acini induced by SALPC is mediated by activation of the signal transduction pathway involving the release of intracellular calcium; arachidonic acid mobilization and prostanoid production may then be linked to the longer-term, cytolytic action of SALPC, especially in tumour cells.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Control of inositol polyphosphate-mediated calcium mobilization by arachidonic acid in pancreatic acinar cells of rats

1. The patch-clamp technique of whole-cell current recording was applied to single, enzymatically isolated, rat pancreatic acinar cells to investigate the current responses evoked by internal perfusion of inositol polyphosphates (InsPx). The InsPx were included in the solution filling the recording pipette and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3; 10 microM) evoked transient current responses generally of less than 1 min duration, inositol 2,4,5-trisphosphate (Ins(2,4,5)P3; 10 microM) evoked smaller current transients while inositol 1,3,4,5-tetrakisphosphate (InsP4; 10 microM) evoked no detectable current response. However, in the presence (in external bathing solution) of the phospholipase A2 inhibitor 4-bromophenacyl bromide (4-BPB; 8 microM) all three of the InsPx now evoked prolonged current responses lasting for several minutes. The current responses to all three InsPx were abolished by inclusion of the Ca2+ chelator EGTA (5 mM) in the internal, pipette-filling solution indicating that the responses are calcium dependent and reflect the effect of the InsPx in increasing intracellular Ca2+. Inositol 1,3,4,5,6-pentophosphate (InsP5) induced no current response when tested up to 20 microM in the presence or absence of 4-BPB. 2. The potentiating effect of 4-BPB on the InsPx-induced current responses was not mimicked by application of arachidonic acid (AA) oxidation inhibitors; indomethacin (20 microM), nordihydroguaiaretic acid (20 microM) or proadifen (SKF525A, 100 microM). The effects of 4-BPB were countered however, by the inclusion of 2 microM AA in the external solution. The results suggest that the 4-BPB potentiates the response by inhibiting the activity of phospholipase A2, thereby reducing the formation of AA. 3. In the presence of 4-BPB (8 microM) the InsPx-evoked responses were dose dependent with an increase in both the amplitude and speed of onset with increasing concentrations. In the presence of 4-BPB InsP4 was as efficient as Ins(1,4,5)P3 both in terms of speed of onset and amplitude of responses; the efficacy and dissociation constant (Kd) for both of these InsPx were the same at 1 microM and 45 nM respectively. Ins(2,4,5)P3 was always less effective, with an efficacy and Kd of 10 microM and 750 nM respectively. 4. If 4-BPB was applied after the current responses evoked by the InsPx were over, or if guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) was included in the recording pipette then the phospholipase inhibitor gave rise to an additional, prolonged, current response.(ABSTRACT TRUNCATED AT 400 WORDS)

Excess divalent cations activate Ca(2+)-mobilizing receptors in pancreatic acinar cells

In single, enzymatically dissociated, rat pancreatic acinar cells, external application of excess divalent cations (Ca2+, Sr2+, Ba2+, Ni2+ and Mg2+ over 50 mM) induced Ca(2+)-dependent current responses monitored with the whole-cell recording technique. Inclusion of either EGTA, heparin or GDP[beta S] in the internal solution or treatment of acinar cells with a phorbol ester abolished the divalent-cation-induced responses. In contrast, internal inositol trisphosphate (InsP3) or GTP[gamma S] potentiated the responses. The results indicate that excess divalent cations activate membrane surface receptors or receptor/effector complexes, thereby inducing InsP3-mediated Ca2+ mobilization. The mechanism may be due to modulation of the receptors by changes in electrical profile through indirect action of divalent cations on membrane surface charges, i.e. neutralization of anionic charges. This proposal was supported by the evidence that the trivalent cation, La3+, and the polyvalent cation, protamine, both at much lower concentrations, could induce Ca(2+)-dependent responses, which were abolished by internal application of heparin, GDP[ beta S] or a high concentration of EGTA or by protein kinase C activation with a phorbol ester.

Arachidonic acid regulates the phosphoinositide signal transduction pathway in submandibular acinar cells

Modulation of the phosphoinositide signal transduction pathway by arachidonic acid (AA) in collagenase-dispersed rat submandibular acinar cells was investigated. The muscarinic agonist, carbachol, stimulated PIP2 hydrolysis and the generation of IP3 to five-fold the control levels. This response was inhibited by 75% on pre-treatment of cells with AA. The AA inhibitory effect was not duplicated by a range of prostaglandins and leukotrienes and was not reversed by blockers of the cyclo-oxygenase and lipoxygenase synthetic pathways, indicating that AA action was not mediated by eicosanoid metabolites. Additional experiments confirmed that the enzyme, protein kinase C, was also not a mediator of the AA effect. Arachidonic acid did not affect the uptake of radioactive inositol into acinar cells, but it did inhibit the incorporation of inositol into inositol phospholipids of the phosphoinositide cycle. In studies on inositol phospholipid turnover, AA alone reduced the level of PIP2 but not of PIP or PI. Under conditions of PI cycle stimulation with carbachol, AA significantly lowered PIP2 and PIP but not PI. These findings suggest that arachidonic acid may regulate the phosphoinositide response by inhibiting the synthetic phase of the cycle at a locus distal to PI generation.

Cyclic nucleotide-dependent regulation of agonist-induced calcium increases in mouse megakaryocytes

1. The regulatory effects of cyclic AMP and cyclic GMP on ADP- and thrombin-induced increases in [Ca2+]i were studied in mouse bone marrow megakaryocytes. Changes in [Ca2+]i were continuously monitored in single Fura-2-loaded cells using microspectrofluorometry, and cyclic nucleotides were directly introduced into the single cells using the whole-cell patch-clamp technique. 2. ADP increased [Ca2+]i in a concentration-dependent fashion, and its threshold concentration was in the order of 0.01 microM. A low dose of ADP (below 0.1 microM) induced a transient response of [Ca2+]i which recovered to original levels during the stimulation. A high dose of ADP (0.3-10 microM) induced a biphasic response of [Ca2+]i with an initial peak and a plateau lasting until the end of the stimulation. Repeated stimulation with the same dose of ADP induced a reduced response, probably as a result of desensitization. 3. Thrombin increased [Ca2+]i in a concentration-dependent manner. The time courses of the responses were different from those caused by ADP. Thrombin-induced responses lacked the initial sharp peak observed in ADP-induced responses, and caused a sustained response. 4. The ADP-induced increase in [Ca2+]i was antagonized by the presence of prostaglandin E1 (PGE1, 100-1000 nM), in the medium, and by direct injection of cyclic AMP (100-500 microM) or cyclic GMP (500 microM) into the megakaryocyte. When 500 microM-cyclic AMP was injected into the cells, the rise of [Ca2+]i induced by ADP was reduced by 85%. Effects of these antagonists were inhibited by treatment with a protein kinase inhibitor, H-8. Thrombin-induced increases in [Ca2+]i were reduced by direct injection of cyclic AMP or cyclic GMP. 5. ADP could induce an increase in [Ca2+]i in the absence of external Ca2+. The time course of the response was essentially similar to that observed in the normal condition (1 mM-CaCl2), but the size of the response was reduced by 33%. Thus, 67% of the rise in [Ca2+]i induced by ADP could be accounted for by calcium mobilization from internal storage pools. The presence of NiCl2 (5 mM) duplicated the effects of external Ca2+ removal, suggesting the involvement of a Ca2+ influx pathway, which could be inhibited by Ni2+ in ADP stimulation. 6. Injection of cyclic AMP or cyclic GMP reduced ADP-induced increases in [Ca2+]i under conditions of inhibited Ca2+ influx by NiCl2 (5 mM).(ABSTRACT TRUNCATED AT 400 WORDS)