Purification and characterization of a luminal cholecystokinin-releasing factor from rat intestinal secretion

Assessment of exocrine pancreatic function in children and adolescents with direct and indirect testing

The exocrine pancreas plays an important role in digestion. Understanding of the physiology and regulation of exocrine function provides insight into disease processes and basis of functional testing. Specifically, exocrine pancreatic insufficiency (EPI) can cause maldigestion and thus a proper assessment of exocrine pancreatic function is important. There are indirect and direct methods for evaluating pancreatic function. Indirect methods are varied and include stool, serum, urine, and breath tests. Fecal elastase is a commonly used indirect test today. Direct methods involve stimulated release of pancreatic fluid that is collected from the duodenum and analyzed for enzyme activity. The most used direct test today is the endoscopic pancreatic function test. Indirect pancreatic function testing is limited in identifying cases of mild to moderate EPI, and as such in these cases, direct testing has higher sensitivity and specificity in diagnosing EPI. This review provides a comprehensive guide to indirect and direct pancreatic function tests as well as an in-depth look at exocrine pancreatic function including anatomy, physiology, and regulatory mechanisms.

Satietogenic Protein from Tamarind Seeds Decreases Food Intake, Leptin Plasma and CCK-1r Gene Expression in Obese Wistar Rats

OBJECTIVE

This study evaluated the effect of a protein, the isolated Trypsin Inhibitor (TTI) from Tamarindus indica L. seed, as a CCK secretagogue and its action upon food intake and leptin in obese Wistar rats.

METHODS

Three groups of obese rats were fed 10 days one of the following diets: Standard diet (Labina®) + water; High Glycemic Index and Load (HGLI) diet + water or HGLI diet + TTI. Lean animals were fed the standard diet for the 10 days. Food intake, zoometric measurements, plasma CCK, plasma leptin, relative mRNA expression of intestinal CCK-related genes, and expression of the ob gene in subcutaneous adipose tissue were assessed.

RESULTS

TTI decreased food intake but did not increase plasma CCK in obese animals. On the other hand, TTI treatment decreased CCK-1R gene expression in obese animals compared with the obese group with no treatment (p = 0.027). Obese animals treated with TTI presented lower plasma leptin than the non-treated obese animals.

CONCLUSION

We suggest that TTI by decreasing plasma leptin may improve CCK action, regardless of its increase in plasma from obese rats, since food intake was lowest.

Importance of the enteric nervous system in the control of the migrating motility complex

The migrating motility complex (MMC), a cyclical phenomenon, represents rudimentary motility pattern in the gastrointestinal tract. The MMC is observed mostly in the stomach and gut of man and numerous animal species. It contains three or four phases, while its phase III is the most characteristic. The mechanisms controlling the pattern are unclear in part, although the neural control of the MMC seems crucial. The main goal of this article was to discuss the importance of intrinsic innervation of the gastrointestinal tract in MMC initiation, migration, and cessation to emphasize that various MMC-controlling mechanisms act through the enteric nervous system. Two main neural regions, central and peripheral, are able to initiate the MMC. However, central regulation of the MMC may require cooperation with the enteric nervous system. When central mechanisms are not active, the MMC can be initiated peripherally in any region of the small bowel. The enteric nervous system affects the MMC in response to the luminal stimuli which can contribute to the initiation and cessation of the cycle, and it may evoke irregular phasic contractions within the pattern. The hormonal regulators released from the endocrine cells may exert a modulatory effect upon the MMC mostly through the enteric nervous system. Their central action could also be considered. It can be concluded that the enteric nervous system is involved in the great majority of the MMC-controlling mechanisms.

Protein Digestion-Derived Peptides and the Peripheral Regulation of Food Intake

The gut plays a central role in energy homeostasis. Food intake regulation strongly relies on the gut-brain axis, and numerous studies have pointed out the significant role played by gut hormones released from enteroendocrine cells. It is well known that digestive products of dietary protein possess a high satiating effect compared to carbohydrates and fat. Nevertheless, the processes occurring in the gut during protein digestion involved in the short-term regulation of food intake are still not totally unraveled. This review provides a concise overview of the current data concerning the implication of food-derived peptides in the peripheral regulation of food intake with a focus on the gut hormones cholecystokinin and glucagon-like peptide 1 regulation and the relationship with some aspects of glucose homeostasis.

Potato protease inhibitors inhibit food intake and increase circulating cholecystokinin levels by a trypsin-dependent mechanism

Objective

To investigate the mechanisms underlying the satiety-promoting effects of a novel protease inhibitors concentrate derived from potato (PPIC).

Methods

Acute and prolonged effects of oral PPIC administration (100 mg/kg per day) on food intake, body weight, and gastric emptying were evaluated in healthy rats. Parameters of body weight, food intake, plasma glucose, insulin, and cholecystokinin (CCK) were measured. Duodenal proteolytic activity and CCK expression were determined in tissue extracts. Intestinal STC-1 cell culture model was used to investigate the direct effect of PPIC on CCK transcript level and secretion.

Results

Acute oral administration of PPIC reduced immediate food intake during the first two hours following the treatment, delayed gastric emptying, and decreased proteolytic activity in the duodenum. Repeated oral ingestion of PPIC reduced weight gain in male rats and significantly elevated the plasma CCK levels. Although duodenal mucosal CCK mRNA levels increased in response to PPIC administration, the concentrate failed to elevate CCK expression or release in STC-1 cells. The 14-day ascending dose range study (33 to 266 mg/kg PPIC per day) showed no adverse side effects associated with PPIC administration.

Conclusion

These findings provided evidence that PPIC is effective in reducing food intake and body weight gain in healthy rats when administered orally by increasing circulating CCK levels through a trypsin-dependent mechanism.

Gastrin, cholecystokinin and gastrointestinal tract functions in mammals. Nutr Res Rev. 2006;19:254-283. [PMID: 19079889 DOI: 10.1017/S0954422407334082]

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.

Negative control of human pancreatic secretion: physiological mechanisms and factors

The negative control of pancreatic exocrine secretion in man occurs during the interdigestive and postprandial periods of the digestive cycle. The physiological mechanisms involved include negative feedback mechanisms, well described and accepted in animals, and controlled by the cholecystokinin- and secretin-releasing factors of pancreatic and duodenal origin, along with the active pancreatic proteases present in the upper gut. The presence of these factors and their efficacy in humans, however, have their supporters and detractors, with a possibility for reconciliation among opponents. Besides these releasing factors, hormones, mostly from the intestine, are also involved in this inhibitory process of pancreatic secretion. Somatostatin, peptide YY, pancreatic polypeptide, glucagon, ghrelin, and leptin were described as potentially involved from studies mostly performed on animals. Finally, bile and bile salts have mixed responses on this inhibition, and their effects seem to be at the intestine level with gastrointestinal hormones involved. Future studies will have to be performed in humans to determine the presence of cholecystokinin- and secretin-releasing factors and their role. Finally, the demonstrated modulatory action of hormones and bile acids in other species needs to be confirmed in humans.

Dietary counseling versus dietary supplements for malnutrition in chronic pancreatitis: a randomized controlled trial

BACKGROUND & AIMS

Up to 50% of patients with chronic pancreatitis (CP) are malnourished. There are limited data on the role of dietary intervention in improving the nutritional status of such patients. The aim was to compare the efficacy of medium chain triglyceride (MCT)-enriched commercial dietary supplements with dietary counseling for homemade food in the management of malnutrition in patients with CP.

METHODS

In a randomized controlled trial, consecutive undernourished patients with CP (body mass index [BMI] <18.5 kg/m(2)) at a tertiary care hospital were randomized to receive either dietary counseling for regular homemade food or commercial MCT-enriched dietary supplements for a period of 3 months to compensate for the dietary calorie deficit. All patients received standard management for CP including pancreatic enzyme supplements. Primary outcome measure was improvement in BMI.

RESULTS

Sixty malnourished patients with CP were randomized to counseling group (n = 29; mean age, 32 +/- 10 years; male, 83%) and supplementation group (n = 31; mean age, 28 +/- 10 years; male, 84%). BMI increased in both the counseling group and supplementation group (17.2 +/- 1.7 vs 18.1 +/- 1.8 kg/m(2), P = .001; 16.7 +/- 1.6 vs 18.2 +/- 1.6 kg/m(2), P = .001). There were similar improvements in triceps skinfold thickness, dietary intake, fecal fat, and pain score during a period of 3 months in both groups. There was, however, no significant difference between the counseling and supplementation groups with regard to any of the outcome measures.

CONCLUSIONS

Dietary counseling for a balanced homemade diet is as good as commercial food supplements in improving malnutrition in patients with CP.

GPR93 activation by protein hydrolysate induces CCK transcription and secretion in STC-1 cells

In the intestinal lumen, protein hydrolysate increases the transcription and release of cholecystokinin (CCK) from enteroendocrine cells of the duodenal-jejunal mucosa. Our recent discovery that a G protein-coupled receptor, GPR93, is activated by dietary protein hydrolysate causing induced intracellular calcium-mediated signaling events in intestinal epithelial cells raises a possibility that GPR93 might be involved in the protein hydrolysate induction of CCK expression and/or secretion. Using the enteroendocrine STC-1 cells as a model, the present study demonstrates that increasing expression of GPR93 amplifies the peptone induction of endogenous CCK mRNA levels. A similar increase in CCK transcription, indicated by the luciferase reporter activity driven by an 820-bp CCK promoter, is also observed in response to peptone at a dose as little as 6.25 mg/ml, but not to lysophosphatidic acid (LPA), an agonist of GPR93. We discovered that the upregulation of CCK transcription involves ERK1/2, PKA, and calmodulin-dependent protein kinase-mediated pathways. Additionally, GPR93 activation by peptone induces a response in CCK release at 15 min, which continues over a 2-h period. The cAMP level in STC-1 cells overexpressing GPR93 is induced at a greater extent by peptone than by LPA, suggesting a possible explanation of the different effects of peptone and LPA on CCK transcription and secretion. Our data indicate that GPR93 can contribute to the observed induction of CCK expression and secretion by peptone and provide evidence that G protein-coupled receptors can transduce dietary luminal signals.

How does cholecystokinin stimulate exocrine pancreatic secretion? From birds, rodents, to humans

The field of cholecystokinin (CCK) stimulation of exocrine pancreatic secretion has experienced major changes in the recent past. This review attempts to summarize the present status of the field. CCK production in the intestinal I cells, the molecular forms of CCK produced and subsequently circulated in the blood, the presence or absence of CCK receptors on the isolated pancreatic acinar cells and the associated signaling for acinar cell secretion, and the actual circuits and sites of action for CCK regulation of exocrine pancreatic secretion in vivo are reviewed in different animal species with an emphasis on birds, rodents, and humans. Clear differences in the relative importance of neural and direct modes of CCK action on pancreatic acinar cells were identified. Rodents seem to be endowed with both modes of action, whereas in humans the neural mode may predominate. In birds, such as duck, the direct mode needs further assistance from pituitary adenylate cyclase-activating peptide/VIP receptors. However, much further work needs to be directed to the neural mode to map out all sites of CCK action and details of the full circuits, and we foresee a major revival for this field of research in the near future.

Arg-Ile-Tyr (RIY) derived from rapeseed protein decreases food intake and gastric emptying after oral administration in mice

We previously reported that a bioactive tripeptide Arg-Ile-Tyr (RIY), which has been isolated as an inhibitor for angiotensin I-converting enzyme from the subtilisin digest of rapeseed protein, decreased blood pressure. In this study, we also found that RIY dose-dependently decreased food intake at a dose of 150 mg/kg after oral administration in fasted ddY male mice. The anorexigenic action of RIY was blocked by a cholecystokinin-1 CCK1 receptor antagonist, lorglumide. RIY also decreased the gastric emptying rate at a dose of 150 mg/kg and the RIY-induced delay of gastric emptying was blocked by lorglumide. However, RIY had no affinity for CCK1 receptor. Taken together, RIY decreased food intake and gastric emptying by stimulating CCK release.

Biochemistry and biology of SPINK-PSTI and monitor peptide

This article summarizes structural and functional properties of pancreatic secretory trypsin inhibitor (PSTI), which has been identified in many species. Its prominent role is to protect the pancreas from prematurely activated trypsinogen before entry into the duodenum. In the rat there are two isoforms, one of which is PSTI-I, a 61-amino acid peptide involved in the feedback regulation of pancreatic enzymes. Independent investigations in neoplastic diseases led to the discovery of tumor-associated trypsin inhibitor,which is identical to PSTI.

Dose-response for inhibition by amylin of cholecystokinin-stimulated secretion of amylase and lipase in rats

BACKGROUND AND AIMS

The neuroendocrine hormone amylin, cosecreted with insulin from pancreatic beta-cells in response to nutrient ingestion, has several physiologic actions to limit the rate of nutrient uptake, including the slowing of gastric emptying.

METHODS

To investigate whether amylin might modulate digestive enzyme secretion from the exocrine pancreas, anesthetized Sprague Dawley rats were cannulated via the pancreatic duct and the secretory response (flow, amylase and lipase) to cholecystokinin (1 microg s.c.) was measured in the absence and in the presence of 0.1, 0.3 and 1 microg s.c. doses of amylin.

RESULTS

Amylin alone did not affect pancreatic secretion, but it dose-dependently inhibited cholecystokinin-stimulated amylase secretion by up to 58% and lipase secretion by up to 67%. The ED50's for these responses were 0.21 microg+/-0.18 log and 0.11 microg+/-0.05 log, respectively, doses that result in excursions of plasma amylin concentration that are within the reported physiological range. Amylin did not evoke cell signalling in the Ar42j model of pancreatic acinar cells, and responses to amylin were not observed in either Ar42j cells or isolated pancreatic acini in a microphysiometer indicating that the effect of amylin was indirect.

CONCLUSIONS

Inhibition of stimulated pancreatic enzyme secretion is likely to be a physiological, extrapancreatic, action of amylin. Amylinergic mechanisms modulating both gastric emptying and pancreatic enzyme secretion may thus match, respectively, the appearance of substrate and enzymes in the gut lumen.

Effects on digestive secretions

Rat amylin subcutaneously injected into rats dose-dependently inhibits pentagastrin-stimulated gastric acid secretion and protects the stomach from ethanol-induced gastritis. The ED50s for these actions (0.050 and 0.036 microg, respectively) are the lowest for any dose-dependent effect of amylin thus far described, and their similar potencies are consistent with a mechanistic (causal) association. At higher amylin doses, inhibition of gastric acid secretion was almost complete (93.4%). Gastric injury (measured by a subjective analog scale) was inhibited by up to 67%. The observation that effective doses of amylin result in plasma concentrations of 7-10 pM (i.e., within the reported range; Pieber et al., 1994) supports the interpretation that inhibition of gastric acid secretion and maintenance of gastric mucosal integrity are physiological actions of endogenous amylin. The pharmacology of these responses fits with one mediated via amylin-like receptors. Rat amylin inhibited CCK-stimulated secretion of pancreatic enzymes,amylase, and lipase by up to approximately 60% without having significant effect in the absence of CCK. ED50s for the effect were in the 0.1-0.2 microg range, calculated to produce plasma amylin excursions within the physiological range. Effects of informative ligands are consistent with the concept of amylin receptor mediation. Amylin was effective in ameliorating the severity of pancreatitis in a rodent model. The amylin analog pramlintide inhibited gallbladder emptying in mice as measured by total weight of acutely excised gallbladders. Amylin inhibition of gastric acid secretion, pancreatic enzyme secretion, and bile secretion likely represents part of an orchestrated control of nutrient appearance. Modulation of digestive function fits with a general role of amylin in regulating nutrient uptake. Rate of ingestion, rate of release from the stomach, and rate of digestion of various food groups appear to be under coordinate control.

Multiple insect resistance in transgenic tomato plants over-expressing two families of plant proteinase inhibitors

Protease inhibitors have been proposed as potential defense molecules for increased insect resistance in crop plants. Compensatory over-production of insensitive proteases in the insect, however, has limited suitability of these proteins in plant protection, with very high levels of inhibitor required for increased plant resistance. In this study we have examined whether combined used of two inhibitors is effective to prevent this compensatory response. We show that leaf-specific over-expression of the potato PI-II and carboxypeptidase inhibitors (PCI) results in increased resistance to Heliothis obsoleta and Liriomyza trifolii larvae in homozygote tomato lines expressing high levels (>1% the total soluble proteins) of the transgenes. Leaf damage in hemizygous lines for these transformants was, however, more severe than in the controls, thus evidencing a compensation response of the larvae to the lower PI concentrations in these plants. Development of comparable adaptive responses in both insects suggests that insect adaptation does not entail specific recognition of the transgene, but rather represents a general adaptive mechanism triggered in response to the nutritional stress imposed by sub-lethal concentrations of the inhibitors. Combined expression of defense genes with different mechanisms of action rather than combinations of inhibitors may then offer a better strategy in pest management as it should be more effective in overcoming this general adaptive response in the insect.

Gastrointestinal satiety signals II. Cholecystokinin

During a meal, ingested nutrients accumulate in the stomach, with a significant portion passing on to the small intestine. The gastrointestinal presence of ingested nutrients initiates a range of physiological responses that serve to facilitate the overall digestive process. Thus peptides and transmitters are released, and various neural elements are activated that coordinate gastrointestinal secretion and motility and can eventually lead to meal termination or satiety. Among the range of gastrointestinal peptides released by ingested nutrients is the brain/gut peptide CCK. CCK plays a variety of roles in coordinating gastrointestinal activity and has been demonstrated to be an important mediator for the control of meal size.

The bifunctional rat pancreatic secretory trypsin inhibitor/monitor peptide provides protection against premature activation of pancreatic juice

BACKGROUND

In the rat, two forms of the pancreatic secretory trypsin inhibitor, PSTI-I and PSTI-II, are secreted into pancreatic juice. It is assumed that their role is to protect the pancreas from premature activation of the protease-rich pancreatic juice. In the small intestine, PSTI-I, also called 'monitor peptide', is thought to have a different role: PSTI-I competes with protein for activated trypsin. In the presence of a protein-rich meal, free PSTI induces a release of cholecystokinine from the intestine.

METHODS

To investigate whether its role as monitor peptide is compatible with the inhibitory, protective function in the pancreas, PSTI-I was chemically synthesized and then renatured.

RESULTS

The peptide was almost completely trypsin resistant and exhibited a dose-dependent inhibitory activity to bovine and partially purified rat trypsin. Furthermore, experiments with trypsin- and endopeptidase-activated pancreatic juice demonstrated that its inhibitory capacity was sufficient to prevent premature activation. Binding studies of (125)I-labeled PSTI-I with the putative intestinal receptor using isolated membranes indicated the presence of high-affinity binding sites (k(d) = 5 x 10(-8)M). Binding of PSTI-I could be competed with excess PSTI-I or trypsin. In a biological assay system, injections of PSTI-I displayed monitor peptide activity by inducing a dose-dependent trypsinogen release from the pancreas.

CONCLUSION

Our experiments support a dual function of PSTI-I: monitoring protein in the gut due to its 'moderate' affinity for trypsin and a protective role in the pancreas.

The soybean beta-conglycinin beta 51-63 fragment suppresses appetite by stimulating cholecystokinin release in rats

We previously demonstrated that soybean beta-conglycinin peptone suppresses food intake and gastric emptying by direct action on rat small intestinal mucosal cells to stimulate cholecystokinin (CCK) release. The aim of the present study was to define the active fragment in beta-conglycinin by using synthetic peptides chosen from the sequence of three beta-conglycinin subunits. We selected the fragments that had multiple nonadjacent arginine residues, and investigated their ability to bind to components of the rat intestinal brush border membrane as well as to stimulate CCK release and appetite suppression. The fragment from 51 to 63 of the beta subunit (beta 51-63) had the strongest binding activity. Intraduodenal infusion of beta 51-63 inhibited food intake and markedly increased portal CCK concentration. The threshold concentration of beta 51-63 to affect food intake was 3 micro mol/L. The CCK-A receptor antagonist abolished the beta 51-63-induced suppression of food intake. Three types of smaller fragments of beta 51-63 (beta 51-59, beta 53-63 and beta 53-59) and two types of fragments similar to beta 51-63 in the beta-conglycinin alpha and alpha' subunits (alpha 212-224 and alpha' 230-240) had less binding ability than did beta 51-63. Model peptides constructed with arginine (R) and glycine (G), such as GRGRGRG, had strong binding affinity, but peptides containing a single R or RR did not. These results indicate that the beta-conglycinin beta 51-63 fragment is the bioactive appetite suppressant in beta-conglycinin, and multiple arginine residues in the fragment may be involved in this effect.

Casein binds to the cell membrane and induces intracellular calcium signals in the enteroendocrine cell: a brief communication

Dietary protein but not amino acids stimulates cholecystokinin (CCK) secretion in rat mucosal cells. However, the dietary protein sensory mechanisms and the intracellular signal pathway in the enteroendocrine cells have not yet been clarified. The relationship between dietary protein binding to cell membrane and intracellular calcium responses were examined in the CCK-producing enteroendocrine cell line STC-1. The binding of solubilized STC-1 cell membrane to proteins was analyzed using a surface plasmon resonance sensor. Intracellular calcium concentrations of STC-1 cell suspensions loaded with Fura-2 AM were measured using a spectrafluorophotometer system with continuous stirring. Intracellular calcium concentrations in STC-1 cells were increased by exposure to alpha-casein or casein sodium, but not to bovine serum albumin. Solubilized STC-1 membranes bound to alpha-casein and casein sodium but did not bind to bovine serum albumin. alpha-Casein demonstrated higher membrane binding and intracellular calcium stimulating activities than casein sodium. Thus, protein binding to the STC-1 cell membrane and intracellular calcium responses were correlated. Intracellular calcium responses to alpha-casein were suppressed by an L-type calcium channel blocker. These results suggest that casein, a dietary protein, binds to a putative receptor on the CCK-producing enteroendocrine cell membrane and elicits the subsequent intracellular calcium response via an L-type calcium channel.

Effects of protease inhibitor ONO-3403 on pancreatic exocrine response to CCK in rats

AIM

To examine the pancreatic exocrine response to CCK-8 and to clarify the mechanism of the pancreatic exocrine hypersecretion after oral administration of synthetic protease inhibitor ONO-3403 in rats.

METHODS

A single oral dose of synthetic protease inhibitor ONO-3403 was given to rats by orogastric tube 6h and 12 h before experiments. The pancreatic juice was collected before test and after stimulation of stepwise increasing doses of CCK-8. The output of protein, amylase, lipase and bicarbonate in pancreatic juice or pancreatic tissue were determinated by Lowry method, Chromogenic method with blue-dyed starch polymer, Whitaker method and by the DST 800 multititration system, respectively.

RESULTS

Oral administration of ONO-3403 had no influence on pancreatic juice flow and output of protein in basal and CCK-8 stimulation at 6h after ONO-3403 pretreated, but it caused a significant increase in pancreatic juice flow (peak level 215±9 ulper 30 min vs 93±6 ulper 30 min, P<0.01) and protein output (peak level 16475±1 801 ug per 30 min vs 5 920±593 ug per 30 min, P<0.01) of the basal and CCK-8 stimulation at 12 h after ONO-3 403 pretreated. The basal pancreatic juice flow and output of amylase (470±32 su per 30 min at 6 h P<0.01, 394±47 su per 30 min at 12 h, P<0.05 vs 251±32 su per 30 min), bicarbonate (2.224±0.333 umolper 30 min at 6 h, P<0.05; 3.148±0.374 umolper 30 min at 12 h, P<0.01 vs 1.428±0.282 umolper 30 min) were significant high after ONO-3403 pretreated than those of control group. There was no change in lipase output compared with control group. The pancreatic weight, pancreatic contents of protein and amylase in ONO-3 403 pretreated rats were similar to those in control rats.

CONCLUSION

ONO-3 403 can increase pancreatic exocrine secretion and sensitivity to CCK-8 stimulation.The mechanism of ONO3 403 induced pancreatic exocrine hypersecretion may be a feedback regulation of the pancreas by increasing CCK secretion.

Soybean beta-conglycinin peptone suppresses food intake and gastric emptying by increasing plasma cholecystokinin levels in rats

Cholecystokinin (CCK) is an important physiologic mediator that regulates satiety and gastric emptying. We demonstrated previously that soybean peptone acts directly on rat small intestinal mucosal cells to stimulate CCK release. In the present study, we examined the effects of beta-conglycinin, a major component of soy protein, and its peptone on food intake and gastric emptying after an intraduodenal infusion of beta-conglycinin peptone in relation to CCK release and interaction with the mucosal cell membrane. Intraduodenal infusion of beta-conglycinin peptone inhibited food intake in a dose-dependent manner, but that of whole soy peptone or camostat did not. The suppression of food intake by beta-conglycinin peptone was abolished by an intravenous injection of devazepide, a selective peripheral CCK receptor antagonist. The beta-conglycinin peptone infusion strongly suppressed gastric emptying with marked increases in portal CCK levels. We also observed that the beta-conglycinin peptone dose dependently and more potently stimulated CCK release from isolated dispersed mucosal cells of the rat jejunum than did beta-conglycinin itself. This stimulation corresponded to the binding activity of the peptide or protein to solubilized components of the rat jejunum membrane as evaluated by surface plasmon biosensor. These results indicate that beta-conglycinin peptone suppresses food intake, and this effect may be due to beta-conglycinin peptone in the lumen stimulating endogenous CCK release with direct acceptance to the intestinal cells.

Effects of evodiamine on gastrointestinal motility in male rats

The effects of evodiamine on gastric emptying, gastrointestinal transit, and plasma levels of cholecystokinin (CCK) were studied in male rats. Evodiamine, isolated from the dry unripened fruit of Evodia rutaecarpa Bentham (a Chinese medicine named Wu-chu-yu), has been recommended for abdominal pain, acid regurgitation, nausea, diarrhea, and dysmenorrhea. Gastrointestinal motility was assessed in rats 15 min after intragastric instillation of a test meal containing charcoal and Na(2)51CrO(4). Gastric emptying was determined by measuring the amount of radiolabeled chromium contained in the small intestine as a percentage of the initial amount received. Gastrointestinal transit was evaluated by calculating the geometric center of distribution of the radiolabeled marker. Blood samples were collected for CCK radioimmunoassay (RIA). After administration of evodiamine (0.67-6.00 mg/kg), both gastric emptying and gastrointestinal transit were inhibited, whereas the plasma concentration of CCK was increased in a dose-dependent manner. The selective CCK(1) receptor antagonists, devazepide and lorglumide, effectively attenuated the evodiamine-induced inhibition of gastric emptying and gastrointestinal transit. L-365,260 (3R-(+)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl)-N'-(3-methylphenyl)-urea), a selective CCK(2) receptor antagonist, did not alter the evodiamine-induced inhibition of gastric emptying and gastrointestinal transit. These results suggest that evodiamine inhibits both gastric emptying and gastrointestinal transit in male rats via a mechanism involving CCK release and CCK(1) receptor activation.

Neural hormonal regulation of exocrine pancreatic secretion

Exocrine pancreatic secretion is regulated by hormone-hormonal and neural-hormonal interactions involving several regulatory peptides and neurotransmitter from the gut, the pancreas and the vagus nerve. The roles of the gastrointestinal peptides including secretin, CCK, neurotensin, motilin, PYY and pancreatic islet hormones including insulin, pancreatic polypeptide and somatostatin have been established. Interactions among secretin, CCK and neurotensin produce synergistic stimulatory effect. Motilin modulates the cyclic pattern of pancreatic secretion while local insulin provides a permissive role for the action of secretin and CCK at physiological concentration. Somatostatin, PYY and pancreatic polypeptide are inhibitory regulators, acting either on the release of secretin and CCK or on the action of the two stimulatory hormones. The vagal afferent-efferent pathway mediates the actions of many of these regulatory peptides, particularly of secretin and CCK. Acetylcholine and nitric oxide are the neurotransmitters known to mediate the actions of secretin and CCK. Serotonin (5-HT) released from enterochromaffin cells in the intestinal mucosa and nerve terminals of the enteric nervous system and intrapancreatic nerves may be involved in both stimulatory and inhibitory mechanism through its various receptor subtypes. 5-HT also mediates the action of secretin and CCK. The regulatory roles of neuropeptides, PACP and GRP, are now established, whereas those of others are being uncovered. Pancreatic juice provides both positive and negative feedback regulation of pancreatic secretion through mediation of both secretin- and CCK-releasing peptides. Three CCK-releasing peptides have been purified: monitor peptide from pancreatic juice, diazepam-binding inhibitor from porcine intestine, and luminal CCK-releasing factor from rat intestinal secretion. All have been shown to stimulate CCK release and pancreatic enzyme secretion. Pancreatic phospholipase A2 from pancreatic juice and intestinal secretion appears to function as a secretin-releasing peptide. However, the detailed map of neurohormonal regulatory pathways of exocrine pancreatic secretion is yet to be constructed.

Inhibitory effects of octreotide on luminal cholecystokinin-releasing factor, plasma cholecystokinin, and pancreatic secretion in conscious rats

INTRODUCTION

Luminal cholecystokinin-releasing factor (LCRF), purified from rat intestinal secretions, is an intraluminal regulator of cholecystokinin (CCK) secretion during bile and pancreatic juice diversion.

AIM

Because the LCRF content was not influenced by intravenous administration of atropine or somatostatin, the inhibitory effect of a potent somatostatin analog octreotide on LCRF content, the plasma CCK level, and pancreatic secretion was examined.

METHODOLOGY

Rats were prepared with bile and pancreatic cannulae and two duodenal cannulae and with an external jugular vein cannula. After 1.5-hour basal collection, bile and pancreatic juice was diverted for 2 hours, during which octreotide was infused intravenously or into the duodenal lumen. The changes in pancreatic secretion were recorded for 2 hours, and the plasma CCK level and LCRF content 2 hours after the treatment were measured.

RESULTS

Bile and pancreatic juice diversion significantly increased pancreatic secretion and plasma CCK and LCRF levels. Intravenous infusion of octreotide (0.2 and 0.5 nmol/kg/hour) inhibited all parameters. Intraduodenal infusion of a lower dose of octreotide (33 nmol/kg/hour) inhibited pancreatic secretion, but did not inhibit CCK release or LCRF content. The higher doses (100 and 300 nmol/kg/hour) inhibited all parameters.

CONCLUSION

Intravenous and intraduodenal administrations of octreotide inhibited CCK release and LCRF content and pancreatic secretion induced by bile and pancreatic juice diversion.

Luminal CCK-releasing factor stimulates CCK release from human intestinal endocrine and STC-1 cells

CCK is secreted into the blood from intestinal endocrine cells following ingestion of a meal. Recently, it has been demonstrated that the ability of certain foods to stimulate CCK release is mediated by endogenously produced CCK-releasing factors. A newly discovered luminal CCK-releasing factor (LCRF) is secreted into the intestine, where it stimulates CCK secretion. However, the mechanism whereby LCRF affects intestinal epithelial cells is unknown. The current study was designed to determine whether LCRF has a direct effect on CCK cells to stimulate hormone secretion. In dispersed human intestinal mucosal cells, LCRF (5-200 nM) significantly stimulated CCK release in a concentration-dependent manner. This stimulatory effect was absent in calcium-free media and was inhibited by the L-type calcium-channel blockers diltiazem and nifedipine. To examine direct cellular effects of LCRF on CCK cells, further studies were conducted in the CCK-containing enteroendocrine cell line STC-1. As in native cells, LCRF significantly stimulated CCK release from STC-1 cells in a calcium-dependent manner. In cells loaded with a calcium-sensitive dye, LCRF stimulation produced a rapid increase in intracellular calcium. To examine the electrophysiological basis for this stimulation, whole cell recordings were made from STC-1 cells. Whole cell calcium currents were identified under basal conditions; moreover, calcium-channel activity was increased by LCRF. These studies demonstrate that 1) LCRF has a direct effect on human intestinal cells to stimulate CCK secretion, 2) stimulated hormone release is calcium dependent, and 3) LCRF activates calcium currents in CCK cells, which leads to CCK secretion.

Dietary protein peptic hydrolysates stimulate cholecystokinin release via direct sensing by rat intestinal mucosal cells

We previously demonstrated that a peptic hydrolysate of guanidinated casein strongly stimulates exocrine pancreatic secretion in chronic bile-pancreatic juice-diverted rats and cholecystokinin (CCK) release from dispersed rat intestinal mucosal cells. These results reveal that the chemically modified protein hydrolysate stimulates CCK secretion and increases pancreatic secretion by a luminal trypsin-independent direct action on the small intestine. In the present study, we examined the direct effect of peptic hydrolysates of naturally occurring dietary proteins, casein, soybean protein isolate (SPI), egg white, and wheat gluten on CCK release under in vitro trypsin-independent conditions. All protein hydrolysates significantly stimulated CCK release from dispersed rat intestinal mucosal cells. Among the hydrolysates treated, SPI hydrolysate was the most effective in stimulating CCK release. The potential of SPI hydrolysate to stimulate CCK release was increased by long-term peptic digestion. However, an SPI-like amino acid mixture did not effect CCK release. In conclusion, peptic hydrolysates of commonly ingested dietary proteins stimulate CCK release via trypsin-independent direct sensing by intestinal mucosal cells.

Inhibitory effect of somatostatin on cholecystokinin release is independent of luminal cholecystokinin-releasing factor content in conscious rats

INTRODUCTION

Exclusion of bile-pancreatic juice from the intestine increases pancreatic secretion via cholecystokinin (CCK) release in conscious rats. Luminal CCK-releasing factor (LCRF), purified from rat intestinal secretions, is an intraluminal regulator of CCK secretion during bile-pancreatic juice diversion.

AIMS

Because somatostatin is a potent inhibitor of CCK release and pancreatic secretion, the inhibitory effect of somatostatin on LCRF was examined.

METHODOLOGY

Rats were prepared with bile and pancreatic cannulae and two duodenal cannulae and with an external jugular vein cannula. The experiments were conducted without anesthesia. After 1.5-hour basal collection of pancreatic juice with bile-pancreatic juice return, bile-pancreatic juice was diverted for 2 hours, during which time somatostatin (2, 10 nmol/kg/h) was infused intravenously. The rats were killed before and 1 and 2 hours after bile-pancreatic juice diversion. To examine the effect of luminal somatostatin, 50 or 200 nmol/kg/h of somatostatin was infused into the duodenum. The plasma CCK and luminal content of LCRF were measured by specific radioimmunoassays.

RESULTS

Bile-pancreatic juice diversion significantly increased pancreatic secretion, plasma CCK, and LCRF levels. Intravenous infusion of somatostatin inhibited CCK release and pancreatic secretion, but not LCRF content. Luminal administration of somatostatin did not show any effect.

CONCLUSION

Inhibitory effect of circulating somatostatin on CCK release and pancreatic secretion is independent of LCRF content.

Pancreatic phospholipase A2 from the small intestine is a secretin-releasing factor in rats

A secretin-releasing activity exists in the upper small intestine and pancreatic juice in the rat and the dog. Group I pancreatic phospholipase A2 (PLA2) in canine pancreatic juice and porcine pancreatic PLA2 stimulate the release of secretin from both STC-1 cells and a secretin-producing cell (S cell)-enriched preparation isolated from rat duodenal mucosa. We investigated the distribution and release of pancreatic PLA2-like immunoreactivity in the gastrointestinal tract and the role of PLA2 on the release of secretin and pancreatic exocrine secretion in response to duodenal acidification in anesthetized rats. PLA2-like immunoreactivity was detected in the mucosa throughout the gastrointestinal tract. High concentrations of PLA2 were found in both the small intestine and the pancreas. Duodenal acidification significantly increased the release of PLA2 from the upper small intestine (385% over basal secretion). Intravenous infusion of an anti-PLA2 serum (anti-PLA2) dose-dependently inhibited the release of secretin and pancreatic exocrine secretion in response to duodenal acid perfusion. Preincubation of the concentrate of intestinal acid perfusate (10-fold) from donor rats with the anti-PLA2 significantly suppressed its stimulation of secretin release and pancreatic exocrine secretion in recipient rats. We conclude that pancreatic PLA2 also functions as a secretin-releasing factor in the small intestine that mediates acid-stimulated release of secretin in rats.

Cefaclor, a cephalosporin antibiotic, delays gastric emptying rate by a CCK-A receptor-mediated mechanism in the rat

Studies in vitro suggest that cephalosporin antibiotics release the gut hormone cholecystokinin. Cholecystokinin is known to inhibit gastric emptying. Here we examine the effects of cefaclor on gastric emptying and intestinal motility. Male Sprague-Dawley rats were fitted with gastric cannulas. Following a 3-week recovery, the rate of gastric emptying of saline, peptone (4.5%) or cefaclor was determined after instillation into the gastric cannula, while intestinal transit was measured by using the propagation of arabic gum + charcoal mixture given intraduodenally. Gastric emptying of saline was significantly delayed by the addition of cefaclor (3, 10, 30 or 100 mM). The CCK-A antagonist SR-27897B (1 mg kg(-1), i.p.) reversed the delay induced by 10 mM cefaclor, whereas the CCK-B antagonist CI-988 (1 mg kg(-1), i.p.) had no significant effect. In capsaicin-treated rats, 10 mM cefaclor emptied more rapidly than in vehicle-treated animals. Thirty-minute intestinal transit was increased at 30 and 100 mM of cefaclor, while the gastric acid secretion following cefaclor instillation was no different than the group which received saline. The cephalosporin antibiotic cefaclor appears to be a potent stimulant of CCK release from gut endocrine cells, resembling the effects of peptone. Cefaclor delays gastric emptying via capsaicin-sensitive afferent pathways, which involve CCK-A receptor interaction.

Involvement of calmodulin and protein kinase C in cholecystokinin release by bombesin from STC-1 cells

The mouse intestinal neuroendocrine tumor cell line STC-1 secretes cholecystokinin (CCK) and other hormones. We investigated the role of Ca2+, calmodulin (CaM), and protein kinase C (PKC) in the regulation of CCK release from STC-1 cells. Phorbol 12-myristate 13-acetate (TPA) significantly stimulated CCK release. Staurosporine significantly inhibited CCK release from STC-1 cells stimulated by TPA in a dose-dependent manner. The absence of extracellular calcium completely inhibited CCK release from TPA-stimulated STC-1 cells. Neurotensin did not stimulate CCK release from these cells. W-7, a CaM antagonist, reduced CCK release from STC-1 cells stimulated by bombesin in a dose-dependent manner. These findings suggest that CaM and PKC play an important role in the regulation of CCK release from STC-1 cells stimulated by bombesin.

Control of CCK gene transcription by PACAP in STC-1 cells

The mechanisms by which neuroendocrine stimulants regulate CCK gene transcription are unclear. We examined promoter activation by pituitary adenylate cyclase-activating polypeptide (PACAP), a known CCK secretagogue, in the enteroendocrine cell line STC-1. The promoter region from -70 to -87 bp, relative to the transcriptional start site, contains a composite calcium/cyclic AMP response element (CRE)/activator protein 1 (AP1) site that may bind CRE binding protein (CREB) and AP1. PACAP (with IBMX) stimulated expression of an 87-bp construct 3.35+/-0.36-fold but had no effect on a -70 construct. The effect was blocked by the protein kinase A inhibitor H-89 and by a dominant-negative CREB plasmid. Mutation of the CRE/AP1 site to a canonical CRE site did not affect the response to PACAP, but mutation to a canonical AP1 site prevented it. CREB phosphorylation was increased after PACAP treatment. Electrophoretic mobility shift assay and supershift analysis revealed that CREB and not AP1 bound to the CRE/AP1 site and that PACAP increased the proportion of phosphorylated CREB that was bound. We conclude that PACAP increases CCK gene expression via a cAMP-mediated pathway involving CREB phosphorylation by protein kinase A and activation of a composite CRE/AP1 site.

Cellular mechanism of sodium oleate-stimulated secretion of cholecystokinin and secretin

Long-chain fatty acids are potent stimulants of secretin and CCK release. The cellular mechanisms of fatty acid-stimulated secretion of these two hormones are not clear. We studied the stimulatory effect and mechanism of sodium oleate (SO) on secretin- and CCK-producing cells. SO stimulated the release of secretin or CCK from isolated rat mucosal cell preparations enriched in either secretin- or CCK-producing cells, respectively. SO also time- and dose-dependently stimulated secretin and CCK release from STC-1 cells. In STC-1 cells, SO-stimulated secretin and CCK release was potentiated by IBMX and inhibited by a protein kinase A-selective inhibitor and a cAMP-specific antagonist. SO-stimulated releases of the two hormones were also inhibited by downregulation or inhibitors of protein kinase C, a calmodulin antagonist and an inhibitor of calmodulin-dependent protein kinase II. Chelating of extracellular Ca(2+) or addition of an L-type calcium channel blocker diminished SO-stimulated hormone releases. SO caused an increase in intracellular Ca(2+) concentration that was partially reversed by diltiazem but had no effect on production of cAMP, cGMP, or inositol-1,4,5-triphosphate. These results indicate that SO acts on secretin- and CCK-producing cells. Its stimulatory effect is potentiated by endogenous protein kinase A and mediated by activation of Ca(2+) influx through the L-type channels and of protein kinase C and Ca(2+)/calmodulin-dependent protein kinase II.

Advances in nutritional management of chronic pancreatitis

Nutrition has an important role in the management of chronic pancreatitis (CP), with two main goals for treatment of patients. The first goal is to provide optimal nutrition support and the second is to decrease pain by minimizing stimulation of the exocrine pancreas. Because cholecystokinin (CCK) stimulates secretion from the exocrine pancreas, one approach is to decrease CCK levels through modulation of diet. If postprandial pain is a limiting factor, alternative enteral therapies that minimally stimulate the pancreas may be beneficial. Nutritional counseling, antioxidants, and pancreatic enzymes may play a role in effective management of CP as well. In addition, because idiopathic CP is associated with cystic fibrosis gene mutations, therapies directed toward cystic fibrosis may also benefit these patients.

Pathophysiological role of cholecystokinin in humans

Cholecystokinin (CCK) is a major gastrointestinal hormone that plays an important role in stimulation of pancreatic secretion and gall-bladder contraction, regulation of gastrointestinal motility and induction of satiety. Ingestion of fat and protein induces significant increases in plasma CCK. Intraluminal mediators of CCK secretion, luminal CCK releasing factor and diazepam-binding inhibitor, were purified from rat intestinal secretion. These CCK-releasing factors (RF) are secreted tonically by the small intestine and stimulate CCK release. Another kind of CCK-RF named 'monitor peptide' was purified from the rat pancreatic juice that stimulates CCK secretion when introduced into rat intestine. Bile exclusion from the duodenum causes an increase in basal CCK and enhances stimulated plasma CCK release, and bile salt replacement reverses these effects. Thus, the CCK-RF are spontaneously secreted into the intestinal lumen in humans, while the CCK-producing cells are under constant suppression by intraduodenal bile acids. In acute pancreatitis, plasma CCK levels are high in patients with gallstone pancreatitis, but not in patients with pancreatitis from other causes, such as alcoholic and idiopathic pancreatitis. A transient disturbance of bile flow into the duodenum by stones or oedema of the pancreas together with impairment of pancreatic exocrine function might cause the increase in plasma CCK release in gallstone pancreatitis. Patients with chronic pancreatitis with mild to moderate impairment of exocrine function and abdominal pain, had significantly higher plasma CCK concentrations, whereas patients with pancreatic insufficiency had a significantly lower plasma CCK response to a test meal than the healthy subjects. The increased CCK may further aggravate pancreatitis and worsen the prognosis of pancreatitis by stimulating the injured pancreas, resulting in the vicious circle via endogenous CCK release. The CCK-A receptor antagonist might be therapeutically useful in acute pancreatitis by stopping the vicious circle.

Diazepam-binding inhibitor mediates feedback regulation of pancreatic secretion and postprandial release of cholecystokinin

Recently, we isolated a trypsin-sensitive cholecystokinin-releasing peptide (CCK-RP) from porcine and rat intestinal mucosa. The amino acid sequence of this peptide was determined to be identical to that of the diazepam-binding inhibitor (DBI). To test the role of DBI in pancreatic secretion and responses to feeding, we used pancreaticobiliary and intestinal cannula to divert bile-pancreatic juice from anesthetized rats. Within 2 hours, this treatment caused a 2-fold increase in pancreatic protein output and a >10-fold increase in plasma CCK. Luminal DBI levels increased 4-fold. At 5 hours after diversion of bile-pancreatic juice, each of these measures returned to basal levels. Intraduodenal infusion of peptone evoked a 5-fold increase in the concentration of luminal DBI. In separate studies, we demonstrated that intraduodenal administration of antiserum to a DBI peptide specifically abolished pancreatic secretion and the increase in plasma CCK levels after diversion of bile-pancreatic juice. To demonstrate that DBI mediates the postprandial rise in plasma CCK levels, we showed that intraduodenal administration of 5% peptone induced dramatic increases in pancreatic secretion and plasma CCK, effects that could be blocked by intraduodenal administration of anti-DBI antiserum. Hence, DBI, a trypsin-sensitive CCK-RP secreted from the proximal small bowel, mediates the feedback regulation of pancreatic secretion and the postprandial release of CCK.

Cholecystokinin: proofs and prospects for involvement in control of food intake and body weight

Evidence that CCK participates in the control of meal size is compelling, but the avenues by which CCK may affect daily food intake and body weight regulation are still uncertain. Although participation of brain CCK in control of food intake is acknowledged, our focus here is on participation of peripheral CCK in the control of food intake. Therefore, in this article we (1) review evidence for CCK's participation in control of meal size, (2) document involvement of CCK-A receptors located on vagal sensory neurons in control of food intake by exogenous and endogenous CCK, (3) point out apparent discrepancies in the experimental record, which auger for non-endocrine sources of CCK and non-vagal sites of CCK action, and (4) summarize recent observations, suggesting mechanisms by which CCK could participate in the control of daily food intake and body weight regulation.

Pancreatic exocrine-endocrine interrelationship. Clinical implications

The exocrine and endocrine pancreata are anatomically and functionally interrelated. As a result, exocrine pancreatic dysfunction often accompanies endocrine pancreatic dysfunction and vice versa. This article delineates the nature of these exocrine and endocrine pancreatic interrelationships and their clinical significance in the management of pancreatic disorders.

Role of cholecystokinin in the anorexia produced by duodenal delivery of peptone in rats

We used the cholecystokinin receptor antagonist devazepide to assess the importance of CCK in mediating the anorexia produced by 2-h duodenal infusions of peptone, a protein digest, at dark onset in nonfasted rats. Peptone alone (0.14-2.24 g/h) suppressed food intake dose dependently by 18-96%, with an approximate half-maximal dose of 1 g/h. Peptone-induced reductions in caloric ingestion were comparable to the caloric loads infused. Devazepide alone (30-1,000 microgram/kg) stimulated food intake dose dependently by 30-73%, with a minimal effective dose of 100 micrograms/kg. Devazepide appeared to reverse the anorexic response to peptone (1.1 g/h) dose dependently by 29-65%, with a minimal effective dose of 30 micrograms/kg. The magnitudes of these devazepide-induced effects were similar to, and in some cases were larger than, those produced when the same doses of devazepide were administered alone. Coadministration of devazepide (1,000 micrograms/kg) and a lower peptone dose (0.8 g/h) produced similar results. These results suggest that an essential CCK mechanism plays a significant role in mediating the satiety response to duodenal delivery of protein.

Porcine pancreatic phospholipase A2 stimulates secretin release from secretin-producing cells

We have isolated, from canine pancreatic juice, two 14-kDa proteins with secretin-releasing activity that had N-terminal sequence homology with canine pancreatic phospholipase A2 (PLA2). In this study we have obtained evidence that secretin-releasing activity is an intrinsic property of pancreatic PLA2. Porcine pancreatic PLA2 from Sigma or Boehringer Mannheim was fractionated into several peaks by reverse phase high performance liquid chromatography. They were tested for stimulation of secretin release from murine neuroendocrine intestinal tumor cell line STC-1 and secretin cells enriched mucosal cell preparations isolated from rat upper small intestine. Each enzyme preparation was found to contain several components of secretin-releasing activity. Each bioactive fraction was purified to homogeneity by rechromatography and then subjected to mass spectral analysis and assays of PLA2 and secretin-releasing activities. It was found that the fraction with highest enzymatic activity also had the highest secretin-releasing activity and the same Mr as porcine pancreatic PLA2. Moreover, it also had the same N-terminal amino acid sequence (up to 30 residues determined) as that of porcine pancreatic PLA2, suggesting that it was identical to the enzyme. Purified porcine pancreatic PLA2 also stimulated secretin release concentration-dependently from both STC-1 cells and a mucosal cell preparation enriched in secretin-containing endocrine cells isolated from rat duodenum. Abolishment of the enzymatic activity by pretreatment with bromophenacyl bromide did not affect its secretin-releasing activity. The stimulatory effect of purified pancreatic PLA2 on secretin secretion from STC-1 cells was inhibited by an L-type Ca2+ channel blocker, by down-regulation of protein kinase C or by pretreatment of the cell with pertussis toxin. It is concluded that porcine pancreatic PLA2 possesses an intrinsic secretin-releasing activity that was independent of its enzymatic activity. This action is pertussis toxin-sensitive and is in part dependent on Ca2+ influx through the L-type channel and activation of protein kinase C.

Diazepam-binding inhibitor33-50 elicits Ca2+ oscillation and CCK secretion in STC-1 cells via L-type Ca2+ channels

We recently isolated and characterized 86-amino acid CCK-releasing peptide from porcine intestinal mucosa. The sequence of this peptide is identical to that of porcine diazepam-binding inhibitor (DBI). Intraduodenal administration of DBI stimulates the CCK release and elicits pancreatic secretion in rats. In this study we utilized a murine tumor cell line (STC-1 cells) that contains CCK to examine if DBI directly acts on these cells to stimulate CCK release. We investigated the cellular mechanisms responsible for this action. We showed that DBI33-50, a biologically active fragment of DBI1-86, significantly stimulated CCK secretion in STC-1 cells. This action was abolished by Ca2+-free medium. The mean basal intracellular Ca2+ concentration ([Ca2+]i) was 52 nM in fura 2-loaded STC-1 cells. DBI33-50 (1-1,000 nM) elicited Ca2+ oscillations; DBI33-50 (10 nM) increased the oscillation frequency to 5 cycles/10 min and elicited a net [Ca2+]i increase (peak - basal) to 157 nM. In contrast, bombesin and forskolin caused an initial transient [Ca2+]i followed by a small sustained [Ca2+]i plateau. Withdrawal of extracellular Ca2+ abolished Ca2+ oscillations stimulated by DBI33-50. L-type Ca2+ channel blockers nifedipine and diltiazem (3-10 microM) markedly attenuated DBI-stimulated Ca2+ oscillations. In other cell types L-type Ca2+ channels are activated by cAMP-protein kinase A. DBI33-50 failed to stimulate cAMP formation in STC-1 cells. Similarly, DBI33-50 had no effect on myo-inositol 1,4, 5-trisphosphate concentration ([IP3]), whereas bombesin caused an eightfold increase in [IP3] over basal. In addition, inhibitors of phospholipase C (U-73122), phospholipase A2 (ONO-RS-082), and protein tyrosine kinase (genistein) did not alter the Ca2+ oscillations elicited by DBI33-50. It appears that DBI33-50 acts directly on STC-1 cells to elicit Ca2+ oscillations via the voltage-dependent L-type Ca2+ channels, resulting in the secretion of CCK. Mediation of this action is by intracellular mechanisms independent of the traditional signal transduction pathways, including phospholipase C, phospholipase A2, protein tyrosine kinase, and cAMP systems.

Atropine-resistant secretion of a putative luminal CCK-releasing peptide in conscious rats

The changes in levels of the newly discovered luminal CCK-releasing factor (LCRF) in the small intestinal lumen before and after bile-pancreatic juice diversion in conscious rats were examined by a specific RIA. Moreover, we also examined whether LCRF secretion was under cholinergic control. Anti-LCRF antiserum was raised in rabbits, and a sensitive RIA was established. The localization of LCRF was examined by immunohistochemistry. The luminal content of LCRF was significantly increased by bile-pancreatic juice diversion, during which luminal trypsin activity was eliminated. The increase in luminal LCRF content was not inhibited by intravenous infusion of atropine. The changes in plasma levels of CCK and pancreatic secretion were similar to those in luminal LCRF contents. LCRF immunostaining was observed in villus tip enterocytes of the small intestine and was most prominent in the duodenal portion. These results support our original hypothesis that LCRF may be released spontaneously into the small intestinal lumen from the villus tip enterocytes and its intraluminal degradation by proteases regulates CCK release. Furthermore, LCRF release was not subject to cholinergic regulation.

Regulation of the action of the novel cholecystokinin-releasing peptide diazepam binding inhibitor by inhibitory hormones and taurocholate

Diazepam binding inhibitor (DBI1-86) has recently been isolated in search for a cholecystokinin (CCK)-releasing peptide in the duodenum that is responsible for the feedback regulation of exocrine pancreatic secretion. Synthetic porcine DBI1-86 stimulates CCK release in vivo and in vitro from isolated intestinal mucosal cells. We postulated that DBI intraduodenally releases CCK in a paracrine fashion and might be the missing link in the feedback regulation of exocrine pancreatic secretion. Somatostatin, peptide YY (PYY) and taurocholate are known to inhibit feedback-stimulated CCK release in the rat. In this study, we investigated the effect of somatostatin, PYY and taurocholate on DBI-stimulated CCK secretion. Dispersed rat intestinal mucosal cells were prepared from the proximal small bowel and continuously perfused. The perfusate was collected and the release of CCK into the medium was measured. DBI1-86 dose-dependently stimulated CCK release, with a maximal effect at 10(-9) M. Somatostatin blocked the DBI-stimulated CCK release. Pretreatment of the cells with pertussis toxin fully reversed the inhibitory effect of somatostatin on DBI-stimulated CCK secretion, suggesting that somatostatin exerts its action by an inhibitory G-protein. In contrast, PYY (10(-6) M) and taurocholate (10(-6) M) did not affect DBI stimulated CCK levels, indicating that they act through different mechanisms to inhibit feedback-stimulated CCK release.

Bombesin stimulates cholecystokinin secretion through mitogen-activated protein-kinase-dependent and -independent mechanisms in the enteroendocrine STC-1 cell line

Bombesin has been reported to stimulate cholecystokinin (CCK) secretion from rat duodeno-jejunal I-cells. Bombesin was shown to activate mitogen-activated protein kinases (MAPKs) in cell types such as Swiss 3T3 fibroblasts and rat pancreatic acinar cells. No information is available on whether MAPK is activated in intestinal endocrine cells upon bombesin stimulation. This was studied by using the CCK-producing enteroendocrine cell line STC-1. Bombesin stimulated markedly and transiently both p42(MAPK) and p44(MAPK), with a maximum at 2 min, and a decrease to basal levels within 10 min. As expected, bombesin stimulated MAPK kinase 1 (MEK-1) activity. Activation of protein kinase C (PKC) with PMA also stimulated p42(MAPK), p44(MAPK) and MEK-1. Treatment of cells with PD 098059 (at 10 microM or 30 microM), which selectively inhibits MEK phosphorylation, blocked bombesin-induced p42(MAPK) and p44(MAPK) activation for at least 90 min. However, PD 098059 inhibited bombesin- and PMA-stimulated CCK secretion during the first 15 min, but failed to significantly reduce CCK release at later times. Inhibition of PKC with staurosporine, or PKC down-regulation by prolonged treatment with PMA, both drastically decreased MEK-1, p42(MAPK) and p44(MAPK) activation upon bombesin stimulation. Additionally, PKC activation appeared to be required for both MAPK-dependent (early) and -independent (late) CCK responses to bombesin. It is concluded that the early CCK secretory response of STC-1 cells to bombesin involves MAPK pathway activation through a PKC-dependent mechanism, whereas the late phase of bombesin-induced CCK secretion, that also requires PKC, appears to result from a MAPK-independent process.

Bioactivity of intraduodenally and intravenously infused fragments of luminal cholecystokinin releasing factor (LCRF)

A luminal cholecystokinin releasing factor (LCRF), has been purified from intestinal secretion and found to have a mass of 8136 daltons. The amino-terminal 41 residues have been sequenced. Previous studies showed that intraduodenal infusion of the synthetic amino-terminal 35 amino acid peptide, LCRF1-35 significantly stimulated pancreatic protein and fluid secretion in conscious rats, but the peptide did not stimulate amylase release from isolated, dispersed pancreatic acini. In the present study, several fragments of LCRF were synthesized and tested for CCK-releasing activity (pancreatic protein secretion) to determine whether shorter fragments of LCRF exhibit the characteristic biological activity of native LCRF and synthetic LCRF1-35. Compounds tested were LCRF1-41, LCRF1-35, LCRF1-65 and LCRF11-25. Of the fragments shorter than LCRF1-35, only LCRF11-25 but not LCRF1-6 had significant CCK releasing activity. LCRF1-41 was equivalent to LCRF1-35 in potency and efficacy. Intravenous and intraduodenal infusion of LCRF1-35 elicited nearly identical dose-response curves.

Cholecystokinin- and secretin-releasing peptides in the intestine--a new regulatory interendocrine mechanism in the gastrointestinal tract

Maintenance of homeostasis in the upper small bowel is a vital process for the body and therefore highly controlled. The enteric nervous system and the endocrine system are the regulators in this process influencing each other. The endocrine system in the gut consists of the classical hormones [cholecystokinin (CCK) secretin] to evoke motility or secretion. They are under control of releasing factors which are probably influenced by the enteric nervous system. Diazepam binding inhibitor and luminal CCK-releasing factor are likely candidates for CCK-releasing peptides in the negative feedback process in the absence of pancreatic juice. Experimental evidence suggests a secretin-releasing peptide. Further studies will be needed to determine the physiological role of each of these peptides. Monitor peptide in the pancreatic juice seems to function as a specific positive enhancement for CCK release. All these peptides are inactivated by the proteolytic enzymes during the interdigestive period. The discovery of additional releasing peptides and factors is very likely.