Regulation of pancreatic enzyme secretion in conscious rats by intraluminal somatostatin: mechanism of action

These studies were undertaken to characterize 1) the immunoreactive somatostatin (SS) forms found in the gastrointestinal lumen and 2) to assess the possible role of luminal SS on pancreatic exocrine function. Results indicate that different forms of SS are secreted into the rat gastric and duodenal lumen in proportions corresponding to their gastric and duodenal contents. Inhibition of diversion-stimulated pancreatic exocrine secretion by intraduodenal infusion of SS (SS-ID) is dose dependent with a maximal effective dose of 24 micrograms/kg-1h-1 for volume and 48 micrograms/kg-1h-1 for protein output. Infusion of supramaximal doses result in a loss of the inhibitory effect of SS-ID on both volume and protein outputs. Comparison between inhibition of pancreatic secretion by iv SS (SS-IV) and SS-ID indicates that SS-ID is about 20 times less potent than SS-IV in its inhibition of stimulated pancreatic exocrine secretion. Intraileal (SS-IL) infusion of SS at 48 micrograms/kg-1h-1 did not inhibit stimulated pancreatic secretion but was rather stimulatory possibly through the inhibition of putative ileal inhibitors. Likewise, passive immunization against circulating SS did not affect the inhibitory effects of SS-ID on pancreatic secretion. These data indicate that SS is secreted into the rat gastrointestinal lumen and that its infusion into the duodenal lumen inhibits stimulated pancreatic secretion in a dose-dependent fashion. The observation that SS-IL did not inhibit pancreatic secretion and that passive immunization against circulating SS did not affect the inhibitory effect of SS-ID suggest that the action of SS-ID on stimulated exocrine pancreatic secretion is probably indirect and may involve a duodenal signal, possibly an inhibition of endogenous release of cholecystokinin and/or secretin.     

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.     

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.     

Effect of atropine on pancreatic secretion in conscious rats

The effect of atropine on exocrine pancreatic secretion was investigated in conscious rats. Intravenous atropine infusion decreased nonstimulated protein secretion during recirculation of pancreatic juice into the duodenum D50 = 15-20 micrograms/kg/h. The maximum inhibition from protein secretion (-89%) was obtained with 600 micrograms/kg/h. With larger doses, the inhibition was less. The response to secretin and cholecystokinin-pancreozymin was not significantly modified by atropine. When pancreatic juice was diverted during the course of an intravenous atropine infusion, the first 1-hour peak of protein output was significantly decreased, but the following 2-hour period was increased, the sum of these 2 periods being similar in both conditions. The response to soybean trypsin inhibitor during recirculation was decreased as well as the first peak after diversion. During atropine infusion fluid secretion decreased more powerfully after 1 h diversion and after soybean trypsin inhibitor than during recirculation of pancreatic juice. It is suggested that during recirculation of pancreatic juice nonstimulated protein secretion is mostly (89%), and water secretion is partially controlled by cholinergic mechanisms. After soybean trypsin inhibitor stimulus and during the early phase following juice diversion protein secretion seems to be partly under the control of cholinergic mechanisms. However, during the latter phase following diversion, it is not so. Parasympathetic stimulation appears also to play a significant, although less important, role in fluid secretion.     

Role of cholecystokinin in bombesin-stimulated pancreatic enzyme secretion in conscious rats

Since bombesin is a potent stimulus of cholecystokinin (CCK) secretion, it is assumed that the stimulatory effect of bombesin on pancreatic protein secretion is mediated by CCK. This study was undertaken to determine in the conscious rat with a cannulated pancreatic duct the role of CCK in the stimulation of pancreatic protein secretion by bombesin. Infusion of 18 pmol/kg/30 min of bombesin into rats stimulated pancreatic protein secretion from 6.7 +/- 1.1 to 9.9 +/- 0.4 mg/30 min (p less than 0.05). This stimulation of pancreatic protein secretion was accompanied by a significant increase in plasma CCK, measured by a specific and sensitive radioimmunoassay, from 3.2 +/- 0.2 to 4.7 +/- 0.2 pM (p less than 0.01). When a similar plasma CCK increment as during infusion of bombesin (1.5 +/- 0.2 pM) was achieved by infusion of 6 pmol/kg/30 min of exogenous CCK (1.6 +/- 0.3 pM), pancreatic protein secretion increased only from 6.9 +/- 0.7 to 7.6 +/- 0.7 mg/30 min (p less than 0.05). To achieve a pancreatic protein secretion similar to that during bombesin, large doses of exogenous CCK (24 pmol/kg/30 min) were needed, resulting in considerably higher plasma CCK concentrations of 10.9 +/- 0.7 pM. It is concluded that CCK is unlikely to play a significant role in the stimulation of pancreatic protein secretion by bombesin in the rat.     

Influence of bile flow obstruction vs bile diversion on pancreatic secretion in the conscious rat

Changes in pancreatic exocrine functions were compared between conscious rats with bile duct ligation and bile diversion from the duodenum on the first, third, fifth, and seventh postoperative days. Body weight was significantly decreased with time in both groups. Basal secretions of fluid, bicarbonate, and protein remained unchanged throughout the experimental period in bile duct ligated rats, whereas in bile diverted rats, the basal bicarbonate secretion with returning of pancreatic juice to the duodenum increased on the third postoperative day, and the basal protein output significantly increased with time. Basal secretions with returning of pancreatic juice to the duodenum in both groups were higher than that in control (bile and pancreatic juice returned to the intestine) rats. Stepwise increases in fluid and bicarbonate outputs responding to the graded doses of secretin were observed in bile duct ligated rats on the first and third postoperative days, as has been observed in bile diverted rats. However, on the fifth and seventh postoperative days, stepwise responses to graded doses of secretin were no longer observed in bile duct ligated rats. The pancreatic response to cerulein was greater in bile diverted rats than in bile duct ligated rats. Plasma CCK concentration in 7-d bile duct ligated rats (4.7 pM) was significantly higher than that in 7-d bile diverted rats (1.6 pM), although the pancreatic wet weight, protein concentration, and total content were comparable for the two groups. It was suggested that the presence of bile in the duodenum is required to maintain normal pancreatic secretion, and that the removal of bile from the intestine has quite different effects, depending on whether the bile flow is obstructed or diverted.     

Influence of bile flow obstruction vs bile diversion on pancreatic secretion in the conscious rat

Changes in pancreatic exocrine functions were compared between conscious rats with bile duct ligation and bile diversion from the duodenum on the first, third, fifth, and seventh postoperative days. Body weight was significantly decreased with time in both groups. Basal secretions of fluid, bicarbonate, and protein remained unchanged throughout the experimental period in bile duct ligated rats, whereas in bile diverted rats, the basal bicarbonate secretion with returning of pancreatic juice to the duodenum increased on the third postoperative day, and the basal protein output significantly increased with time. Basal secretions with returning of pancreatic juice to the duodenum in both groups were higher than that in control (bile and pancreatic juice returned to the intestine) rats. Stepwise increases in fluid and bicarbonate outputs responding to the graded doses of secretin were observed in bile duct ligated rats on the first and third postoperative days, as has been observed in bile diverted rats. However, on the fifth and seventh postoperative days, stepwise responses to graded doses of secretin were no longer observed in bile duct ligated rats. The pancreatic response to cerulein was greater in bile diverted rats than in bile duct ligated rats. Plasma CCK concentration in 7-d bile duct ligated rats (4.7 pM) was significantly higher than that in 7-d bile diverted rats (1.6 pM), although the pancreatic wet weight, protein concentration, and total content were comparable for the two groups. It was suggested that the presence of bile in the duodenum is required to maintain normal pancreatic secretion, and that the removal of bile from the intestine has quite different effects, depending on whether the bile flow is obstructed or diverted.     

Effect of bile duct obstruction on pancreatic enzyme secretion and intestinal proteolytic enzyme activity in the rat

The role of bile in regulation of intestinal proteolytic activity in rats was investigated by studying the effects of bile diversion and bile duct obstruction on pancreatic protease secretion and on recovery of protease from the intestine. Diversion of bile and pancreatic juice from the intestine caused a large increase in pancreatic enzyme secretion; replacement of bile partially suppressed this response. Bile duct obstruction resulted (3–4 days postobstruction) in a threefold increase in pancreatic juice chymotrypsin but caused large decreases inintestinal trypsin and chymotrypsin activities and total proteolytic activity. Recovery of pancreatic juice protein (labeled with¹⁴C) from intestinal contents was markedly decreased in bile duct obstruction, indicating a more rapid rate of degradation and absorption of pancreatic juice protein. The evidence suggests that interruption of bile flow results in an accelerated rate of degradation of pancreatic proteolytic enzymes, and that the increase in pancreatic enzyme secretion is an adaptation to decreased intestinal proteolytic activity.     

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.     

Effect of intravenous ethanol on pancreatic secretion in the conscious rat

To study whether the secretory state of thein vivo pancreas affects its response to ethanol, we administered ethanol intravenously to conscious rats in the presence and absence of exogenous secretin. In the absence of secretin when pancreatic secretion was maintained at basal levels, intravenous ethanol produced a significant stimulation of the flow rate, bicarbonate concentration, and protein concentration. This response persisted despite elimination of gastric acid production by infusion of the H2 receptor antagonist cimetidine. In contrast, intravenous ethanol inhibited the flow rate and bicarbonate concentration when pancreatic secretion was augmented by the infusion of GIH secretin. Thus, the rat pancreas responds differently to intravenous ethanol in the basal and secretin-stimulated states. These results may account for the lack of agreement regarding the pancreatic effects of systemic ethanol.This work was supported by USPHS grant AA-03508.     

Effect of somatostatin on pancreatic enzyme secretion

The effect of somatostatin (SS) on the pancreatic enzyme secretion was studied in a perfusion system using dispersed pancreatic rat acini in vitro. In addition the effect of SS on pancreatic secretion in vivo was also studied in conscious rats for comparison. In an in vitro study, 6 x 10(-7) M SS-14 caused no significant change in amylase release when added 20 min before stimulation by 10(-5) M carbamylcholine (Cch), 10(-6) M A23187, 5 x 10(-7) M secretin and 2 mM dibutyryl cyclic AMP. The addition of 6 x 10(-7) M SS-28 also caused no significant change in amylase release stimulated by 10(-5) M Cch. High performance liquid chromatographic examination indicated that no degradation of either SS-14 or SS-28 occurred after reaction with dispersed acini. In an in vivo study SS-14 caused marked inhibition of basal pancreatic secretion and stimulated pancreatic secretion by bile-pancreatic juice diversion. These results indicate that SS has no direct inhibitory action on rat pancreatic secretion, and that SS may inhibit the pancreatic secretion by indirect mechanisms.     

Action of intragastric ethanol on the pancreatic secretion of conscious rats.

A surgical procedure has been disigned which permits injection in the stomach and the duodenum by separate catheters, collection of the pancreatic juice during the experiments, recirculation of the pancreatic juice into the duodenum between experiments, and a normal circulation of bile in rats. Experiments were performed in conscious rats given either 20% ethanol or water. In rats submitted to daily ethanol consumption for 13 months, the intragastric injection of 2 g/kg 20% ethanol considerably increased the pancreatic secretion of protein and, to a lesser extent, of water. In control non-alcoholic rats, a short period of increased secretion is followed by a major inhibition of pancreatic secretion, this reverse reaction to ethanol of pancreatic secretion according to whether or not rats are adapted to regular ethanol consumption is similar to what has been previously observed in dog. In chronic alcoholic rats, the release of secretin is probably not very different from controls.     

Effect of somatostatin on pancreatic enzyme secretion

The effect of somatostatin (SS) on the pancreatic enzyme secretion was studied in a perfusion system using dispersed pancreatic rat acini in vitro. In addition the effect of SS on pancreatic secretion in vivo was also studied in conscious rats for comparison. In an in vitro study, 6×10^-7M SS-14 caused no significant change in amylase release when added 20 min before stimulation by 10^-5M carbamylcholine (Cch), 10^-6M A23187, 5×10^-7M secretin and 2mM dibutyryl cyclic AMP. The addition of 6×10^-7M SS-28 also caused no significant change in amylase release stimulated by 10^-5M Cch. High performance liquid chromatographic examination indicated that no degradation of either SS-14 or SS-28 occurred after reaction with dispersed acini. In an in vivo study SS-14 caused marked inhibition of basal pancreatic secretion and stimulated pancreatic secretion by bile-pancreatic juice diversion. These results indicate that SS has no direct inhibitory action on rat pancreatic secretion, and that SS may inhibit the pancreatic secretion by indirect mechanisms.     

Circulating ethanol does not stimulate pancreatic secretion in conscious rats

The purpose of this study was to test the hypothesis that circulating ethanol at concentrations of approximately 0.1 mg% stimulates pancreatic secretion. Awake rats recovered from surgery were used in these experiments. Intravenous infusion protocols were established that produced blood ethanol concentrations 0.1 mg% for over an hour. Maintenance of 0.1 mg% blood ethanol concentration or transient concentrations as high as 0.17 mg% did not cause significant increases in pancreatic protein or fluid secretion. To test whether elevated blood ethanol would augment stimulated pancreatic secretion, the trypsin inhibitor camostat was infused intraduodenally at doses of 0.05, 0.2, and 0.5 mg/hr, each dose level infused for 2 hours. Elevated blood ethanol concentrations (0.1 mg%) did not significantly affect camostat-stimulated pancreatic protein or fluid secretion. In contrast to intravenous infusion, intraduodenal infusion of ethanol significantly stimulated pancreatic protein and fluid secretion, which was associated with blood ethanol concentrations of > or =0.19 mg%. The increases in pancreatic secretion were completely blocked by intravenous infusion of the cholecystokinin (CCK) receptor antagonist CR1409. We conclude that circulating ethanol does not stimulate pancreatic secretion in awake, recovered rats and that intraduodenal ethanol-stimulated pancreatic secretion is mediated by CCK.     

Effect of circulating somatostatin on exocrine pancreatic secretion in conscious dogs

We determined the effects of exogenous somatostatin-14 (100 and 200 ng/kg/h; mimicking postprandial somatostatin concentrations) on pancreatic responses to a background infusion of secretion in combination with graded doses of CCK-8 in conscious dogs with chronic gastric and duodenal fistulas. The lower dose of somatostatin-14 (S-14), which produced S-14 plasma levels lower than measured after a meal, did not change basal or stimulated pancreatic secretion. The upper dose of S-14, which produced plasma S-14 concentrations slightly above the postprandial range, caused inhibition of pancreatic fluid and protein secretion to low doses of CCK-8 (p less than 0.05). The inhibition was surmountable with higher doses of CCK-8. We interpret these data as indicating that circulating S-14 is not an important hormonal regulator of exocrine pancreatic secretion.     

Effect of tetramethylpyrazine on exocrine pancreatic and bile secretion

AIM: To investigate the effect of tetramethylpyrazine ?A (ligustrazine, TMP) on the secretion of exocrine pancreas(and biliary). METHODS: In in vivo study, we investigated the effect of TMP on the secretion of pancreatic-bile juice (PBJ) in rats. Using human pancreatic duct cell line, CAPAN-1, combined with the short-circuit current (Isc) technique we further studied the effect of TMP on the pancreatic anion secretion. RESULTS: Administration of TMP (80mg/kg, ip) significantly increased the secretion of PBJ (P<0.05), but the pH of PBJ and the secretion of pancreatic protein were not significantly affected. Basolateral addition of TMP produced a dosedependent increase in Isc(EC50=1.56mmol/L), which contained a fast transient Isc response followed by a slow decay. Apical application of CI- channel blockers, DPC (1mmol/L), decreased the response by about 67.1% (P<0.001), whereasamiloride (100μmol/L), a epithelial sodium channel blockers, had no effect. Removal of extracellular HCO3^- abolished TMP-induced increase in Isc by about 74.4% (P<0.001),but the removal of external Cl^- did not. Pretreatment with phosphodiesterase inhibitor, IBMX(0.5mmol/L), decreasedthe TMP-induced Isc by 91% (P<0.001). CONCLUSION: TMP could stimulate the secretion of PBJ, especially pancreatic ductal HCO3- secretion via cAMP or cGMP-dependent pathway. It need further study to investigate the roles of cAMP or cGMP in the effect of TMP on the secretion of exocrine pancreas.     

Effect of short- and long-term alcohol feeding in rats on pancreatic enzyme content and enzyme secretion in isolated pancreatic lobules in vitro

Summary The effect of short-and long-term ethanol intake on digestive enzyme secretion was determined in isolated pancreatic lobules of rats. Groups of male Wistar rats were fed a modified Lieber-DeCarli diet containing either 5% (w/v) óf ethanol, isocaloric amounts of a liquid diet in which ethanol was substituted by starch, or solid rat chow; for 3 days, 1, 2, 4, 8 and 12 weeks. Basal and caerulein-stimulated secretion of lipase, amylase, chymotrypsin and trypsin and the enzyme content in the tissue were studied. Feeding the liquid control diet decreased the tissue content of the four enzymes as compared with the values obtained in the group receiving solid rat chow. While basal and stimulated amylase secretion was markedly reduced in the former group, the secretion pattern of the other enzymes exhibited only transient changes. Caerulein-stimulated secretion of lipase and the proteases was increased by ethanol, the effect being more pronounced during the initial phase of the experiment. Alcohol feeding stimulated the basal secretion of these enzymes only in weeks 1–4. In contrast to the other enzymes, basal and stimulated amylase secretion was not enhanced by ethanol feeding. The results suggest that the enzyme secretion of the rat pancreas is distinctly altered by chronic ethanol feeding. However, the response of the pancreatic enzymes is non-parallel, and changes with the duration of alcohol intake.