Acetylcholine and cholecystokinin receptors functionally couple by different G-proteins to phospholipase C in pancreatic acinar cells

Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Protein kinase C-delta (PKC-delta) is involved in growth, differentiation, tumor suppression, and regulation of other cellular processes. PKC-delta activation causes translocation, tyrosine phosphorylation, and serine-threonine kinase activity. However, little is known about the ability of G protein-coupled receptors to activate these processes or the mediators involved. In the present study, we explored the ability of the neurotransmitter/hormone, CCK, to stimulate these changes in PKC-delta and explored the mechanisms. In rat pancreatic acini under basal conditions, PKC-delta is almost exclusively located in cytosol. CCK and TPA stimulated a rapid PKC-delta translocation to membrane and nuclear fractions, which was transient with CCK. CCK stimulated rapid tyrosine phosphorylation of PKC-delta and increased kinase activity. Using tyrosine kinase (B44) and a tyrosine phosphatase inhibitor (orthovanadate), changes in both CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation were shown to correlate with changes in its kinase activity but not translocation. Both PKC-delta tyrosine phosphorylation and activation occur exclusively in particulate fractions. The Src kinase inhibitors, SU6656 and PP2, but not the inactive related compound, PP3, inhibited CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation and activation. In contrast, PP2 also had a lesser effect on CCK- but not TPA-stimulated PKC-delta translocation. CCK stimulated the association of Src kinases with PKC-delta, demonstrated by co-immunoprecipitation. These results demonstrate that CCKA receptor activation results in rapid translocation, tyrosine phosphorylation, and activation of PKC-delta. Stimulation of PKC-delta translocation precedes tyrosine phosphorylation, which is essential for activation to occur. Activation of Src kinases is essential for the tyrosine phosphorylation and kinase activation to occur and plays a partial role in translocation.

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Cholecystokinin activates CCKB-receptor-mediated Ca-signaling in hippocampal astrocytes

Cholecystokinin-8S (CCK-8S) is the most abundant neuropeptide in the mammalian cortex and the limbic system; however, its physiological functions remained largely obscure. We studied effects of CCK on astrocytic Ca signaling, which has met considerable interest as a second messenger in astrocytic-neuronal signaling, by digital ratio-imaging of Fura-2/AM loaded rat and mouse hippocampal astrocytes in dissociated culture. Superfusion of CCK-8S (5-50 nM for 2 min) evoked repetitive Ca increases of several hundred nanomolar in a subpopulation of astrocytes. Mouse astrocytes appeared to be more responsive to CCK than rat cells with respect to the fraction of cells responding as well as to the amplitudes of Ca increases. The Ca responses persisted in the absence of extracellular Ca, indicating that release of Ca from intracellular stores is the primary source of these Ca increases. The CCK-8S-induced Ca increases were blocked by the CCKB receptor antagonist PD135158 (100 nM) but not by the CCKA antagonist lorglumide (100 nM). We surmise that astrocytes might be a major primary target for CCK in the CNS.

Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice

Hypersecretion of insulin from the pancreas is among the earliest detectable metabolic alterations in some genetically obese animals including the ob/ob mouse and in some obesity-prone humans. Since the primary cause of obesity in the ob/ob mouse is a lack of leptin due to a mutation in the ob gene, we tested the hypothesis that leptin targets a regulatory pathway in pancreatic islets to prevent hypersecretion of insulin. Insulin secretion is regulated by changes in blood glucose, as well as by peptides from the gastrointestinal tract and neurotransmitters that activate the pancreatic islet adenylyl cyclase (e.g., glucagon-like peptide-1) and phospholipase C (PLC) (e.g., acetylcholine) signaling pathways to further potentiate glucose-induced insulin secretion. Effects of leptin on each of these regulatory pathways were thus examined. Leptin did not influence glucose or glucagon-like peptide-1-induced insulin secretion from islets of either ob/ob or lean mice, consistent with earlier findings that these regulatory pathways do not contribute to the early-onset hypersecretion of insulin from islets of ob/ob mice. However, leptin did constrain the enhanced PLC- mediated insulin secretion characteristic of islets from ob/ob mice, without influencing release from islets of lean mice. A specific enhancement in PLC-mediated insulin secretion is the earliest reported developmental alteration in insulin secretion from islets of ob/ob mice, and thus a logical target for leptin action. This action of leptin on PLC-mediated insulin secretion was dose-dependent, rapid-onset (i.e., within 3 min), and reversible. Leptin was equally effective in constraining the enhanced insulin release from islets of ob/ob mice caused by protein kinase C (PKC) activation, a downstream mediator of the PLC signal pathway. One function of leptin in control of body composition is thus to target a PKC-regulated component of the PLC-PKC signaling system within islets to prevent hypersecretion of insulin.

Involvement of G proteins in the effect of carbachol and cholecystokinin in rat pancreatic islets

Phospholipase C is involved in the insulinotropic effect of carbachol (CCh) and cholecystokinin octapeptide (CCK-8). The involvement of the type of G protein was investigated in rat pancreatic islets. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S; a nonhydrolyzable GTP analogue) increased insulin release in electrically permeabilized islets. Both CCh and CCK-8 increased the GTP gamma S effect indicative of an involvement of G proteins. Pretreatment of the islets with pertussis toxin (PT) impaired the CCh-induced insulin secretion in the presence of 3.0 mM glucose and inhibited the stimulatory CCh effect on inositol 1,4,5-trisphosphate (IP3) levels at low and high glucose. In contrast to CCh, the CCK-8 effect on both insulin release and IP3 levels of islets was not modified by a PT pretreatment at various glucose concentrations. Two types of experiments indicate the type of G protein involved: first, long-term agonistic stimulation by either CCh or CCK-8 led to a downregulation of alpha o and alpha q/11, respectively; second, introduction of specific anti-alpha o or -alpha q/11 antibodies into electrically permeabilized islets nearly completely abolished the effects of CCh and CCK-8, respectively. The data indicate that both CCh and CCK-8 act as insulinotropic agents via the phospholipase C system; in the effect of CCh the PT-sensitive alpha o and in the effect of CCK-8 the PT-insensitive alpha q/11 is involved.

Intracellular calcium signals in response to bradykinin in individual neuroblastoma cells

The Ca indicator fura 2 was used to study the modulation of cytoplasmic Ca by bradykinin (Bk) in single N1E-115 murine neuroblastoma cells. Increases in cytoplasmic Ca in response to Bk were mediated by the B2 receptor subtype. Responses to high concentrations of Bk (1-100 nM) were homogeneous and characterized by a rapidly rising transient that decayed to baseline in the continued presence of agonist, with a half-time of 15 s. Responses to low concentrations of Bk (100-500 pM) were more heterogeneous, with longer latencies and often with oscillations. Pretreatment with thapsigargin for 20 min prevented the Ca response, showing that the Ca change results from intracellular Ca release. Removal of external Ca had little effect on the response to Bk, indicating that the agonist does not activate Ca influx. The extent of Ca release and refilling after Bk was tested with ionomycin. A saturating dose of Bk (20 nM) mobilizes > 90% of stored Ca within 30 s, and this is replaced slowly. Replacement of external Na by N-methyl-D-glucamine to block Na/Ca exchange affected the Ca response, causing decreases in latency and in the period of Ca oscillations and increases in overall duration and peak amplitude of the response.

Epidermal growth factor inhibits hormone- and fibroblast growth factor-induced activation of phospholipase C in rat pancreatic acini

Epidermal growth factor (EGF) inhibits cholecystokinin-octapeptide-stimulated amylase release and inositol 1,4,5-trisphosphate (1,4,5-IP3) production in isolated rat pancreatic acini. In the present study, pancreatic acini were used to investigate the effect of EGF on amylase release and 1,4,5-IP3 production induced by secretagogues that activate either phospholipase C-beta (carbachol, bombesin) or phospholipase C-gamma [basic fibroblast growth factor (bFGF)]. The results show that EGF (100 ng/ml) inhibited bombesin (0.1 nM-1 microM)-induced amylase release almost completely. Similarly, the effect of EGF on carbachol-stimulated amylase release was substantial at submaximal (0.1 microM: 44% inhibition), maximal (1 microM: 75% inhibition), and supramaximal (100 microM: 33% inhibition) carbachol concentrations. EGF reduced amylase release at submaximal bFGF concentrations (0.1 nM: 40% inhibition), but not at supramaximal bFGF concentrations (1 and 10 nM). EGF decreased the peak increase of 1,4,5-IP3 in response to bombesin and carbachol (5 s after beginning of the incubation) and bFGF (15 s after beginning of the incubation) by 81 +/- 19%, 65 +/- 15%, and 56 +/- 18%, respectively. Receptor binding characteristics for secretagogues that activate phospholipase C were not influenced by coincubation with EGF excluding heterologous transmembrane receptor modulation. These results suggest that EGF inhibits the action of phospholipase C-beta- and gamma-isoenzyme-activating secretagogues in the exocrine pancreas by a postreceptor mechanism.

Collagen type IV stimulates an increase in intracellular Ca2+ in pancreatic acinar cells via activation of phospholipase C

Intracellular Ca2+ responses to extracellular matrix molecules were studied in suspensions of pancreatic acinar cells loaded with Fura-2. Collagen type I, laminin, fibrinogen and fibronectin were unable to raise cytosolic free Ca2+ concentration ([Ca2+]i), whereas collagen type IV, at concentrations from 5 to 50 micrograms/ml, significantly increased it. The effect of collagen type IV was not due to possible contamination with type-I transforming growth factor beta or plasminogen, as neither of these agents was able to increase [Ca2+]i. Using highly specific mass assays, concentrations of inositol lipids, 1,2-diacylglycerol (DAG) and Ins(1,4,5) P3 were measured in pancreatic acinar cells stimulated with collagen type IV. A decrease in the concentrations of PtdIns(4,5) P2 and PtdIns4 P with a concomitant increase in the concentrations of DAG and InsP3 mass were observed, showing that collagen type IV increases [Ca2+]i by activation of phospholipase C. The observed [Ca2+]i signals had two components, the first resulting from Ca2+ release from the intracellular stores, and the second resulting from Ca2+ flux from the extracellular medium through the verapamil-insensitive channels. A tyrosine kinase inhibitor (tyrphostine) was able to block inositol lipid signalling caused by collagen type IV, which together with the insensitivity of this pathway to cholera toxin and pertussis toxin or to preactivation of protein kinase C, the longer duration of the increase in [Ca2+]i and a longer lag period needed for observation of increases in DAG and InsP3 concentration with collagen type IV than with carbachol (50 mM) suggest that activation of phospholipase C by collagen type IV is caused by tyrosine kinase activation. Inositol lipid signalling and increases in [Ca2+]i were also observed with Arg-Gly-Asp (RGD)-containing peptide but not with Arg-Asp-Gly (RDG)-containing peptide. Collagen type IV and RGD-containing peptide, but not carbachol, competed in increasing [Ca2+]i and DAG concentration, suggesting that the binding site of collagen type IV responsible for phospholipase C activation contains the RGD sequence. Together the present results suggest that, in pancreatic acinar cells, RGD sequence(s) within collagen type IV molecules cause activation of tyrosine kinase, probably through one of the integrin receptors, which then stimulates phospholipase C and increases [Ca2+]i.

Regulation of gap junctional coupling in isolated pancreatic acinar cell pairs by cholecystokinin-octapeptide, vasoactive intestinal peptide (VIP) and a VIP-antagonist

Cholecystokinin-octapeptide (CCK-OP) induces a time- and dose-dependent decrease of gap junctional conductance in isolated pairs of pancreatic acinar cells. In double whole-cell experiments, the time course could be described by the latency and the half-life time (t1/2) of cell-to-cell uncoupling. The latency shows a biphasic dependence on [CCK-OP] with a minimum of about 50 sec at 10(-9) M CCK-OP. In the presence of vasoactive intestinal peptide (VIP), the biphasic relationship is shifted to lower CCK-OP concentrations. The increase of latency at high concentrations of CCK-OP (> 10(-9) M) was blocked by addition of a VIP-antagonist. t1/2 decreases monophasically with increasing [CCK-OP]. Addition of GTP gamma S to the pipette solution suppresses the [CCK-OP] dependence of the latency and potentiates the uncoupling phase. The kinetic data are discussed in terms of CCK binding to receptors of high and low affinity. Evidence is presented that secretion and cell-to-cell coupling are not related by an all-or-none process, but that for physiological CCK-OP concentrations, gap junctional uncoupling follows secretion.

G-proteins coupled to phosphoinositide hydrolysis in the cochlear and vestibular sensory epithelia of the rat are insensitive to cholera and pertussis toxins

In the cochlear (CSE) and vestibular sensory epithelia (VSE), phosphoinositides are hydrolyzed in response to stimulation of phospholipase C (PLC) by cholinergic muscarinic and purinergic P2y agonists. Such receptor-mediated activation of PLC is expected to be coupled through guanine nucleotide-binding proteins (G-proteins). Although several classes of G-proteins have been identified in the inner ear, nothing is known about the type of G-proteins associated with the phosphoinositide second messenger system in CSE and VSE. Phosphoinositide hydrolysis was determined by the release of radiolabeled inositol phosphates (InsPs). Ten mM NaF plus 10 microM AlCl3 increased basal InsPs accumulation 2-fold in both CSE and VSE of the rat. Release of InsPs was also enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) in saponin-permeabilized tissues. Furthermore, release of InsPs stimulated by both carbamylcholine (CCh) and adenosine 5'-O-[3-thiotriphosphate] (ATP-gamma-S) was significantly inhibited by 100 microM guanosine 5'-O-[2-thiodiphosphate] (GDP-beta-S). These results strongly suggest the involvement of G-proteins in the receptor-PLC coupling in CSE and VSE. ADP-ribosylation in membrane fractions of CSE and VSE in the presence of cholera toxin (CTX) or pertussis toxin (PTX) indicated the existence of Gs- and G(i)-type G-proteins. However, neither CTX nor PTX affected basal or agonist-stimulated release of InsPs. These observations suggest that muscarinic and P2y purinergic receptors are coupled to PLC via CTX- and PTX-insensitive G-proteins in CSE and VSE.

Ca2+ oscillations in pancreatic acinar cells: spatiotemporal relationships and functional implications

The pancreatic acinar cells are of particular interest for the study of cytosolic Ca2+ signals, since they are morphologically polarized and generate agonist-specific Ca2+ oscillation patterns. Recent data obtained by combining digital video imaging of Fura-2 fluorescence with patch-clamp whole-cell current recording have provided new information on the spatiotemporal relationships of the cytosolic Ca2+ signals and the Ca(2+)-activated ionic currents. Low agonist concentrations evoke repetitive short-lasting local Ca2+ spikes in the secretory pole region that activate shortlasting current spikes. In the case of acetylcholine stimulation the spikes are confined to this region. When cholecystokinin is used the shortlasting local spikes precede longer Ca2+ transients that spread to the whole of the cell. Infusion of non-metabolizable inositol trisphosphate analogues can mimic these responses. The shortlasting local Ca2+ spikes are particularly sensitive to blockade by the inositol trisphosphate receptor antagonist heparin. These results show that the secretory pole region has a particularly high sensitivity to inositol trisphosphate probably due to clustering of high affinity receptors.

Local and global cytosolic Ca2+ oscillations in exocrine cells evoked by agonists and inositol trisphosphate

Submaximal stimulation with agonists generating inositol 1,4,5-trisphosphate (IP3) evokes cytosolic Ca2+ oscillations in many different cell types. In general, each Ca2+ rise is initiated from a specific region near the plasma membrane and then spreads as a wave throughout the cell. We now demonstrate that low (physiological) agonist concentrations evoke local cytosolic Ca2+ spikes in the secretory pole of single mouse pancreatic acinar cells that are particularly sensitive to blockade by the IP3 receptor antagonist heparin. These spikes can occur alone or repetitively or can precede longer lasting Ca2+ signals that spread throughout the cell. Intracellular IP3 application mimics these agonist actions. The short-lasting local Ca2+ spikes provide an economical signaling mechanism and are of physiological significance since they activate Ca(2+)-dependent Cl- and cation currents important for control of fluid secretion.

Mast cell activation by pedicellarial toxin of sea urchin, Toxopneustes pileolus

Pedicellarial toxin, partially purified from the sea urchin Toxopneustes pileolus, dose-dependently and time-dependently caused histamine release from rat peritoneal mast cells. Pedicellarial toxin induced a rapid initial rise in [Ca2+]i within several seconds which was followed by a further slower increase of [Ca2+]i (second rise). The toxin induced a dose-dependent formation of inositol 1,4,5-triphosphate (IP3) as well as the histamine release in mast cells. Furthermore, the toxin stimulated phosphoinositide-specific phospholipase C (PI-PLC) activity in mast cell membranes. 2-Nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC), a PLC inhibitor, inhibited the activation of PI-PCL induced by pedicellarial toxin. Cholera toxin inhibited pedicellarial toxin-induced histamine release, whereas pretreatment of pertussis toxin failed to inhibit it. These results suggest that pedicellarial toxin from T. pileolus activates PI-PCL and the stimulation of PI turnover may lead to the release of IP3 into the cytoplasm, resulting in histamine release from rat mast cells.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Intracellular calcium 'signatures' evoked by different agonists in isolated bovine aortic endothelial cells

Agonist induced increases in intracellular free calcium, [Ca2+]i, were measured in single Fura-2 loaded bovine aortic endothelial (BAE) cells by dual wavelength microspectrofluorimetry. Low doses of ATP (less than 10 microM) induced complex changes in [Ca2+]i. These changes usually consisted of a large initial transient peak with subsequent fluctuations superimposed upon a maintained rise in [Ca2+]i. Higher doses of ATP (greater than 50 microM) produced much simpler biphasic increases in [Ca2+]i in individual cells. Acetylcholine and bradykinin also elicited increases in [Ca2+]i in single cells in confluent monolayers of endothelial cells. However, only acetylcholine produced complex fluctuations. High doses of acetylcholine evoked simple rises in [Ca2+]i similar to those seen with high doses of ATP. In contrast, bradykinin evoked relatively simple rises in [Ca2+]i at all doses used. These results indicate that the mechanisms responsible for generating agonist induced increases in [Ca2+]i in BAE cells are not homogeneous. ATP and acetylcholine produced more complex Ca2+ changes or 'signatures' in single confluent bovine aortic endothelial cells than bradykinin. All three agonists appeared to release Ca2+ from intracellular stores as well as stimulating Ca2+ influx. The possible mechanisms underlying these phenomena are discussed.

Epidermal growth factor inhibits both cholecystokinin octapeptide-induced inositol 1,4,5-trisphosphate production and [CA2+]i increase in rat pancreatic acinar cells

We have studied the effects of epidermal growth factor (EGF) on both cholecystokinin octapeptide (CCK-OP)-induced inositol-1,4,5 trisphosphate (IP3) production and on cytosolic free calcium concentrations [Ca2+]i by fluorescence measurements in fura-2-loaded pancreatic acini. Our data show that EGF inhibits CCK-OP induced IP3 production by 40 +/- 9% and decreases CCK-OP induced rise in cytosolic Ca2+ by 41 +/- 9%. These data indicate that activation of EGF receptors leads to inhibition of CCK-OP induced stimulation of phospholipase C (PLC).

Effects of cholecystokinin, cholecystokinin JMV-180 and GTP analogs on enzyme secretion from permeabilized acini and chloride conductance in isolated zymogen granules of the rat pancreas

Previous studies have shown that hormonal activation of the Cl- conductance in pancreatic zymogen granules (ZG) is closely related to enzyme secretion from acinar cells. We have now examined the role of guanine nucleotides in stimulated and unstimulated protein secretion from isolated digitonin-permeabilized pancreatic acini and in the Cl- conductance of isolated ZG. Protein secretion from permeabilized isolated acini, measured at 0.1 mM Ca2+, increased with increasing cholecystokinin octapeptide (CCK-8) concentrations and decreased at high CCK-8 concentrations. The maximum secretion, approximately twice the control level, was reached at 1 nM CCK-8. The CCK analog, CCK JMV-180, which supposedly acts as an agonist on high-affinity CCK receptors and as an antagonist on low-affinity CCK receptors, stimulated maximum enzyme secretion at a CCK JMV-180 concentration of 0.1 microM and no decrease in secretion was observed at higher CCK JMV-180 concentrations, 0.1 mM guanosine 5'-[gamma-thio]triphosphate (GTP [S]) also increased the protein release by approximately twice that of the control and shifted the CCK-8 concentration causing maximum stimulation from 1 nM to 0.01 nM. GTP[S] concentrations greater than 0.1 mM inhibited protein release evoked by an optimal concentration of 1 nM CCK-8, 0.1 mM GTP[S] had no pronounced effect on the protein secretion stimulated by low concentrations of CCK JMV-180, but inhibited protein secretion evoked by CCK JMV-180 concentrations greater than 0.1 microM. This indicates that guanosine-nucleotide-binding proteins [G protein(s)] coupling to CCK receptors also mediate both CCK-induced increases and CCK-induced decreases of enzyme secretion at low and high CCK concentrations, respectively. ZG were prepared on a Percoll gradient from CCK-8-stimulated or CCK-JMV-180-stimulated and unstimulated acini. Their Cl- conductances were estimated in the absence of Ca2+ and in the presence of 1 mM EGTA from the rate of decrease in absorbance following addition of the K+ ionophore valinomycin as a measure of ZG osmotic lysis. The Cl- conductance in ZG from CCK-8-stimulated and CCK-JMV-180-stimulated acini was maximally activated at 1 pM and 10 nM respectively. At higher agonist concentrations, Cl- conductance was decreased. Direct addition of 10 microM GTP[S] to isolated ZG from unstimulated acini increased the rate of lysis by approximately 40% of the control value. This effect was approximately additive to that of CCK-8 or of CCK JMV-180 prestimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Biphasic kinetics of inositol 1,4,5-trisphosphate accumulation in gastrin-stimulated parietal cells. Effects of pertussis toxin and extracellular calcium

The effects of Pertussis toxin (PTx) and extracellular Ca2+ were investigated on gastrin-induced Ins(1,4,5)P3 mass level in isolated gastric parietal cells. Basal Ins(1,4,5)P3 content was 5.48 +/- 0.49 pmol/500,000 cells. Gastrin (10 nM) induced a rapid increase in Ins(1,4,5)P3 content which was maximal after 15 s and corresponded to 2-2.5-fold basal level; this Ins(1,4,5)P3 content then decreased within 30 s. After a longer time of gastrin exposure (greater than 1 min), a sustained and unexpected increase in Ins(1,4,5)P3 accumulation was observed which was maximal at 7.5 min (corresponding to 2.3-2.8-fold basal value) and slightly decreased thereafter. PTx treatment of cells (200 ng/ml) for 3 h or removal of extracellular Ca2+ did not affect the rapid rise, but drastically reduced the sustained increase in Ins(1,4,5)P3 content (60-100% inhibition); this inhibition was not evident after 10 min of hormone stimulation. Furthermore, diltiazem, a Ca2+ channel blocker, led to a similar inhibition of the sustained increase. We concluded that: (i) gastrin induced a rapid increase in Ins(1,4,5)P3 content via a mechanism insensitive to PTx and to extracellular Ca2+, and (ii) gastrin induced a sustained increase in Ins(1,4,5)P3 level via a mechanism sensitive to PTx and to extracellular Ca2+. Even though the rapid rise in Ins(1,4,5)P3 content may be involved in the intracellular Ca2+ mobilization occurring after the first seconds of hormone stimulation, the physiological role of the sustained Ins(1,4,5)P3 increased level remains to be elucidated.

Receptors for insulin interact with Gi-proteins and for epidermal growth factor with Gi- and Gs-proteins in rat pancreatic acinar cells

In rat pancreatic acinar cells epidermal growth factor (EGF) and insulin increase both basal and cholecystokinin (CCK-OP) stimulated amylase release in vitro (1) as a long term function of this tissue. Here we show that preincubation of isolated plasma membranes with EGF or with insulin leads to increased incorporation of the GTP-photoaffinity analogue [alpha-32P]GTP-gamma-azidoanilide into 40/41 kDa proteins and to reduction of pertussis toxin- (PT) catalyzed [alpha-32P]ADP-ribosylation of three 40/41 kDa proteins which had been previously identified as Gi1, Gi2 and Gi3 (2). In the presence of GTP gamma S, EGF- and insulin-induced inhibition of PT-mediated [alpha-32P]ADP-ribosylation of 40/41 kDa proteins is eliminated. EGF enhances cholera toxin- (CT) mediated ADP-ribosylation of all three 40/41 kDa Gi-proteins as well as of five 45 and four 48/50 kDa proteins, which had been previously identified as Gs-proteins (2), whereas insulin has no effect. We conclude from our data that both EGF and insulin interact with the same Gi-proteins as CCK-OP does, whereas EGF additionally interacts with nine Gs-proteins. It is likely that one, two or all three 40/41 kDa Gi-proteins are involved in insulin- and EGF-induced potentiation of CCK-OP-stimulated enzyme secretion. In addition interaction of EGF with Gs-protein could play a role in the potentiation of CCK-OP-induced enzyme secretion from pancreatic acinar cells.

Acetylcholine and cholecystokinin induce different patterns of oscillating calcium signals in pancreatic acinar cells

Receptor-activated cytoplasmic calcium (Ca2+) oscillations have been investigated in single pancreatic acinar cells by microfluorimetry (Fura-2 as indicator). At submaximal concentrations of the agonists acetylcholine (ACh) and cholecystokinin octapeptide (CCK-8), both give rise to oscillatory changes in the cytosolic free calcium concentration ([Ca2+]i). The patterns of oscillations are markedly and consistently different for each of these two agonists. The ACh induced oscillations are superimposed upon a median elevation in background [Ca2+]i. The CCK-8 induced oscillations are of longer duration with [Ca2+]i returning to prestimulus levels between the discrete spikes. The ACh induced oscillations are rapidly abolished upon removal of extracellular Ca2+ while the CCK-8 induced oscillations persist for many minutes in the absence of external Ca2+. The CCK-8, but not the ACh, induced oscillations are increased in duration by the protein kinase C (PKC) inhibitor staurosporine and abolished by the PKC activating phorbol ester PMA. It is clear that CCK-8 and ACh do not activate receptor transduction mechanisms in an identical manner to generate oscillating [Ca2+]i signals.

Gap junctional permeability is affected by cell volume changes and modulates volume regulation

Isolated pancreatic acinar cell pairs became electrically uncoupled by exposure to a mild hypotonic shock. Reduction of bath osmolarity caused a delayed closure of gap junctional channels in the minute range. Dialysis of cell pairs by GTP[S] in the double whole-cell patch-clamp mode shortened the latency and shifted the hypotonically induced electrical uncoupling to lower osmolarity changes. Cellular treatment with cytochalasin B catalyzed electrical uncoupling by a hypotonic shock. In all cases, electrical uncoupling could be blocked completely by the protein kinase C (PKC) inhibitor polymyxin B. These results provide the first evidence suggesting that changes of cell volume and gap junctional permeability are correlated and that a G-protein dependent mechanism is involved. Evidence is presented that gap junctional coupling modulates volume regulation.

Receptor-activated cytoplasmic Ca2+ spiking mediated by inositol trisphosphate is due to Ca2(+)-induced Ca2+ release

Receptor-mediated inositol 1,4,5-trisphosphate (Ins-(1,4,5)P3) generation evokes fluctuations in the cytoplasmic Ca2+ concentration ([Ca2+]i). Intracellular Ca2+ infusion into single mouse pancreatic acinar cells mimicks the effect of external acetylcholine (ACh) or internal Ins(1,4,5)P3 application by evoking repetitive Ca2+ release monitored by Ca2(+)-activated Cl- current. Intracellular infusion of the Ins(1,4,5)P3 receptor antagonist heparin fails to inhibit Ca2+ spiking caused by Ca2+ infusion, but blocks ACh- and Ins(1,4,5)P3-evoked Ca2+ oscillations. Caffeine (1 mM), a potentiator of Ca2(+)-induced Ca2+ release, evokes Ca2+ spiking during subthreshold intracellular Ca2+ infusion. These results indicate that ACh-evoked Ca2+ oscillations are due to pulses of Ca2+ release through a caffeine-sensitive channel triggered by a small steady Ins(1,4,5)P3-evoked Ca2+ flow.

The molecular basis of enzyme secretion

Acinar cells are one of the best studied models of exocytotic secretion. A number of different hormones and neurotransmitters interact with specific membrane receptors, and it is commonly held that pancreatic secretagogues stimulate enzyme release via the elevation of either cytosolic free Ca2+ or cellular cyclic adenosine monophosphate. The discovery of the pivotal role played by phospholipid metabolism in the chain of events leading to secretion, together with the introduction of sensitive techniques to monitor cytosolic free Ca2+, has generated a series of studies that have challenged this classical model. Thus, several observations in pancreatic acini as well as other cell types have argued against the notion that a generalized increase in cytosolic free Ca2+ represents a sufficient and necessary stimulus for exocytosis in nonexcitable cells. Furthermore, the demonstration that a single agonist activates multiple transduction pathways has served to refute the schematic view that receptor agonists activate only one second messenger system. The aim of this article is to review the recent advances in understanding the molecular and cellular mechanisms of signal transduction, with particular emphasis on the inositol lipid pathway, and to integrate this information into a new working model of enzyme secretion from acinar cells.

Cholecystokinin activates Gi1-, Gi2-, Gi3- and several Gs-proteins in rat pancreatic acinar cells

On separation of rat pancreatic plasma membrane proteins by two-dimensional gel electrophoresis, 15 GTP-binding protein (G-protein) alpha-subunits could be detected immunochemically using an alpha common antibody. These consisted of five 48 kDa proteins (pI 5.70, 5.80, 5.90, 6.10 and 6.25) and five 45 kDa proteins (pI 5.90, 6.05, 6.25, 6.30 and 6.70), presumably corresponding to low- and high-molecular mass forms of the Gs-protein, as well as three 40/41 kDa proteins (pI 5.50, 5.70 and 6.00) and two 39 kDa proteins (pI 5.50 and 6.00). All of these proteins except for the more acidic 39 kDa protein were ADP-ribosylated by cholera toxin (CT). In addition, the three 40/41 kDa proteins and the more alkaline 39 kDa protein were also ADP-ribosylated by pertussis toxin (PT). CT- and PT-induced ADP-ribosylation changed the pI values of G-protein alpha-subunits by 0.2 pI units to more acidic values. Preincubation of isolated pancreatic membranes with cholecystokinin octapeptide (CCK-OP), which stimulates phospholipase C in acinar cells, decreased CT-induced as well as PT-induced ADP-ribosylation of the three 40/41 kDa proteins, whereas CT-induced ADP-ribosylation of one 45 kDa (pI 5.80) and all 48 kDa proteins was enhanced in the presence of CCK. Carbachol, another stimulant of phospholipase C, had no effect. The three 40/41 kDa proteins and one 48 kDa protein could be labelled with the GTP analogue [alpha-32P]GTP-gamma-azidoanilide. CCK, but not carbachol, stimulated incorporation of the GTP analogue into all of these four proteins. Using different anti-peptide antisera specific for alpha-subunits of G-proteins we identified the three 40/41 kDa Gi-proteins as Gi1 (pI 6.00), Gi2 (pI 5.50) and Gi3 (pI 5.70). The Gi3-protein was found to be the major Gi-protein of pancreatic plasma membranes. One of the 39 kDa proteins (pI 6.0) was identified as Go. These results indicate that CCK receptors functionally interact with six Gs-proteins and with Gi1, Gi2 and Gi3-proteins. Since evidence suggests that a 40/41 kDa CT substrate is involved in the stimulation of phospholipase C in pancreatic acinar cells, it is likely that one, two or all three 40/41 kDa Gi-proteins are involved in the coupling of CCK receptors with phospholipase C.

Rhodopsin activation causes retinal degeneration in Drosophila rdgC mutant

Drosophila rdgC (retinal degeneration-C) mutants show normal retinal morphology and photoreceptor physiology at young ages. Dark-reared rdgC flies retain this wild-type phenotype, but light-reared mutants undergo retinal degeneration. rdgC photoreceptors with low levels of rhodopsin as a result of vitamin A deprivation or a mutant rhodopsin (ninaE) gene fail to show rdgC-induced degeneration even after prolonged light treatment, demonstrating that degeneration occurs as a result of light stimulation of rhodopsin. Analysis of norpA; rdgC flies shows that the norpA-encoded phospholipase C, the target enzyme of the G protein activated by rhodopsin, is not required for rdgC-induced degeneration. Thus the rdgC+ gene product is required to prevent retinal degeneration that results from a previously unrecognized consequence of rhodopsin stimulation.

A G-protein mediates secretagogue-induced gap junctional channel closure in pancreatic acinar cells

Using the double whole-cell patch-clamp technique, we determined that dialysis of cell pairs by GTP[S] potentiated electrical uncoupling induced by extracellular addition of carbamylcholine (CCh). An inhibitor of diglyceride lipase, RHC 80267, further potentiated CCh/GTP[S]-induced junctional channel closure, probably by accumulation of diacylglycerol. Moreover, the protein kinase C inhibitor polymyxin B completely blocked uncoupling elicited by CCh/GTP[S]. These results provide the first evidence suggesting that gap junction channel closure by cholinergic stimulation is mediated by a G-protein, which acts by increasing phosphatidylinositol biphosphate breakdown and protein kinase C activity.

Muscarinic receptor-stimulated phosphoinositide turnover in human SK-N-SH neuroblastoma cells: differential inhibition by agents that elevate cyclic AMP

The possibility that an increased intracellular concentration of cyclic AMP (cAMP) can regulate the extent of muscarinic receptor-stimulated phosphoinositide (PPI) turnover in the human neuroblastoma cell line SK-N-SH was examined. Addition of either forskolin (or its water-soluble analog, L-85,8051), theophylline, isobutylmethylxanthine, or cholera toxin, agents that interact with either the catalytic unit of adenylate cyclase, cAMP phosphodiesterase, or the guanine nucleotide binding protein linked to adenylate cyclase activation, resulted in a 45-181% increase in cAMP concentration and a 27-70% inhibition of carbachol-stimulated inositol phosphate release. Through the use of digitonin-permeabilized cells, the site of inhibition was localized to a step at, or distal to, the guanine nucleotide binding protein that regulates phospholipase C activity. In contrast, when intact SK-N-SH cells were exposed to prostaglandin E1, the ensuing increases in cAMP were not accompanied by an inhibition of stimulated PPI turnover. These differential effects of increased cAMP concentrations on stimulated PPI turnover may reflect the compartmentation of cAMP within SK-N-SH cells.

Excitation of locus coeruleus neurons by vasoactive intestinal peptide: evidence for a G-protein-mediated inward current

Vasoactive intestinal polypeptide (VIP) caused a reversible increase in the firing rate of locus coeruleus (LC) neurons. Voltage-clamp at -60 mV revealed that VIP induced an inward current associated with a small increase in conductance. The inward current persisted in the presence of Co2+ (to block Ca2+ channels) or tetrodotoxin (to block fast voltage-dependent Na+ channels). Substitution (80%) of Na+ with choline or Tris reduced the VIP-elicited inward current by approximately 75%. Changing external K+ concentrations did not alter the effect of VIP. The inward current induced by VIP became irreversible after the intracellular administration of GTP gamma S, a hydrolysis-resistant analog of GTP which can cause a prolonged activation of G-proteins. The intracellular application of GDP beta S, which can interfere with G-protein activation, attenuated the effect of VIP. Pertussis toxin, an inactivator of certain G-proteins, did not block the effect of VIP. We conclude that VIP directly excites LC neurons by inducing a largely Na-dependent inward current. As this effect became irreversible in the presence of intracellular GTP gamma S, was attenuated by GDP beta S, and was not eliminated by pertussis toxin, mediation through a pertussis toxin-insensitive G-protein is suggested.

Characterization of inositol 1,4,5-trisphosphate-sensitive (IsCaP) and -insensitive (IisCaP) nonmitochondrial Ca2+ pools in rat pancreatic acinar cells

We have measured Ca2+ uptake and Ca2+ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca2+ uptake into cells was monitored with a Ca2+ electrode, whereas Ca2+ uptake into membrane vesicles was measured with 45Ca2+. Using inhibitors of known action, such as the H+ ATPase inhibitors NBD-Cl and NEM, the Ca2+ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP3) and its analog inositol 1,4,5-trisphosphorothioate (IPS3), we could functionally differentiate two nonmitochondrial Ca2+ pools. Ca2+ uptake into the IP3-sensitive Ca2+ pool (IsCaP) occurs by a MgATP-dependent Ca2+ uptake mechanism that exchanges Ca2+ for H+ ions. In the absence of ATP Ca2+ uptake can occur to some extent at the expense of an H+ gradient that is established by a vacuolar-type MgATP-dependent H+ pump present in the same organelle. The other Ca2+ pool takes up Ca2+ by a vanadate-sensitive Ca2+ ATPase and is insensitive to IP3 (IisCaP). The IsCaP is filled at "higher" Ca2+ concentrations (approximately 10(-6) mol/liter) which may occur during stimulation. The low steady-state [Ca2+] of approximately 10(-7) mol/liter is adjusted by the IisCaP. It is speculated that both Ca2+ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.

Inhibition of electrical coupling in pairs of murine pancreatic acinar cells by OAG and isolated protein kinase C

Gap junctional coupling was studied in pairs of murine pancreatic acinar cells using the double whole-cell patch-clamp technique. During stable electrical coupling, addition of OAG (1-oleoyl-2-acetyl-sn-glycerol) induced a progressive reduction of the junctional conductance to the detectable limit (approximately 3 pS). Prior to complete electrical uncoupling, various discrete single channel conductances between 20 and 100 pS could be observed. Polymyxin B, a potent inhibitor of the protein kinase C (PKC) system, completely suppressed OAG-stimulated electrical uncoupling. Dialysis of cell pairs with solutions containing PKC, isolated from rat brain, also caused electrical uncoupling. The presence of 0.1 mM dibutyryl cyclic AMP and 5 mM ATP in the pipette solution, which serves to stabilize the junctional conductance, did not suppress the effects of OAG or isolated PKC. We conclude that an increase of protein kinase C activity leads to the closure of gap junction channels, presumably via a PKC-dependent phosphorylation of the junctional peptide, and that this mechanism is dominant over cAMP-dependent upregulatory effects in the experimental time range (less than or equal to 1 hr). A correlation of the observed single channel conductances with the appearance of channel subconductance states or various channel populations is discussed.

Diffusion into the patch-clamp recording pipette of a factor necessary for muscarinic current response

Ca-dependent K and Cl currents were measured in isolated cells from rat lacrimal glands using the tight-seal whole-cell recording method. Upon application of acetylcholine (ACh), both K and Cl-selective currents were activated. The size of the ACh-activated currents declined after a few minutes of whole-cell recording. The rundown curve was composed of an initial stable period followed by a rather rapid decline. Both the length of the initial plateau and the speed of the falling phase were dependent on cell size and recording pipette resistance. The results suggest that the rundown was due to washout of an unknown cytosolic substance. Another manifestation of washout was an increase in the delay of the response. Plots of the inverse of the delay as a function of time in whole-cell recording showed again an initial plateau and a falling phase, but the stable period lasted less than in amplitude plots. Analysis of the washout time course suggested that the cytosolic substance has a diffusion coefficient of 5.4 x 10(-6) cm2/s, corresponding to a molecular weight of 350. Washed-out cells were insensitive to GTP-gamma-S, but responded normally to an internal application of inositol-trisphosphate (InsP3), introduced through the pipette. Thus, the step of the response which is sensitive to washout is closely related to the production of InsP3. Addition of various exogenous water soluble substances failed to halt washout. Among the inactive substances were GTP (or a combination of Mg and GTP) and small water soluble precursors of InsP3. The results imply that the production of InsP3 by muscarinic agonists in exocrine glands requires the presence of a small molecular weight, water soluble substance. It is suggested that this substance is an unknown co-factor of phospholipase C or of Gp, the GTP binding protein governing the production of InsP3.

Evidence for a cholera-toxin-sensitive G-protein involved in the regulation of phosphatidylinositol 4-phosphate kinase of rat liver membranes

Studies on the phosphorylation of inositol phospholipids of rat liver membranes have shown that [gamma S]pppG stimulates 32P incorporation from [gamma-32P]ATP into PI and PIP. This effect appeared specific for stable GTP analogues and could not be reproduced by other compounds. ADP-ribosylation of the membranes with cholera toxin resulted in a large decrease of PIP2 without changes in the level of PIP. Since an activation of phospholipase C can be ruled out, the lowering of PIP2 is explained on the basis of an inhibition of PIP kinase (EC 2.7.1.68). From these results it appears that a novel cholera-toxin-sensitive G-protein is involved in the regulation of PIP kinase.

Photoaffinity labeling with GTP-gamma-azidoanilide of a cholera toxin-sensitive 40 kDa protein from pancreatic acinar cells

In isolated pancreatic acinar plasma membranes a 40 kDa protein was labeled with the photoreactive GTP-analogue [alpha 32P] GTP-gamma-azidoanilide. Increased incorporation of the photolabel into the 40 kDa protein was obtained in the presence of increasing concentrations of cholecystokinin-octapeptide (10(-8) - 10(-5) M) but not with carbachol. Adenylyl cyclase activating hormones such as vasoactive intestinal polypeptide and secretin had no effect. Pretreatment of plasma membranes with cholera toxin reduced incorporation of GTP-gamma-azidoanilide into the 40 kDa protein by about 30%. This reduction was reversed if ADP-ribosylation by cholera toxin was performed in the presence of cholecystokinin, whereas carbachol had no effect. The data indicate that a cholera toxin-sensitive 40 kDa GTP-binding protein is involved in functionally coupling cholecystokinin- but not muscarinic acetylcholine-receptors to phospholipase C.

[Guanidine nucleotide binding proteins as membrane signal transduction components and regulators of enzymatic effectors]

The vast majority of extracellular signals alters cell function by activating cell surface receptors. The transmembranous signalling process initiated by an activated receptor leads to the generation of an intracellular signal and eventually to a cellular response. In contrast to receptors that are permanently coupled to an enzyme or an ion channel representing the effector, a large number of surface receptors for hormones, neurotransmitters and receptors for exogenous chemical or physical stimuli reversibly interacts with membranous signal transduction components which, in turn, regulate intracellular messenger-generating effectors. The transducer molecules isolated so far form a family of guanine nucleotide-binding proteins (G- or N-proteins). All isolated G-proteins are composed of three different subunits (alpha, beta, gamma). The alpha-subunit, which is specific for the individual G-protein, binds and hydrolyzes GTP and is target of ADP-ribosylating bacterial toxins. Hormone-induced activation of a receptor causes interaction with the alpha-subunit of a G-protein and the exchange of bound GDP with GTP. The GTP-bound form of the alpha-subunit represents the active form of the G-protein, which is capable of stimulating or inhibiting the respective effector. The active state of the alpha-subunit is terminated by its inherent GTPase activity causing hydrolysis of bound GTP. The beta gamma-complexes of G-proteins are structurally very similar and functionally interchangeable; they appear to dissociate from the alpha-subunits during receptor activation of the G-protein. Possible functions of the beta gamma-complex are to anchor the non-activated G-protein in the membrane, to facilitate G-protein-receptor interaction, and to promote the inactive state of the alpha-subunit. G-protein-regulated effectors include enzymes, ion channels and probably transporters. The best studied G-protein-regulated enzyme is the retinal cyclic GMP-phosphodiesterase which is activated by bleached rhodopsin via the tissue-specific G-protein, termed transducin. The ubiquitously occurring membrane-bound adenylate cyclase is under dual control by families of stimulatory and inhibitory receptors, acting via G-proteins called Gs and Gi, respectively. Moreover, the receptor control of phospholipases A2 and C and probably of phospholipase D most likely involves G-proteins which have not yet been identified. Finally, the activity of NADPH oxidase of neutrophils and that of cyclic AMP phosphodiesterases in liver and fat cells may be regulated via G-proteins. Modulations of non-enzymatic effectors are reviewed elsewhere.