Two hypotheses on the feedback regulation of pancreatic enzyme secretion

Factors Influencing Proteolysis and Protein Utilization in the Intestine of Pigs: A Review

Pigs are among the most important farm animals for meat production worldwide. In order to meet the amino acid requirements of the animals, pigs rely on the regular intake of proteins and amino acids with their feed. Unfortunately, pigs excrete about two thirds of the used protein, and production of pork is currently associated with a high emission of nitrogen compounds resulting in negative impacts on the environment. Thus, improving protein efficiency in pigs is a central aim to decrease the usage of protein carriers in feed and to lower nitrogen emissions. This is necessary as the supply of plant protein sources is limited by the yield and the cultivable acreage for protein plants. Strategies to increase protein efficiency that go beyond the known feeding options have to be investigated considering the characteristics of the individual animals. This requires a deep understanding of the intestinal processes including enzymatic activities, capacities of amino acid transporters and the microbiome. This review provides an overview of these physiological factors and the respective analyses methods.

Health Risks and Benefits of Chickpea (Cicer arietinum) Consumption

Chickpeas (CPs) are one of the most commonly consumed legumes, especially in the Mediterranean area as well as in the Western world. Being one of the most nutritional elements of the human diet, CP toxicity and allergy have raised health concerns. CPs may contain various antinutritional compounds, including protease inhibitors, phytic acid, lectins, oligosaccharides, and some phenolic compounds that may impair the utilization of the nutrients by people. Also, high consumption rates of CPs have enhanced the allergic problems in sensitive individuals as they contain many allergens. On the other hand, beneficial health aspects of CP consumption have received attention from researchers recently. Phytic acid, lectins, sterols, saponins, dietary fibers, resistant starch, oligosaccharides, unsaturated fatty acids, amylase inhibitors, and certain bioactive compounds such as carotenoids and isoflavones have shown the capability of lowering the clinical complications associated with various human diseases. The aim of this paper is to unravel the health risks as well as health-promoting aspects of CP consumption and to try to fill the gaps that currently exist. The present review also focuses on various prevention strategies to avoid health risks of CP consumption using simple but promising ways.

Dietary levels of protein and free amino acids affect pancreatic proteases activities, amino acids transporters expression and serum amino acid concentrations in starter pigs

The dietary contents of crude protein and free amino acids (AA) may affect the protein digestion and AA absorption in pigs. Trypsin and chymotrypsin activities, AA serum concentrations and expression of AA transporters in the small intestine of pigs fed a low protein, AA-supplemented (19.2%, LPAA) or a high protein (28.1%, HP), wheat-soybean meal diet were measured in two 14-d trials. The LPAA diet contained free L-Lys, L-Thr, DL-Met, L-Leu, L-Ile, L-Val, L-His, L-Trp and L-Phe. All pigs were fed the same amount of feed (890 and 800 g/d for trial 1 and 2 respectively). In trial 1, samples of mucosa (duodenum, jejunum and ileum) and digesta (duodenum and jejunum) were collected from 14 pigs (17.2 ± 0.4 kg); in trial 2, blood samples were collected from 12 pigs (12.7 ± 0.3 kg). The trypsin and chymotrypsin activities in both intestinal segments were higher in pigs fed the HP diet (p < 0.01). Trypsin activity was higher in jejunum than in duodenum regardless the dietary treatment (p < 0.05). Pigs fed the LPAA diet expressed more b^0,+ AT in duodenum, B⁰ AT1 in ileum (p < 0.05), and tended to express more y⁺ LAT1 in duodenum (p = 0.10). In pigs fed the LPAA diet, the expression of b^0,+ AT was higher in duodenum than in jejunum and ileum (p < 0.01), but no difference was observed in pigs fed the HP diet. Ileum had the lowest b^0,+ AT expression regardless the diet. The serum concentrations of Lys, Thr and Met were higher in LPAA pigs while serum Arg was higher in HP pigs (p < 0.05). Serum concentrations of AA appear to reflect the AA absorption. In conclusion, these data indicate that the dietary protein contents affect the extent of protein digestion and that supplemental free AA may influence the intestinal site of AA release and absorption, which may impact their availability for growth of young pigs.

Adaptive regulation of digestive serine proteases in the larval midgut of Helicoverpa armigera in response to a plant protease inhibitor

Protease inhibitors (PIs) are direct defenses induced by plants in response to herbivory. PIs reduce herbivore digestive efficiency by inhibiting insects' digestive proteases; in turn insects can adapt to PIs by generally increasing protease levels and/or by inducing the expression of PI-insensitive proteases. Helicoverpa armigera, a highly polyphagous lepidopteran insect pest, is known for its ability to adapt to PIs. To advance our molecular and functional understanding of the regulation of digestive proteases, we performed a comprehensive gene expression experiment of H. armigera exposed to soybean Kunitz trypsin inhibitor (SKTI) using a custom-designed microarray. We observed poor larval growth on the SKTI diet until 24 h, however after 48 h larvae attained comparable weight to that of control diet. Although initially the expression of several trypsins and chymotrypsins increased, eventually the expression of some trypsins decreased, while the number of chymotrypsins and their expression increased in response to SKTI. Some of the diverged serine proteases were also differentially expressed. The expression of serine proteases observed using microarrays were further validated by qRT-PCR at different time points (12, 24, 48, 72 and 96 h) after the start of SKTI ingestion. There were also large changes in transcriptional patterns over time in the control diet. Carbohydrate metabolism and immune defense genes were affected in response to SKTI ingestion. Enzyme assays revealed reduced trypsin-specific activity and increased chymotrypsin-specific activity in response to SKTI. The differential regulation of trypsins and chymotrypsins at the transcript and protein levels accompanying a rebound in growth rate indicates that induction of SKTI-insensitive proteases is an effective strategy of H. armigera in coping with this protease inhibitor in its diet.

Two-year carcinogenicity studies with the oral direct thrombin inhibitor ximelagatran in the rat and the mouse

The results of 18 months mouse and 24 months rat carcinogenicity studies with the oral direct thrombin inhibitor ximelagatran are presented. In the mouse, gavage doses of ximelagatran up to 180 μmol/kg per d produced no neoplastic changes in any of the tissues examined. In the rat, gavage doses up to 240 μmol/kg per d produced multiple macroscopically detectable nodules in the pancreas, which are seen to be focal/multifocal acinar cell hyperplasia and focal/multifocal acinar cell adenoma upon histological evaluation. There were no other treatment-related effects on tumor incidence or distribution in the rat. The studies show a clear species difference in pancreatic effects between the rat and the mouse to long-term treatment with ximelagatran.

Initiation of acute pancreatitis by heparan sulphate in the rat

OBJECTIVE

The initiating events in the onset of pancreatitis are poorly understood. Possible candidates may be endogenous ligands, acting on receptors within ductal, acinar or stellate cells, which have previously been shown to cause a systemic inflammatory response syndrome. The aim of this study was to investigate whether acute pancreatitis could be induced by heparan sulphate (HS)infused into the pancreatic ducts in the rat.

MATERIAL AND METHODS

Retrograde biliary-pancreatic infusion of heparan sulphate of different structures, taurodeoxycholate (TDC) or phosphate buffered saline (PBS) was performed. Local pancreatic inflammation was evaluated after 6 h by means of morphological evaluation, neutrophil and macrophage infiltration and levels of plasma amylase. Systemic inflammation was evaluated by measuring plasma IL-6, MCP-1 and CINC-1 concentrations.

RESULTS

Heparan sulphate induced a local inflammatory response visualized as a rapid infiltration of neutrophils and macrophages into the pancreas. Heparan sulphate induced inflammation and oedema without causing damage to acinar cells, as measured by morphological changes and plasma amylase concentrations. Furthermore, an increase in serum concentrations of CINC-1 and IL-6 was seen. The positive control (TDC) had increased levels of all variables analysed and the negative control (heparan sulphate administered intraperitoneally) was without effects.

CONCLUSIONS

Our findings suggest a receptor-mediated innate immune response of the pancreatic cells induced by heparan sulphate. This finding may be helpful in elucidating some of the mechanisms involved during the initiation of pancreatitis, as well as in the search for a potential future therapeutic application.

Purification and identification of a binding protein for pancreatic secretory trypsin inhibitor: a novel role of the inhibitor as an anti-granzyme A

Pancreatic secretory trypsin inhibitor (PSTI) is a potent trypsin inhibitor that is mainly found in pancreatic juice. PSTI has been shown to bind specifically to a protein, distinct from trypsin, on the surface of dispersed cells obtained from tissues such as small intestine. In the present study, we affinity-purified the binding protein from the 2% (w/v) Triton X-100-soluble fraction of dispersed rat small-intestinal cells using recombinant rat PSTI. Partial N-terminal sequencing of the purified protein gave a sequence that was identical with the sequence of mouse granzyme A (GzmA), a tryptase produced in cytotoxic lymphocytes. We confirmed the formation of an affinity-cross-linked complex between (125)I-labelled PSTI and recombinant rat GzmA (rGzmA). In situ hybridization and immunostaining revealed the existence of GzmA-expressing intraepithelial lymphocytes in the rat small intestine. We concluded that the PSTI-binding protein isolated from the dispersed cells is GzmA that is produced in the lymphocytes of the tissue. The rGzmA hydrolysed the N -alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester (BLT), and the BLT hydrolysis was inhibited by PSTI. Sulphated glycosaminoglycans, such as fucoidan or heparin, showed almost no effect on the inhibition of rGzmA by PSTI, whereas they decreased the inhibition by antithrombin III. In the present paper, we propose a novel role of PSTI as a GzmA inhibitor.

Pancreatic exocrine secretion in steers infused postruminally with casein and cornstarch

Our objective was to evaluate the effect of postruminal protein infusion on pancreatic exocrine secretions. One Holstein, two crossbred, and five Angus steers (305 +/- 5 kg) with pancreatic pouch-duodenal reentrant cannulas and abomasal infusion catheters were used in a replicated 4 x 4 Latin square. All steers were abomasally infused with 1,050 g/d of raw cornstarch with treatments of 0, 60, 120, or 180 g/d of sodium casein suspended in water to yield 6,000 g/d of infusate daily. Steers were limit-fed (1.5 x NEm; 12 equal portions daily) a 90% corn silage, 10% supplement diet formulated to contain 12.5% CP. Periods consisted of 3 d of adaptation to infusion, 7 d of full infusion, 1 d of collection, and 7 d of rest. Pancreatic juice was collected in 30-min fractions continuously for 6 h. Total juice secreted and the pH of individual fractions were recorded, a 10% subsample was retained to form a composite sample, and remaining fluid was returned to the duodenum. Juice composite samples were stored (-30 degrees C) until analyzed for total protein and activities of alpha-amylase, trypsin, and chymotrypsin. Casein infusion linearly increased alpha-amylase concentration (182 to 271 units/mL; P < 0.02; 17.5 to 24.6 units/mg of protein; P < 0.03) and secretion rate (26,847 to 41,894 units/h; P < 0.01). Total juice secretion (155 g/h), pH of pancreatic juice (8.13), secretion rate of protein (1,536 mg/h), and concentration of protein (10.2 mg/mL) in pancreatic secretions were not affected (P > 0.05) by casein infusion. Similarly, casein infusion did not change 0.05) trypsin and chymotrypsin concentrations (1,379 and 349 units/L or 0.134 and 0.033 units/mg of protein, respectively) or secretion rates (206 and 52 units/h, respectively). Abomasal infusion of protein with starch stimulated a greater pancreatic secretion of alpha-amylase activity into the intestine than infusion of starch alone.

Pathological evaluation, clinical chemistry and plasma cholecystokinin in neonatal and young miniature swine fed soy trypsin inhibitor from 1 to 39 weeks of age

The potential toxicity of dietary soy trypsin inhibitor (TI) was evaluated in neonatal miniature swine. From 1 to 6 weeks of age, two groups of male piglets were artificially reared in an Autosow and automatically fed either TI or control liquid diet. From 6 to 39 weeks of age, these two groups were fed either TI or control chow diet. A third group, sow control (SC), suckled from birth to 6 weeks of age, were also weaned to control chow from 6 to 39 weeks of age. Clinical chemistry and plasma cholecystokinin (CCK) determined at 6, 18, 30 and 39 weeks of age, and serum amylase activity with gross and histopathological analyses of major organs at 6 and 39 weeks of age are reported. TI had no effect on plasma CCK, serum amylase activity, or numerous clinical chemistry values. TI-fed piglets had a larger relative liver weight at 6 weeks of age. Relative pancreas weight decreased with age but was not affected by TI. Gross and histopathological analyses of major organs, except the spleen, were within normal limits. Increased incidence of extramedullary hematopoiesis was noted in the spleen of the TI group at 6 but not at 39 weeks of age. There was no consistent pattern in immunohistochemical foci for secretin, gastrin releasing polypeptide or CCK, and no change in DNA, RNA, mitotic index or nuclear density of pancreatic cells. At 6 weeks of age, TI increased pancreatic protein and amylase activity but not trypsin or chymotrypsin activity. None of the effects suggested that this dose of TI was toxic to either the neonatal or sexually mature miniature male swine.

Dietary soya beans and kidney beans stimulate secretion of cholecystokinin and pancreatic digestive enzymes in 400-day-old Hooded-Lister rats but only soya beans induce growth of the pancreas

The effects of age on cholecystokinin (CCK) release, pancreatic enzyme secretion, and growth of the pancreas mediated by dietary kidney beans or soya beans were evaluated in trials with 30-, 90-, 250-, and 400-day-old rats. Soya beans increased blood CCK and caused hypersecretion of digestive enzymes and rapid pancreatic growth in all rats. Kidney beans also elevated circulating CCK and stimulated enzyme secretion. However, with 90-, 250-, and 400-day-old rats, the secretory responses were attenuated. Furthermore, kidney beans did not induce pancreatic growth in 250- and 400-day-old rats.

Effects of soybean diet on the beta cells in the streptozotocin treated rats for induction of diabetes

Certain dietary components have been reported to potentially suppress the initiation of experimental insulin-dependent diabetes mellitus (IDDM) in animal models. In the present study, we showed that dietary soybean prevents induction of experimental hyperglycemia by retaining beta cell activity. In rats fed raw soybean, expression of insulin mRNA in pancreatic beta cells was significantly increased compared to those fed with normal diet. In those rats and upon injection of streptozotocin, only few beta cells underwent cell death, most of them demonstrating active viability with enhanced mRNA expression and insulin content. This is consistent with the fact that blood glucose level was normalized (72.51 +/- 1.54 mg/dl) after a transitory hyperglycemic state (> 300 mg/dl). It implies the dietary soybean can prevent beta cell injury by streptozotocin. Moreover, a prolonged hyperglycemia was not observed in rats fed raw soybean even when this was substituted by the normal diet after streptozotocin injection. In contrast, in streptozotocin-treated rats fed normal diet, most beta cells were destroyed and severe hyperglycemia was observed. Although the protective effect was not recorded in the rats fed with heated soybean, some beta cells were found to retain their cell organelles for insulin secretion indicating that some heat-stable components of soybean might prevent the cytotoxic insult. Therefore, we suggest that dietary soybean protected the streptozotocin-induced beta cell damage and restrained the development of hyperglycemia in rats.

Chemopreventive agents: protease inhibitors

Certain protease inhibitors, called the anticarcinogenic protease inhibitors in this review, are capable of preventing carcinogenesis in a wide variety of in vivo and in vitro model systems. The anticarcinogenic protease inhibitors are extremely potent agents with the ability to prevent cancer, with some unique characteristics as anticarcinogenic agents. The anticarcinogenic protease inhibitors have the ability to irreversibly suppress the carcinogenic process. They do not have to be continuously present to suppress carcinogenesis. They can be effective when applied in both in vivo and in vitro carcinogenesis assay systems at long time periods after carcinogen exposure, and are effective as anticarcinogenic agents at extremely low molar concentrations. While several different types of protease inhibitors can prevent the carcinogenic process, the most potent of the anticarcinogenic protease inhibitors on a molar basis are those with the ability to inhibit chymotrypsin or chymotrypsin-like proteases. The soybean-derived protease inhibitor, Bowman-Birk inhibitor (BBI), is a potent chymotrypsin inhibitor that has been extensively studied for its ability to prevent carcinogenesis in many different model systems. Much of this review is focused on the characteristics of BBI as the anticarcinogenic protease inhibitor, as this is the protease inhibitor that has risen to the human trial stage as a human cancer chemopreventive agent. Part of this review hypothesizes that the Bowman-Birk family of protease inhibitors plays a role in plants similar to that of alpha1-antichymotrypsin in people. Both BBI and alpha1-antichymotrypsin are potent inhibitors of chymotrypsin and chymotrypsin-like enzymes, are highly anti-inflammatory, and are thought to play important roles in the defense of their respective organisms. It is believed that BBI will be shown to play a major role in the prevention and/or treatment of several different diseases, in addition to cancer.

Cholecystokinin cells

Cholecystokinin (CCK) is an important hormonal regulator of the digestive process. CCK cells are concentrated in the proximal small intestine, and hormone is secreted into the blood upon the ingestion of food. The physiological actions of CCK include stimulation of pancreatic secretion and gallbladder contraction, regulation of gastric emptying, and induction of satiety. Therefore, in a highly coordinated manner, CCK regulates the ingestion, digestion, and absorption of nutrients. CCK is produced by two separate cell types: endocrine cells of the small intestine and various neurons in the gastrointestinal tract and central nervous system. Accordingly, CCK can function as either a hormone or a neuropeptide. This review focuses on the physiology of the CCK cell in the intestine and, in particular, on how the CCK cell is regulated to secrete its hormone product. The effects of ingested nutrients on the CCK cell and the intracellular messenger systems involved in controlling secretion are reviewed. A summary is provided of recent studies examining the electrophysiological properties of CCK cells and newly discovered proteins that act as releasing factors for CCK, which mediate feedback pathways critical for regulated secretion in the intact organism.

Mechanisms of desensitization and resensitization of proteinase-activated receptor-2

Proteinase-activated receptor-2 (PAR-2) is a G-protein-coupled receptor that is expressed by intestinal epithelial cells, which are episodically exposed to pancreatic trypsin in the intestinal lumen. Trypsin cleaves PAR-2 to expose a tethered ligand, which irreversibly activates the receptor. Thus, PAR-2 may desensitize and resensitize by novel mechanisms. We examined these mechanisms in kidney epithelial cells, stably expressing human PAR-2, and intestinal epithelial cells, which naturally express PAR-2. Trypsin stimulated a prompt increase in [Ca2+]i, due to mobilization of intracellular Ca2+, followed by a sustained plateau, due to influx of extracellular Ca2+. Repeated application of trypsin caused marked desensitization of this response, which is due in part to (a) irreversible cleavage of the receptor by trypsin and (b) protein kinase C-mediated termination of signaling. Trypsin exposure resulted in internalization of PAR-2 into early endosomes and then lysosomes; but endocytosis was not the mechanism of rapid desensitization. Thus, activated PAR-2 is endocytosed and degraded. The Ca2+ response to trypsin resensitized by 60-90 min. Brefeldin A, which disrupted Golgi stores of PAR-2, and cycloheximide, which inhibited protein synthesis, markedly attenuated resensitization. Thus, PAR-2-mediated Ca2+ mobilization desensitizes by irreversible receptor cleavage, protein kinase C-mediated termination of signaling, and PAR-2 targeting to lysosomes. It resensitizes by mobilization of large Golgi stores and synthesis of new receptors.

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

Cholecystokinin (CCK) secretion in rats and humans is inhibited by pancreatic proteases and bile acids in the intestine. It has been hypothesized that the inhibition of CCK release caused by pancreatic proteases is due to proteolytic inactivation of a CCK-releasing peptide present in intestinal secretion. To purify the putative luminal CCK-releasing factor (LCRF), intestinal secretions were collected by perfusing a modified Thiry-Vella fistula of jejunum in conscious rats. From these secretions, the peptide was concentrated by ultrafiltration followed by low-pressure reverse-phase chromatography and purified by reverse-phase high-pressure liquid chromatography. Purity was confirmed by high-performance capillary electrophoresis. Fractions were assayed for CCK-releasing activity by their ability to stimulate pancreatic protein secretion when infused into the proximal small intestine of conscious rats. Partially purified fractions strongly stimulated both pancreatic secretion and CCK release while CCK receptor blockade abolished the pancreatic response. Amino acid analysis and mass spectral analysis showed that the purified peptide is composed of 70-75 amino acid residues and has a mass of 8136 Da. Microsequence analysis of LCRF yielded an amino acid sequence for 41 residues as follows: STFWAYQPDGDNDPTDYQKYEHTSSPSQLLAPGDYPCVIEV. When infused intraduodenally, the purified peptide stimulated pancreatic protein and fluid secretion in a dose-related manner in conscious rats and significantly elevated plasma CCK levels. Immunoaffinity chromatography using antisera raised to synthetic LCRF-(1-6) abolished the CCK releasing activity of intestinal secretions. These studies demonstrate, to our knowledge, the first chemical characterization of a luminally secreted enteric peptide functioning as an intraluminal regulator of intestinal hormone release.

Molecular cloning, expression and potential functions of the human proteinase-activated receptor-2

We used PCR to amplify proteinase activated receptor-2 (PAR-2) from human kidney cDNA. The open reading frame comprised 1191 bp and encoded a protein of 397 residues with 83% identity with mouse PAR-2. In KNRK cells (a line of kirsten murine sarcoma virus-transformed rat kidney epithelial cells) transfected with this cDNA, trypsin and activating peptide (AP) corresponding to the tethered ligand exposed by trypsin cleavage (SLIGKV-NH2) induced a prompt increase in cytosolic calcium ion concentration ([Ca2+]i). Human PAR-2 (hPAR-2) resided both on the plasma membrane and in the Golgi apparatus. hPAR-2 mRNA was highly expressed in human pancreas, kidney, colon, liver and small intestine, and by A549 lung and SW480 colon adenocarcinoma cells. Hybridization in situ revealed high expression in intestinal epithelial cells throughout the gut. Trypsin and AP stimulated an increase in [Ca2+]i in a rat intestinal epithelial cell line (hBRIE 380) and stimulated amylase secretion in isolated pancreatic acini. In A549 cells, which also responded to trypsin and AP with mobilization of cytosolic Ca2+, AP inhibited colony formation. Thus PAR-2 may serve as a trypsin sensor in the gut. Its expression by cells and tissues not normally exposed to pancreatic trypsin suggests that other proteases could serve as physiological activators.

Development and regulation of porcine pancreatic function

A surgical and experimental procedure was developed to enable the collection of pure and inactivated pancreatic juice during the growth of the pig. Studies have shown that, during the suckling period, both the basal and the secretory responses to suckling are low, if present at all. After weaning, basal levels of the total exocrine secretion, total protein, amylase, and trypsin, respectively, increase slightly, while the postprandial levels of total protein, amylase, trypsin, lipase, colipase, and carboxylester lipase, respectively, increase markedly. The pancreatic juice enzyme composition changes qualitatively and the antibacterial activity of the pancreatic juice also significantly increases. Piglet age appeared to be of minor importance, since weaning at either 4 or 6 wk of age gave the same results. Secretin and CCK administered together in supraphysiological doses only significantly affect exocrine function from 3-4 wk of age. However, CCK may also affect the exocrine pancreas indirectly via reflexes initiated intraduodenally. Milk consumption in the suckling pig leads to a postprandial increase in glucose levels but not insulin. Milk appears to be able to regulate the exocrine pancreas to produce only the amount and type of enzymes required for digestion. Thus, milk components or digestive products may affect pancreas function regulation. Studies show that enterostatin, the procolipase activation peptide, may inhibit pancreatic secretion mediated indirectly through the GI tract. Pancreastatin, an endocrine peptide, inhibits both insulin secretion and protein and trypsin secretion to pancreatic juice. In hypoinsulinemic (alloxan+streptozotocin diabetes) pigs (15-20 kg), no postprandial pancreatic juice response is seen, although CCK 33 + secretin can stimulate pancreatic secretion. Hypoinsulinemic pigs have a reduced capacity for glucose tissue utilization, suggesting that tissue metabolism and exocrine pancreas secretion are related.

Feedback inhibition of cholecystokinin secretion by bile acids and pancreatic proteases

Feedback inhibition of CCK release by bile acids and pancreatic proteases is well established in the rat. The question of whether these mechanisms are important in humans has not been completely resolved, but current evidence strongly suggests that feedback regulation of CCK release by bile acids is present in humans and is physiologically significant, whereas the existence and importance of feedback regulation of CCK release by pancreatic proteases in humans are still highly controversial.

Pancreatic enlargement is evident in rats fed diets containing raw soybeans (Glycine max) or cowpeas (Vigna unguiculata) for 800 days but not in those fed diets based on kidney beans (Phaseolus vulgaris) or lupinseed (Lupinus angustifolius)

Pancreatic weights and composition were studied with rats fed diets containing raw legume seeds for up to 800 d. Rapid pancreatic enlargement was induced by dietary soybeans (Glycine max) (high Kunitz and Bowman-Birk trypsin inhibitor contents, moderate lectin content) during the initial 150 d. Over the next 200 d the rate of pancreatic growth was similar to that in controls. After 350 d a second period of rapid pancreatic growth occurred. Macroscopic pancreatic nodules were evident in a number of rats fed soybeans for 500 d or more. A similar pattern of pancreatic growth was observed in rats fed dietary cowpeas (Vigna unguiculata) (high Bowman-Birk inhibitor content, low lectin content). Extensive pancreatic growth was also found in young rats fed moderate dietary levels of kidney beans (Phaseolus vulgaris) (low Bowman-Birk inhibitor content, high lectin content). However, the trophic effects diminished with time, and from 100 d onwards, little enlargement was evident. Consumption of a lupinseed (Lupinus angustifolius) diet (low trypsin inhibitor, low lectin content) did not cause pancreatic enlargement. The initial pancreatic growth induced by dietary soybeans seemed to be due to the lectins and trypsin inhibitors, whereas the second period of pancreatic growth was possibly due primarily to the trypsin inhibitors.

The effect of trypsin inhibitor on the pancreas and small intestine of mice

Pancreatic and intestinal growth rates were measured in mice fed on raw soya-bean flour (RSF) for up to 24 weeks. Control animals were fed on standard chow. The effects of RSF on the mouse pancreas resembled that seen in rats, showing hypertrophy with some hyperplasia. A marked increase in small intestinal weight was also found in mice fed on RSF but not in rats fed on this diet. Histological studies showed an increase in both villous and crypt thicknesses in the small intestine from these mice, and DNA, RNA and protein measurements indicated that the increase in intestinal weight was due to hypertrophy and hyperplasia of the mucosal layer. To determine whether the intestinal growth in mice fed on RSF was purely a response to the trypsin inhibitor (TI) component of the diet, pancreatic and intestinal growth rates were also determined in mice fed on the synthetic trypsin inhibitor camostate, at levels of 0.5 or 2 g/kg in rat chow, for periods of 1-8 weeks. Control animals were fed on standard chow. RSF and 0.5 g camostate/kg had similar trypsin inhibitor activities (measured against bovine trypsin), and both caused similar increases in pancreatic weight, DNA, RNA and protein content. However, 0.5 g camostate/kg did not affect small intestinal weight. Chow containing 2 g camostate/kg contained twice as much TI activity as the RSF diet but produced only a small increase in small intestinal weight at 2 and 8 weeks. This intestinal growth was significantly less than that seen with RSF. The present study shows that, in the mouse, RSF or a diet containing camostate in the appropriate dose produces pancreatic growth comparable to that seen in the rat. RSF also causes intestinal growth, but camostate-containing diets have little or no effect on the growth of the intestine.

A specific binding of the cholecystokinin-releasing peptide (monitor peptide) to isolated rat small-intestinal cells

A specific binding of the cholecystokinin (CCK)-releasing peptide (monitor peptide) to isolated rat jejunal mucosal cells was investigated. The 125I-labelled purified monitor peptide bound to the rat jejunal cells, and a large excess amount of the non-labelled monitor peptide inhibited the binding. The binding was completed within 60 min at 37 degrees C. The optimum pH for the binding was 8-9. A Scatchard plot of the specific binding was linear, and the dissociation constant was 50 nM. The density of the monitor-peptide-binding sites was high in duodenum but low in ileal and absent in colonic mucosa. A recombinant monitor peptide and four kinds of point mutants of it were prepared. The binding of the mutant monitor peptides to the cells indicated that only a trypsin inhibitor of the mutants could bind to the mucosal cells. Human pancreatic secretory trypsin inhibitor inhibited the specific binding, but other trypsin inhibitors, i.e. bovine basic pancreatic trypsin inhibitor, soybean trypsin inhibitor, egg-white trypsin inhibitor, leupeptin, antipain and FOY-305, did not affect the specific binding at all. These findings suggested that the specific binding site for the monitor peptide on the jejunal mucosal cells has a trypsin-like specificity, exhibiting a special specificity for the pancreatic-secretory-trypsin-inhibitor family. Autoradiography of an affinity-cross-linked complex of the 125I-labelled intact monitor peptide and the binding site suggested that its molecular mass was 33 kDa or 53 kDa in the presence or absence of 2-mercaptoethanol respectively.

Trypsin inhibitor and maternal reunion increase plasma cholecystokinin in neonatal rats

Intragastric soybean trypsin inhibitor increased plasma CCK bioactivity by 87% in nondeprived, 9-12-day-old rat pups. Reunion with the dam for 1 h after overnight maternal deprivation also increased plasma CCK significantly. These results demonstrate that CCK can be released from the small intestine of rats as early as postnatal day 9.

Cholecystokinin cells purified by fluorescence-activated cell sorting respond to monitor peptide with an increase in intracellular calcium

Cholecystokinin (CCK) is secreted from specific enteroendocrine cells of the upper small intestine upon ingestion of a meal. In addition to nutrients, endogenously produced factors appear to act within the gut lumen to stimulate CCK release. One such factor is a trypsin-sensitive CCK-releasing peptide found in pancreatic juice, known as monitor peptide. This peptide is active within the intestinal lumen and is hypothesized to stimulate CCK secretion by interacting directly with the CCK cell. We have found that monitor peptide releases CCK from isolated rat intestinal mucosal cells and that this effect is dependent upon extracellular calcium. In the present study, we used monitor peptide as a tool for isolating CCK cells from a population of small intestinal mucosal cells. Dispersed rat intestinal mucosal cells were loaded with the calcium-sensitive fluorochrome Indo-1, and CCK secretory cells were identified spectrofluorometrically by their change in fluorescence when stimulated with monitor peptide. Cells demonstrating a change in their emission fluorescence ratio were sorted using a fluorescence-activated cell sorter. More than 90% of the sorted cells stained positively for CCK with immunohistochemical staining. Furthermore, sorted cells secreted CCK when stimulated with membrane-depolarizing concentrations of potassium chloride, dibutyryl cAMP, calcium ionophore, and monitor peptide. These findings indicate that functional intestinal CCK cells can be highly enriched using fluorescence-activated cell sorting. Furthermore, monitor peptide appears to interact directly with CCK cells to signal CCK release through an increase in intracellular calcium.

Influence of dietary protein concentrations or of duodenal amino acid infusion on cholecystokinin release in goats

1. Whether dietary protein levels or duodenal infusion of amino acids alters the release of cholecystokinin (CCK) in blood plasma of goats was investigated in three experiments. The CCK determination was done by radioimmunoassay with specific CCK-8 antibody. The male adult Shiba goat, a miniature Japanese native goat, was used. 2. In Experiment 1, four goats were offered a diet containing 4.94 g crude protein (CP)/kg BW0.75 for the first 7 days. They were then given a diet containing 5.86 g CP/kg BW0.75 for 7 days and thereafter 6.79 g CP/kg BW0.75 for the following 7 days. On the last day of each experimental period, blood samples were taken from the jugular vein at zero (before feeding), 30, 60, 120, 240 or 360 min after the feeding of the diet. Plasma CCK levels were not affected by dietary protein levels and time after feeding. 3. Influence of phenylalanine or tryptophan (2 mmol/20 ml) infusion into the duodenum was investigated by a 3 x 3 latin square in Experiment 2. Plasma CCK level was determined at 15 min intervals for 1 hr. Both phenylalanine and tryptophan gradually enhanced plasma CCK concentrations with time after infusion. 4. Branched-chain amino acids such as leucine and isoleucine were supplemented intraduodenally in Experiment 3 as in Experiment 2. No significant change in plasma CCK levels was observed.

The importance of gastric secretion in the feedback control of interdigestive and postprandial pancreatic secretion in rats

Previous studies demonstrated that pancreatic enzyme secretion in rats is stimulated by the diversion of pancreatic juice from the duodenum or by the inhibition of pancreatic proteinases in the intestinal lumen but little attention has been paid to the role of gastric secretion in this stimulation. This study, carried out on conscious rats with large gastric (GF) and pancreatic fistulas, confirms that diversion of pancreatic juice in rats with the GF closed results in the progressive stimulation of pancreatic secretion reaching the maximum similar to that induced by exogenous CCK. When the GF was kept open, the diversion resulted in only small increment in pancreatic secretion and this was accompanied by progressive increase in gastric acid outputs. Similar amounts of HCl (25-400 mumol/h) instilled intraduodenally (i.d.) in rats with the GF open fully reproduced the increase in pancreatic secretion observed after the diversion of pancreatic juice and this effect was completely abolished by the pretreatment with L-364,718, a specific CCK receptor antagonist. Pretreatment with omeprazole to suppress completely gastric acid secretion in the diverted state resulted in a decline in pancreatic secretion similar to that observed after opening the GF. Camostate given in graded doses (6-200 mg/kg) either i.d. or s.c. in rats with pancreatic juice returned to the duodenum caused a dose-dependent increase in pancreatic secretion, but after opening the GF or after omeprazole this increase was reduced by about 50% while after L-364,718 it was abolished. This study provides evidence that gastric secretion plays an important role in the pancreatic response to diversion of pancreatic juice or inhibition of luminal proteinases (but not to feeding) and the elimination of gastric acid reduces this response.

Effect of a high-protein diet on the gene expression of a trypsin-sensitive, cholecystokinin-releasing peptide (monitor peptide) in the pancreas

The adaptation to a high protein diet of the concentration and mRNA level of a trypsin-sensitive, cholecystokinin-releasing peptide (monitor peptide), which was proposed to be the mediator of the cholecystokinin release in response to protein intake, was investigated in the rat pancreas. Adult rats were placed on one of two isocaloric diets. One group was fed a 22% casein diet (control diet) and the other a 64% casein diet (high-protein diet) for 14 days. In order to quantify the monitor peptide separately from pancreatic secretory trypsin inhibitor (PSTI-II), which is highly similar in its amino acid and mRNA nucleotide sequences to the monitor peptide but has less cholecystokinin-releasing activity, we used specific assay methods: HPLC was used for determining the monitor peptide concentration in zymogen granules and a synthetic oligonucleotide probe for determining the mRNA of the monitor peptide in the pancreas. The concentrations in the zymogen granules and the mRNA levels in the pancreas of the two peptides increased in parallel during the adaptation to the high protein diet, indicating that these two peptides were under the same control during the adaptation. The concentration and mRNA level of the monitor peptide, which were measured after 0, 3, and 14 days, increased throughout the experiment period, as did the concentration of trypsin. This suggested that the monitor peptide and trypsin may respond to similar signals during the adaptation to a high protein diet and that this apparent coordination may facilitate the adaptation of the pancreas to the diet.

Meal-stimulated exocrine pancreatic secretion and release of GI peptides in normal and nicotine-treated rats

In rats, treated chronically with saline and nicotine, we studied the postprandial release of gastrin and cholecystokinin by specific radioimmunoassays and simultaneously measured secretory outputs of the exocrine pancreas. Rats were prepared surgically with gastric and pancreatic fistulas. Meal-stimulated release of peptides and exocrine secretory outputs were measured 24 h postoperatively in conscious rats. Infusion of food via intragastric cannula significantly stimulated plasma gastrin levels in both control and nicotine treated rats. Postprandial gastrin levels in nicotine treated rats were significantly higher compared to gastrin levels obtained after food in untreated control rats. Plasma CCK levels were increased in both groups after food. These levels remained significantly elevated from the basal values only for a transient period following infusion of the liquid meal. There were no differences in postprandial plasma CCK levels between the two groups. Outputs of exocrine pancreatic volume, protein and trypsin increased significantly after food in both control and nicotine treated groups of rats. The differences in outputs of volume and protein between the two groups of rats were not significant; however, the trypsin outputs in the nicotine rats were decreased significantly when compared to control rats. The data indicate that in rats, administration of food stimulated the release of immunoreactive gastrin and CCK with concomitant increase in exocrine pancreatic secretions of volume, protein and trypsin. Chronic nicotine treatment and its effect on food, however, appeared to have induced hyperfunction of G-cells that resulted in increased gastrin secretion and a decrease in trypsin secretion by exocrine pancreas. These data may have important implications in the etiology of the development of exocrine pancreatic dysfunction in chronic smokers.

Residue trypsin inhibitor: data needs for risk assessment

Trypsin inhibitor (TI) occurs naturally in many foods from plants, notably soybean protein products. Heat treatment inactivates TI and improves nutritional quality, but residual TI activity of 5 to 20% remains after typical commercial treatments. Chronic feeding of TI or products that contain TI can inhibit trypsin and chymotrypsin, stimulate their secretion, cause hypertrophy and hyperplasia of the pancreas, and lead to adenomas and carcinomas of the exocrine pancreas. In the rat, TI promotes pancreatic carcinogenesis initiated by azaserine. Data needed for possible risk assessment on TI would include 2-year bioassays from animals treated with TI and fed diets carefully controlled for type and amount of fat (which also promotes pancreatic carcinogenesis). The effects of TI on protein nutrition would have to be considered when identifying the maximum tolerated dose. Major reductions in human dietary TI exposure may not be feasible because of the multiple sources of TI, the substantial promotion by other factors such as fat, and the adverse effects of excessive heat on food products. For risk assessment of TI in a particular food, other promotors and the feasibility of decreasing TI intake must be considered.

Interactions of pancreatic secretory trypsin inhibitor in small intestinal juice: its hydrolysis and protection by intraluminal factors

It has been proposed that modulation of cholecystokinin (CCK) release by proteinases, proteinase inhibitors and protein is mediated by a pancreatic secretory trypsin inhibitor (PSTI), also called monitor peptide, in the rat. When human [125I]-PSTI was incubated with fasting small bowel juice or activated pancreatic juice greater than 88% of tracer eluted from gel chromatography in the characteristic position of hydrolysed PSTI. However, when the small bowel juice had been pre-incubated with soybean trypsin inhibitor 3 g/l, casein 5 g/l or lactalbumin 30 g/l, the hydrolysis of PSTI diminished so that 95%, 32%, and 33% respectively, now eluted in the characteristic position of free (i.e. intact and not bound to an enzyme) PSTI. When [125I]-PSTI was incubated with pure trypsin, chymotrypsin, elastase or enterokinase greater than 95% of tracer eluted in the position of PSTI-enzyme complex. Incubation of PSTI with trypsin plus one other enzyme was required to produce hydrolysis. The degree of protection of PSTI from hydrolysis in duodenal juice produced by these substances correlates with their affects on CCK release. Our findings support the hypothesis that PSTIs mediate the modulation of CCK release by intraluminal proteinases, proteinase inhibitors and proteins.

Feedback regulation of pancreatic exocrine secretion in animal and man

This chapter focuses on studies dealing with the feedback mechanism of pancreatic exocrine secretion in animal and man. Clear evidence is presented that this feedback mechanism is working in the rat and the pig and that this feedback is mediated in the rat by the gastrointestinal hormones pancreozymin (enzyme secretion) and secretin (volume and bicarbonate secretion). Two novel peptides have been described--the 'CCK-releasing peptide' originating from the small intestinal mucosa, and the 'monitor peptide' cosecreted together with the enzymes in the pancreatic juice--to account for the stimulation of pancreatic enzyme secretion by the release of CCK. A similar feedback regulation of pancreatic secretion is working in man. It remains as yet controversial whether the feedback in humans is regulated via hormonal or neural pathways. It is also a matter of debate whether this feedback regulation of pancreatic enzyme secretion could be utilized for therapeutic aims in the treatment of pain in patients with chronic pancreatitis.

Effects of glutamine-enriched parenteral nutrition on the exocrine pancreas

Total parenteral nutrition (TPN) is associated with intestinal and pancreatic atrophy and pancreatic exocrine insufficiency. Recent investigations have demonstrated that the addition of glutamine to intravenous feedings attenuates TPN-associated intestinal atrophy. However, the effect of glutamine-supplemented intravenous feedings on the pancreas of intact animals is unknown. This study compared the effects of an intravenous infusion of a 2% glutamine-enriched diet (GLN) with an isonitrogenous, isocaloric diet without glutamine (CONT) on the composition and structure of the exocrine pancreas in laboratory rats with and without a 60% small bowel resection. In nonresected, TPN-fed animals, pancreatic weight was significantly increased in the GLN group when compared to CONT (645 +/- 33 g vs 554 +/- 20 g, p less than 0.05). Nonresected GLN animals also had increased pancreatic DNA (3.82 +/- 0.19 mg vs 2.91 +/- 0.49 mg, p less than 0.005) and protein contents (93.0 +/- 5.9 mg vs 76.6 +/- 7.0 mg, p = 0.08) compared to control. Similar significant increases in pancreatic weight, DNA, and protein were observed in intestinally resected animals fed the glutamine diet. When data from CONT and GLN animals were pooled and analyzed together, glutamine significantly increased total pancreatic trypsinogen and lipase contents (p less than 0.05). The increase in trypsinogen in resected GLN animals was significantly greater than in CONT animals (283 +/- 22 vs 139 +/- 23, p less than 0.005). Biochemical and morphometric observations demonstrated that the trophic effects of glutamine on the exocrine pancreas were manifest by acinar hyperplasia and not hypertrophy. Glutamine appears to be an important nutrient for pancreatic exocrine tissue during TPN.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of cholinergic control in feedback regulation of pancreatic secretion in the rat

The effect of atropine (100 micrograms/kg/h, i.v.) on plasma cholecystokinin and pancreatic secretion during diversion of bile and pancreatic juice from the intestine was studied in 8 conscious rats equipped with jugular vein, pancreatic, biliary, and duodenal cannulas, and with pyloric ligation and gastric drainage. Diversion of bile and pancreatic juice to the exterior for 4 h significantly increased pancreatic protein and fluid secretion. Atropine delayed the pancreatic response to diversion, but during 4 h of diversion, neither total nor incremental pancreatic protein or fluid secretion was inhibited by atropine. Plasma cholecystokinin levels were elevated after diverting bile and pancreatic juice and were not significantly reduced by atropine (23.0 +/- 6.6 pM vs. 16.0 +/- 3.9 pM at 1.5 h and 17.3 +/- 5.4 pM vs. 13.1 +/- 2.9 pM at 4 h after bile and pancreatic juice diversion; atropine-treated vs. controls, respectively). These results indicate that cholinergic nerves play no important role in feedback regulation of cholecystokinin release and that the previously reported suppressive effect of atropine on the pancreatic response to diversion of bile and pancreatic juice from the intestine was secondary to inhibition of gastric secretion.

Pancreatic response in rats and mice to trypsin inhibitors from soy and potato after short- and long-term dietary exposure

The effects on the pancreas of chronic (95 wk) dietary exposure to protease inhibitors from soy and potato were compared in rats and mice. Soy and potato trypsin inhibitor (TI) concentrates were prepared from defatted raw soy flour and potato juice, respectively, by selective precipitation and ultrafiltration. Animals were fed a diet in which casein supplied approximately 20% protein. Each concentrate (less than 1% of the diet) was added to provide 100 and 200 mg of trypsin inhibitor activity per 100 g of diet. In short-term (28 d) experiments in rats, both sources of TI decreased the apparent nutritional quality of casein and produced pancreatic hypertrophy consistent with a hormonally mediated feedback mechanism for pancreatic adaptation to diet that is interactive with the nutritional status of the animal. After long-term feeding (95 wk), soy and potato TI produced dose-related pancreatic pathology in rats consisting of nodular hyperplasia and acinar adenoma, which was typical of that associated with raw soy flour. Although mice responded similarly to rats to soy TI in short-term (28-d) feeding experiments, they were resistant to the formation of these lesions following long-term feeding. This considerable species variation in propensity to develop preneoplastic and neoplastic lesions of the pancreas is not predicted by the short-term hypertrophic and hyperplastic response of the pancreas to TI.