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

CCK activates RhoA and Rac1 differentially through Galpha13 and Galphaq in mouse pancreatic acini

Cholecystokinin (CCK) has been shown to activate RhoA and Rac1, as well as reorganize the actin cytoskeleton and, thereby, modify acinar morphology and amylase secretion in mouse pancreatic acini. The aim of the present study was to determine which heterotrimeric G proteins activate RhoA and Rac1 upon CCK stimulation. Galpha(13), but not Galpha(12), was identified in mouse pancreatic acini by RT-PCR and Western blotting. Using specific assays for RhoA and Rac1 activation, we showed that only active Galpha(13) activated RhoA. By contrast, active Galpha(13) and Galpha(q), but not Galpha(s), slightly increased GTP-bound Rac1 levels. A greater increase in Rac1 activation was observed when active Galpha(13) and active Galpha(q) were coexpressed. Galpha(i) was not required for CCK-induced RhoA or Rac1 activation. The regulator of G protein signaling (RGS) domain of p115-Rho guanine nucleotide exchange factor (p115-RGS), a specific inhibitor of Galpha(12/13)-mediated signaling, abolished CCK-stimulated RhoA activation. By contrast, both RGS-2, an inhibitor of Galpha(q), and p115-RGS abolished CCK-induced Rac1 activation, which was PLC pathway-independent. Active Galpha(q) and Galpha(13), but not Galpha(s), induced morphological changes and actin redistribution similar to 1 nM CCK. CCK-induced actin cytoskeletal reorganization was inhibited by RGS-2, but not by p115-RGS, whereas CCK-induced amylase secretion was blocked by both inhibitors. Together, these findings indicate that, in mouse pancreatic acini, Galpha(13) links CCK stimulation to the activation of RhoA, whereas both Galpha(13) and Galpha(q) link CCK stimulation to the activation of Rac1. CCK-induced actin cytoskeletal reorganization is mainly mediated by Galpha(q). By contrast, Galpha(13) and Galpha(q) signaling are required for CCK-induced amylase secretion.

Smooth muscle function and dysfunction in gallbladder disease

The gallbladder epithelium and smooth muscle layer are exposed to concentrated biliary solutes, including cholesterol and potentially toxic hydrophobic bile salts, which are able to influence muscle contraction. Physiologically, gallbladder tone is regulated by spontaneous muscle activity, hormones, and neurotransmitters released into the muscle from intrinsic neurons and extrinsic sympathetic nerves. Methods to explore gallbladder smooth muscle function in vitro include cholecystokinin (CCK) receptor-binding studies and contractility studies. In human and animal models, studies have focused on cellular and molecular events in health and disease, and in vitro findings mirror in vivo events. The interplay between contraction and relaxation of the gallbladder muscularis leads in vivo to appropriate gallbladder emptying and refilling during fasting and postprandially. Defective smooth muscle contractility and/or relaxation are found in cholesterol stone-containing gallbladders, featuring a type of gallbladder leiomyopathy; defects of CCKA receptors and signal transduction may coexist with abnormal responses to oxidative stress and inflammatory mediators. Abnormal smooth musculature contractility, impaired gallbladder motility, and increased stasis are key factors in the pathogenesis of cholesterol gallstones.

CCK-A receptor activates RhoA through G alpha 12/13 in NIH3T3 cells

Cholecystokinin (CCK) is a major regulator of pancreatic acinar cells and was shown previously to be capable of inducing cytoskeletal changes in these cells. In the present study, using NIH3T3 cells stably transfected with CCK-A receptors as a model cell, we demonstrate that CCK can induce actin stress fibers through a G13- and RhoA-dependent mechanism. CCK induced stress fibers within minutes similar to those induced by lysophosphatidic acid (LPA), the active component of serum. The effects of CCK were mimicked by active RhoV14 and blocked by dominant-negative RhoN19, Clostridium botulinum C3 transferase, and the Rho-kinase inhibitor Y-27632. CCK rapidly induced active Rho in cells as shown with a pull-down assay using the Rho binding domain of rhotekin and by a serum response element (SRE)-luciferase reporter assay. To evaluate the G protein mediating the action of CCK, cells were transfected with active alpha-subunits; Galpha13 and Galpha12 but not Galphaq induced stress fibers and in some cases cell rounding. A p115 Rho guanine nucleotide exchange factor (GEF) regulator of G protein signaling (RGS) domain known to interact with G12/13 inhibited active alpha12/13-and CCK-induced stress fibers, whereas RGS2 and RGS4, which are known to inhibit Gq, had no effect. Cotransfection with plasmids coding for the G protein alpha-subunit carboxy-terminal peptide from alpha13 and, to a lesser extent alpha12, also inhibited the effect of CCK, whereas the peptide from alphaq did not. These results show that in NIH3T3 cells bearing CCK-A receptors, CCK activates Rho primarily through G13, leading to rearrangement of the actin cytoskeleton.

Influence of cholecystitis state on pharmacological response to cholecystokinin of isolated human gallbladder with gallstones

We studied the influence of the inflammatory state of the gallbladder with gallstones on its response to cholecystokinin (CCK). Responses to CCK were evaluated in isolated human gallbladder strips incubated with pharmacological antagonists. Gallbladders from patients with gallstones were classified as having mild and severe chronic cholecystitis. Healthy gallbladders were collected from liver donors. In donor gallbladders, the CCK contraction was abolished with the CCK-A receptor antagonist, L-364718, and significantly reduced by indomethacin. In gallbladders with gallstones, only mild cholecystitis showed a decreased contraction to CCK. In gallbladders with gallstones, no involvement of prostaglandins in the CCK response was observed. In severe cholecystitis, CCK contractile effect was reduced by the serotonin receptor antagonist methysergide. In healthy gallbladder, the contraction provoked by CCK is mediated by CCK-A receptors and modulated by prostaglandins. The presence of gallstones in the gallbladder is correlated with a loss of prostaglandins-modulated CCK contraction. However, the excessive release of serotonin in advanced cholecystitis normalizes the contraction to CCK, suggesting that the state of cholecystitis affects the pool of inflammatory mediators responsible for gallbladder CCK-altered motility.

Cholecystokinin stimulates extracellular signal-regulated kinase through activation of the epidermal growth factor receptor, Yes, and protein kinase C. Signal amplification at the level of Raf by activation of protein kinase Cepsilon

Cholecystokinin (CCK) and related peptides are potent growth factors in the gastrointestinal tract and may be important for human cancer. CCK exerts its growth modulatory effects through G(q)-coupled receptors (CCK(A) and CCK(B)) and activation of extracellular signal-regulated protein kinase 1/2 (ERK1/2). In the present study, we investigated the different mechanisms participating in CCK-induced activation of ERK1/2 in pancreatic AR42J cells expressing both CCK(A) and CCK(B). CCK activated ERK1/2 and Raf-1 to a similar extent as epidermal growth factor (EGF). Inhibition of EGF receptor (EGFR) tyrosine kinase or expression of dominant-negative Ras reduced CCK-induced ERK1/2 activation, indicating participation of the EGFR and Ras in CCK-induced ERK1/2 activation. However, compared with EGF, CCK caused only small increases in tyrosine phosphorylation of the EGFR and Shc, Shc-Grb2 complex formation, and Ras activation. Signal amplification between Ras and Raf in a CCK-induced ERK cascade appears to be mediated by activation of protein kinase Cepsilon (PKCepsilon), because 1) down-modulation of phorbol ester-sensitive PKCs inhibited CCK-induced activation of Ras, Raf, and ERK1/2 without influencing Shc-Grb2 complex formation; 2) PKCepsilon, but not PKCalpha or PKCdelta, was detectable in Raf-1 immunoprecipitates, although CCK activated all three PKC isoenzymes. In addition, the present study provides evidence that the Src family tyrosine kinase Yes is activated by CCK and mediates CCK-induced tyrosine phosphorylation of Shc. Furthermore, we show that CCK-induced activation of the EGFR and Yes is achieved through the CCK(B) receptor. Together, our data show that different signals emanating from the CCK receptors mediate ERK1/2 activation; activation of Yes and the EGFR mediate Shc-Grb2 recruitment, and activation of PKC, most likely PKCepsilon, augments CCK-stimulated ERK1/2 activation at the Ras/Raf level.

A role for phosphorylation of inositol 1,4,5-trisphosphate receptors in defining calcium signals induced by Peptide agonists in pancreatic acinar cells

Stimulation of pancreatic acinar cells with acetylcholine (ACh) and cholecystokinin (CCK) results in an elevation of cytosolic calcium ([Ca(2+)](c)) through activation of inositol 1,4,5-trisphosphate receptors (InsP(3)R). The global temporal pattern of the [Ca(2+)](c) changes produced by ACh or CCK stimulation differs significantly. The hypothesis was tested that CCK stimulation results in a protein kinase A (PKA)-mediated phosphorylation of InsP(3)R and this event contributes to the generation of agonist-specific [Ca(2+)](c) signals. Physiological concentrations of CCK evoked phosphorylation of the type III InsP(3)R, which was blocked by pharmacological inhibition of PKA. Imaging of fura-2-loaded acinar cells revealed that the rate of [Ca(2+)](c) rise during CCK-evoked oscillations slows with each subsequent oscillation, consistent with a developing modulation of release, whereas the kinetics of ACh-evoked oscillations remain constant. Stimulation of cells with ACh following activation of PKA resulted in a slowing of the ACh-evoked [Ca(2+)](c) rise, which now resembled a time-matched CCK response. PKA activation also resulted in a slowing of [Ca(2+)](c) increases elicited by photolysis of caged InsP(3). Targeted, PKA-mediated phosphorylation of type III InsP(3)R is involved in a physiological CCK response, as disruption of the targeting of PKA with the peptide HT31 resulted in marked changes in the CCK-evoked [Ca(2+)](c) signal but had no effect on ACh-evoked responses. Stimulation of cells with bombesin, which evokes [Ca(2+)](c) oscillations indistinguishable from those produced by CCK, also results in PKA-mediated phosphorylation of type III InsP(3)R. Thus, we conclude that PKA-mediated phosphorylation of type III InsP(3)R is a general mechanism by which the patterns of [Ca(2+)](c) oscillations are shaped in pancreatic acinar cells.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

Neuropeptide FF receptors couple to a cholera toxin-sensitive G-protein in rat dorsal raphe neurones

In rat dorsal raphe neurones, nociceptin (300 nM) reduced the peak [Ca(2+)](i) transient, triggered by depolarization, by 36.7+/-1.8% (n=46). This effect of nociceptin decreased to 16.7+/-2.9% (n=18) after pre-treatment of the neurones with pertussis toxin (5 microg/ml, 2-6 h) but was unchanged (37.4+/-2.1%, n=44) after pre-incubation with cholera toxin (5 microg/ml, 2-6 h). This suggests that, in dorsal raphe neurones, the ORL1 receptor couples to inhibitory (G(i/o)) G-proteins. The neuropeptide FF analogue, [D-Tyr1, (N-Me)Phe(3)]neuropeptide FF (10, 100, 1000 nM), acted as an anti-opioid and reduced the effect of nociceptin (300 nM, 30 s) by 62.0+/-3.3% (n=28). Following pre-incubation with cholera toxin (5 microg/ml, 2-6 h) [D-Tyr1, (N-Me)Phe3] neuropeptide FF was unable, at the three concentrations tested, to block nociceptin activity. We conclude that, in rat dorsal raphe neurones, neuropeptide FF receptors couple to stimulatory G-proteins (Gs).

Targeted phosphorylation of inositol 1,4,5-trisphosphate receptors selectively inhibits localized Ca2+ release and shapes oscillatory Ca2+ signals

The current study provides biochemical and functional evidence that the targeting of protein kinase A (PKA) to sites of localized Ca(2+) release confers rapid, specific phosphoregulation of Ca(2+) signaling in pancreatic acinar cells. Regulatory control of Ca(2+) release by PKA-dependent phosphorylation of inositol 1,4, 5-trisphosphate (InsP(3)) receptors was investigated by monitoring Ca(2+) dynamics in pancreatic acinar cells evoked by the flash photolysis of caged InsP(3) prior to and following PKA activation. Ca(2+) dynamics were imaged with high temporal resolution by digital imaging and electrophysiological methods. The whole cell patch clamp technique was used to introduce caged compounds and to record the activity of a Ca(2+)-activated Cl(-) current. Photolysis of low concentrations of caged InsP(3) evoked Cl(-) currents that were inhibited by treatment with dibutryl-cAMP or forskolin. In contrast, PKA activators had no significant inhibitory effect on the activation of Cl(-) current evoked by uncaging Ca(2+) or by the photolytic release of higher concentrations of InsP(3). Treatment with Rp-adenosine-3',5'-cyclic monophoshorothioate, a selective inhibitor of PKA, or with Ht31, a peptide known to disrupt the targeting of PKA, largely abolished forskolin-induced inhibition of Ca(2+) release. Further evidence for the targeting of PKA to the sites of Ca(2+) mobilization was revealed using immunocytochemical methods demonstrating that the R(IIbeta) subunit of PKA was localized to the apical regions of acinar cells and co-immunoprecipitated with the type III but not the type I or type II InsP(3) receptors. Finally, we demonstrate that the pattern of signaling evoked by acetylcholine can be converted to one that is more "CCK-like" by raising cAMP levels. Our data provide a simple mechanism by which distinct oscillatory Ca(2+) patterns can be shaped.

Pertussis toxin inhibits cholecystokinin- and epidermal growth factor-induced mitogen-activated protein kinase activation by disinhibition of the cAMP signaling pathway and inhibition of c-Raf-1

Pertussis toxin (PTx), which inactivates G(i/o) type G proteins, is widely used to investigate the involvement of G(i/o) proteins in signal transduction. Activation of extracellular-regulated kinases 1 and 2 (ERK1/2) by G protein-coupled receptors has been described to occur either through a PTx-insensitive pathway involving activation of phospholipase C and protein kinase C (PKC), or through a PTx-sensitive pathway involving G(i)betagamma-mediated activation of Src. Cholecystokinin (CCK) activates ERK1/2 by a PKC-dependent, and thus presumably PTx-insensitive, pathway. However, CCK has recently been shown to induce activation of G(i) proteins in addition to G(q/11). In the present study, PTx partially inhibited CCK-induced ERK1/2 activation in pancreatic AR42J cells, although activation of phospholipase C was not reduced. PTx also inhibited ERK1/2 activation in response to the PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor (EGF) as well as activation of c-Raf-1 by EGF and CCK. In contrast, PTx, CCK, and EGF had only minor effects on A-Raf and B-Raf activity. Forskolin, a direct activator of adenylyl cyclase, inhibited CCK- and EGF-induced activation of c-Raf-1 and ERK1/2 in a manner similar to that of PTx. In PTx-treated cells, the cAMP content was increased and forskolin did not further inhibit CCK- and EGF-induced activation of c-Raf-1 or ERK1/2. In conclusion, the present study shows that PTx-sensitivity of receptor-induced ERK1/2 activation could be a consequence of disinhibition of the adenylyl cyclase signaling pathway, which in turn causes inhibition of c-Raf-1 activation rather than indicating involvement of a PTx-sensitive G protein in this signaling pathway.

Glucagon-like peptide-1 does not mediate amylase release from AR42J cells

In this study, AR42J pancreatic acinar cells were used to investigate if glucagon-like peptide-1 (GLP-1) or glucagon might influence amylase release and acinar cell function. We first confirmed the presence of GLP-1 receptors on AR42J cells by reverse trasncriptase-polymerase chain reaction (RT-PCR), Western blotting, and partial sequencing analysis. While cholecystokinin (CCK) increased amylase release from AR42J cells, GLP-1, alone or in the presence of CCK, had no effect on amylase release but both CCK and GLP-1 increased intracellular calcium. Similar to GLP-1, glucagon increased both cyclic adenosine monophosphate (cAMP) and intracellular calcium in AR42J cells but it actually decreased CCK-mediated amylase release (n = 20, P < 0.01). CCK stimulation resulted in an increase in tyrosine phosphorylation of several cellular proteins, unlike GLP-1 treatment, where no such increased phosphorylation was seen. Instead, GLP-1 decreased such protein phosphorylations. Genestein blocked CCK-induced phosphorylation events and amylase secretion while vanadate increased amylase secretion. These results provide evidence that tyrosine phosphorylation is necessary for amylase release and that signaling through GLP-1 receptors does not mediate amylase release in AR42J cells. J. Cell. Physiol. 181:470-478, 1999. Published 1999 Wiley-Liss, Inc.

Mechanisms of gallbladder hypomotility in pregnant guinea pigs

BACKGROUND & AIMS

Gallbladder muscle contraction becomes impaired during pregnancy. This study was designed to investigate the mechanisms of gallbladder hypomotility induced by pregnancy in guinea pigs.

METHODS

Gallbladder muscle cells were obtained by enzymatic digestion. Cell contraction was expressed as percent shortening of initial control cell length.

RESULTS

Contraction induced by cholecystokinin (CCK)-8 or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) was reduced in muscle cells from pregnant guinea pigs. The response to KCl or D-myo-inositol 1,4, 5-trisphosphate was not different between controls and pregnant animals. These findings suggest that impaired contraction in pregnancy might be caused by defective G protein activation. The function and content of G proteins were examined by using [35S]GTPgammaS binding and G protein subunit quantitation. In female controls, CCK-8 at 1 micromol/L caused increased [35S]GTPgammaS binding to Galphai3 but not to Galphaq/11, Galphai1-2, or Galphas. GTPgammaS binding to Galphai3 induced by CCK-8 was reduced in gallbladder muscle from pregnant guinea pigs. Measurements of basal G proteins showed that the content of Galphai3 was significantly lower and the Galphas content was higher in muscles from pregnant guinea pigs than in controls.

CONCLUSIONS

Pregnancy may cause down-regulation of contractile G proteins such as Galphai3 and up-regulation of Galphas that mediates relaxation, resulting in impaired gallbladder muscle contraction.

Signal transduction pathways mediating CCK-induced gallbladder muscle contraction

The signal transduction that mediates CCK-induced contraction of gallbladder muscle was investigated in the cat. Contraction was measured by scanning micrometry in single muscle cells isolated enzymatically with collagenase. Production of D-myo-inositol 1,4, 5-trisphosphate (IP3) and sn-1,2-diacylglycerol (DAG) was quantitated using HPLC and TLC, respectively. Protein kinase C (PKC) activity was determined by measuring the phosphorylation of a specific substrate peptide from myelin basic protein, Ac-MBP-(4-14). CCK-induced contraction was blocked by incubation in strontium medium, pertussis toxin (PTx), and antibodies against Gialpha3 or betagamma-subunits but was not blocked by Ca2+-free medium or by antibodies against Gq/11alpha, Gialpha1-2, or Goalpha. The contraction induced by CCK was inhibited by the phospholipase C (PLC) inhibitor U-73122, anti-PLC-beta3 antibody, and the IP3 receptor antagonist heparin but was not inhibited by the the phospholipase D inhibitor propranolol or antibodies against PLC-beta1 or PLC-beta2. Western blot analysis of gallbladder muscle revealed the presence of PLC-beta2 and PLC-beta3 but not PLC-beta1. CCK caused a 94% increase in IP3 generation and an 86% increase in DAG generation. A low dose of CCK caused PKC translocation, and CCK-induced contraction was blocked by the PKC inhibitor H-7. A high dose of CCK, however, caused no PKC translocation, and its contraction was blocked by the calmodulin antagonist CGS9343B. In conclusion, CCK contracts cat gallbladder muscle by stimulating PTx-sensitive Gi 3 protein coupled with PLC-beta3, producing IP3 and DAG. Low doses activate PKC, whereas high doses activate calmodulin.

Coupling of the PTH/PTHrP receptor to multiple G-proteins. Direct demonstration of receptor activation of Gs, Gq/11, and Gi(1) by [alpha-32P]GTP-gamma-azidoanilide photoaffinity labeling

Parathyroid hormone (PTH) elicits many of its physiological effects by activating distinct, G-protein-coupled signaling cascades that lead to synthesis of cyclic AMP and hydrolysis of phosphatidylinositol 4,5-bisphosphate. Using the nonhydrolyzable photo-reactive GTP analog [alpha-32P]GTP-gamma-azidoanilide (GTP-AA) and peptide antisera raised against G-protein alpha-subunits, we studied coupling of the PTH receptor to G-proteins in rat osteoblast-like cells (ROS 17/2.8), and in human embryonal kidney cells expressing the cloned human PTH/parathyroid hormone-related peptide (PTHrP) receptor at 40,000 receptors/cell (C20) or 400,000 receptors/cell (C21). Incubation of C21 membranes (but not C20 membranes) with [Nle8,18, Tyr34]-bovine PTH(1-34) amide (bPTH[1-34]) led to concentration-dependent incorporation of GTP-AA into the two isoforms of G alpha s, into G alpha q/11, and to a much lesser extent into G alpha i(1). In ROS 17/2.8 cells, bPTH(1-34) increased the incorporation of GTP-AA into G alpha s, but not into G alpha q/11 or G alpha i. The ability of bPTH(1-34) to increase labeling of G alpha s and G alpha q/11 was correlated with the receptor-dependent sensitivity of the adenylyl cyclase and phospholipase C signaling pathways to the hormone.

Impaired G protein function in gallbladder muscle from progesterone-treated guinea pigs

This study was designed to elucidate the mechanism of action of progesterone on gallbladder smooth muscle in guinea pigs. Adult male guinea pigs were treated with either progesterone (2 mg.kg-1.day-1) or saline for 7 days. Gallbladder muscle cells were isolated by enzymatic digestion with collagenase. Contractile responses to agonists were expressed as percent shortening from control cell length. [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTP gamma S)-binding properties of G proteins were assessed in crude membranes of gallbladder muscle with or without cholecystokinin octapeptide (CCK-8) stimulation. Gallbladder muscle cells from progesterone-treated guinea pigs exhibited an impaired contractile response to CCK-8, GTP gamma S, or aluminum fluoride but a normal response to potassium chloride or D-myo-inositol 1,4,5-trisphosphate compared with controls. Western blot analysis of gallbladder muscle revealed the presence of Gi1-2, Gi3, Gq/11, and Gs proteins. The maximal contraction induced by CCK-8 was blocked by pertussis toxin and Gi alpha 3-specific antibodies, but not by Gi alpha 1-2 or Gq/11 alpha antibodies. CCK-8 caused a significant increase in [35S]GTP gamma S binding to Gi alpha 3, but not to Gq/11 alpha or Gi alpha 1-2. The stimulation of Gi alpha 3 binding, however, was significantly reduced in gallbladder muscle membranes from progesterone-treated guinea pigs compared with that in control animals. In conclusion, progesterone might cause gallbladder hypomotility by downregulating Gi3 proteins.

Characterization of the antiproliferative signal mediated by the somatostatin receptor subtype sst5

We investigated cell proliferation modulated by cholecystokinin (CCK) and somatostatin analogue RC-160 in CHO cells bearing endogenous CCKA receptors and stably transfected by human subtype sst5 somatostatin receptor. CCK stimulated cell proliferation of CHO cells. This effect was suppressed by inhibitor of the soluble guanylate cyclase, LY 83583, the inhibitor of the cGMP dependent kinases, KT 5823, and the inhibitor of mitogen-activated protein (MAP) kinase kinase, PD 98059. CCK treatment induced an increase of intracellular cGMP concentrations, but concomitant addition of LY 83583 virtually suppressed this increase. CCK also activated both phosphorylation and activity of p42-MAP kinase; these effects were inhibited by KT 5823. All the effects of CCK depended on a pertussis toxin-dependent G protein. Somatostatin analogue RC-160 inhibited CCK-induced stimulation of cell proliferation but it did not potentiate the suppressive effect of the inhibitors LY 83583 and KT 5823. RC-160 inhibited both CCK-induced intracellular cGMP formation as well as activation of p42-MAP kinase phosphorylation and activity. This inhibitory effect was observed at doses of RC-160 similar to those necessary to occupy the sst5 recombinant receptor and to inhibit CCK-induced cell proliferation. We conclude that, in CHO cells, the proliferation and the MAP kinase signaling cascade depend on a cGMP-dependent pathway. These effects are positively regulated by CCK and negatively influenced by RC-160, interacting through CCKA and sst5 receptors, respectively. These studies provide a characterization of the antiproliferative signal mediated by sst5 receptor.

Possible involvement of G-proteins in the regulation of striatal dopamine D2 receptor affinity by cholecystokinin octapeptide

A G(i)-protein antibody AS/7 at 1:10 dilution significantly increased the K(d) values of the D2 agonist [3H]N-propylnorapomorphine (NPA) binding sites in the rat striatal membranes, and coincubation with sulphated cholecystokinin octapeptide (CCK-8; 1 nM) did not further increase the K(d) values. A GTP analogue guanylyl-imidodiphosphate (GMP-PNP) at 100 microM markedly increased the K(d) values of the [3H]NPA binding sites in the rat forebrain sections, and coincubation with CCK-8 (1 nM) again did not produce a further increase in the K(d) values. The present results indicate that abnormal activity of G-proteins abolished the ability of CCK-8 to reduce the D2 receptor affinity in the brain.

Differential distribution of alpha subunits and beta gamma subunits of heterotrimeric G proteins on Golgi membranes of the exocrine pancreas

Heterotrimeric G proteins are well known to be involved in signaling via plasma membrane (PM) receptors. Recent data indicate that heterotrimeric G proteins are also present on intracellular membranes and may regulate vesicular transport along the exocytic pathway. We have used subcellular fractionation and immunocytochemical localization to investigate the distribution of G alpha and G beta gamma subunits in the rat exocrine pancreas which is highly specialized for protein secretion. We show that G alpha s, G alpha i3 and G alpha q/11 are present in Golgi fractions which are > 95% devoid of PM. Removal of residual PM by absorption on wheat germ agglutinin (WGA) did not deplete G alpha subunits. G alpha s was largely restricted to TGN-enriched fractions by immunoblotting, whereas G alpha i3 and G alpha q/11 were broadly distributed across Golgi fractions. G alpha s did not colocalize with TGN38 or caveolin, suggesting that G alpha s is associated with a distinct population of membranes. G beta subunits were barely detectable in purified Golgi fractions. By immunofluorescence and immunogold labeling, G beta subunits were detected on PM but not on Golgi membranes, whereas G alpha s and G alpha i3 were readily detected on both Golgi and PM. G alpha and G beta subunits were not found on membranes of zymogen granules. These data indicate that G alpha s, G alpha q/11, and G alpha i3 associate with Golgi membranes independent of G beta subunits and have distinctive distributions within the Golgi stack. G beta subunits are thought to lock G alpha in the GDP-bound form, prevent it from activating its effector, and assist in anchoring it to the PM. Therefore the presence of free G alpha subunits on Golgi membranes has several important functional implications: it suggests that G alpha subunits associated with Golgi membranes are in the active, GTP-bound form or are bound to some other unidentified protein(s) which can substitute for G beta gamma subunits. It further implies that G alpha subunits are tethered to Golgi membranes by posttranslational modifications (e.g., palmitoylation) or by binding to another protein(s).

Pertussis toxin-sensitive G-proteins inhibit fibroblast growth factor-induced signaling in pancreatic acini

Signal transduction of fibroblast growth factor (FGF) receptors is known to involve tyrosine phosphorylation of several substrates, including Grb2, phospholipase C-gamma, and phosphatidylinositol 3-kinase, whereas the role of G-proteins in FGF receptor signaling is controversial. In the present study we investigated the role of G-proteins in FGF receptor signaling in rat pancreatic acini. Immunological analysis revealed the presence of FGF receptor and phospholipase C-gamma1 in rat pancreatic acini. Both basic fibroblast growth factor (FGF-2) and guanosine 5'-(gamma-O-thio)triphosphate (GTPgammaS) caused an increase in inositol 1,4,5-trisphosphate (1,4,5-IP3) production and amylase release. Combined stimulation of the acini with GTPgammaS and FGF-2 led to a decrease of these responses as compared to the effect of the single substances. When pancreatic acini were preincubated with FGF-2 (1 nM) or vehicle (water) ADP-ribosylation of the alpha-subunit of Gi-type G-proteins by pertussis toxin was reduced in membranes prepared from FGF-2 pretreated acini as compared to control acini, suggesting functional interaction of FGF receptors with Gi-proteins. Pretreatment of acini with pertussis toxin which inhibits Gi-type G-proteins abolished the inhibitory effect of GTPgammaS on FGF-induced 1,4,5-IP3 production and amylase release, whereas the stimulatory effects of FGF-2 and GTPgammaS on these parameters remained unchanged. In conclusion, these results show communication of FGF receptors and Gi-type G-proteins and that Gi-type G-proteins exert an inhibitory influence on FGF-induced activation of phosphoinositide-specific phospholipase C in pancreatic acinar cells.

Human blood platelet protein map established by two-dimensional polyacrylamide gel electrophoresis

Two-dimensional (2-D) maps of cytosol and enriched-membrane platelet proteins has allowed the identification of more than 25 spots by three different methods: matching of the platelet gels with other 2-D reference maps, immunoblotting with chemiluminescence detection, and N-terminal sequencing. Different G protein (guanosine triphosphate-binding protein) subunits, cytoskeletal proteins, and proteins common to the human liver, red blood cells and plasma were identified. The two platelet protein maps presented here contribute to the project of identification of human cell and body fluid proteins. They may serve as working tools since platelets are popular models for the study of central nervous system neurotransmitter systems and stimulus-response coupling mechanisms.

Loss of sensitivity to cholecystokinin stimulation of isolated pancreatic acini from genetically diabetic rats

Pancreatic exocrine function of a new inbred strain Otsuka Long-Evans Tokushima Fatty (OLETF) rat that develops spontaneous persistent hyperglycemia was evaluated in in vitro isolated pancreatic acini and compared with that in the control Long-Evans Tokushima Otsuka (LETO) rat. Serum glucose and insulin concentrations in the OLETF rats were significantly high (glucose: 270 +/- 12 vs. 208 +/- 10 mg/100 ml, P < 0.01; insulin: 12.4 +/- 1.7 vs. 4.9 +/- 0.6 ng/ml, P) < 0.01), whereas pancreatic wet weight was significantly low (803 +/- 20 vs. 1,138 +/- 17 mg, P < 0.01) compared with those in the LETO rat. Pancreatic acini isolated from the OLETF rat were totally insensitive to cholecystokinin (CCK)-8 stimulation at concentrations of up to 100 nM. However, neither the responsiveness nor the sensitivity to carbamylcholine, bombesin, and secretin of the acini from the OLETF rat was altered or even increased, probably due to the larger amylase content in the OLETF rat acini compared with those of the LETO rat acini (31.5 +/- 2.0 vs. 13.0 +/- 1.1 Somogyi units/micrograms DNA, P < 0.01). The responsiveness to fluoride, a direct activator of guanine nucleotide-binding protein, in the OLETF rat acini was similar to that in the LETO rat, suggesting that the transmembrane signaling and effectors and subsequent intracellular signal transduction molecules in the OLETF rat acini are normal. Moreover, 125I-CCK binding to the acini prepared from the OLETF rat was totally absent. These present results indicate that the OLETF rat has a selective defect in the binding of CCK to its receptors on the acinar cell surface.

Inhibition by somatostatin of amylase secretion induced by calcium and cyclic AMP in rat pancreatic acini

It has recently been shown that somatostatin inhibits amylase secretion from isolated pancreatic acini by reducing cyclic AMP (cAMP) production [Matsushita, Okabayashi, Hasegawa, Koide, Kido, Okutani, Sugimoto and Kasuga (1993) Gastroenterology 104, 1146-1152]. To date, however, little is known as to the other mechanism(s) by which somatostatin inhibits amylase secretion in exocrine pancreas. To investigate the action of somatostatin independent of cAMP generation, we examined the effect of somatostatin in isolated rat pancreatic acini stimulated by 1 microM calcium ionophore A23187 and 1 mM 8-bromo-cyclic AMP (8Br-cAMP). Somatostatin inhibited amylase secretion evoked by a combination of A23187 and 8Br-cAMP in a dose-dependent manner. The maximum inhibition was obtained by 10(-7) M somatostatin, and at this concentration somatostatin inhibited the effect of A23187 and 8Br-cAMP by approximately 30%. In electrically permeabilized acini, an elevation of free calcium concentration resulted in an increase in amylase secretion and cAMP enhanced the secretion evoked by calcium. cAMP shifted the dose-response curve for calcium-induced secretion leftwards and elevated the peak value of secretion. Somatostatin inhibited the effect of cAMP on calcium-induced amylase secretion by shifting the dose-response curve to the right. To determine the involvement of a G-protein(s), we examined the effect of somatostatin in acini pretreated with pertussis toxin. Pretreatment of acini with pertussis toxin completely blocked somatostatin-inhibition of amylase-secretion evoked by A23187 and 8Br-cAMP. These results indicate that somatostatin decreases amylase secretion induced by cAMP and calcium by reducing the calcium sensitivity of exocytosis. A pertussis toxin-sensitive G-protein is also involved in this step.

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.

Effects of guanine nucleotides on bombesin-stimulated signal transduction in rat pancreatic acinar cells

To study the role of guanine nucleotide binding proteins (G proteins) in bombesin receptor signal transduction, we investigated the effects of guanine nucleotide analogues and of the G protein activator NaF on bombesin-induced amylase release, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) production and release of intracellular Ca2+ in rat pancreatic acini. In digitonin-permeabilized acini, guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), a well-known activator of G proteins, potentiated bombesin-induced Ins(1,4,5)P3 production and increased amylase release at low bombesin concentrations (< 10 nM). By contrast, GTP gamma S decreased bombesin-stimulated amylase release at high bombesin concentrations (> 10 nM). Fluoride (10 mM), another G protein activator, had similar effects to GTP gamma S on amylase release. However, unlike GTP gamma S it had no effect on Ins(1,4,5)P3 production and release of intracellular Ca2+ induced by high bombesin concentrations. GDP and its analogues, such as 2'-desoxyguanosine 5'-diphosphate (dGDP) or guanosine 5'-[beta-thio]diphosphate (GDP beta S), inhibit activation of G proteins. GDP and dGDP both inhibited amylase release and Ins(1,4,5)P3 production at all bombesin concentrations tested. In contrast, GDP beta S mimicked the effects of GTP gamma S on bombesin-stimulated amylase release and Ins(1,4,5)P3 accumulation. In conclusion, we suggest that bombesin receptor-mediated signal transduction involves G proteins in pancreatic acini. The correlation between inhibition of maximum-stimulated enzyme secretion and further increase in Ins(1,4,5)P3 production in response to GTP gamma S at high bombesin concentrations suggests that overstimulation of phospholipase C inhibits amylase release. The discrepant effects of GDP and of GDP beta S on phospholipase C activity and amylase release might be due to the ability of GDP beta S, but not of GDP to activate G proteins persistently after phosphorylation by G protein-associated GDP kinases.

Two different spatiotemporal patterns for Ca2+ oscillations in pancreatic acinar cells: evidence of a role for protein kinase C in Ins(1,4,5)P3-mediated Ca2+ signalling

The oscillations in cytosolic Ca2+ evoked in pancreatic exocrine acinar cells by submaximal concentrations of the two phosphoinositidase-coupled agonists acetylcholine (ACh) and cholecystokinin octapeptide (CCK-8) have very different temporal patterns. In the present study we use digital video imaging of Fura-2 fluorescence to map the spatial distribution of Ca2+ during the oscillating responses to these two agonists. The spatial patterns induced are very different for each of these agonists. ACh oscillations are sinusoidal and initiated at the secretory pole of these morphologically and functionally polarized cells. As they spread across the cell, pronounced gradients in Ca2+ develop that persist throughout the oscillating response. CCK-8 induces a series of discrete Ca2+ transients of longer duration and lower frequency. These elevations in Ca2+ arise slowly, throughout the cells and without any detectable gradients in Ca2+. We consider that the different spatiotemporal patterns can be explained on the basis of a physiologically relevant interaction between Ins(1,4,5)P3 and protein kinase C in second messenger-mediated Ca2+ signalling.

Inhibitory interactions between stimulus-secretion pathways in the exocrine rat pancreas

In many tissues the cellular responses mediated through different intracellular messenger systems are mutually interactive. In the exocrine pancreas the secretagogues acting via adenosine cyclic monophosphate (cAMP) and those acting via calcium-phosphoinositides can potentiate one another. On the other hand, protein kinase C (PK-C) modulates receptor-induced responses in exocrine pancreatic cells and other cell types. Recording total protein output, monitored on-line at 280 nm, from superfused rat pancreatic segments, we demonstrate that secretin (a cAMP-acting hormone) reduces the efficacy of the calcium-mediated secretagogue cholecystokinin-octapeptide (CCK-8). Likewise, the PK-C activator 12,O,tetradecanoyl phorbol 13 acetate (TPA) reduces both the efficacy of secretin and the potency of cholecystokinin. Thus, the hypothesis of potentiation between different stimulus-secretion coupling mechanisms must be revised, and receptor-activated responses in the exocrine pancreas must be considered a complex model with multiple inhibitory and stimulatory interactions.

Identification and localization of G proteins in exocrine glands

GTP-binding proteins were identified in rat parotid acinar plasma-enriched membranes (PM) by immunoblot analysis and localized immunohistochemically in the parotid gland as well as in other exocrine glands by using affinity-purified antisera specific for alpha subunits of the G proteins. Isolated rat parotid acinar PM immunoreacted strongly to antisera directed against Gs alpha, Gi alpha 1/alpha 2, Gi alpha 3, and Go alpha; the signal for Go alpha, however, was weak with crude Go antisera. Immunohistochemical studies to identify and localize Go in rat parotid tissue revealed that antisera to Go alpha immunoreacted with ductal cells. In addition, strong immunoreactivity to Go alpha antisera was noted in ductal cells of other salivary glands including rat submandibular, mouse parotid, and mouse submandibular glands. Light labeling of rat parotid and submandibular gland acinar cells was also noted. In contrast, in the rat and mouse pancreas, Go antisera immunoreacted primarily with islet cells. Ductal cells were negative, but there was light labeling of rat pancreatic acinar cells. The apparent ductal specificity of Go alpha staining was further verified by demonstrating that Go alpha antisera immunoreacted strongly with HSG-PA cells, a human transformed salivary ductal cell line. The results demonstrate that rat parotid acinar plasma membranes express a number of G proteins including Go and that Go appears to be selectively expressed in the ductal cells of rat parotid gland and other salivary glands. The selective enrichment of Go in ductal cells suggests that this G protein may play an important role in ductal cell physiology.

Study of the states and populations of the rat pancreatic cholecystokinin receptor using the full peptide antagonist JMV 179

The full peptide antagonist of the pancreatic cholecystokinin (CCK) receptor, JMV 179, [Boc-Tyr(SO3H)-Ahx-Gly-dTrp-Ahx-Asp phenylethyl ester, where Tyr(SO3H) = sulfated tyrosine, Ahx = 6-aminohexanoic acid] was modified at its N-terminus by incorporation of p-hydroxyphenyl propionate (Bolton-Hunter reagent, BH) and was subsequently radioiodinated. After HPLC purification, 125I-BH-JMV-179, a CCK antagonist radioligand of high specific activity (2000 Ci/mmol) was obtained. 125I-BH-JMV-179 bound to a single population of sites on rat pancreatic plasma membranes, (Kd = 3.9 nM, Bmax = 40 pmol/mg protein). Binding was dependent on time, temperature, and protein concentration, and was fully reversible. JMV 179 radioligand detected four times as many sites as an agonist radioligand [C. Hadjiivanova, M. Dufresne, S. Poirot, P. Sozzani, N. Vaysse, L. Moroder and D. Fourmy (1992) Eur. J. Biochem. 204, 273-279]. Agonists and antagonists of the A- and B-subtype CCK/gastrin receptors inhibited 125I-BH-JMV-179 binding with an order of potency compatible with the A-subtype CCK receptor pharmacology. Moreover, the sulfate group on the tyrosine residue of the CCK peptides appeared to be of much less importance for antagonist affinity than for agonist affinity. Inhibition of 125I-BH-JMV-179 binding by agonists (except JMV 180), demonstrated the presence of two affinity classes of binding sites. The population of sites having an apparent high affinity for CCK represented 30 pmol/mg protein and threefold the number of high-affinity sites previously identified by an agonist radioligand. In presence of non-hydrolyzable GTP, all the sites bound CCK agonists with a low affinity. Moreover, saturation analysis of JMV 179 radioligand binding in the presence of CCK indicated that CCK interacted competitively with all JMV 179 sites and demonstrated binding of JMV 179 radioligand to two distinct affinity classes of sites. In the presence of GTP[S] a single affinity class of sites for JMV 179 radioligand was found as in the control experiments without CCK. This study, with the first CCK peptide antagonist radioligand, demonstrates that CCK receptors exist in two interconvertible affinity states regulated by guanine-nucleotide-binding regulatory protein(s) in rat pancreatic plasma membranes. JMV 179 radioligand does not induce receptor coupling but distinguishes the two affinity states of the CCK receptors. JMV 179 reveals the existence of populations of high-affinity and low-affinity sites for CCK which had not previously been detected by agonist radioligand binding, thus suggesting heterogeneity of CCK receptor sites in membranes.

Characterization of adenosine receptors in brush-border membranes from pig kidney

1. The adenosine receptors from pig kidney proximal tubules have been studied in membrane vesicle preparations derived from either luminal (brush-border membranes-BBM-) or basolateral (BL) sides. There was a substantial amount of A2-like NECA binding in both preparations, but the A1 subtype of adenosine receptors was not found in either BBM or BL membranes. The use of [3H]-CGS21680 which is a more specific ligand for A2a receptors revealed true adenosine receptors in the BBM. 2. The kinetic parameters for [3H]-CGS21680 binding to pig renal BBM were: Bmax = 1.48 pmol mg-1 protein and Kd = 150 nM. In the presence of Gpp(NH)p the affinity decreased (Kd = 220 nM), whereas the addition of Mg2+ induced a marked increase in affinity (Kd = 83 nM). These equilibrium constants are higher than those found for the A2a adenosine receptors present in pig brain striatal membranes (Kd = 12 nM), and are close to those found in rat renal BBM (Kd = 90 nM). 3. The order of potency of agonist and antagonists was not consistent with the presence of either A1 or A2 receptors, but it was very similar to the agonist order of potency for the A3 receptor subtype. Furthermore, the blockade of the [3H]-CGS21680 binding by both cholera and pertussis toxin further supports the view that the subtypes present in BBM are neither A1 nor A2. 4. Overall the results suggest the presence in BBM of an A3 receptor, or of a new subtype of adenosine receptor, which is linked to G proteins sensitive to both cholera and pertussis toxins.

Intravesicular acidification correlates with binding of ADP-ribosylation factor to microsomal membranes

The ADP-ribosylation factor (ARF), a highly conserved low molecular weight GTP-binding protein, has been implicated to function in intracellular protein transport to and within the Golgi complex. In pancreatic acinar cells the ARF is confined to the cytoplasmic faces of trans-Golgi stack membranes, a compartment known to maintain a low intravesicular pH, which is established by a chloride-dependent MgATP-driven proton pump. The present study shows that MgATP (2mM), but neither adenosine 5'-[gamma-thio]triphosphate in the presence of Mg2+ nor ATP in the absence of Mg2+, increases transfer of ARF from the surrounding medium into the vesicle membranes. The specific vacuolar-type proton pump inhibitor bafilomycin B1 (10 nM), the protonophore carbonylcyanide m-chlorophenylhydrazone (10 microM), and replacement of chloride in the incubation buffer by acetate or nitrate resulted in an almost complete inhibition of the MgATP-dependent association of ARF to the vesicle membranes. The results demonstrate that redistribution of ARF to the vesicle membrane correlates with the intravesicular pH established by a vacuolar-type H(+)-ATPase. The intravesicular pH appears to be one mechanism by which certain low molecular weight GTP-binding proteins become relocated from the cytosol to their specific membrane vesicles.

Evidence for G proteins in rat parotid plasma membranes and secretory granule membranes

G proteins were identified in rat parotid plasma membrane-enriched fractions and in two populations of isolated secretory granule membrane fractions. Both [32P]ADP-ribosylation analysis with bacterial toxins and immunoblot analysis with crude and affinity-purified antisera specific for alpha subunits of G proteins were utilized. Pertussis toxin catalysed the ADP-ribosylation of a 41 kDa substrate in the plasma membrane fraction and both secretory granule membrane fractions. Cholera toxin catalysed the ADP-ribosylation of two substrates with molecular masses of 44 kDa and 48 kDa in the plasma membrane fraction but not in the secretory granule fractions. However, these substrates were detected in the secretory granule fractions when recombinant ADP-ribosylating factor was present in the assay medium. Immunoblot analysis of rat parotid membrane fractions using both affinity-purified and crude antisera revealed strong immunoreactivity of these membranes with anti-Gs alpha, -Gi alpha 1/alpha 2 and -Gi alpha 3 sera. In contrast Gs alpha was the major substrate found in both of the secretory granule fractions. Granule membrane fractions also reacted moderately with anti-Gi alpha 3 antiserum, and weakly with anti-Gi alpha 1/alpha 2 and -G(o) alpha sera. The results demonstrate that the parotid gland membranes express a number of G proteins. The presence of G proteins in secretory granule membranes suggests that they may play a direct role in regulating exocytosis in exocrine glands.

Pharmacological and biochemical characterization of cholecystokinin/gastrin receptors in developing rat pancreas. Age-related expression of distinct receptor glycoforms

Cholecystokinin/gastrin receptors in the pancreas of newborn (3-day-old) rats are of type A, as in control mature rats, revealed by pharmacological analysis of specific 125I-Bolton-Hunter-reagent-labelled [Thr34,Ahx37]cholecystokinin(31-39) (Ahx, aminohexanoic acid) binding. Also, by 1 day post-partum, pancreatic cholecystokinin receptors were shown to be coupled to guanine-nucleotide-binding regulatory (G) proteins. Scatchard analysis of 125I-Bolton-Hunter-reagent-labelled [Thr34,Ahx37]cholecystokinin(31-39) binding to pancreatic membranes from rats at different times after birth showed a slight increase in the binding capacity of cholecystokinin receptors between days 3 and 14 and a sixfold increase in 21-day-old rats, with no change in receptor affinity during development. SDS/PAGE analysis of pancreatic membranes affinity labelled with the photoactivable ligand 125I-[2-(p-azidosalicylamido)-1,3'-dithiopropionate]-labelled [Thr34,Ahx37]cholecystokinin-(31-39) identified cholecystokinin receptors of 100-135 kDa in 3-day-old rats, 96-130 kDa in 7-day-old rats, 90-125 kDa in 10-day-old rats and 85-100 kDa in 14-day-old and 21-day-old rats, as found in control adult rats. Endo-beta-N-acetylglucosaminidase F treatment yielded a core protein of 42 kDa in all developmental stages. These findings are consistent with an age-related postnatal expression of distinct glycoforms of pancreatic cholecystokinin receptors. Furthermore, it was observed that the period 2-3 weeks after birth, characterized by stabilization of the mass of the cholecystokinin receptor, precedes the dramatic increase in the receptor number.

Association of a 19- and a 21-kDa GTP-binding protein to pancreatic microsomal vesicles is regulated by the intravesicular pH established by a vacuolar-type H(+)-ATPase

Evidence suggests that certain ras-related small molecular weight GTP-binding proteins (smg-proteins) are involved in intracellular membrane trafficking and vesicle fusion. We have previously shown that intravesicular acidification due to a vacuolar-type H(+)-ATPase, which is Cl- dependent and highly sensitive to the specific inhibitor bafilomycin, enhances GTP-induced fusion of pancreatic microsomal vesicles (Hampe, W., Zimmermann, P., Schulz, I. 1990. FEBS Lett. 271:62-66). This process may involve function of smg-proteins. The present study shows that MgATP (2 mM), but neither MgATP gamma S nor ATP in the absence of Mg2+, increases association of 19- and 21-kDa smg-proteins to the vesicle membrane as monitored by their [ alpha-32P]GTP binding. The affinity of smg-proteins for [ alpha-32P]GTP was not altered by MgATP. Bafilomycin B1 (10(-8) M), the protonophore CCCP (10(-5) M), and replacement of Cl- in the incubation buffer by CH3COO- or NO3- resulted in an almost complete inhibition of the MgATP-dependent association of the 19- and 21-kDa smg-proteins to the vesicle membranes. Furthermore, the MgATP effect on both smg-proteins was found to be due to the intravesicular pH and not to the H+ gradient over the vesicle membrane. We conclude that association of a 19-kDa (immunologically identified as the ADP-ribosylation factor, arf) and a yet unidentified 21-kDa GTP-binding protein to vesicle membranes is regulated by the intravesicular pH established by a vacuolar-type H(+)-ATPase.

Cytoplasmic Ca2+ signals evoked by activation of cholecystokinin receptors: Ca(2+)-dependent current recording in internally perfused pancreatic acinar cells

The effects on the cytosolic Ca2+ concentration of activating cholecystokinin receptors on single mouse pancreatic acinar cells have been investigated using patch-clamp whole-cell recording of Ca(2+)-dependent Cl- current. We used the nonsulphated octapeptide of cholecystokinin (CCK8-NS) since the effects of even high concentrations were rapidly reversible which was not the case for the sulphated octapeptide. A submaximal concentration of CCK8-NS (10 nM) evoked a current response consisting of short-lasting (a few seconds) spikes, and some of these spikes were seen to trigger larger and longer (about half a minute) current pulses. At a higher concentration (100 nM) CCK8-NS evoked smooth and sustained responses. The effect of CCK8-NS was almost abolished when the internal perfusion solution contained a high concentration of the Ca2+ chelator EGTA (5 mM). The responses evoked by CCK8-NS were independent of the presence of Ca2+ in the external solution at least for the first 5 min of stimulation. Internal perfusion with GTP-gamma-S markedly potentiated the effect of CCK8-NS or at a higher concentration itself induced responses very similar to those normally evoked by CCK8-NS. Caffeine added to the external solution at a low concentration (0.2-1 mM) enhanced weak CCK8-NS responses, whereas high caffeine concentrations always inhibited the CCK8-NS-evoked responses. These inhibitory caffeine effects were quickly reversible. Forskolin evoked a similar inhibitory effect. Intracellular heparin (200 micrograms/ml) infusion markedly inhibited the response to CCK8-NS stimulation. We conclude that the primary effect of activating CCK receptors is to induce inositoltrisphosphate (IP3) production.(ABSTRACT TRUNCATED AT 250 WORDS)

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).

Guanosine nucleotides modulate the inhibitory effect of brefeldin A on protein secretion

Brefeldin A (BFA) causes rapid redistribution of Golgi proteins into the endoplasmic reticulum (ER), leaving no definable Golgi-apparatus, and blocks transport of proteins from the ER to distal secretory compartments of the cell. Using pulse-chase experiments the present study shows that BFA (1 microgram/ml) inhibits basal and CCK-stimulated protein secretion in isolated pancreatic acinar cells by 65 +/- 6% and 84 +/- 5%, respectively. In isolated permeabilized cells higher concentrations of BFA (30 micrograms/ml) were necessary to obtain inhibition of protein secretion. In parallel experiments protein secretion was stimulated by GTP (1 mM). BFA had no inhibitory effect on protein secretion in the presence of GTP, indicating that BFA might act on a GTP-binding protein. Investigating the effect of BFA on small molecular weight GTP-binding proteins we observed that [alpha-32P]GTP binding to a 21 kDa protein in a subcellular fraction enriched in ER was increased in the presence of BFA. We conclude that this 21 kDa and possibly also other GTP-binding proteins may be the molecular target of Brefeldin A in pancreatic acinar cells.

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.

Receptor-activated cytoplasmic Ca2+ oscillations in pancreatic acinar cells: generation and spreading of Ca2+ signals

Receptor-activated cytoplasmic Ca2+ oscillations have been investigated using both single cell microfluorometry and voltage-clamp recording of Ca(2+)-dependent Cl- current in single internally perfused acinar cells. In these cells there is direct experimental evidence showing that the ACh-evoked [Ca2+]i fluctuations are due to an inositol trisphosphate-induced small steady Ca2+ release which in turn evokes repetitive Ca2+ spikes via a caffeine-sensitive Ca(2+)-induced Ca2+ release process. There is indirect evidence suggesting that receptor-activation in addition to generating the Ca2+ releasing messenger, inositol trisphosphate, also produces another regulator involved in the control of Ca2+ signal spreading. Intracellular inositol trisphosphate or Ca2+ infusion produce short duration repetitive spikes confined to the cytoplasmic area close to the plasma membrane, but these signals can be made to progress throughout the cell by addition of caffeine or by receptor activation.

Receptor regulation of phosphoinositidase C

Numerous hormones, neurotransmitters and growth factors regulate intracellular events by acting at cell surface receptors which are coupled to the generation of inositol phospholipid-derived intracellular messengers. Receptors trigger the hydrolysis of inositol phospholipids by activating phosphoinositidase C (PIC) enzymes. At least four families of genes encode structurally distinct PIC enzymes and it is likely that distinct PIC isoenzymes participate in different pathways of signal transduction. Two different modes of receptor regulation have been identified and these involve distinct PIC isoenzymes. In the first of these, PIC-gamma is a substrate for growth factor receptor protein-tyrosine kinases. The second of these pathways involves PIC-beta plus other isoenzymes whose activities are regulated by G proteins in response to agonist binding to G protein-linked receptors. At least two types of G proteins regulate PIC activity and each may control the activity of different PIC isoenzymes.