Physiological role of cholecystokinin in meal-induced insulin secretion in conscious rats. Studies with L 364718, a specific inhibitor of CCK-receptor binding

Free fatty acid receptors in the endocrine regulation of glucose metabolism: Insight from gastrointestinal-pancreatic-adipose interactions

Glucose metabolism is primarily controlled by pancreatic hormones, with the coordinated assistance of the hormones from gastrointestine and adipose tissue. Studies have unfolded a sophisticated hormonal gastrointestinal-pancreatic-adipose interaction network, which essentially maintains glucose homeostasis in response to the changes in substrates and nutrients. Free fatty acids (FFAs) are the important substrates that are involved in glucose metabolism. FFAs are able to activate the G-protein coupled membrane receptors including GPR40, GPR120, GPR41 and GPR43, which are specifically expressed in pancreatic islet cells, enteroendocrine cells as well as adipocytes. The activation of FFA receptors regulates the secretion of hormones from pancreas, gastrointestine and adipose tissue to influence glucose metabolism. This review presents the effects of the FFA receptors on glucose metabolism via the hormonal gastrointestinal-pancreatic-adipose interactions and the underlying intracellular mechanisms. Furthermore, the development of therapeutic drugs targeting FFA receptors for the treatment of abnormal glucose metabolism such as type 2 diabetes mellitus is summarized.

The Cholecystokinin Type 2 Receptor, a Pharmacological Target for Pain Management

Over the past decades, accumulating evidence has demonstrated a pivotal role of cholecystokinin type 2 receptor (CCK2R) in pain modulation. The established role of CCK2R activation in directly facilitating nociception has led to the development of several CCK2R antagonists, which have been shown to successfully alleviate pain in several rodent models of pain. However, the outcomes of clinical trials are more modest since they have not demonstrated the expected biological effect obtained in animals. Such discordances of results between preclinical and clinical studies suggest reconsidering our knowledge about the molecular basis of the pharmacology and functioning of CCK2R. This review focuses on the cellular localization of CCK2R specifically in the sensory nervous system and discusses in further detail the molecular mechanisms and signal transduction pathways involved in controlling pain perception. We then provide a comprehensive overview of the most successful compounds targeting CCK2R and report recent advances in pharmacological strategies used to achieve CCK2R modulation. We purposely distinguish between CCK2R benefits obtained in preclinical models and outcomes in clinical trials with different pain etiologies. Lastly, we emphasize the biological and clinical relevance of CCK2R as a promising target for the development of new treatments for pain management.

Cholecystokinin is involved in triglyceride fatty acid uptake by rat adipose tissue

The incorporation of plasma triglyceride (TG) fatty acids to white adipose tissue (WAT) depends on lipoprotein lipase (LPL), which is regulated by angiopoietin-like protein-4 (ANGPTL-4), an unfolding molecular chaperone that converts active LPL dimers into inactive monomers. The production of ANGPTL-4 is promoted by fasting and repressed by feeding. We hypothesized that the postprandial hormone cholecystokinin (CCK) facilitates the storage of dietary TG fatty acids in WAT by regulating the activity of the LPL/ANGPTL-4 axis and that it does so by acting directly on CCK receptors in adipocytes. We report that administration of CCK-8 (a bioactive fragment of CCK) to rats: (i) reduces plasma ANGTPL-4 levels; (ii) represses Angptl-4 expression in WAT and (iii) simultaneously enhances LPL activity in this tissue without inducing Lpl expression. In vivo CCK-8 effects are specifically antagonized by the CCK-2 receptor (CCK-2R) antagonist, L-365,260. Moreover, CCK-8 downregulates Angptl-4 expression in wild-type pre-adipocytes, an effect that is not observed in engineered pre-adipocytes lacking CCK-2R. These effects have functional consequences as CCK-8 was found to promote the uptake of dietary fatty acids by WAT, as demonstrated by means of proton nuclear magnetic resonance (¹H-NMR). The efficacy of acute CCK-8 administration was not reduced after chronic CCK-8 treatment. Moreover, the effects of CCK-8 on WAT were not associated to the increase of circulating insulin. Our results show that cholecystokinin promotes lipid storage in WAT by acting on adipocyte CCK-2R, suggesting a pivotal role for CCK in TG homeostasis.

Potential roles of GPR120 and its agonists in the management of diabetes

Free fatty acids (FFAs) serve not only as nutrients that provide energy but also as extracellular signaling molecules that manipulate intracellular physiological events through FFA receptors (FFARs) such as FFAR4. FFAR4 is also known as G-protein coupled receptor 120 (GPR120). The main role of GPR120 is to elicit FFA regulation on metabolism homeostasis. GPR120 agonism correlates with prevention of the occurrence and development of metabolic disorders such as obesity and diabetes. GPR120 activation directly or indirectly inhibits inflammation, modulates hormone secretion from the gastrointestinal tract and pancreas, and regulates lipid and/or glucose metabolism in adipose, liver, and muscle tissues, which may help prevent obesity and diabetes. This review summarizes recent advances in physiological roles of GPR120 in preventing insulin resistance and protecting pancreatic islet function, and examines how resident GPR120 in the pancreas may be involved in modulating pancreatic islet function.

Cholecystokinin-producing (I) cells of intestinal mucosa in dexamethasone-treated rats

The aim of this study was to investigate the morphological, immunohistochemical and ultrastructural changes of cholecystokinin-producing (I) cells of gastrointestinal mucosa in dexamethasone-treated rats (D). After 12-daily intraperitoneal administration of 2mg/kg dexamethasone, rats developed diabetes similar to human diabetes mellitus type 2. The mean diameter of the duodenum was significantly decreased due to significant reduction of volume fraction and profile area of lamina propria. There was a decrease in volume fraction and number of cholecystokinin (CCK)-producing cells per mm(2) of mucosa, as well as their numerical density, but without statistical significance. Also, dexamethasone induced appearance of hyperactive duodenal I-cells with small number of granules and dilated endoplasmic reticulum. In conclusion, the present study showed that morphological changes in duodenum cholecystokinin-producing (I) cells occurred in diabetic rats, in a manner which, suggests compensatory effort of CCK cells in diabetic condition.

Roles of dorsomedial hypothalamic cholecystokinin signaling in the controls of meal patterns and glucose homeostasis

A role for dorsomedial hypothalamus (DMH) cholecystokinin (CCK) signaling in feeding control has been proposed. Administration of CCK into the DMH reduces food intake and OLETF rats lacking CCK1 receptors (CCK1R) become hyperphagic and obese. We hypothesized that site specific replenishment of CCK1R in the DMH of OLETF rats would attenuate aspects of their feeding deficits. Recombinant vectors of adeno-associated viral (AAV)-mediated expression of CCK1R (AAVCCK1R) were bilaterally delivered into the DMH of OLETF. OLETF rats with AAVCCK1R injections demonstrated a 65% replenishment of Cck1r mRNA expression in the DMH relative to lean LETO control rats. Although this level of replenishment did not significantly affect overall food intake or body weight through 14 weeks following viral injections, meal patterns were partially normalized in OLETF rats receiving AAVCCK1R with a significant decrease in dark cycle meal size and a small but significant decrease in daily food intake in the meal analysis chambers. Importantly, the elevation in blood glucose level of OLETF rats was attenuated by the AAVCCK1R injections (p=0.03), suggesting a role for DMH CCK signaling in glucose homeostasis. In support of this role, administration of CCK into the DMH of intact rats enhanced glucose tolerance, as this occurred through activation of CCK1R but not CCK2R signaling. In conclusion, partial replenishment of CCK1R in the DMH of OLETF rats, although insufficient for altering overall food intake and body weight, normalizes meal pattern changes and reduces blood glucose levels. Our study also shows a novel role of DMH CCK signaling in glucose homeostasis.

Impaired insulin secretion and enhanced insulin sensitivity in cholecystokinin-deficient mice

OBJECTIVE

Cholecystokinin (CCK) is released in response to lipid intake and stimulates insulin secretion. We hypothesized that CCK deficiency would alter the regulation of insulin secretion and glucose homeostasis.

RESEARCH DESIGN AND METHODS

We used quantitative magnetic resonance imaging to determine body composition and studied plasma glucose and insulin secretion of CCK gene knockout (CCK-KO) mice and their wild-type controls using intraperitoneal glucose and arginine infusions. The area of anti-insulin staining in pancreatic islets was measured by immunohistochemistry. Insulin sensitivity was assessed with euglycemic-hyperinsulemic clamps.

RESULTS

CCK-KO mice fed a low-fat diet had a reduced acute insulin response to glucose but a normal response to arginine and normal glucose tolerance, associated with a trend toward greater insulin sensitivity. However, when fed a high-fat diet (HFD) for 10 weeks, CCK-KO mice developed glucose intolerance despite increased insulin sensitivity that was associated with low insulin secretion in response to both glucose and arginine. The deficiency of insulin secretion in CCK-KO mice was not associated with changes in β-cell or islet size.

CONCLUSIONS

CCK is involved in regulating insulin secretion and glucose tolerance in mice eating an HFD. The impaired insulin response to intraperitoneal stimuli that do not typically elicit CCK release suggests that this hormone has chronic effects on β-cell adaptation to diet in addition to acute incretin actions.

Control of somatostatin (SS) secretion by CCK-1 and CCK-2 receptors' occupation in RIN-14B cells, a rat pancreatic islet cell line

OBJECTIVES

This study evaluated the role played by cholecystokinin (CCK) receptors' occupation in the control of somatostatin (SS) secretion in RIN-14B cells.

METHODS

The presence of the CCK receptors 1 and 2 was confirmed by immunofluorescence, and SS secretion was evaluated by enzyme-linked immunosorbent assay.

RESULTS

By immunofluorescence, 95% of the cell population was composed of SS cells bearing both CCK-R subtypes with 5% of beta cells (data not shown). Cerulein (Cae), a CCK-1R agonist, and pentagastrin, a CCK-2R agonist, dose-dependently increased SS release, 3-fold at 1 mumol/L Cae, 2.5-fold at 10 mumol/L pentagastrin, with occupation of both CCKRs confirmed by L-364,178 and L-365,260 inhibition of CCK receptors 1 and 2. The occupation of high-affinity CCK-1R by Cae was confirmed on SS release with JMV-180, a high-affinity CCK-1R agonist, and absence of SS release inhibition at high Cae concentration occupying the low-affinity CCK-1R. These cells release more than 60% of their SS content by constitutive secretion, confirmed by cycloheximide and brefeldin inhibiting SS synthesis and intracellular trafficking, respectively.

CONCLUSIONS

Both CCKR subtypes occupy RIN-14B cells and initiate SS secretion through constitutive secretion controlled at SS synthesis level. Somatostatin secretion via the CCK-1R occupation mobilizes its high-affinity sites.

Gastrin, cholecystokinin and gastrointestinal tract functions in mammals

The aim of the present review is to synthesise and summarise our recent knowledge on the involvement of cholecystokinin (CCK) and gastrin peptides and their receptors in the control of digestive functions and more generally their role in the field of nutrition in mammals. First, we examined the release of these peptides from the gut, focusing on their molecular forms, the factors regulating their release and the signalling pathways mediating their effects. Second, general physiological effects of CCK and gastrin peptides are described with regard to their specific receptors and the role of CCK on vagal mucosal afferent nerve activities. Local effects of CCK and gastrin in the gut are also reported, including gut development, gastrointestinal motility and control of pancreatic functions through vagal afferent pathways, including NO. Third, some examples of the intervention of the CCK and gastrin peptides are exposed in diseases, taking into account intervention of the classical receptor subtypes (CCK1 and CCK2 receptors) and their heterodimerisation as well as CCK-C receptor subtype. Finally, applications and future challenges are suggested in the nutritional field (performances) and in therapy with regards to the molecular forms or in relation with the type of receptor as well as new techniques to be utilised in detection or in therapy of disease. In conclusion, the present review underlines recent developments in this field: CCK and gastrin peptides and their receptors are the key factor of nutritional aspects; a better understanding of the mechanisms involved may increase the efficiency of the nutritional functions and the treatment of abnormalities under pathological conditions.

Immunohistochemical analysis of cholecystokinin A receptor distribution in the rat pancreas

Cholecystokinin (CCK) plays a major role in the regulation of pancreatic enzyme secretion based on its binding to the CCK-A receptor (CCK-AR). While CCK-AR is known to be expressed in rat islet B cells, the localization of CCK-AR in rat pancreatic A and D cells remains poorly understood. The aim of this study was to identify the localization of CCK-AR in rat pancreatic islets by means of double immunofluorescence straining with antibodies against CCK-AR, glucagon, insulin and somatostatin and with in situ hybridization to detect its transcript. CCK-AR-like immunoreactive cells were found to overlap both with glucagon-like immunoreactive cells and insulin-like immunoreactive cells but not with somatostatin-like immunoreactive cells. An in situ hybridization study using a cRNA probe for CCK-AR revealed that CCK-AR mRNA was expressed in the center and periphery of the pancreatic islets. Further to this, immunofluorecsence staining using anti-glucagon antibody was carried out after in situ hybridization using the CCK-AR cRNA probe in order to identify CCK-AR mRNA expressing cells. CCK-AR mRNA exhibited a distribution pattern almost identical to that of glucagon-like immunoreactive cells. These results show clearly that CCK-AR exists not only in B but also in A cells of the rat pancreas, suggesting that CCK regulates the secretion of insulin and glucagon at least partly via CCK-AR.

Regulation of leptin distribution between plasma and cerebrospinal fluid by cholecystokinin receptors

Cholecystokinin (CCK) is a postprandial hormone that elicits a satiating effect and regulates feeding behaviour. CCK has been shown to enhance the effect of leptin in several experimental paradigms. The goal of this work was to characterize the effect of endogenous CCK on plasma leptin content by using CCK receptor antagonists. Therefore, we administered SR-27897, a selective CCK1 receptor antagonist, and L-365260, a selective CCK2 receptor antagonist, to fed and food-deprived rats and determined plasma leptin concentration by enzyme immunoassay. Plasma insulin and glucose concentration as well as food intake were also determined. Under our conditions, SR-27897 increased plasma concentration of leptin both in fed and food-deprived rats. It also increased food intake as well as plasma concentration of insulin in fed animals. L-365260 increased plasma leptin concentration only in fed rats. In animals receiving exogenous leptin, CCK-8 increased the ratio between the concentration of leptin in cerebrospinal fluid and plasma. These results show that CCK receptor antagonists increases plasma concentration of leptin and suggest that endogenous CCK may facilitate the uptake of plasma leptin to the cerebrospinal fluid.

Localization of cholecystokinin receptor subtypes in the endocine pancreas

This study was undertaken to clarify the controversy in the literature about pancreatic localization of the cholecystokinin (CCK) CCK(A) and CCK(B) receptors. With antibodies used by other investigators, we first established their specificity by Western blotting, indirect immunofluorescence, and confocal microscopy with each antibody's peptide antigen. Co-localization assays between the CCK receptors and the pancreatic hormones insulin, glucagon, and somatostatin revealed that the CCK(A) RAbs 1122 and R1-2 recognized insulin and glucagon cells in rat, pig, and human pancreas but not in the somatostatin cells. Conversely, the three CCK(B) RAbs tested, 9262, 9491, and GR4, identified the somatostatin cells. Abs 9491 and GR4 occasionally co-localized with glucagon, a feature that never occurred with Ab 9262. Finally, the specificity of Ab 9262 for the pancreatic CCK(B) R was confirmed in six different species. It co-localized with somatostatin but never with glucagon in these species. Our data suggest the use of Abs 1122 and 9262 to specifically identify and localize pancreatic CCK(A) and CCK(B) receptors, respectively. Confusion in the literature may result from the lack of specificity of most antibodies used, as established in this study.

Direct and indirect effects of amino acids on hepatic glucose metabolism in humans

AIM/HYPOTHESIS

The study was designed to examine the contribution of direct (substrate-mediated) and indirect (hormone-mediated) effects of amino acids on hepatic glucose metabolism in healthy men.

METHODS

The protocols were: (i) CON+S (n=7): control conditions with somatostatin to inhibit endogenous hormone release resulting in fasting plasma concentrations of amino acids, insulin (approximately 28 pmol/l) and glucagon (approximately 65 ng/l), (ii) AA+S ( n=7): amino acid infusion-fasting insulinaemia-fasting glucagonaemia, (iii) GLUC+S ( n=6): fasting amino acids-fasting insulinaemia-hyperglucagonaemia (approximately 99 ng/l) and (iv) AA-S (n=5): amino acid infusion without somatostatin resulting in amino acid-induced hyperinsulinaemia (approximately 61 pmol/l)-hyperglucagonaemia (approximately 147 ng/l). Net glycogenolysis was calculated from liver glycogen concentrations using (13)C nuclear magnetic resonance spectroscopy. Total gluconeogenesis (GNG) was calculated by subtracting net glycogenolysis from endogenous glucose production (EGP) which was measured with [6,6-(2)H(2)]glucose. Net GNG was assessed with the (2)H(2)O method.

RESULTS

During AA+S and GLUC+S, plasma glucose increased by about 50% (p<0.01) due to a comparable rise in EGP. This was associated with a 53-% (p<0.05) and a 65% increase (p<0.01) of total and net GNG during AA+S, whereas net glycogenolysis rose by 70% (p<0.001) during GLUC+S. During AA-S, plasma glucose remained unchanged despite nearly-doubled (p<0.01) total GNG.

CONCLUSION/INTERPRETATION

Conditions of postprandial amino acid elevation stimulate secretion of insulin and glucagon without affecting glycaemia despite markedly increased gluconeogenesis. Impaired insulin secretion unmasks the direct gluconeogenic effect of amino acids and increases plasma glucose.

Pancreatic capillary blood flow during caerulein-induced pancreatitis evaluated by a laser-doppler flowmeter in rats

PURPOSE: The pancreatic capillary blood flow (PCBF) was studied to determine its alterations during caerulein-induced pancreatitis in rats. METHODS: Twenty rats were divided in groups: control and caerulein. A laser-Doppler flowmeter to measure PCBF continuously was used. Blood pressure (BP) and heart rate (HR) were monitored. Serum biochemistry analyses were determined. Histopathological study was performed. RESULTS: The PCBF measured a mean of 109.08 ± 14.54% and 68.24 ± 10.47% in control group and caerulein group, respectively. Caerulein group had a mean decrease of 31.75 ± 16.79%. The serum amylase was 1323.70 ± 239.10U.I-1 and 2184.60 ± 700.46U.I-1 in control and caerulein groups, respectively. There was a significant difference in the PCBF (p<0.05) and serum amylase (p<0.05) when compared to control and caerulein groups. Although micro and microvacuolization were seen in 30% in caerulein group, no significant difference was seen between the groups. CONCLUSION: A decrease in the PCBF may be one of the leading events and it is present before histopathological tissue injury had been established in this model of acute pancreatitis.

Genetic, pharmacological and functional analysis of cholecystokinin-1 and cholecystokinin-2 receptor polymorphism in type 2 diabetes and obese patients

Cholecystokinin (CCK) and gastrin (G) and their receptors (CCK1 and CCK2) are involved in multiple physiological functions. Notably, CCK1R plays a role in the regulation of food intake whereas both CCK1R and CCK2R play a role in the regulation of pancreatic endocrine function. CCK1R and CCK2R may therefore serve as pharmacological targets in diabetes and obesity and genes encoding these receptors may be candidate genes in the pathogenesis of the diseases. In this study, we used single nucleotide polymorphism analysis and allele specific amplification for mutation screening of the CCK2 receptor gene and family linkage study. Mutated receptors were constructed, expressed in COS-7 cells for analysis of their binding and functional properties. V125I-CCK2 receptor variant was found in 2 out of 18 type 2 diabetes mellitus families tested. V125I mutation co-segregated in those 2 initial families, but further association studies showed that this mutation was not associated with diabetes or early age at diagnosis of the disease. V125I-CCK2 receptor high affinity sites exhibited a 2-fold enhanced binding affinity for CCK which was correlated to a slightly increased potency in coupling to inositol phosphate production. Since CCK2 receptor is expressed in pancreatic glucagon-producing cells in humans and is involved in secretion of glucagon, an increase of binding affinity of the mutated CCK2 receptor could enhance glucagon secretion in patients bearing V125I mutation. We also characterized a mutant of the CCK1 receptor which was previously identified in an obese patient. This mutant, V365I-CCK1, demonstrated a decreased level of expression (26%) and efficacy (25%) to stimulate inositol phosphates. It can therefore be expected that in humans bearing V365I mutation, decreases in CCK1 receptor expression and coupling efficiency may affect CCK-induced regulation of satiety. Polymorphism or mutations in the CCK receptors may be involved in type 2 diabetes mellitus and obesity. However, further studies are necessary to precisely evaluate this role in humans.

Cell-specific localization of the cholecystokininA receptor in the porcine pancreas

Cholecystokinin (CCK) produced in the mucosa of the upper small intestine exerts several biological functions. Its secretion in physiological amounts is modulated by the interaction of extracellular regulators and by binding to intracellular receptors of the target cells. The relative affinity of CCK to its receptor has been characterized in various biological and pharmacological studies and it is now well established that CCK has a higher affinity to the CCKA than to the CCKB receptor. Furthermore CCK influences the secretion of pancreatic enzymes in several species but very little is known about the relationship between CCK and the islet hormone-producing cells in the pig pancreas. The localization of this receptor at the cellular level showed conflicting results in animal studies and has not been described in pigs. The aim of the present study was to characterize the precise cellular location of the CCKA receptor in the porcine pancreas. Polyclonal antiserum was raised against the N-terminal epitope of the CCKA receptor molecule and used for localization studies. Using immunohistochemistry on methanol/acetic acid-fixed, paraffin-embedded pancreas, the CCKA receptor could successfully be localized in islet cells. Parallel staining of serial sections with antibodies directed against insulin and glucagon revealed colocalization with glucagon in alpha cells. No immunoreaction was found in the exocrine pancreas. Our results support the concept that in the porcine species the stimulation of the exocrine pancreas is mediated by the CCKB rather than the CCKA receptor, as it is known for the rat species.

Antidiabetogenic action of cholecystokinin-8 in type 2 diabetes

Cholecystokinin (CCK) is a gut hormone and a neuropeptide that has the capacity to stimulate insulin secretion. As insulin secretion is impaired in type 2 diabetes, we explored whether exogenous administration of this peptide exerts antidiabetogenic action. The C-terminal octapeptide of CCK (CCK-8) was therefore infused i.v. (24 pmol/kg x h) for 90 min in six healthy postmenopausal women and in six postmenopausal women with type 2 diabetes. At 15 min after start of infusion, a meal was served and ingested during 10 min. On a separate day, saline was infused instead of CCK-8. In both healthy subjects and subjects with type 2 diabetes, CCK-8 reduced the increase in circulating glucose after meal ingestion and potentiated the increase in circulating insulin. The ratio between the area under the curves for serum insulin and plasma glucose during the 15- to 75-min period after meal ingestion was increased by CCK-8 by 198 +/- 18% in healthy subjects (P = 0.002) and by 474 +/- 151% (P = 0.038) in subjects with type 2 diabetes. In contrast, the increase in the circulating levels of gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (GLP-1), or glucagon after meal ingestion was not significantly affected by CCK-8. The study therefore shows that CCK-8 exerts an antidiabetogenic action in both healthy subjects and type 2 diabetes through an insulinotropic action that most likely is exerted trough a direct islet effect. As at the same time, CCK-8 was infused without any adverse effects, the study suggests that CCK is a potential treatment for type 2 diabetes.

Duodenal loading with glucose induces fos expression in rat brain: selective blockade by devazepide

The role of CCK in mediating neuronal activity in the brain in response to dietary carbohydrate was measured by detecting Fos immunoreactivity in response to duodenal glucose load in rats after administration of the CCK-A receptor antagonist devazepide. In adult, male Sprague-Dawley rats, infusion for 30 min of 545 mg (2.18 kcal) dextrose through a duodenal cannula induced Fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), lateral division of the central nucleus of the amygdala (CeAL), and the external subnucleus of the lateral parabrachial nucleus (LPBE). Devazepide treatment (1 mg/kg) attenuated Fos expression in the NTS and AP by 81 and 78%, respectively, but not in the CeAL or LPBE. These results indicate that central neuronal activation is elicited by dietary glucose in the intestinal lumen and that activation of neurons in the NTS and AP is mediated by CCK-A receptors.

CCK receptor subtype in insulin-producing cells: a combined functional and in situ hybridization study in rat islets and a rat insulinoma cell line

Cholecystokinin (CCK) stimulates insulin secretion. It is, however, not established whether CCK receptors are expressed in insulin-producing cells. We therefore investigated, by in situ hybridization, whether CCK-A or CCK-B receptor mRNA could be detected in normal rat pancreatic islets and in the rat insulinoma cell line, RINm5F. The CCK-A, but not the CCK-B, receptor transcript was detected in both islets and RINm5F cells. Islet CCK-A receptors were mostly confined to the center of the islets corresponding to the distribution of the B cells. In RINm5F cells, insulin release was not significantly affected by cholecystokinin (CCK)-8-S (10(-13) to 10(-7) M), which is in contrast to the insulinotropic effect of CCK-8-S in normal rat islets. Similarly, in FURA-2AM-loaded cells, CCK-8-S (10(-11) to 10(-7) M) was without effect on the intracellular Ca2+ concentration ([Ca2+]ic) in RINm5F cells, whereas CCK-8-S (10(-7) M) markedly increased [Ca2+]ic (by 366+/-2 nM (P < 0.001) in normal rat islet cells. We conclude that the CCK-A, but not the CCK-B, receptor subtype is expressed in both normal rat islets and in the rat insulinoma-derived cell line RINmS5F. There is, however, a functional difference between normal islets and the RINm5F cells with respect to effects of CCK-8-S on insulin release and [Ca2+]ic.

Effects of CCK on pancreatic function and morphology

This report reviews the effects of CCK on the pancreas and in particular analyzes recent studies in which CCK antagonists were used to evaluate the physiological role of CCK in modulating pancreatic function and morphology. CCK is released from endocrine cells of the small intestine in response to a meal. In various animal species there are CCK receptors on pancreatic acinar cells with two sites; occupation of the high affinity site is thought to mediate pancreatic secretion and growth, whereas occupation of the low affinity site by high CCK concentrations is thought to be responsible for supramaximal inhibition of secretion and pancreatitis. Recently, CCK receptors were also found on postganglionic cholinergic neurons in the gastrointestinal tract. Administration of CCK agonists stimulates pancreatic secretion and growth. Although in some previous studies CCK was given at doses that mimic its postprandial increase in plasma, these studies did not prove that the actions of exogenous CCK were physiologically important. In addition, it was unclear if CCK primarily acts as a true hormone or as a neurotransmitter. The development of specific CCK receptor antagonists made it possible to better evaluate the physiological role of CCK. In humans, CCK-A antagonists like loxiglumide or L-364,718 at doses that completely inhibited the action of supraphysiological doses of exogenous CCK reduced meal-stimulated pancreatic enzyme secretion only by approximately 50%. On the other hand, atropine abolished the postprandial increase in pancreatic secretion and in addition markedly reduced the increase in pancreatic secretion due to infusion of "physiological" doses of CCK (i.e., CCK doses that mimic its postprandial increase in plasma). The increase in pancreatic bicarbonate secretion was only slightly reduced by CCK blockade. CCK antagonists failed to reduce the postprandial increase in plasma insulin, but markedly reduced the postprandial PP release. CCK-A antagonists caused slight hypotrophy and hypoplasia of the exocrine pancreas. However, even after 9 months of effective blockade of the CCK-A-receptor, mice had normal body weight and an almost normal pancreas. CCK antagonists were unable to alter short-term changes in pancreatic growth due to feeding and fasting. In some species, CCK agonists induced development of pancreatic nodules and increased the growth of malignant tumors. Studies about the effects of CCK antagonists on induction and growth of pancreatic tumors showed controversial results. In conclusion, CCK may act on the pancreas by three pathways: (1) At low doses it serves as a neurotransmitter by acting on cholinergic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological role of cholecystokinin on postprandial insulin secretion and gastric meal emptying in man. Studies with the cholecystokinin receptor antagonist loxiglumide

Cholecystokinin was previously proposed to play an important role in the regulation of postprandial insulin secretion either indirectly, by inhibiting gastric meal emptying, or directly, by acting as an incretin promoting the release of insulin. The aim of this investigation was therefore to clarify the role of endogenous cholecystokinin in the regulation of insulin release and gastric emptying applying the highly potent and specific cholecystokinin receptor antagonist loxiglumide. Five healthy volunteers were examined after an overnight fast. Gastric meal emptying was measured by the double indicator technique using a multiple lumen tube in the duodenum and 99mTc-diethylenetriamine pentaacetate as a meal marker and polyethylene glycol 4000 as a duodenal perfusion marker. Postprandial insulin, C-peptide, cholecystokinin and glucose levels were measured after ingestion of two isocaloric meals of a) Ensure (containing fat, protein and glucose), and b) a pure glucose meal (1.11 mol/l). The meals were given either with an intravenous infusion of loxiglumide (22 mumol.kg-1.h-1) or placebo. The infusion of loxiglumide markedly accelerated the gastric emptying of the mixed meal (area under curve, 5576 +/- 352 min vs 3498 +/- 109 min; p less than 0.001) and the pure glucose meal (area under curve 5662 +/- 537 min vs 3551 +/- 534 min; p less than 0.05). Simultaneously, loxiglumide induced a more rapid rise in postprandial insulin levels after both meals resulting in significantly higher (p less than 0.05) insulin levels during the first postprandial hour, but similar insulin levels in the second postprandial hour. Accordingly, we found a close correlation between meal emptying and insulin release (r = 0.748, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cholecystokinin-receptor blockade on pancreatic and biliary function in healthy volunteers

This study used the specific cholecystokinin (CCK)-receptor antagonist loxiglumide to evaluate whether endogenous CCK, which is released after a meal, regulates pancreatic and biliary functions. Eight healthy volunteers were studied twice on separate days. The subjects received a continuous intraduodenal infusion of a 750-kcal liquid test meal for 2 hours either with or without IV infusion of 5 mg.kg-1.h-1 of loxiglumide. Loxiglumide at this dose abolishes the actions of CCK at various target organs including gallbladder and pancreas, when given at doses that mimic postprandial plasma concentrations of CCK. Loxiglumide markedly decreased the meal-stimulated outputs of amylase, trypsin, and chymotrypsin by 55%-70% of control values but only slightly decreased duodenal volume (25% inhibition of mean integrated secretion). Loxiglumide abolished gallbladder emptying induced by infusion of nutrients and even increased gallbladder volumes when compared with prior fasting values. Correspondingly, loxiglumide almost abolished the output of bilirubin after infusion of nutrients. However, loxiglumide failed to alter the increase in circulating concentrations of glucose, insulin, and C peptide after infusion of nutrients. The present results show that CCK is one of several factors that regulate pancreatic protein secretion after absorption of nutrients. However, CCK is probably not involved in regulation of pancreatic secretion of fluid. In contrast, gallbladder function is mainly regulated by CCK, both in terms of its emptying after intestinal absorption of nutrients and in terms of maintenance of its fasting volume. Cholecystokinin does not play a major physiological role as an insulinotropic factor.

Cholecystokinin antagonism by anthramycin, a benzodiazepine antibiotic, in the central nervous system in mice

Anthramycin (ATM) which is a product of some streptomyces micro-organisms was shown to antagonize the central effects of cholecystokinin (CCK) such as antinociception and satiety and to displace CCK bound to the slices from the brains of mice. Sulfated octapeptide CCK (CCK8) was administered intracisternally to mice at doses of 1 microgram/mouse for inducing antinociception and 200 ng/mouse for satiety. ATM was administered intraperitoneally to mice at doses such as 0.3 and 0.5 mg/kg. CCK8-induced antinociception and satiety were significantly reversed by ATM in those doses. [125I]CCK8 binding to the brain slices was observed autoradiographically. The autoradiograms from the slices were converted to false color images by using a microcomputer. The radioactivity in the autoradiograms was expressed by color spectra in the false color images. Comparison of the binding of [125I]CCK8 to the brain slices in the presence and the absence of ATM revealed that ATM (10(-6) M) clearly displaced the CCK8 binding in the various regions, especially in the cortex, of the brain. These findings suggest that ATM acts as an potent antagonist of CCK in the central nervous system in mice.

Effects of three different cholecystokinin receptor antagonists on basal and stimulated insulin and glucagon secretion in mice

Cholecystokinin (CCK) receptor antagonists may be valuable tools for investigating the physiological role of CCK in islet function. In this study, the effects of the three different CCK receptor antagonists, proglumide, CR 1409 and L-364,718, on basal and stimulated insulin and glucagon secretion were investigated in vivo in the mouse. Each of the CCK antagonists was injected intravenously, either alone or together with one of the secretagogues CCK-8 (5.3 nmol kg-1), carbachol (0.16 mumol kg-1) or glucose (2.8 mmol kg-1). At a low dose level, proglumide (28 mumol kg-1) inhibited selectively CCK-8-induced insulin and glucagon secretion. However, at a higher dose level (280 mumol kg-1), proglumide inhibited also carbachol- and glucose-induced insulin secretion. Furthermore, proglumide elevated basal plasma levels of both glucagon and glucose. CR 1409 inhibited CCK-8-induced insulin secretion at a high (21 mumol kg-1) but not at a low (0.21 mumol kg-1) dose level. In contrast, CCK-8-induced glucagon secretion was not affected by CR 1409. L-364,718 (2.4 mumol kg-1) inhibited both CCK-8-induced insulin and glucagon secretion. In contrast, L-364,718 did not affect basal plasma levels of insulin, glucagon or glucose or those levels after stimulation with carbachol or glucose. We conclude that, of these three CCK antagonists, L-364,718 is the most specific CCK receptor antagonist for studies of both insulin and glucagon secretion in the mouse.

Mechanism of actions of cholecystokinin octapeptide on food intake and insulin and pancreatic polypeptide release in the dog

We investigated the mechanism by which CCK-8 injected into the third cerebral ventricle (ITV administration) inhibits food intake and stimulates insulin and pancreatic polypeptide (PP) secretion in the dog. ITV administration of CCK-8 (4.08 micrograms/5 min) resulted in a significant elevation of plasma insulin and PP concentrations. This effect was abolished by truncal vagotomy and promptly inhibited by ITV administration of atropine (20 micrograms) and proglumide (10 mg). CCK-8 was less effective in increasing insulin and PP concentrations than in reducing feeding. Thus, 1.36 micrograms of ITV CCK-8 markedly reduced food intake to 14, 15, 29 and 31% of control values at 10, 30, 60 and 120 min, respectively. Atropine and naloxone (50 micrograms) had no blocking effect on CCK-8-induced satiety, whereas proglumide antagonized it. These results indicate that ITV CCK-8 effects the endocrine pancreas and food intake through atropine-sensitive and atropine-insensitive mechanisms, respectively, both of which are likely to be mediated by CNS CCK receptors. Intravenous CCK-8 also stimulated PP and insulin release, through mechanisms that were atropine-sensitive and atropine-insensitive, respectively. However, its mode of action, especially on insulin secretion, was quite different from that of ITV CCK-8. Therefore, exogenous CCK appears to act in the brain and the periphery in concert with and independently from cholinergic systems.

Lack of insulinotropic effect of endogenous and exogenous cholecystokinin in man

Intraduodenal phenylalanine administration (333 mg/min over 60 min) released endogenous cholecystokinin in healthy young subjects as demonstrated radioimmunologically and by intraduodenal bilirubin and pancreatic enzyme output. Concomitantly, there was only a small increase over basal in circulating immunoreactive-insulin and immunoreactive-C-peptide concentrations. In healthy volunteers intraduodenal infusions of saline (10 ml/min), glucose (333 mg/min) or phenylalanine (333 mg/min) were performed for 60 min when plasma glucose was clamped at approximately 8 mmol/l. Phenylalanine enhanced immunoreactive-insulin and immunoreactive-C-peptide responses three-fold more than did the same amount of glucose. Immuno-reactive gastric inhibitory polypeptide responses were small and not different after glucose and phenylalanine administration. Immunoreactive cholecystokinin was significantly stimulated to 9.4 +/- 1.4 pmol/l only by intraduodenal phenylalanine. Plasma phenylalanine concentrations increased into the supraphysiological range (approximately 1.5 mmol/l). Intravenous infusions of phenylalanine achieving plasma concentrations of 1.2 mmol/l stimulated insulin secretion at elevated plasma glucose concentrations (approximately 8 mmol/l clamp experiments), but had no effect at basal plasma glucose concentrations. A small increase in cholecystokinin also was observed. Intravenous infusions of synthetic sulphated cholecystokinin-8 leading to plasma concentrations in the upper postprandial range (8-12 pmol/l) did not augment the immunoreactive-insulin or immunoreactive-C-peptide levels during hyperglycaemic clamp experiments, in the absence or presence of elevated plasma phenylalanine concentrations. It is concluded that the augmentation of the glucose-induced insulin release by intraduodenal administration of phenylalanine cannot be related to cholecystokinin release, but rather is explained by the combined effects of elevated glucose and phenylalanine concentrations. In man, cholecystokinin does not augment insulin secretion caused by moderate hyperglycaemia, elevations of phenylalanine concentrations, or combinations thereof.