Preparation of enriched plasma membranes from bovine gallbladder muscularis for characterization of cholecystokinin receptors

Gastrin, Cholecystokinin, Signaling, and Biological Activities in Cellular Processes

The structurally-related peptides, gastrin and cholecystokinin (CCK), were originally discovered as humoral stimulants of gastric acid secretion and pancreatic enzyme release, respectively. With the aid of methodological advances in biochemistry, immunochemistry, and molecular biology in the past several decades, our concept of gastrin and CCK as simple gastrointestinal hormones has changed considerably. Extensive in vitro and in vivo studies have shown that gastrin and CCK play important roles in several cellular processes including maintenance of gastric mucosa and pancreatic islet integrity, neurogenesis, and neoplastic transformation. Indeed, gastrin and CCK, as well as their receptors, are expressed in a variety of tumor cell lines, animal models, and human samples, and might contribute to certain carcinogenesis. In this review, we will briefly introduce the gastrin and CCK system and highlight the effects of gastrin and CCK in the regulation of cell proliferation and apoptosis in both normal and abnormal conditions. The potential imaging and therapeutic use of these peptides and their derivatives are also summarized.

Role of caveolae in the pathogenesis of cholesterol-induced gallbladder muscle hypomotility

Muscle cells from human gallbladders (GB) with cholesterol stones (ChS) exhibit a defective contraction, excess cholesterol (Ch) in the plasma membrane, and lower binding of CCK-1 receptors. These abnormalities improved after muscle cells were incubated with Ch-free liposomes that remove the excess Ch from the plasma membrane. The present studies were designed to investigate the role of caveolin-3 proteins (Cav-3) in the pathogenesis of these abnormalities. Muscle cells from GB with ChS exhibit higher Ch levels in the plasma membrane that were mostly localized in caveolae and associated with parallel increases in the expression of Cav-3 in the caveolae compared with that in GB with pigment stones (PS). The overall number of CCK-1 receptors in the plasma membrane was not different between muscle cells from GB with ChS and PS, but they were increased in the caveolae in muscle cells from GB with ChS. Treatment of muscle cells from GB with ChS with a Galpha(i3) protein fragment increased the total binding of CCK-1 receptors (from 8.3 to 11.2%) and muscle contraction induced by CCK-8 (from 11.2 to 17.3% shortening). However, Galpha(q/11) protein fragment had no such effect. Moreover, neither fragment had any effect on muscle cells from GB with PS. We conclude that the defective contraction of muscle cells with excessive Ch levels in the plasma membrane is due to an increased expression of Cav-3 that results in the sequestration of CCK-1 receptors in the caveolae, probably by inhibiting the functions of Galpha(i3) proteins.

Prostaglandins mediate tonic contraction of the guinea pig and human gallbladder

The gallbladder (GB) maintains tonic contraction modulated by neurohormonal inputs but generated by myogenic mechanisms. The aim of these studies was to examine the role of prostaglandins in the genesis of GB myogenic tension. Muscle strips and cells were treated with prostaglandin agonists, antagonists, cyclooxygenase (COX) inhibitors, and small interference RNA (siRNA). The results show that PGE2, thromboxane A2 (TxA2), and PGF(2alpha) cause a dose-dependent contraction of muscle strips and cells. However, only TxA2 and PGE2 (E prostanoid 1 receptor type) antagonists induced a dose-dependent decrease in tonic tension. A COX-1 inhibitor decreased partially the tonic contraction and TxB2 (TxA2 stable metabolite) levels; a COX-2 inhibitor lowered the tonic contraction partially and reduced PGE2 levels. Both inhibitors and the nonselective COX inhibitor indomethacin abolished the tonic contraction. Transfection of human GB muscle strips with COX-1 siRNA partially lowered the tonic contraction and reduced COX-1 protein expression and TxB2 levels; COX-2 siRNA also partially reduced the tonic contraction, the protein expression of COX-2, and PGE2. Stretching muscle strips by 1, 2, 3, and 4 g increased the active tension, TxB2, and PGE2 levels; a COX-1 inhibitor prevented the increase in tension and TxB2; and a COX-2 inhibitor inhibited the expected rise in tonic contraction and PGE2. Indomethacin blocked the rise in tension and TxB2 and PGE2 levels. We conclude that PGE2 generated by COX-2 and TxA2 generated by COX-1 contributes to the maintenance of GB tonic contraction and that variations in tonic contraction are associated with concomitant changes in PGE2 and TxA2 levels.

Impaired cytoprotective function of muscle in human gallbladders with cholesterol stones

Acute cholecystitis develops in gallbladders (GB) with excessive bile cholesterol (Ch). Increased membrane Ch content affects membrane function and may affect PGE(2) receptors involved in the cytoprotection against acute inflammation. This study was aimed at determining whether the cytoprotective response to PGE(2) is affected by lithogenic bile with Ch. Muscle cells from human GB with cholesterol stones (ChS) or pigment stones (PS) were obtained by enzymatic digestion. PGE(2) levels were measured by radioimmunoassay, and activities of superoxide dismutase (SOD) and catalase were assayed by spectrophotometry. The contraction in response to H(2)O(2) in muscle cells from PS was 14 +/- 0.3%, not different from normal controls, and decreased after the cells were incubated with Ch-rich liposomes (P < 0.05), which increase the Ch content in the plasma membranes. In muscle cells from GB with ChS, H(2)O(2)-induced contraction was only 9.2 +/- 1.3% and increased to 14 +/- 0.2% after Ch-free liposome treatment to remove Ch from the plasma membranes (P < 0.01). H(2)O(2) caused a similar increase in the levels of lipid peroxidation and PGE(2) content in muscle cells from GBs with ChS and PS. However, the activities of SOD and catalase were significantly lower in muscle cells from GBs with ChS compared with those with PS. The binding capacity of PGE(2) receptors was also significantly lower in muscle cells from GBs with ChS compared with those with PS. In conclusion, the cytoprotective response to reactive oxygen species is reduced in muscle cells from GBs with ChS despite a normal increase in the cellular levels of PGE(2). This impaired cytoprotective response may be due to a dysfunction of PGE(2) receptors with decreased binding capacity resulting from excessive Ch levels in the plasma membrane.

Role of PGE2 on gallbladder muscle cytoprotection of guinea pigs

H2O2 and taurochenodeoxycholic acid (TCDC) impair the contraction induced by CCK-8, ACh, and KCl without affecting the actions of PGE2 and damage functions of membrane proteins except for PGE2 receptors. The aim of this study was to examine whether the preserved PGE2 actions contribute to cytoprotective mechanisms against reactive oxygen species. Muscle cells from guinea pig gallbladder were obtained by enzymatic digestion. Levels of lipid peroxidation and activities of SOD and catalase were determined by spectrophotometry. Pretreatment with PGE2 prevented the inhibition of H2O2 or TCDC on agonist (CCK-8, ACh, and KCl)-induced contraction and reduced the expected increase in lipid peroxidation and activities of catalase and SOD caused by H2O2 and TCDC. Incubation with CCK-8 for 60 min desensitized CCK-1 receptors up to 30 min, whereas no receptor desensitization was observed after PGE2 pretreatment. Cholesterol-rich liposome treatment enhanced the inhibition of H2O2 and TCDC on agonists-induced contraction, including that of PGE2. Pretreatment with PGE2 before H2O2 and TCDC did not completely block their inhibition on agonist-induced contraction. Cholesterol-rich liposome treatment impaired the expected increase in catalase activities in response to PGE2. We conclude that pretreatment with PGE2 prevents the muscle cell damage caused by H2O2 and TCDC due to the resistance of PGE2 receptors to agonist-induced desensitization. The preservation of PGE2 receptors may be designed to conserve these cytoprotective functions that are, however, impaired by the presence of excess cholesterol in the plasma membrane.

PAF-like lipids- and PAF-induced gallbladder muscle contraction is mediated by different pathways in guinea pigs

H2O2 stimulates gallbladder muscle contraction and scavengers of free radicals through the generation of PGE2. Oxidative stress causes lipid peroxidation and generation of platelet-activating factor (PAF) or PAF-like lipids. The present studies therefore were aimed at determining whether either one induced by H2O2 mediates the increased generation of PGE2. Dissociated muscle cells of guinea pig gallbladder were obtained by enzymatic digestion. Both PAF-like lipids and PAF-induced muscle contraction was blocked by the PAF receptor antagonist CV-3988. This antagonist also blocked the increased PGE2 production caused by PAF-like lipids or PAF. Actions of PAF-like lipids were completely inhibited by indomethacin, but those of PAF were only partially reduced by indomethacin or by nordihydroguaiaretic acid and completely blocked by their combination. PAF-like lipids-induced contraction was inhibited by AACOCF3 (cystolic phospholipase A2 inhibitor), whereas the actions of PAF were blocked by MJ33 (secretory phospholipase A2 inhibitor). Receptor protection studies showed that pretreatment with PAF-like lipids before N-ethylmaleimide protected the contraction induced by a second dose of PAF-like lipids or PGE2 but not by PAF. In contrast, pretreatment with PAF protected the actions of PAF and PGE2 but not that of PAF-like lipids. Both PAF-like lipids and PAF-induced contractions were inhibited by anti-Galphaq/11 antibody and by inhibitors of MAPK and PKC. In conclusion, PAF-like lipids seem to activate a pathway different from that of PAF probably by stimulating a different PAF receptor subtype.

Hydrophilic but not hydrophobic bile acids prevent gallbladder muscle dysfunction in acute cholecystitis

The pathogenesis of acute cholecystitis (AC) is controversial. Bile acids may be involved in the pathogenesis of AC because the hydrophobic chenodeoxycholic acid (CDCA) reproduced in vitro the muscle dysfunction observed in AC and was prevented by the hydrophilic ursodeoxycholic acid (UDCA). The present study examined the in vivo effects of UDCA or CDCA on gallbladder muscle dysfunction caused by AC. Guinea pigs were treated with placebo, UDCA, or CDCA for 2 weeks before sham operation or induction of AC by bile duct ligation (BDL) for 3 days. Pretreatment with oral UDCA prevented the defective contraction in response to agonists (acetylcholine [ACh], cholecystokinin 8 [CCK-8], and KCl) that occurs after BDL. Prostaglandin (PG) E(2)-induced contraction remained normal in the placebo and UDCA-treated groups but was impaired in the CDCA-treated group. Treatment with UDCA also prevented the expected increase in the levels of H(2)O(2), lipid peroxidation, and PGE(2) content in the placebo-treated AC group, whereas CDCA caused further increases in these oxidative stress markers. The binding capacity of PGE(2) to its receptors and the activity of catalase were reduced after treatment with CDCA. Treatment with UDCA enriched gallbladder bile acids with its conjugates and reduced the percentage of CDCA conjugates. In contrast, treatment with CDCA significantly decreased the percentage of UDCA in bile. In conclusion, oral treatment with UDCA prevents gallbladder muscle damage caused by BDL, whereas oral treatment with CDCA worsens the defective muscle contractility and the oxidative stress.

Effects of bile acids on the muscle functions of guinea pig gallbladder

Hydrophobic bile acids impair gallbladder emptying in vivo and inhibit gallbladder muscle contraction in response to CCK-8 in vitro. This study was aimed at determining the mechanisms of muscle cell dysfunction caused by bile acids in guinea pig gallbladders. Muscle cells were obtained by enzymatic digestion. Taurochenodeoxycholic acid (TCDC), a hydrophobic bile acid, caused a contraction of up to 15% and blocked CCK-induced contraction. Indomethacin abolished the TCDC-induced contraction. Hydrophilic bile acid tauroursodeoxycholic acid (TUDC) had no effect on muscle contraction but prevented the TCDC-induced contraction and its inhibition on CCK-induced contraction. Pretreatment with NADPH oxidase inhibitor PH2I, xanthine oxidase inhibitor allopurinol, and free-radical scavenger catalase also prevented TCDC-induced contraction and its inhibition of the CCK-induced contraction. TCDC caused H2O2 production, lipid peroxidation, and increased PGE2 synthesis and activities of catalase and SOD. These changes were significantly inhibited by pretreatment of PH2I or allopurinol. Inhibitors of cytosolic phospholipase A2 (cPLA2), protein kinase C (PKC), and mitogen-activating protein kinase (MAPK) also blocked the TCDC-induced contraction. It is concluded that hydrophobic bile acids cause muscle cell dysfunction by stimulating the formation of H2O2 via activation of NADPH and xanthine oxidase. H2O2 causes lipid peroxidation and activates cPLA2 to increase PGE2 production, which, in turn, stimulates the synthesis of free-radical scavengers through the PKC-MAPK pathway.

Reactive oxygen species (H(2)O(2)): effects on the gallbladder muscle of guinea pigs

Reactive oxygen species (ROS) have been implicated in the pathogenesis of muscle dysfunction in acute inflammatory processes. The aim of these studies was to determine the effects of ROS on gallbladder muscle function in vitro. Single muscle cells were obtained by enzymatic digestion. H(2)O(2) (70 microM) caused maximal contraction of up to 14% and blocked the response to CCK-8, ACh, and KCl. It did not affect the contractions induced by guanosine 5'-O-(3-thiotriphosphate), diacylglycerol, and inositol 1,4,5-trisphosphate that circumvent membrane receptors. The contraction induced by H(2)O(2) was inhibited by AACOCF(3) [cytosolic phospholipase A(2) (cPLA(2)) inhibitor], indomethacin (cyclooxygenase inhibitor), chelerythrine [protein kinase C (PKC) inhibitor], or PD-98059 [mitogen-activated protein kinase (MAPK) inhibitor]. H(2)O(2) also reduced the CCK receptor binding capacity from 0.36 +/- 0.05 pmol/mg protein (controls) to 0.17 +/- 0.03 pmol/mg protein. The level of lipid peroxidation as well as the PGE(2) content was significantly increased after H(2)O(2) pretreatment. Unlike superoxide dismutase, the free radical scavenger catalase prevented the H(2)O(2) induced contraction, and its inhibition of the CCK-8 induced contraction. It is concluded that ROS cause damage to the plasma membrane of the gallbladder muscle and contraction through the generation of PGE(2) induced by cPLA(2)-cyclooxygenase and probably mediated by the PKC-MAPK pathway.

Abnormalities of gallbladder muscle associated with acute inflammation in guinea pigs

Muscle strips from experimental acute cholecystitis (AC) exhibit a defective contraction. The mechanisms responsible for this impaired contraction are not known. The present studies investigated the nature of these abnormalities. AC was induced by ligating the common bile duct of guinea pigs for 3 days. Contraction was studied in enzymatic dissociated muscle cells. Cholecystokinin (CCK) and prostaglandin E2 (PGE2) receptor binding studies were performed by radioreceptor assay. The levels of lipid peroxidation, cholesterol, phospholipid, and H2O2 as well as the catalase and superoxide dismutase (SOD) activities were determined. PGE2 content was measured by radioimmunoassay. Muscle contraction induced by CCK, ACh, or KCl was significantly reduced in AC, but PGE2-induced contraction remained normal. GTPgammaS, diacyglycerol (DAG), and 1,4,5-trisphosphate (IP3), which bypass the plasma membrane, caused a normal contraction in AC. The number of functional receptors for CCK was significantly decreased, whereas those for PGE2 remained unchanged in AC. There was a reduction in the phospholipid content and increase in the level of lipid peroxidation as well as H2O2 content in the plasma membrane in AC. The PGE2 content and the activities of catalase and SOD were also elevated. These data suggest that AC cause damage to the constituents of the plasma membrane of muscle cells. The preservation of the PGE2 receptors may be the result of muscle cytoprotection.

Gallbladder muscle dysfunction in patients with chronic acalculous disease

BACKGROUND & AIMS

The mechanisms responsible for the abnormalities of gallbladder emptying in patients with chronic acalculous gallbladder disease (AGD) have not been elucidated. This study was designed to determine whether a muscle defect could explain this gallbladder dysfunction.

METHODS

Gallbladder contraction induced by a continuous intravenous cholecystokinin octapeptide (CCK-8) infusion was determined by ultrasonography in control subjects, patients with AGD, pigment stones, and cholesterol stones. Muscle cells were obtained by enzymatic digestion. (125)I-CCK-8 binding and [(35)S]guanosine triphosphate gamma S (GTP gamma S) binding studies were performed.

RESULTS

In vivo gallbladder contraction induced by CCK-8 was significantly lower in AGD (29.4%) and cholesterol stones (28.8%) than in pigment stones (59.8%) and normal controls (57.8%; P < 0.01). In vitro muscle cell contraction induced by CCK-8 was also lower in AGD than in pigment stones. It remained impaired in AGD after stimulation with the G-protein activators GTP gamma S and AlF(4) and with the second messenger 1,2-dioctanoyl-sn-glycerol. However, GTP gamma S binding induced by CCK-8 and vasoactive intestinal polypeptide and the binding capacity of CCK receptors were not different between AGD and pigment stones.

CONCLUSIONS

These findings suggest that there is a good correlation between in vivo and in vitro gallbladder response to CCK-8 in patients with AGD. Unlike those found in cholesterol stones, the muscle defects in AGD appear to reside in the contractile apparatus.

Review article: in vitro studies of gall-bladder smooth muscle function. Relevance in cholesterol gallstone disease

The interplay between contraction and relaxation in the gall-bladder muscularis leads to appropriate gall-bladder emptying and refilling during fasting and in the postprandial state in vivo. Several studies in both human and animal models have focused on cellular and molecular events in the gall-bladder wall in health and disease in vitro. Principal methods to study gall-bladder smooth muscle function include receptor binding studies (at the level of plasmamembranes or histological sections), phase contrast microscopy (at the level of isolated smooth muscle cells), and tensiometry (at the level of smooth muscle strips or the whole gall-bladder). At a very early stage, cholesterol gallstone disease is characterized by exposure of the gall-bladder wall to excess of biliary cholesterol and the cytotoxic effect of the bile salt deoxycholate. On a long-term basis, a form of gall-bladder leiomyopathy develops with defects involving the mechanisms of signal transduction at the level of plasmamembranes. The end-stage result is pathological contraction and/or relaxation of smooth musculature, impaired gall-bladder motility and gall-bladder stasis, all key factors in the pathogenesis of biliary cholesterol crystallization and gallstones.

Defect of receptor-G protein coupling in human gallbladder with cholesterol stones

Human gallbladders with cholesterol stones (ChS) exhibit an impaired muscle contraction and relaxation and a lower CCK receptor-binding capacity compared with those with pigment stones (PS). This study was designed to determine whether there is an abnormal receptor-G protein coupling in human gallbladders with ChS using (35)S-labeled guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding, (125)I-labeled CCK-8 autoradiography, immunoblotting, and G protein quantitation. CCK and vasoactive intestinal peptide caused significant increases in [(35)S]GTPgammaS binding to Galpha(i-3) and G(s)alpha, respectively. The binding was lower in ChS than in PS (P < 0.01). The reduced [(35)S]GTPgammaS binding in ChS was normalized after the muscles were treated with cholesterol-free liposomes (P < 0.01). Autoradiography and immunoblots showed a decreased optical density (OD) for CCK receptors, an even lower OD value for receptor-G protein coupling, and a higher OD for uncoupled receptors or Galpha(i-3) protein in ChS compared with PS (P < 0.001). G protein quantitation also showed that there were no significant differences in the Galpha(i-3) and G(s)alpha content in ChS and PS. We conclude that, in addition to an impaired CCK receptor-binding capacity, there is a defect in receptor-G protein coupling in muscle cells from gallbladder with ChS. These changes may be normalized after removal of excess cholesterol from the plasma membrane.

Cholesterol inhibits spontaneous action potentials and calcium currents in guinea pig gallbladder smooth muscle

Elevated cholesterol decreases agonist-induced contractility and enhances stone formation in the gallbladder. The current study was conducted to determine if and how the electrical properties and ionic conductances of gallbladder smooth muscle are altered by elevated cholesterol. Cholesterol was delivered as a complex with cyclodextrin, and effects were evaluated with intracellular recordings from intact gallbladder and whole cell patch-clamp recordings from isolated cells. Cholesterol significantly attenuated the spontaneous action potentials of intact tissue. Furthermore, calcium-dependent action potentials and calcium currents were reduced in the intact tissue and in isolated cells, respectively. However, neither membrane potential hyperpolarizations induced by the ATP-sensitive potassium channel opener, pinacidil, nor voltage-activated outward potassium currents were affected by cholesterol. Hyperpolarizations elicited by calcitonin gene-related peptide were reduced by cholesterol enrichment, indicating potential changes in receptor ligand binding and/or second messenger interactions. These data indicate that excess cholesterol can contribute to gallbladder stasis by affecting calcium channel activity, whereas potassium channels remained unaffected. In addition, cholesterol enrichment may also modulate receptor ligand behavior and/or second messenger interactions.

CCK receptor dysfunction in muscle membranes from human gallbladders with cholesterol stones

Human gallbladders with cholesterol stones exhibit impaired muscle contraction induced by agonists that act on transmembrane receptors, increased membrane cholesterol content, and abnormal cholesterol-to-phospholipid ratio compared with those with pigment stones. The present study was designed to investigate the functions of the CCK receptor of gallbladder muscle membranes by radioreceptor assay and cross-linking. 125I-labeled CCK-8 binding was time-dependent, competitive, and specific. Scatchard analysis showed that the maximum specific binding (Bmax) was significantly decreased in cholesterol compared with pigment stone gallbladders (0.18 +/- 0. 07 vs. 0.38 +/- 0.05 pmol/mg protein, P < 0.05). In contrast, the affinity for CCK was higher in cholesterol than pigment stone gallbladders (0.18 +/- 0.06 vs. 1.2 +/- 0.23 nM). Similar results were observed in binding studies with the CCK-A receptor antagonist [3H]L-364,718. Cross-linking and saturation binding studies also showed significantly less CCK binding in gallbladders with cholesterol stones. These abnormalities were reversible after incubation with cholesterol-free liposomes. The Bmax increased (P < 0.01) and the dissociation constant decreased (P < 0.001) after incubation with cholesterol-free liposomes. In conclusion, human gallbladders with cholesterol stones have impaired CCK receptor binding compared with those with pigment stones. These changes are reversed by removal of the excess membrane cholesterol. These receptor alterations may contribute to the defective contractility of the gallbladder muscle in patients with cholesterol stones.

Excess membrane cholesterol alters human gallbladder muscle contractility and membrane fluidity

BACKGROUND & AIMS

The relationship between muscle contractility, plasma membrane cholesterol, and fluidity was investigated in human gallbladders with gallstones.

METHODS

Isolated gallbladder muscle cells were used to measure contraction. Plasma membranes of gallbladder muscle were purified in a sucrose gradient and measured for cholesterol content and cholesterol/phospholipid mole ratio. Membrane fluidity was determined by using fluorescence polarization and was expressed as the reciprocal of anisotropy.

RESULTS

The maximal contraction induced by cholecystokinin octapeptide was significantly less in gallbladders with cholesterol stones than in those with pigment stones. The membrane cholesterol content and cholesterol/phospholipid mole ratio were significantly higher in gallbladders with cholesterol stones than in those with pigment stones. Membrane anisotropy was also higher than in gallbladders with pigment stones, reflecting lower membrane fluidity in gallbladders with cholesterol stones. After muscle cells from cholesterol stone gallbladders were incubated with cholesterol-free liposomes for 4 hours, cholecystokinin octapeptide-induced contraction, membrane cholesterol content and cholesterol/phospholipid ratio, and membrane fluidity returned to normal levels.

CONCLUSIONS

Gallbladder muscle from patients with cholesterol stones has increased membrane cholesterol/phospholipid mole ratio and decreased membrane fluidity resulting in impaired muscle contractility. These abnormalities are corrected by removing the excess cholesterol from the plasma membranes.

Expression of two different cholecystokinin receptors in Xenopus oocytes injected with mRNA from rabbit pancreas and rat hippocampus

Electrophysiological responses to cholecystokinin (CCK) were studied in Xenopus oocytes injected with mRNA from rabbit pancreas or rat hippocampus. CCK-octapeptide(26-33) (sulfated form) (CCK-8) elicited inward currents in both groups. In oocytes injected with pancreatic mRNA, CCK-8-induced currents were composed of two components, fast and slow. However, in oocytes injected with hippocampal mRNA, fast currents disappeared. The potency ranking of the agonists and the antagonist indicated that the receptors expressed by pancreatic and hippocampal mRNA were CCKA- and CCKB-subtypes, respectively. Extracellular application of EGTA had little effect on the CCKB-mediated response, but attenuated the CCKA-mediated one. Intracellular injection of EGTA abolished the CCKB-mediated response, whereas small smooth currents remained in oocytes expressing the CCKA-receptor. The reversal potentials of the CCKA- and CCKB-receptor-mediated responses were consistent with that for CI- currents. However, the reversal potential of the small smooth currents in EGTA-loaded oocytes expressing the CCKA-receptors was close to that for a non-selective cation channel. These results suggest that CCK-8 activates at least two different channels, a Ca(2+)-dependent Cl- channel and a non-selective cation channel in oocytes expressing the CCKA-receptor, while the CCKB-receptor elicits only a Ca(2+)-dependent Cl- channel.

[3H]-SR 27897B: a selective probe for autoradiographic labelling of CCK-A receptors in the brain

The binding and distribution of radiolabelled SR27897B, a potent CCK-A antagonist, was characterized using in vitro receptor autoradiography. Rapid imaging and quantitative analysis of [3H]SR27897B binding was obtained in a very short period of time (5 days) with a highly sensitive radioimager ensuring very short exposure times for isotopes such as tritium. Tritiated SR27897B binding sites are localized almost exclusively in the area postrema and the medical part of the nucleus tractus solitarius and in this nucleus the rostral-caudal distribution of CCK-A sites differed from that of sulphated CCK8 receptors. Receptor binding properties analyzed on 15 microns serial coronal sections showed on site receptor occupancy in these two regions with high affinity and selectivity characteristic of the CCK-A receptor. These results precisely locate the SR27897B binding sites and provide further support for the absence of heterogeneity of the CCK-A receptors in the rat brain.

Role of CCK in gallbladder function

Cholecystokinin may play a role in regulation of interdigestive motility, but this still remains to be investigated. CCK constitutes the major hormonal stimulus for postprandial gallbladder emptying. CCK exerts its contractile effects mainly through interaction directly with receptors on the gallbladder smooth muscle cells in the muscle layer, but also through interaction with cholinergic nerves extrinsic and/or intrinsic in nature. Furthermore, CCK can enhance ongoing nicotinic ganglionic transmission occurring in the serosal layer by release of acetylcholine. CCK interaction with the gallbladder smooth muscle CCKA receptor was studied in further detail. CCK contracts strips of gallbladder muscle in a concentration-dependent way with a potency in the nanomolar range in all tested species. The potency is 1,000-fold better than that of gastrin; thus, the receptor is of type CCKA. CCK binding to this receptor is specific and of high affinity, 1,000-fold better than that of gastrin with no differences between the tested species including bovine, porcine, and human. Also, CCK binding affinity was independent of age, gender, or weight of the person and pathology of the human gallbladder. The biochemistry of the CCKA receptor varies between the tested species (bovine and human). Both CCKA receptors are heavily glycosylated, but of different size and carbohydrate content. The bovine CCKA receptor is of apparent size M(r) = 70-85 kD with N-linked complex carbohydrates and sialic acids. The human CCKA receptor is of M(r) = 85-95 kD, with N-linked complex carbohydrates, but no sialic acids. They both have a protein core of apparent size M(r) = 43 kD, with almost identically sized fragments after enzymatic cleavage. Probably the protein cores contain the receptor binding region, which seems well preserved between species. CCK and the CCKA gallbladder muscularis receptor are main regulators of postprandial gallbladder emptying. The biochemistry of the CCKA gallbladder smooth muscle receptor is in accord with newly generated data of purification and cloning of the rat pancreatic CCKA receptor.

Receptors for gut regulatory peptides

Receptors for regulatory peptides (hormones or neurotransmitters) play a pivotal role in the ability of cells to taste the rich neuroendocrine environment of the gut. Recognition of low concentration of peptides with a high specificity and translation of the peptide-receptor interaction into a biological response through different signalling pathways (adenylyl cyclase-cAMP or phospholipase C-phosphatidylinositol) are crucial properties of receptors. While many new receptors have been identified and thereafter characterized functionally during the 1980s, molecular biology now emerges as the privileged way for the structural characterization and discovery of receptors. Different strategies of receptor cloning have been developed which may or may not require prior receptor purification. Among cloning strategies that do not require receptor purification, homology screening of cDNA libraries, expression of receptor cDNA or mRNA in Xenopus laevis oocytes or in COS cells, and the polymerase chain reaction method achieved great success, e.g. cloning of receptors for cholecystokinin, gastrin, glucagon-like peptide 1, gastrin-releasing peptide/bombesin, neuromedin K, neuropeptide Y, neurotensin, opioids, secretin, somatostatin, substance K, substance P and vasoactive intestinal peptide. All these receptors belong to the superfamily of G-protein-coupled receptors which consist of a single polypeptide chain (350-450 amino acids) with seven transmembrane segments, an N-terminal extracellular domain and a C-terminal cytoplasmic domain. In this chapter, we have detailed the properties of three receptors which play an important role in digestive tract physiology and illustrate various signal transduction pathways: pancreatic beta-cell galanin receptors which mediate inhibition of insulin release and intestinal epithelial receptors for vasoactive intestinal peptide and peptide YY, which mediate the stimulation and inhibition of water and electrolyte secretion, respectively.

Characterization of muscarinic receptors on guinea pig gallbladder smooth muscle

BACKGROUND

Cholinergic agonists are of major importance for the regulation of gallbladder motility. However, the gallbladder muscarinic receptors have not been localized or characterized directly using radioligands, and it has not been clearly established which subtype of muscarinic receptor mediates contraction. The aim of the present study was to characterize the gallbladder muscarinic receptors.

METHODS

Binding studies to guinea pig gallbladder sections were performed using 1-[N-methyl-3H] scopolamine methyl chloride. Carbachol-induced contraction was measured using muscle strips.

RESULTS

Binding of 1-[N-methyl-3H] scopolamine methyl chloride was reversible, dependent on time, temperature, and pH. Autoradiography showed binding only over the smooth muscle. Binding and carbachol-induced contractions were inhibited by muscarinic receptor antagonists with the following potencies: atropine > N-methyl-scopolamine > silahexocyclium-methylsulfate > AF-DX 384 [(+-)-5,11-dihydro-11-([(2-(2-[(dipropylamino)-methyl]-1- piperidinyl)ethyl)amino]carbonyl)-6H-pyrido (2,3b) (1,4)-benzodiazepine-6-one] > hexahydro-siladifenidol hydrochloride > AF-DX 116 [(+-)-11-([2-[(diethylamino)methyl]-1- piperidinyl]-acetyl)-5,11-dihydro-6H-pyrido (2,3b)(1,4)benzodiazepine-6-one] > pirenzepine. Carbachol inhibited binding to gallbladder sections over the same range of concentrations that caused contractions. The concentration-contraction curves for carbachol were not altered by tetrodotoxin.

CONCLUSIONS

Gallbladder smooth muscle cells possess muscarinic receptors of the M3 type. These receptors mediate carbachol-induced contraction.

Characterization of cholecystokinin receptors on guinea pig gastric chief cell membranes

The binding of cholecystokinin (CCK) to its receptors on guinea pig gastric chief cell membranes were characterized by the use of 125I-CCK-octapeptide (CCK8). At 30 degrees C optimal binding was obtained at acidic pH in the presence of Mg2+, while Na+ reduced the binding. In contrast to reports on pancreatic and brain CCK receptors, scatchard analysis of CCK binding to chief cell membranes revealed two classes of binding sites. Whereas, in the presence of a non-hydrolyzable GTP analog, GTP gamma S, only a low affinity site of CCK binding was observed. Chief cell receptors recognized CCK analogs, with an order of potency of: CCK8 greater than gastrin-I greater than CCK4. Although all CCK receptor antagonists tested (dibutyryl cyclic GMP, L-364718 and CR1409) inhibited labeled CCK binding to chief cell membranes, the relative potencies of these antagonists in terms of inhibiting labeled CCK binding were different from those observed in either pancreatic membranes or brain membranes. The results indicate, therefore, that on gastric chief cell membranes there exist specific CCK receptors, which are coupled to G protein. Furthermore, chief cell CCK receptors may be distinct from pancreatic or brain type CCK receptors.

Gallbladder CCK receptors: species differences in glycosylation of similar protein cores

Receptors for cholecystokinin (CCK) on gallbladder muscularis smooth muscle have different apparent sizes in man (Mr = 85,000-95,000) and cow (Mr = 70,000-85,000). In this work, these receptors were demonstrated to represent N-linked complex glycoproteins with Mr = 43,000 protein cores, based on lectin-affinity chromatography and the deglycosylation of bands affinity labeled with 125I-D-Tyr-Gly-[(Nle28,31, pNO2-Phe33)CCK-26-33] using neuraminidase, O-glycanase and endoglycosidases H and F. Similarities in the core proteins were further demonstrated by Staphylococcus aureus V8 protease peptide mapping, in which both proteins yielded similar fragment patterns. Thus, gallbladder CCK receptors present in man and cow are both N-linked complex glycoproteins, with different carbohydrate domains and similar protein cores.

Protease peptide mapping of affinity-labeled rat pancreatic cholecystokinin-binding proteins

Affinity-labeling probes with sites of cross-linking distributed along the ligand have been used to biochemically characterize the pancreatic cholecystokinin (CCK) receptor. Probes with photolabile sites spanning the receptor-binding domain have labeled a Mr = 85,000-95,000 plasma membrane protein, while a probe cross-linked via the amino terminus of CCK-33, far removed from the carboxyl-terminal receptor-binding domain, has labeled a distinct Mr = 80,000 protein. In this work, protease peptide mapping of the pancreatic proteins labeled by each of these probes has been performed to gain insight into the identities of the bands and to define domains of the labeled proteins. Photolabile decapeptide probes with sites of cross-linking at the amino terminus, mid region, and carboxyl terminus of the receptor-binding domain each labeled a Mr = 85,000-95,000 glycoprotein with a Mr = 42,000 core protein and similar Staphylococcus aureus V8 protease peptide maps. This confirms that each probe labels the same binding protein and the same domain of that protein. Serial slices through the broad labeled band were separately deglycosylated and protease-treated, demonstrating a single protein core with differential glycosylation. The CCK-33-based probe, however, labeled predominantly two proteins, one having similar sizes in its native and deglycosylated forms to that labeled by the decapeptide probes and a distinct Mr = 80,000 protein. Of note, the peptide map of the protein believed to be the same as that labeled by the shorter probes was different, suggesting that this probe labeled the binding subunit at a site distinct from that which was labeled by the short probes.

Functional and biochemical characterization of the human gallbladder muscularis cholecystokinin receptor

Gallbladders removed at cholecystectomy are a potentially useful source of human receptor for the gastrointestinal peptide hormone cholecystokinin (CCK). Seven healthy gallbladders (removed incidentally at time of resection of hepatic metastases) and 50 diseased gallbladders were studied. Cholecystokinin radioligand binding to an enriched plasma membrane preparation from these tissues was shown to be rapid, reversible, temperature-dependent, saturable, specific, and high-affinity. Computer analysis of equilibrium binding data using the Ligand program best fit a single class of binding sites with Kd = 1.0 +/- 0.1 nM (mean +/- SEM). This was similar in health and disease, with no apparent differences related to age, gender, or body habitus. The structural specificity for binding to this site correlated well with relative potencies for CCK-gastrin peptides to stimulate gallbladder contraction. To biochemically characterize this receptor, we used a battery of reagents, including "long" (125I-Bolton Hunter-CCK-33) and "short" 125I-D-Try-Gly-[(Nle28,31)CCK-26-33] probes that were cross-linkable through their amino terminus and a monofunctional probe with a photolabile group at its carboxyl terminus 125I-D-Tyr-Gly[(Nle28,31,pNO2-Phe33)CCK-26-33]. All probes specifically labeled a human gallbladder muscularis protein of Mr = 85,000-95,000, which was also independent of diagnosis. Labeling of this band was inhibited in a concentration-dependent manner by CCK-8 and by L-364,718. Thus, the CCK receptor present on the very common surgically removed human gallbladder is functionally and biochemically intact and is useful for further characterization.