Stimulatory effect of an endogenous peptide in rat pancreatic juice on pancreatic enzyme secretion in the presence of atropine: evidence for different mode of action of stimulation from exogenous trypsin inhibitors

A new factor which activated the secretion of pancreatic enzymes was discovered and purified from rat bile-pancreatic juice. A fraction below M.W.10,000 of rat bile-pancreatic juice enhanced trypsinogen secretion by injection into anesthetized rat duodenum. The factor was purified from this fraction using its biological activity as an index by Sephadex G-50, SP Sephadex C-50 and HPLC. This factor was a peptide of which molecular weight was about 6,000 and had trypsin inhibitory activity. From these and some other findings, it was suggested that the peptide was identical with the "Kazal type" inhibitor. In the anesthetized and atropine-treated rat, of which intestinal trypsin was removed by thoroughly washing with saline containing 5 microM soybean trypsin inhibitor (SBTI), pancreatic secretion became basal state, and was not stimulated by injection of SBTI into its duodenum any longer. Under this condition, however, injection of this purified peptide brought about markedly stimulation of pancreatic enzyme secretion. These results suggest that this peptide has a certain function which enhances pancreatic enzyme secretion by the different manner from exogenous trypsin inhibitors such as SBTI.     

Influence of neural blockages and proglumide on rat pancreatic enzyme secretion in response to intraluminal fatty acid

Effects of neural blockages on pancreatic enzyme secretion in response to infusing fatty acid into the lumen were investigated using anesthetized rats, equipped with bile-pancreatic and duodenal cannulae, to evaluate the relative contribution of the neural and the hormonal mediations in the pancreatic response. Oleate (0.2 ml) was injected as a bolus into the rat duodenum, and the trypsin output in bile-pancreatic juice was monitored to determine the pancreatic enzyme secretion response with continuous return of bile-pancreatic juice to the intestine. When anticholinergic agents such as atropine sulfate and scopolamine were administrated, although basal level in pancreatic enzyme secretion fell, the increase of pancreatic enzyme secretion from basal level after stimulation by oleate was not significantly different from that of the control (no blockage). However, the ganglion blocker, hexamethonium, inhibited the pancreatic enzyme secretion in response to oleate by 94%. An adrenergic blocker, guanethidine, also led to as much of a decrease as the ganglion blocker-induced decrease. Adrenergic alpha- and beta-blockers partially, but not completely, inhibited the enzyme secretion. Adrenergic blockage also suppressed the basal level in pancreatic enzyme secretion. On the other hand, a specific CCK antagonist, proglumide, significantly inhibited pancreatic enzyme secretion induced by oleate in the presence of scopolamine. These observations suggest that pancreatic enzyme secretion in response to oleate is primarily mediated by CCK and that adrenergic modulation may play an important role in the CCK-mediated pancreatic enzyme secretion in response to oleate, although interpretation of these results may have some restriction related to anesthesia.     

Factors involved in the intestinal feedback regulation of pancreatic enzyme secretion in the rat.

Further studies on the feedback regulation of pancreatic enzyme secretion by trypsin were conducted in conscious rats, surgically prepared so that pancreatic juice could be collected or returned. Suppression of enzyme secretion by trypsin as well as its stimulation by SBTI occurred only in the upper part of the small intestine, where the hormone CCK is known to be released. Over a limited range, trypsin suppression of pancreatic secretion was proportional to the dose of trypsin. Higher concentrations had no further effect, suggesting "saturation" of the intestine. Trypsin which had its active center blocked by DFP did not suppress enzyme output. These results supported the concept that only trypsin (or chymotrypsin) with an exposed active center suppressed pancreatic enzyme secretion in the rat by somehow suppressing the release of CCK from the intestinal cell. Presumably CCK is released from the intestine following "removal" of trypsin from the intestine either by diverting the juice or by feeding SBTI which binds the enzyme. All of the evidence supported the view that the effect of trypsin or SBTI on pancreatic secretion was mediated at the intestinal level and not in the blood as has been suggested.     

Effects of some gastrointestinal peptides on pancreatic growth in rats (preliminary results)

The purpose of this study was to estimate the effects of cholecystokinin (CCK), somatostatin (SS) pancreatic polypeptide (PP) and their interaction with each other, given them in single doses, on pancreatic secretion and pancreatic growth after long-term treatment in rats. The acute secretory effects of the above mentioned peptides were studied on conscious rats supplied with pancreatic, gastric and jugular vein cannulae. The pancreatic growth was characterized by measurements of pancreatic weight, desoxyribonucleic acid (DNA), protein, trypsin and amylase content after 5 days treatment. Amylase output was increased by caerulein alone, and given it in combination with somatostatin (SS), while its value decreased by SS alone. After 5 days treatment, the pancreatic weight, trypsin and amylase activity (hypertrophy) was increased by caerulein, and these values were not altered by S alone. In combinative administration of caerulein with somatostatin, the stimulatory effect by caerulein was decreased. PP given alone or in combination with caerulein decreased both the basal and stimulated amylase output. PP given for 5 days decreased pancreatic trypsin and amylase contents and counteracted the stimulatory effect by caerulein to these enzymes' contents. It has been concluded that: 1. caerulein stimulates both pancreatic enzyme secretion and pancreatic growth; 2. somatostatin inhibits the pancreatic secretion and caerulein induced pancreatic growth, but it does not affect the spontaneous growth of pancreas; 3. pancreatic polypeptide inhibits the pancreatic secretion and decreases pancreatic trypsin and amylase contents.     

The ghrelin pentapeptide inhibits the secretion of pancreatic juice in rats.

Ghrelin, a 28 amino acids polypeptide was recognized as an endogenous ligand for the growth hormone secretagogue receptor. It turned out that the entire sequence of ghrelin is not necessary for performing the above-mentioned functions. It was suggested that 5 residues (Gly-Ser-Ser(n-octanoyl)-Phe, pentaghrelin) constituted functionally active part of the full-length polypeptide. Ghrelin-28 was found to inhibit pancreatic enzyme output in rats, though the effect of pentaghrelin was not studied so far. The study aimed to determine the involvement of pentaghrelin in pancreatic juice secretion in anaesthetized rats. Male Wistar rats (220 +/- 20 g body weight, b. wt.) were anesthetized, the external jugular vein and common biliary-pancreatic duct were cannulated. Pentaghrelin boluses (i.v., 1.2, 12, and 50 nmol kg(-1) b. wt.) were injected every 30 min with or without CCK-8 infusion, duodenal mucosal CCK(1) receptor blockade with tarazepide, vagotomy and capsaicin pretreatment. Pentaghrelin boluses reduced the volume of pancreatic-biliary juice, protein and trypsin outputs both under basal and CCK-8-stimulated conditions in a dose-dependent manner. However, exogenous pentaghrelin failed to affect the pancreatic secretion in rats subjected to vagotomy, capsaicin deactivation of afferents or pretreatment with Tarazepide. In conclusion, pentaghrelin may control exocrine pancreas secretion by affecting duodenal neurohormonal mechanism(s) involving CCK and vagal nerves in rats.     

Stimulative effect of a casein hydrolysate on exocrine pancreatic secretion that is independent of luminal trypsin inhibitory activity in rats.

We have previously demonstrated that proteins could stimulate pancreatic secretion independently of luminal bile-pancreatic juice (BPJ) in a BPJ-diverted rat. To determine whether luminal protease-independent pancreatic secretion occurs in normal rats with BPJ returned to the upper small intestine, we investigated the pancreatic secretory response to intraduodenal instillation of a casein hydrolysate or the synthetic trypsin inhibitor, FOY 305, at concentrations which could almost equally inhibit hydrolysis of the synthetic substrate for trypsin with the luminal content. FOY 305 at 10 micrograms/ml and casein hydrolysate solutions at both 100 and 200 mg/ml similarly inhibited approx. 80% of the tryptic activity in the luminal contents of the proximal small intestine. Intraduodenal administration of casein hydrolysate solutions (100 and 200 mg/ml) significantly increased pancreatic secretion in a dose-dependent manner. However, intraduodenal administration of FOY 305 (10 micrograms/ml) was ineffective for stimulating pancreatic secretion. These results demonstrate that dietary protein enhances pancreatic secretion independently of the masking of luminal trypsin activity in rats.     

Stimulative Effect of a Casein Hydrolysate on Exocrine Pancreatic Secretion That is Independent of Luminal Trypsin Inhibitory Activity in Rats

We have previously demonstrated that proteins could stimulate pancreatic secretion independently of luminal bile-pancreatic juice (BPJ) in a BPJ-diverted rat. To determine whether luminal protease-independent pancreatic secretion occurs in normal rats with BPJ returned to the upper small intestine, we investigated the pancreatic secretory response to intraduodenal instillation of a casein hydrolysate or the synthetic trypsin inhibitor, FOY 305, at concentrations which could almost equally inhibit hydrolysis of the synthetic substrate for trypsin with the luminal content. FOY 305 at 10 μg/ml and casein hydrolysate solutions at both 100 and 200 mg/ml similarly inhibited approx. 80% of the tryptic activity in the luminal contents of the proximal small intestine. Intraduodenal administration of casein hydrolysate solutions (100 and 200 mg/ml) significantly increased pancreatic secretion in a dose-dependent manner. However, intraduodenal administration of FOY 305 (10 μg/ml) was ineffective for stimulating pancreatic secretion. These results demonstrate that dietary protein enhances pancreatic secretion independently of the masking of luminal trypsin activity in rats.     

A secretin releasing peptide exists in dog pancreatic juice

Canine pancreatic juice has been shown to stimulate exocrine pancreatic secretion in the dog. In the present study we investigated whether there is a secretin-releasing peptide in canine pancreatic juice. Pancreatic juice was collected from the dogs with Thomas gastric and duodenal cannulas while pancreatic secretion was stimulated by intravenous administration of secretin at 0.5 microg/kg/h and CCK-8 at 0.2 microg/kg/h, respectively. The pancreatic juice was separated into three different molecular weight (MW) fractions (Fr) by ultrafiltration (Fr 1; MW > 10,000, Fr 2; MW=10,000-4,000 and Fr 3; MW < 4,000), respectively. All the fractions were bioassayed in anesthetized rats. Fraction 3 dose-dependently and significantly stimulated pancreatic juice flow volume from 78.0% to 99.4% (p<0.05) and bicarbonate output from 128.9% to 202.1% (p<0.01), respectively. Plasma secretin concentration also increased from 1.2 +/- 0.5 pM to 5.0 +/- 0.8 pM and 6.0 +/- 1.0 pM (p<0.05). None of these fractions increased pancreatic protein secretion or plasma CCK level. The stimulatory effect of Fraction 3 on pancreatic secretion and the release of secretin was completely abolished by treatment with trypsin (1 mg/ml for 60 min at 37 degrees C) but not by heating (100 degrees C, 10 min). Intravenous injection of a rabbit anti-secretin serum, which rendered plasma secretin almost undetectable in rat plasma, also abolished Fr 3-stimulated pancreatic secretion of fluid and bicarbonate secretion. These observations suggest that a secretin-releasing peptide exists in the canine pancreatic juice. It is trypsin-sensitive and heat-resistant. This peptide may play a significant physiological role on the release of secretin and regulation of exocrine pancreatic secretion.     

Diverse effects of phytohaemagglutinin on gastrointestinal secretions in rats.

Kidney bean lectin phytohaemagglutinin (PHA) is known for its binding capacity to the small intestinal surface inducing marked hyperplasia and hypertrophy and an increased pancreatic function. Recent observations indicate that PHA is able to attach to gastric mucosal and parietal cells. Therefore, we compared the effects of PHA on gastric acid secretion, and pancreatic amylase secretion in rats. To study gastric secretion in conscious animals, rats were surgically prepared with chronic stainless steel gastric cannula and with indwelling polyethylene jugular vein catheter. Acid secretion was determined by titration of the collected gastric juice to pH 7.0. Similar studies were performed to investigate the effect of PHA on pancreatic enzyme secretion in conscious rats supplied with pancreatic cannula. Pancreatic enzyme secretion was also studied in rats anesthetized with either halothane or urethane. In conscious rats PHA significantly inhibited basal acid secretion when compared to vehicle-treated controls. The effect was dose-dependent and reversible. On the other hand, given in the same doses as in the acid-secretory studies, PHA stimulated pancreatic amylase secretion in rats prepared with chronic pancreatic cannula. This effect was blocked by devazepide, a CCK-A receptor antagonist. In halothane-anesthetized rats PHA administration increased pancreatic amylase secretion, too. During urethane anesthesia, however, the stimulatory effect of PHA was not observed. These results provide evidence that intragastric PHA treatment induces opposite effects on gastric acid secretion and pancreatic enzyme secretion: it is a potent inhibitor of acid output, and a stimulator of pancreatic enzyme discharge. Our data also show that the stimulatory effect of PHA on pancreatic enzyme secretion can be blocked by urethane, an anaesthetic that is known to turn off the negative pancreatic feedback control of pancreatic function in rats.     

Exogenous leptin inhibits the secretion of pancreatic juice via a duodenal CCK1-vagal-dependent mechanism in anaesthetized rats

Leptin originally described as product of the ob gene has been shown to be expressed in various tissues including the gastrointestinal tract. In this study, we investigated the influence of leptin on the secretion of pancreatic juice in biliary-pancreatic duct cannulated anaesthetised rats and in dispersed rat pancreatic acini in vitro. Exogenous leptin was given in boluses intravenously with or without CCK-8 (12 pmol kg(-1) body weight) in the presence or absence pharmacological CCK(1) receptor blockade, cervical vagotomy, and capsaicin pre-treatment. Administration of leptin (0.1, 1 and 10 microg kg(-1) body weight) did not affect the volume of bile and pancreatic juice while the protein and trypsin outputs were reduced in a dose-dependent manner. In the rats, leptin inhibited CCK-8 stimulated protein and trypsin outputs stronger than the basal pancreatic secretion. The inhibition by leptin was abolished by the pharmacological CCK(1) receptor blockade, cervical vagotomy, and capsaicin pre-treatment. In contrast, leptin did not affect basal and CCK-8-stimulated amylase release from the dispersed rat pancreatic acini in vitro. In conclusion, the results of the present study suggest that leptin does not act directly on the rat pancreatic acinar cells but inhibits the secretion of pancreatic enzymes acting indirectly via a neurohormonal CCK-vagal-dependent mechanism.     

Stimulation of rat pancreatic exocrine secretion by urocortin and corticotropin releasing factor

Neural and hormonal mechanisms control pancreatic secretion. The effects of the corticotropin releasing factor (CRF) related neuropeptide urocortin (UCN) on pancreatic exocrine secretion were examined. In anesthetized male rats, pancreatic secretion volume and total protein were assayed. UCN increased pancreatic secretory volume and protein secretion and potentiated cholecytokinin-stimulated protein secretion. Astressin, a non-specific CRF receptor antagonist, inhibited UCN-stimulated protein output while CRF(2) receptor antagonist, antisauvagine-30, was without effect. Atropine, but not subdiaphragmatic vagotomy, inhibited UCN-mediated secretion. In acinar cells, UCN did not stimulate release of amylase nor intracellular cAMP. UCN is a pancreatic exocrine secreatagogue with effects mediated through cholinergic intrapancreatic neurons.     

Food and secretagogue stimulation decrease the digestive enzyme content remaining in the rat pancreas

The aim of the study reported here was to investigate changes in the digestive enzyme content in the pancreas after food and secretagogue stimulation. Rats from which food had been withheld overnight were either fed (between 6 and 8 a.m.) or not before euthanasia and pancreatic excision (at 8 a.m.: 21 not fed and 21 fed) and at 4 (12 p.m.: six not fed and six fed) and 8 h later (4 p.m.: six not fed and six fed). Another 16 rats were anesthetized, fitted with jugular vein and pancreatic duct catheters, and infused with the secretagogues, CCK-33 and secretin, during 1.5 h of pancreatic juice collection before euthanasia and pancreatic excision. The pancreata were homogenized, and total soluble protein and individual enzyme (trypsin and amylase) tissue contents were analyzed. Results indicated lower amounts of protein and enzymes remaining in the pancreata of the fed, compared with non-fed rats. Enzyme values indicated recovery within four hours in fed rats, but non-fed rats also had increased values during daytime. High enzyme secretion during the high dose of hormonal stimulation was reflected in lower enzyme values remaining in the pancreas, compared with that in response to low-dose stimulation. Results indicated that stimulation of the pancreas, either by food ingestion or exogenous secretagogues, lowers the amounts of digestive enzymes remaining in the pancreas, and imply that stimulation and circadian rhythms influence the pancreatic enzyme content at euthanasia. This finding should be borne in mind in interpretation of data from pancreatic studies.     

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.     

全身照射后大鼠胰腺分泌及其酶活性的变化

本文观察了电离辐射后大鼠胰淀粉酶和胰蛋白酶活性变化,并在离体胰腺腺泡水平探讨了各种促分泌物对腺泡分泌机能的影响。实验结果表明,电离辐射可引起大鼠胰淀粉酶和胰蛋白酶活性明显降低,照射后胰酶活性变化与照射剂量有关。在一定范围内,随照射剂量增加胰酶活性降低程度亦增加,利用离体胰腺腺泡制备方法,进一步观察电离辐射后胰腺腺泡对Carbachol、CCK-OP和VIP刺激的淀粉酶分泌反应亦明显降低,这提示,照射后大鼠胰腺腺泡分泌机能发生了改变。综上所述,电离辐射后大鼠胰外分泌酶活性降低及胰酶分泌减少可能是胰腺外分泌功能障碍的原因之一。     

Pancreatic somatostatin-like immunoreactivity suppresses gastric acid secretion in rats.

Somatostatin-like immunoreactivity (SLI) was extracted from the canine pancreas and purified by ion exchange, affinity chromatography and gel filtration. The 1600 dalton fraction, which is physicochemically similar to synthetic somatostatin was infused into the peripheral circulation of anesthetized rats and its effect upon gastric acid secretion was compared with that of synthetic somatostatin. Both synthetic somatostatin and pancreatic SLI in a dose of 7–8 μg/kg/h suppressed pentagastrin-stimulated gastric acid secretion. It is concluded that the highly purified 1600 dalton fraction of canine pancreatic SLI, like synthetic somatostatin, can exert biological activity upon the stomach of rats.     

Intestinal fat suppresses protein-induced exocrine pancreatic secretion in chronically bile-pancreatic juice-diverted rats

Previously, we showed that the increase in pancreatic enzyme secretion was lower after feeding a casein diet containing fat than that after feeding a fat-free casein diet in chronically bile-pancreatic juice (BPJ)-diverted rats. In the present study, we determined whether the suppressive effects of fats on flow volume of BPJ and pancreatic enzyme secretion depend on delaying gastric emptying and examined the characteristics of the suppression with intraduodenal instillation of soybean oil or lecithin in BPJ-diverted rats. The study was conducted as three separate experiments using conscious rats with chronic BPJ diversion by means of a common bile-pancreatic duct catheter. The flow volume of BPJ and the secretion of pancreatic amylase and trypsin were determined after intraduodenal instillation of the test solution. Exocrine pancreatic secretion was strongly stimulated by administration of guanidinated casein hydrolysate (HGC, 150 mg/ml) in chronic BPJ-diverted rats. However, pancreatic secretion after administration of an emulsion containing HGC with either soybean oil (100 mg/ml) or mixed fat (50 mg/ml soybean oil + 50 mg/ml lecithin) was much lower than that after administration of HGC alone. In contrast, administration of the soybean oil emulsion without HGC resulted in a small, but significant increase in the volume of BPJ. The suppressive effects of soybean oil (100 mg/ml) on the increases in the BPJ flow and enzyme secretion were similar to those of sodium taurocholate (10 mg/ml), and there was no additive effect of soybean oil on taurocholate suppression. In conclusion, duodenally instilled soybean oil suppressed increases in flow volume of BPJ and pancreatic enzyme secretion induced by HGC in chronic BPJ-diverted rats, showing that the suppressive effect of the fat does not depend on delaying gastric emptying.     

Appearance of New Protein Species on the Cell Surface during Sexual Differentiation in the Heterobasidiomycetous Yeast,Tremella mesenterica

The effects of neural blockers on the pancreatic enzyme secretion in response to an intraluminal infusion of soybean trypsin inhibitor and HCl were investigated. The stimulation of pancreatic enzyme secretion upon the intraluminal infusion of soybean trypsin inhibitor was not blocked by atropine, but was completely blocked by guanethidine. The intraluminal infusion of 0.08 n HCl, which is known as a potent secretagogue of secretin, caused a rapid augmentation of trypsin output, which was not blocked by atropine or guanethidine. Preinjection of CR-1392 (1.5 mg/kg, i.p.), which is a strong cholecystokinin receptor antagonist, completely blocked the pancreatic response to soybean trypsin inhibitor, but not that to 0.08 n HCl. This inferred that guanethidine specifically suppressed the CCK-release from the small intestine.

These findings suggest that the pancreatic enzyme secretion in response to soybean trypsin inhibitor is mainly mediated by CCK, and that adrenergic modulation would be involved in the CCK-mediated pancreatic enzyme secretion in response to soybean trypsin inhibitor.     

Differential mechanism and site of action of CCK on the pancreatic secretion and growth in rats

Recent studies demonstrated that cholecystokinin (CCK) at physiological levels stimulates pancreatic enzyme secretion via a capsaicin-sensitive afferent vagal pathway. This study examined whether chemical ablation of afferent vagal fibers influences pancreatic growth and secretion in rats. Bilateral subdiaphragmatic vagal trunks were exposed, and capsaicin solution was applied. Pancreatic wet weight and pancreatic secretion and growth in response to endogenous and exogenous CCK were examined 7 days after capsaicin treatment. Perivagal application of capsaicin increased plasma CCK levels and significantly increased pancreatic wet weight compared with those in the control rats. Oral administration of CCK-1 receptor antagonist loxiglumide prevented the increase in pancreatic wet weight after capsaicin treatment. In addition, continuous intraduodenal infusion of trypsin prevented the increase in plasma CCK levels and pancreatic wet weight after capsaicin treatment. There were no significant differences in the expression levels of CCK-1 receptor mRNA and protein in the pancreas in capsaicin-treated and control rats. Intraduodenal administration of camostat or intravenous infusion of CCK-8 stimulated pancreatic secretion in control rats but not in capsaicin-treated rats. In contrast, repeated oral administrations of camostat or intraperitoneal injections of CCK-8 significantly increased pancreatic wet weight in both capsaicin-treated and control rats. Present results suggest that perivagal application of capsaicin stimulates pancreatic growth via an increase in endogenous CCK and that exogenous and endogenous CCK stimulate pancreatic growth not via vagal afferent fibers but directly in rats.     

Two hypotheses on the feedback regulation of pancreatic enzyme secretion

We review the mechanisms underlying the feedback regulation of pancreatic enzyme secretion in response to a meal. Pancreatic enzyme secretion in the rat and pig is known to be regulated by a negative feedback mechanism mediated by intestinal trypsin and chymotrypsin. Such a mechanism has recently been noted in humans. The presence of these enzymes in the small intestine suppresses pancreatic enzyme secretion, whereas their removal increases it. Two novel peptides have been proposed to account for the stimulation of pancreatic enzyme secretion in response to feeding trypsin inhibitor. One was assumed to be present in rat pancreatic juice and the other to be spontaneously secreted from the rat small intestine. In either case, trypsin and trypsin inhibitors do not directly interact with the luminal surface of the small intestine, but their actions are mediated by a trypsin-sensitive, cholecystokinin-releasing peptide. This is a novel explanation of the well-recognized stimulation of pancreatic enzyme secretion in response to dietary protein intake.     

Localization of pancreatic enzyme secretion-stimulating activity and trypsin inhibitory activity in zymogen granule of the rat pancreas

The intracellular localization of pancreatic enzyme secretion-stimulating activity in rat pancreas was investigated. We found and purified a pancreatic enzyme secretion-stimulating peptide from rat bile/pancreatic juice. The peptide is trypsin-sensitive (showing temporary trypsin inhibitory activity), and it is hypothesized that it acts as a trypsin-sensitive mediator in the feedback regulation of diet-induced pancreatic enzyme secretion. The zymogen granule fraction was purified 5-fold by ultracentrifugation by the Percoll density gradient method. The purity of the zymogen granule fraction was determined from the specific amylase activity and electron microscopic morphology. The specific enzyme activities of amylase and trypsin and the trypsin inhibitory activity increased in parallel during the purification, and the pancreatic enzyme secretion-stimulating activity was also localized in the zymogen granule fraction. These results suggest that the pancreatic enzyme secretion-stimulating peptide originates from the acinar cells, and that it is secreted through exocytosis of zymogen granules into the small intestine, its ratio to trypsin thus remaining constant. This idea supports our hypothesis that the stimulating peptide acts as a mediator for the feedback regulation of pancreatic enzyme secretion by trypsin.