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

The entero-insular axis: a journey in the physiopathology of diabetes

Glycemic homeostasis is an essential mechanism for the proper working of an organism. However, balance in blood lipid and protein levels also plays an important role. The discovery of the hormone insulin and the description of its function for glycemic control made fundamental scientific progress in this field. However, since then our view of the problem has been deeply influenced only in terms of glucose and insulin (in an insulin-centric and glucose-centric way). Based on recent scientific discoveries, a fine and sophisticated network of hormonal and metabolic interactions, involving almost every apparatus and tissue of the human body, has been theorized. Efficient metabolic homeostasis is founded on these intricate interactions. Although it is still not fully defined, this complex network can undergo alterations that lead to metabolic disorders such as diabetes mellitus (DM). The endocrine pancreas plays a crucial role in the metabolic balance of an organism, but insulin is just one of the elements involved and each single pancreatic islet hormone is worthy of our concern. Moreover, pancreatic hormones need to be considered in a general view, concerning both their systemic function as direct mediators and as hormones, which, in turn, are regulated by other hormones or other substances. This more complex scenario should be taken into account for a better understanding of the pathophysiology and the therapeutic algorithms of DM. As a consequence, improvements in modern medicine could help to contemplate this new perspective. This review is focused on some aspects of gut-pancreas interaction, aiming to integrate this synergy into a wider context involving other organs and tissues.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

The production and role of gastrin-17 and gastrin-17-gly in gastrointestinal cancers

The gastrointestinal peptide hormone gastrin is responsible for initiating the release of gastric acid in the stomach in response to the presence of food and/or humoral factors such as gastrin releasing peptide. However, it has a role in the growth and maintenance of the gastric epithelium, and has been implicated in the formation and growth of gastric cancers. Hypergastrinemia resulting from atrophic gastritis and pernicious anemia leads to hyperplasia and carcinoid formation in rats, and contributes to tumor formation in humans. Additionally, gastrin has been suspected to play a role in the formation and growth of cancers of the colon, but recent studies have instead implicated gastrin processing intermediates, such as gastrin-17-Gly, acting upon a putative, non-cholecystokinin receptor. This review summarizes the production and chemical structures of gastrin and of the processing intermediate gastrin-17-Gly, as well as their activities in the gastrointestinal tract, particularly the promotion of colon cancers.

Therapeutic potential for novel drugs targeting the type 1 cholecystokinin receptor

Cholecystokinin (CCK) is a physiologically important gastrointestinal and neuronal peptide hormone, with roles in stimulating gallbladder contraction, pancreatic secretion, gastrointestinal motility and satiety. CCK exerts its effects via interactions with two structurally related class I guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs), the CCK(1) receptor and the CCK(2) receptor. Here, we focus on the CCK(1) receptor, with particular relevance to the broad spectrum of signalling initiated by activation with the natural full agonist peptide ligand, CCK. Distinct ligand-binding pockets have been defined for the natural peptide ligand and for some non-peptidyl small molecule ligands. While many CCK(1) receptor ligands have been developed and have had their pharmacology well described, their clinical potential has not yet been fully explored. The case is built for the potential importance of developing more selective partial agonists and allosteric modulators of this receptor that could have important roles in the treatment of common clinical syndromes.

Cholecystokinin and gastrin receptors

Cholecystokinin and gastrin receptors (CCK1R and CCK2R) are G protein-coupled receptors that have been the subject of intensive research in the last 10 years with corresponding advances in the understanding of their functioning and physiology. In this review, we first describe general properties of the receptors, such as the different signaling pathways used to exert short- and long-term effects and the structural data that explain their binding properties, activation, and regulation. We then focus on peripheral cholecystokinin receptors by describing their tissue distribution and physiological actions. Finally, pathophysiological peripheral actions of cholecystokinin receptors and their relevance in clinical disorders are reviewed.

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.

Effects of proglumide on cholecystokinin-8-induced exocrine and endocrine pancreatic responses in conscious sheep

The effects of cholecystokinin (CCK)-8, either alone or together with the CCK antagonist, proglumide on both exocrine and endocrine pancreatic responses were examined in conscious sheep. Intravenous infusions of CCK-8 (120 pmol/kg/min for 40 min) with vehicle (0 mumol/kg/min proglumide) significantly increased both amylase output in pancreatic juice and plasma insulin concentrations (P < 0.05). Concomitant infusions of proglumide (5-40 mumol/kg/min for 50 min) inhibited both amylase and insulin secretory responses induced by CCK-8 infusion. The antagonistic effects of proglumide occurred in a dose-dependent manner, and proglumide infusion at dose of 20 mumol/kg/min or above simultaneously inhibited CCK-8-induced amylase and insulin responses. In conclusion, although the type of receptor involved is not characterized at present, exogenously infused CCK-8 acts on B cells via a CCK-receptor-mediated mechanism and induces insulin secretion in sheep.

Cholecystokinin-A and cholecystokinin-B/gastrin receptor mRNA expression in the gastrointestinal tract and pancreas of the rat and man. A polymerase chain reaction study

BACKGROUND

Gastrin and cholecystokinin (CCK) are thought to exert trophic effects on the gastrointestinal tract and pancreas. Two types of receptors have been cloned, CCK-A and CCK-B/ gastrin. We have examined the occurrence of CCK-A and CCK-B receptor mRNA in the brain, digestive tract, pancreas, and kidney of the rat and man by Northern blot and reverse transcribed polymerase chain reaction (RT-PCR).

METHODS

Total RNA was isolated from rat tissues and reverse transcribed into cDNA. cDNA from brain, kidney, and pancreas of the rat and man and from human whole stomach were commercially available. Northern blot and a PCR technique based on Taq polymerase-antibody interaction and using CCK-A and CCK-B receptor-specific primers, followed by Southern blot analysis, were the methods used.

RESULTS

By means of Northern blots, CCK-A receptor mRNA was detected in rat fundus mucosa and pancreas but not in the remaining GI tract or brain. By means of RT-PCR, CCK-A receptor mRNA was demonstrated in the brain and the mucosa of the fundus, antrum, duodenum, and colon, kidney, pancreas and pancreatic islets. CCK-B receptor mRNA was detected by Northern blot analysis in the brain and the fundus mucosa but not in the rest of the digestive tract and not in the pancreas, pancreatic islets, or kidney. By RT-PCR, expression of CCK-B receptor mRNA could also be detected in antrum mucosa. In man, CCK-A receptor mRNA was detected in the brain, stomach, pancreas, and kidney, whereas CCK-B receptor mRNA was found in the brain, stomach, and pancreas but not in the kidney. Cloning and DNA-sequence analysis of the PCR-amplified rat and human CCK-A and CCK-B receptor DNA fragments, which cover the protein-encoding regions of the intracellular loop C3, showed complete sequence homology as compared with published rat and human sequences.

CONCLUSIONS

It appears unlikely that CCK will have effects in the ileum, at least not effects mediated by CCK-A receptors. It also appears unlikely that physiologic concentrations of gastrin in the circulation will promote growth (or exert other effects) in the pancreas, duodenum, ileum, and colon, since CCK-B receptor mRNA is not expressed or is poorly expressed in these tissues.

Effects of KSG-504, a new cholecystokinin-A-receptor antagonist, on pancreatic exocrine and endocrine secretions in rats

The effects of KSG-504 ((S)-arginium (R)-4-[N-(3-methoxypropyl)-N-pentylcarbamoyl]-5-(2-naphthylsulf onyl) pentanoate monohydrate), a new cholecystokinin (CCK)-A-receptor antagonist, on pancreatic exocrine secretion in anesthetized rats and endocrine secretion in conscious rats were studied. Intravenous injection of KSG-504 inhibited the pancreatic amylase output stimulated by intravenous infusion of CCK-8 in a dose-dependent manner (ED50: 18 micrograms/kg/min). Moreover, KSG-504 significantly reduced the CCK-8-stimulated increases in pancreatic juice volume and outputs of protein, trypsin and lipase. Intraduodenal infusion of casein increased the plasma CCK concentration and the pancreatic amylase output. KSG-504 significantly inhibited the pancreatic amylase output stimulated by casein. Pancreatic juice volume and bicarbonate output were significantly stimulated by intravenous infusion of secretin, but were not changed by KSG-504. When pancreatic exocrine secretion was stimulated by secretin plus CCK-8, KSG-504 suppressed the increases in juice volume and bicarbonate output to the level stimulated by secretin alone. Basal pancreatic amylase output was decreased by KSG-504. KSG-504 decreased the level of plasma immunoreactive insulin (IRI) stimulated by glucose plus CCK-8, but had no effect on IRI stimulated by glucose alone and the basal IRI. These in vivo studies suggest that KSG-504 has significant inhibitory effects both on the pancreatic exocrine and endocrine secretion stimulated by CCK, but has no effect on the exocrine secretion stimulated by secretin.

Effects of cholecystokinin and glucagon-like peptide 1 on the secretion of pancreatic polypeptide in mice

The gut hormones, cholecystokinin (CCK) and truncated glucagon-like peptide 1 (GLP-1(7-36)amide or GLP-1) both stimulate insulin secretion and affect glucagon secretion in mice, but their effects on the secretion of other islet hormones have not been established in rodents. In the present study, we have examined the influence of the C-terminal octapeptide of CCK, CCK-8, and GLP-1 on the secretion of pancreatic polypeptide (PP) in the mouse by the use of a radioimmunoassay for rodent PP. Mice were injected intravenously with CCK-8 (doses in the range of 0.053-5.3 nmol/kg) or with GLP-1 (doses in the range of 1-32 nmol/kg) and blood was sampled at 2, 6 or 10 min after the injection. Controls were injected with saline. It was found that CCK-8 at 5.3 nmol/kg increases plasma levels of both PP and insulin when the sample was taken at 2 min, but not at 6 or 10 min, after injection. These effects were blocked by the selective CCKA-receptor antagonist, L-364,718 (2.4 micromol/kg). GPL-1 increased plasma insulin levels at 32 nmol/kg at 2 and 6 min after the injection, but plasma PP levels were unaltered. In conclusion, this study, using a newly developed radioimmunoassay for PP in rodents, shows that CCK-8 but not GPL-1 stimulated PP secretion in mice at dose levels where both peptides stimulate insulin secretion. Furthermore, PP secretion in response to CCK-8 showed a similarity with that of insulin in terms of dose- and time-response characteristics as well as sensitivity to CCKA-receptor antagonism.

A cholecystokinin receptor antagonist, loxiglumide, stimulates biliary secretion in conscious rats

The effects of the CCK receptor antagonists loxiglumide [D,L-4-(3,4-dichlorobenzoylamino)-5-(N-3-methoxy-propylpentylam ino)-5-oxo- pentanoic acid, CR 1505] and MK-329 [3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzo-diazepine-3-y l)-1H - indole-2-carboxamide, L-364,718], on bile flow were investigated in conscious rats. The bile duct of male Wistar rats was cannulated to directly collect pure bile, and the second cannula was inserted into the duodenum for reinfusion of bile. On the 4th through 7th postoperative days loxiglumide (25, 50 or 100 mg/kg body weight), MK-329 (1 mg/kg body weight) or the respective solvent (saline and 80% dimethyl sulfoxide) was injected subcutaneously. Loxiglumide caused dose-dependent increases in bile flow and bile acid output with a slight non-dose-dependent increase in bilirubin output. The integrated increments of bile flow during a 3-h period after saline and 100 mg/kg body weight loxiglumide were -14 +/- 71 and 982 +/- 61 microliters/100 g body weight, respectively, and those of bile acids were 2.5 +/- 1.4 and 23.1 +/- 4.1 mumol/100 g body weight, respectively. In contrast, MK-329 markedly decreased the bile flow (-439 +/- 76 vs. control; -32.8 +/- 76 microliters/100 g body weight/3 h, P < 0.001) and bile acids output (-16.3 +/- 6.8 vs. control; 3.4 +/- 3.8 mumol/100 g body weight/3 h, P < 0.001), while it significantly increased bilirubin output (86.4 +/- 15.6 vs. 43.5 +/- 1.1 mg/100 g body weight/3 h, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

CCK-8-stimulated insulin secretion in vivo is mediated by CCKA receptors

The effects of the cholecystokinin A (CCKA) receptor antagonist, L-364,718, and the CCKB receptor antagonist, L-365,260, on CCK-8-stimulated insulin secretion were studied in vivo in the mouse. It was found that CCK-8-stimulated insulin secretion was suppressed by L-364,718 at a low dose level (0.078 mumol/kg). In contrast, L-365,260 caused a partial inhibition of CCK-8-stimulated insulin release only at the high dose level (24 mumol/kg). It is concluded that the CCK-8-stimulated insulin release in vivo is mediated by CCK receptors of the CCKA subtype.

CCKA receptor antagonism inhibits mechanisms underlying CCK-8-stimulated insulin release in isolated rat islets

The influence of the cholecystokinin (CCK)A receptor antagonist, L-364,718, on beta-cell activation was examined in isolated perifused prelabelled rat islets. Insulin secretion and 3H efflux from myo-[2-3H]inositol-prelabelled islets (reflecting phosphoinositide hydrolysis) stimulated by CCK-8 (100 nM) were both inhibited by L-364,718, partially at 1 nM and totally at 10 nM. 45Ca2+ efflux from prelabelled islets was markedly stimulated by CCK-8. This stimulation was inhibited equally by 1 and 10 nM L-364,718. CCK-8 stimulated the 86Rb+ efflux (reflecting K+ movements) from prelabelled islets, which probably reflects an indirect effect of CCK-8 due to opening of Ca(2+)-activated K+ channels. This 86Rb+ efflux was inhibited by L-364,718 at 10 nM but not affected by L-364,718 at 1 nM. It is concluded that insulin secretion, phosphoinositide hydrolysis, Ca2+ and K+ movements stimulated by CCK-8 in isolated islets are all events mediated by CCKA receptors. The L-364,718-induced inhibition of phosphoinositide hydrolysis was most closely correlated to the inhibition of insulin secretion. This suggests that induction of cellular events activated through stimulation of phosphoinositide hydrolysis is a major mechanism underlying CCK-8-stimulated insulin secretion.

Peptide receptor antagonists in the study of insulin and glucagon secretion in mice

Peptide antagonists might be useful tools in elucidating a potential role for neuropeptides in islet hormone secretion. We therefore investigated the effects of three different peptide receptor antagonists on insulin and glucagon secretion in vivo in mice. The gastrin-releasing peptide (GRP) antagonist N-acetyl GRP-(20-26) amide (P less than 0.01) and the vasoactive intestinal polypeptide (VIP) antagonist [4Cl-D-Phe 6,Leu17] VIP (P less than 0.001) specifically inhibited insulin secretion induced by GRP and VIP, respectively. Furthermore, N-acetyl GRP-(20-26) amide specifically inhibited GRP-stimulated glucagon secretion (P less than 0.01) and the cholecystokinin (CCK) antagonist L-364,718 inhibited CCK-8-mediated insulin and glucagon secretion (P less than 0.001). However, neither of these three antagonists affected insulin or glucagon secretion stimulated by autonomic nerve activation (induced by 2-deoxy-glucose (2-DG)). We conclude that these peptide antagonists can be used in studies on the physiological involvement of GRP, VIP and CCK in islet hormone secretion. Furthermore, neither GRP or CCK seems involved in the insulin and glucagon response to 2-DG in the mouse.