Role of gastric juice in feedback regulation of rat pancreatic secretion by luminal proteases

The role of gastric juice in the intestine on the pancreatic secretory response to intraduodenal infusion of trypsin inhibitors or to diversion of bile and pancreatic juice from the intestine was studied in conscious rats with pylorus ligation and gastric juice drainage. In absence of gastric juice in the intestine, diversion of bile and pancreatic juice from the intestine stimulated pancreatic secretion, but the incremental protein and fluid secretory responses to diversion of bile and pancreatic juice were increased approximately 2.9-fold and 2.5-fold, respectively, by intraduodenal infusion of HCl (60 microEq/h). Intraduodenal infusion of HCl (240 microEq/h) had no effect on the pancreatic secretory response to infusion of lima bean trypsin inhibitor (20 mg). These results support the hypothesis that the inhibitory effect of atropine on the pancreatic secretory response to diversion of pancreatic juice or bile and pancreatic juice is secondary to inhibition of gastric acid secretion. The lack of effect of HCl on the pancreatic response to trypsin inhibitor contradicts the hypothesis that acid in the intestine is important or necessary for the feedback response to loss of intraluminal protease activity. It is proposed that acid in the intestine augments the pancreatic response to diversion of pancreatic juice or bile and pancreatic juice by reducing intraluminal pH and thereby inactivating residual pancreatic proteases.     

Effect of atropine on feedback regulation of pancreatic enzyme secretion in rats

The effect of atropine on the feedback regulatory mechanism of pancreatic enzyme secretion exerted by intraluminal trypsin was investigated in conscious rats. Intravenous atropine infusion (50 micrograms/kg/h) suppressed pancreatic enzyme secretion to the same extent in both the presence and the absence of pancreatic juice in the intestine. However, with or without atropine infusion, pancreatic secretory rate was higher throughout diversion of pancreatic juice than during intraduodenal return of the juice. Atropine also inhibited the stimulatory response to intraduodenal trypsin inhibitor and intravenous caerulein. The atropine-induced inhibitory effect was not significantly different between the two experimental conditions. Regardless of atropine administration, both trypsin inhibitor and caerulein caused a significant increase in pancreatic secretion. The results suggest that cholinergic mechanisms have little influence on the feedback regulation. Cholinergic mechanisms may play an important role in maintaining the physiologically basal secretion because the basal secretion is atropine-sensitive.     

Relative importance of cholecystokinin and cholinergic nerve mechanisms in the regulation of pancreatic secretion in rats

Several substances modulate the function of pancreatic acinar cells in vitro through specific receptors. All of them may also act in vivo, but the relative importance of their effect under physiological conditions is highly variable. The aim of this study was to assess the respective participation of cholinergic nerves and cholecystokinin (CCK) in the interdigestive pancreatic exocrine secretion and in the pancreatic response to intragastric or intraduodenal nutrients. Unanesthetized rats, equipped with a semichronic (5 days) pancreatic fistula, were used. They received a test-meal in the stomach (42 kJ) or the duodenum (21 kJ), and were infused intravenously with an antagonist of muscarinic (atropine, pirenzepin), nicotinic (hexamethonium), or CCK (L364718, lorglumide, Boc-Tyr(SO3)-Nle-Gly-D-Trp-Nle-Asp-NH-CH2-CH2-C6H5) receptors. Basal interdigestive pancreatic exocrine secretion decreased significantly after atropine, pirenzepine or hexamethonium. Protein output decreased more (peak inhibition 60 to 80 percent according to the drugs, p less than 0.01 to p less than 0.001) than volume or bicarbonate output (peak inhibition 25 to 65 percent according to the drugs, p less than 0.05 to p less than 0.01). CCK antagonists did not change the interdigestive pancreatic secretion. The cumulated response to the intraduodenal meal did not change after hexamethonium or pirenzepin, but increased after atropine (by approximately 50 percent, p less than 0.001 for volume and bicarbonate output, and nearly 100 percent, p less than 0.05 for protein output). The CCK antagonists entirely suppressed the protein response to the intraduodenal meal, decreased the volume response by 70 percent (p less than 0.01), and the bicarbonate response by 50 percent (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Peptide-YY, a new partner in the negative feedback control of pancreatic secretion

Peptide YY (PYY), a newly discovered ileocolonic peptide, is released by nutrients in the proximal and distal intestine and inhibits pancreatic secretion. However, it is not clear whether PYY can be released in the absence of nutrients in the intestine or whether a physiological role exists for endogenous PYY in negative feedback regulation of pancreatic secretion by pancreatic proteases. In the present study we measured plasma PYY concentrations and determined the effects of anti-PYY serum during stimulation of pancreatic secretion by pancreatic juice diversion (PJD). The effect of SMS 201-995 (SMS; an analog of somatostatin), another inhibitor of pancreatic secretion, on regulation of PYY release induced by PJD was also investigated. Male Wistar rats equipped with pancreatic, biliary, duodenal, and jugular venous cannulas were studied 4-6 days postoperatively. After 90 min of basal collection, pancreatic juice was diverted for 4 h with or without infusion of SMS (2 micrograms/kg.h), given either iv or intraduodenally (ID). Plasma PYY concentrations were significantly increased from a basal level of 177 +/- 15 pg/ml to a peak level of 328 +/- 43 pg/ml 2 h after PJD. These increases in PYY concentration paralleled those in pancreatic protein and fluid outputs. Both iv and ID infusion of SMS during the first 2 h of PJD markedly decreased the plasma PYY concentration to 134 +/- 27 pg/ml and 156 +/- 19 pg/ml, respectively; the total incremental PYY release during 4 h of PJD was inhibited by 100% and 84% by iv and ID SMS, respectively. One milliliter of anti-PYY serum given iv significantly augmented the increment in protein and fluid output during PJD. These results suggest that endogenous PYY released by PJD may play a physiological role in negative feedback regulation of pancreatic secretion in rats.     

Kinesin is involved in regulation of rat pancreatic amylase secretion

BACKGROUND & AIMS: Kinesin has recently been localized to zymogen granules of pancreatic acini and is suggested to participate in exocytosis of exocrine pancreas. We examined the function of kinesin in regulated exocytosis of pancreatic acini in this study. METHODS: Kinesin function in exocytosis was examined by introducing hexahistidine-tagged recombinant kinesin protein and antikinesin monoclonal antibody into streptolysin-O-permeabilized acini. Intracellular localization of introduced recombinant kinesin was investigated by immunohistochemistry. Interaction between recombinant kinesin and the microtubule network was confirmed by nocodazole pretreatment of acini. Kinesin regulation by secretagogues was investigated by examining their effect on adenosine triphosphatase (ATPase) activity of endogenous kinesin. RESULTS: Recombinant kinesin enhanced calcium-stimulated amylase release from streptolysin-O-permeabilized acini. Introduced recombinant kinesin was localized to both the microtubule network and zymogen granule. Nocodazole pretreatment of acini abolished the enhancing effect of recombinant kinesin on calcium-stimulated amylase release. Antikinesin antibody inhibited amylase release stimulated by the combination of calcium and cyclic adenosine monophosphate (cAMP) but not that stimulated by calcium alone. Secretin and 8-bromo-cAMP increased ATPase activity of endogenous kinesin. CONCLUSIONS: Kinesin plays a stimulatory role in regulated exocytosis of pancreatic acini and is involved in stimulus-secretion coupling through a cAMP-dependent pathway.     

Role of cholecystokinin in the negative feedback control of pancreatic enzyme secretion in conscious rats

Using a specific radioimmunoassay for cholecystokinin (CCK) we have studied the relation between circulating CCK concentrations and the feedback regulation of pancreatic enzyme secretion in conscious rats. Recirculation of diverted bile-pancreatic juice into the duodenum or intraduodenal perfusion of trypsin during biliary-pancreatic juice diversion produced basal output of amylase and trypsin and low portal CCK levels (less than 10 pmol/L). Biliary-pancreatic juice diversion or inactivation of trypsin caused increased CCK concentrations (peak values 50-100 pmol/L) and enzyme outputs. During biliary-pancreatic juice diversion, infusion of the CCK receptor antagonist proglumide suppressed the enzyme response without altering the increase in CCK. Measurement of portal and peripheral CCK during biliary-pancreatic juice diversion yielded values of 131 +/- 37 and 32 +/- 5 pmol/L, respectively. The peripheral CCK levels corresponded to concentrations achieved during exogenous CCK-8 infusion which resulted in similar enzyme outputs. Gel chromatography of portal plasma during diversion of biliary-pancreatic juice revealed one peak of CCK corresponding to CCK-8, and a larger peak eluted between CCK-33 and CCK-8, probably representing CCK-22. Similar CCK components were found in water extracts of jejunal mucosa, whereas the acetic acid extracts mainly contained CCK-33/39. We conclude that the negative feedback regulation of pancreatic enzyme secretion in rats is mediated by the release of CCK from the intestine and that the major molecular form of CCK in plasma is probably CCK-22.     

Effects of secretin and caerulein on luminal feedback regulation of pancreatic enzyme secretion in rats

The stimulatory pancreatic response to exclusion of pancreatic proteases from the intestine was compared with the response to stepwise increasing doses of secretin and caerulein in conscious rats. Secretin stimulated pancreatic fluid secretion in a dose-related manner with or without intraduodenal return of pancreatic juice, while it could not significantly affect enzyme secretion. The dose response curve for enzyme secretion to caerulein was smooth during return of the juice. However, the already increased enzyme secretion by pancreatic juice diversion was only stimulated with the smallest dose of caerulein. The maximal dose of caerulein for enzyme secretion during return had been supramaximal dose during diversion. Intraduodenal trypsin inhibitor failed to stimulate enzyme secretion during diversion but induced the same stimulatory effect as the submaximal dose of caerulein during return. Different doses of intraduodenal trypsin caused an almost dose-related inhibition. It is concluded that a submaximal level of endogenous CCK might participate in the feedback regulation of pancreatic enzyme secretion in rats.     

Involvement of pancreatic polypeptide (PP) in luminal feedback regulation in the conscious rat

The possibility of the involvement of pancreatic polypeptide (PP) release in luminal feedback regulation in the conscious rat was examined. Pancreatic secretion in the intestinal phase in the rat is regulated by negative feedback control so that a decrease in luminal protease activities produced by a diversion of bile-pancreatic juice (BPJ) from the intestine stimulates pancreatic secretion. Plasma concentration of rat PP and the effect of exogenous infusion of rat PP on pancreatic secretions during BPJ diversion were determined. Plasma PP concentration significantly increased with BPJ diversion and peaked at 90 min after BPJ diversion began, almost paralleling changes in protein output. Exogenous PP infusion (1, 2, and 10 g/kg/hr) inhibited pancreatic protein and fluid outputs but not the bicarbonate output during BPJ diversion. PP was shown to be physiologically released in the intestinal phase of pancreatic secretion; however, the physiological role of endogenous PP remains unknown.This study was supported in part by the Uehara Memorial Foundation and a grant in aid from the Japanese Ministry of Education.     

Release of pancreatic polypeptide and its mechanism in luminal feedback regulation in the conscious rat

Exclusion of bile and pancreatic juice (BPJ) from the proximal intestine increases the release of pancreatic polypeptide (PP) from 4.4 to 14.3 pM and its increase was diminished by the intravenous infusion of atropine (100 micrograms/kg/h) in conscious rats. Neither intravenous bolus injection nor continuous infusion of cerulein did increase plasma PP concentration. It is suggested that the increase in plasma PP concentration produced by BPJ diversion is regulated by cholinergic mechanism, but not by cholecystokinin (CCK) released despite the known fact that BPJ diversion increases plasma CCK concentration.     

Interaction of the cholinergic system and cholecystokinin in the regulation of endogenous and exogenous stimulation of pancreatic secretion in humans.

Pancreatic enzyme secretion is regulated in humans by the cholinergic system and by cholecystokinin (CCK). The interaction between both regulatory systems in response to exogenous and endogenous stimulation was analyzed in the present study using the cholinergic antagonist atropine and the CCK antagonist loxiglumide. A dose-dependent stimulation of pancreatic enzyme output was achieved either by duodenal perfusion of graded caloric loads or by IV infusion of increasing doses of cerulein. Prestimulated pancreatic secretion was inhibited by atropine and loxiglumide. Atropine furthermore almost completely blocked meal-stimulated pancreatic secretion, whereas loxiglumide caused 60% inhibition. The enzyme response to graded doses of exogenous CCK was significantly inhibited by atropine and loxiglumide. Plasma levels of CCK were not altered by atropine but increased with infusion of loxiglumide. This study supports the concept that pancreatic enzyme secretion is predominantly dependent on a cholinergic tone and that CCK modulates the enzyme-secretory response.     

In vivo action of bombesin on exocrine pancreatic secretion in the rat: independent of cholecystokinin and cholinergic mediation

This study evaluates the effect of bombesin on pancreatic enzyme secretion in the rat and determines whether the stimulatory action of bombesin is mediated through the release of cholecystokinin (CCK) or via a cholinergic pathway. We performed in vivo experiments on conscious rats prepared with cannulae inserted in the pancreatic duct, in the external jugular vein, and in the duodenum. Intravenous infusion of bombesin stimulated pancreatic protein output in a dose-dependent fashion. Bombesin infused at 5 micrograms/kg/h stimulated pancreatic protein secretion from a basal of 12 +/- 5 to 42 +/- 10 mg/h. Infusion of proglumide (400 mg/kg/h) did not affect the stimulatory effect of bombesin on pancreatic protein secretion (38 +/- 5 mg/h). In contrast, infusion of proglumide abolished the pancreatic protein output elicited by intravenous infusion of CCK8 (500 ng/kg/h). This suggests that bombesin does not act through CCK to mediate exocrine pancreatic secretion. In separate studies we intravenously infused rats with atropine (100 micrograms/kg/h) prior to infusion with bombesin. Administration of atropine slightly decreased secretory volume but did not affect the action of bombesin. Combined administration of atropine and proglumide also did not affect pancreatic protein output stimulated by bombesin. Since infusion of neither proglumide nor atropine inhibited the stimulatory action of bombesin, the action of bombesin in the rat is probably direct and not through the release of CCK or via a cholinergic pathway.     

Interactions between cholinergic agonists and enteric factors in the regulation of insulin secretion from isolated perifused rat islets

The ability of the cholinergic agonist carbachol to sensitize islets to the action of combined glucose, cholecystokinin and gastric inhibitory polypeptide was determined in isolated rat islets. In response to this combination, peak first phase insulin secretion from control islets averages 85 +/- 5 pg.islet-1.min-1 (mean +/- SEM) and the insulin secretory rates measured 35-40 min after the onset of stimulation averages 127 +/- 34 pg.islet-1.min-1. A prior 20 min exposure to 1 mmol/l carbachol potentiates the modest insulin stimulatory response to this combination of stimulants: peak first phase release is 354 +/- 61 pg.islet-1.min-1, and release measured 35-40 min after the onset of stimulation is 179 +/- 34 pg.islet-1.min-1. This sensitizing effect of carbachol lasts for at least 40 min and can be duplicated by the natural in vivo agonist acetylcholine. These results demonstrate that cholinergic stimulation of isolated islets primes them to the subsequent stimulatory effect of a moderate increase in the circulating glucose level and to several postulated incretin factors. If operative in vivo, this communications network between cephalic and enteric factors represents a remarkable control system to ensure the release of insulin in amounts commensurate to meet the anticipated and actual insulin requirements for insulin-mediated fuel disposition.     

Feedback regulation of basal pancreatic secretion in humans

The effect of intrajejunal infusion of pancreatic juice on basal pancreatic secretion was studied in patients who had received pancreatoduodenectomy for pancreatic, biliary, or duodenal malignancy. Pure pancreatic juice was obtained through a drainage tube inserted into the main pancreatic duct. There was little fibrosis in the pancreatic remnant and daily pancreatic juice output was more than 200 ml. After intraluminal infusion of pancreatic juice, water, protein, bicarbonate, and enzyme outputs were decreased significantly by about 30%. Intraluminal trypsin also reduced pancreatic secretion. Trypsin inhibitor (aprotinin) suppressed the significant decrease caused by autopancreatic juice or trypsin solution. We conclude that basal pancreatic secretion in humans is under negative feedback control by intestinal pancreatic juice or tryptic activity.     

Specific end-product feedback regulation of pancreatic protein synthesis

The potential for negative feedback regulation of the synthesis of individual enzymes was tested in the rabbit pancreas. Tissues were preincubated with individual pancreatic enzymes under conditions which have been shown to allow significant tissue absorption of these enzymes. The rates of synthesis of total proteins and amylase were then measured by following incorporation of labeled amino acids. Preincubation of lobules with amylase in the bath caused amylase synthesis to be inhibited by 50% without significantly inhibiting total protein synthesis. Preincubation of lobules with chymotrypsinogen significantly inhibited total protein synthesis without significantly inhibiting amylase synthesis. These data demonstrate specific end-product feed-back inhibition of protein synthesis in eukaryotes and may represent a pattern for regulation of protein synthesis in general.