T. M. Chang

A physiological role of peptide YY on exocrine pancreatic secretion in rats.

BACKGROUND Peptide YY (PYY) given intravenously was shown to inhibit pancreatic exocrine secretion both in the dog and the rat. However, a possible physiological role of PYY on the pancreatic secretion has not been clarified. The present study was undertaken to investigate its physiological role on the exocrine pancreas. METHODS In conscious rats, plasma PYY was determined in response to oral ingestion of a 6-mL meal and intravenous infusion of PYY; small intestinal transit time was measured by phenol red as a nonabsorbable marker, and pancreatic secretory studies were performed in rats with pancreatic fistulas and jugular vein catheters. RESULTS Oral ingestion of the meal (containing phenol red, 1.6 mg/100 mL) significantly increased plasma PYY within 30 minutes. During this period, most (97%) of the phenol red was detected in the proximal two-thirds of the small intestine. Intravenous infusion of PYY in 25, 50, and 100 pmol.kg-1 x h-1 produced a dose-dependent increase in plasma PYY. The dose of PYY that simulated the peak postprandial level was 50 pmol.kg-1 x h-1, and this dose of PYY significantly inhibited the pancreatic secretion stimulated by physiological doses of secretin and cholecystokinin-8 (CCK-8). After the meal, pancreatic secretion of bicarbonate and protein significantly increased in rats pretreated with normal rabbit serum, whereas this increase was significantly augmented in rats pretreated with an anti-PYY serum because the postprandial increase in plasma PYY was abolished. CONCLUSIONS PYY plays a regulatory role in the postprandial pancreatic exocrine secretion in rats.

Mechanism of Increased Exocrine Pancreatic Secretion in Pancreatic Juice-Diverted Rats

We investigated a possible role of endogenous cholecystokinin-pancreozymin (CCK-PZ) in the mechanism of exocrine pancreatic secretion after excluding pancreatic juice from the intestine in rats. Fasting plasma immunoreactive CCK-PZ was determined in normal rats, in rats with pancreatic duct ligation, and in sham-operated rats. The mean fasting plasma CCK-PZ concentration of rats with pancreatic duct ligation, 25.1 +/- 2.0 pM, was significantly greater (p less than 0.001) than those of normal and sham-operated rats, 14.3 +/- 1.7 and 11.5 +/- 2.2 pM, respectively. Whereas mean postprandial plasma CCK-PZ concentrations of normal and sham-operated rats were significantly greater (p less than 0.001) than their fasting levels, no significant increase occurred in the rats with pancreatic duct ligation after a meal. The mean fasting plasma CCK-PZ concentration of rats with pancreatic duct ligation was comparable to the mean postprandial CCK-PZ level of normal and sham-operated rats. To determine a possible role of circulating endogenous CCK-PZ on the pancreatic secretion, anesthetized rats were prepared with ligation of pylorus and cannulation of pancreatic duct. After diversion of pancreatic juice began, pancreatic secretion including protein significantly increased, which coincided with a significant increase in plasma CCK-PZ concentration. The increases in both pancreatic secretion and plasma CCK-PZ were reversed by intraduodenal administration of bovine trypsin or rat pancreatic juice. Furthermore, the increase in pancreatic secretion was abolished by intravenous infusion of proglumide or an intravenous bolus injection of a rabbit anti-CCK-PZ serum, which also blocked clearly the increase in the pancreatic secretion stimulated by exogenous CCK-PZ8 (0.125 micrograms X kg-1 X h-1) in rats. Thus we conclude that the increase in pancreatic secretion resulting from elimination of pancreatic juice from the intestine is attributable, in part, to increased release of CCK-PZ, and thus it is suggested that trypsin in the intestinal lumen plays a significant role in release of CCK-PZ.

Secretin is released by digestive products of fat in dogs

We investigated the effect of fat or digestive products of fat on the release of endogenous secretin in 15 gastric fistula dogs with either pancreatic fistulas or duodenal fistulas. In 4 dogs with both gastric and duodenal cannulas, intraduodenal administration of corn oil (Lipomul) at a dose of 15 mmol resulted in a significant increase in plasma secretin concentration, whereas in another group of 4 dogs with complete pancreatic duct ligation, the same amount of triglyceride failed to increase the secretin concentration. When Lipomul incubated with pancreatic enzymes was administered in the duodenum, a marked increase in plasma secretin concentration occurred in the 4 dogs with pancreatic duct ligations. In the 7 dogs with chronic pancreatic fistulas, intraduodenal administration of Lipomul resulted in a significant increase in both plasma secretin concentration and pancreatic secretion of bicarbonate when the pancreatic juice was allowed to flow into the duodenum, whereas no increase in either the secretin concentration or bicarbonate output was apparent using the same amount of Lipomul when the pancreatic juice was diverted from the duodenum. In 4 of these 7 dogs so studied, intraduodenal administration of oleic acid emulsion, with pH adjusted to 5.0 in graded doses, resulted in a dose-related increase in the secretin concentration that paralleled pancreatic bicarbonate output. The increases in both secretin concentration and pancreatic bicarbonate secretion were completely abolished by intravenous infusion of a rabbit antisecretin serum in the 4 dogs. Thus we conclude that release of endogenous secretin plays an important role in the mechanism of exocrine pancreatic secretion stimulated by digestive products of fat in dogs.

Effect of pancreatic juice diversion on secretin release in rats

We investigated a possible role of secretin in the mechanism of exocrine pancreatic secretion after exclusion of pancreatic juice from the intestine in anesthetized rats. Diversion of pancreatic juice from the duodenum resulted in a significant increase in plasma secretin concentration from 0.76 +/- 0.39 pM at 0 time to 3.09 +/- 0.30 pM at 4 h after diversion. This increase in secretin coincided with a steady but significant increase in pancreatic secretion of volume and bicarbonate. Intraduodenal administration of fresh pancreatic juice completely reversed the diversion-induced increases in both plasma secretin and pancreatic secretion. Intravenous injection of a rabbit-antisecretin serum blocked the increase of pancreatic secretion during diversion of pancreatic juice from the duodenum. Thus, we conclude that endogenous secretin is involved in a hormonal mechanism regulating increased pancreatic exocrine secretion in pancreatic juice-diverted rats.

Effect of Pancreatic juice and Trypsin on Oleic Acid-Stimulated Pancreatic Secretion and Plasma Secretin in Dogs

We have investigated a negative feedback mechanism in the intestinal phase of pancreatic exocrine secretion in dogs with gastric cannulas and Thomas duodenal cannulas in whom pancreatic juice was collected by cannulation of the main pancreatic duct. Intraduodenal infusion of oleic acid emulsion in a dose of 18 mmol/h resulted in a significant increase in pancreatic secretion of water, bicarbonate, and protein, which was accompanied by increased plasma concentrations of both secretin and cholecystokinin. Infusion of pancreatic juice or bovine trypsin into the duodenum significantly inhibited the oleic acid-stimulated pancreatic secretion. This inhibition coincided with a significant decrease in plasma secretin level, whereas plasma cholecystokinin concentration was not affected by either pancreatic juice or trypsin. Neither pancreatic secretion nor plasma secretin concentration was affected by intraduodenal administration of NaHCO3 solution. The trypsin-induced suppression of pancreatic secretion was prevented by intravenous administration of secretin in a dose that achieved a plasma secretin level comparable to that during the oleic acid administration. This study indicates that a negative feedback mechanism is operative in the intestinal phase of pancreatic exocrine secretion in dogs, and endogenous secretin plays a significant role in the mechanism.

Effect of pancreatic proteases on plasma cholecystokinin, secretin, and pancreatic exocrine secretion in response to sodium oleate

The effect of pancreatic proteases or juice on the sodium oleate-stimulated pancreatic secretion and plasma concentrations of secretin and cholecystokinin in anesthetized rats was investigated. Each rat received sodium oleate in a dose of 0.12 mmol.h-1 via a duodenal tube. Sodium oleate infusion significantly increased pancreatic secretion (volume and protein output) compared with the saline given the control group. The increase in pancreatic secretion paralleled significant elevations of plasma concentrations of secretin and cholecystokinin. To determine a possible role of pancreatic proteases on the responses induced by sodium oleate, saline, chymotrypsin, and trypsin, a combination of chymotrypsin and trypsin or pancreatic juice was infused into the duodenum. The pancreatic secretion was significantly reduced by pancreatic proteases or pancreatic juice, and the reduction paralleled the decreases in plasma concentrations of the two hormones. These agents suppressed both pancreatic secretion and plasma hormone levels in the following order of magnitude: (pancreatic juice or chymotrypsin + trypsin) greater than (trypsin) greater than (chymotrypsin). The reduction of pancreatic secretion by pancreatic proteases was reversed by intravenous administration of secretin and cholecystokinin in physiological doses. It is concluded that negative-feedback regulation of pancreatic secretion is operative in the intestinal phase in rats and that both secretin and cholecystokinin are involved in the regulation.

Endogenous Nitric Oxide Mediates Pancreatic Exocrine Secretion Stimulated by Secretin and Cholecystokinin in Rats

Nitric oxide (NO) is one of the important biologic mediators in regulation of gastrointestinal (GI) functions, but the influence of NO on the release of secretin and cholecystokinin (CCK) and exocrine pancreatic secretion has not been adequately investigated in the rat. The aim of this study was to determine the role of NO on endogenous and exogenous secretin-or CCK-stimulated pancreatic exocrine secretion both in anesthetized and conscious rats. Experiments were carried out in four different groups of rats with duodenal pancreatobiliary cannulas and jugular vein catheters. Group 1: During duodenal infusion of 0.05N HCl or 15% casein (pH 7.0), N-nitro-l-arginine (NNA), an inhibitor of NO-synthase in graded doses (2.5, 5, 10 mg/kg/h), was infused intravenously. Group 2: One hour after starting intravenous secretin at 5 pmol/kg/h or intravenous CCK-8 at 0.06 &mgr;g/kg/h, NNA in graded doses was administered intravenously. Group 3: In conscious rats, NNA (5 mg/kg/h) was given intravenously for 1 hour after a meal. Group 4: l-Arginine at 100 mg/kg/h was infused intravenously during the period of NNA (5 mg/kg/h) infusion in groups 1, 2, and 3. Pancreatic juice was collected at 30-minute intervals to measure volume, as well as output of bicarbonate and protein. At the end of the experiment, plasma secretin, vasoactive intestinal polypeptide (VIP) and CCK levels were determined by radioimmunoassay (RIA). NNA dose dependently inhibited the pancreatic secretion of fluid and bicarbonate stimulated by duodenal acidification, exogenous secretin, and a meal. NNA dose dependently inhibited the pancreatic secretion of protein stimulated by duodenal infusion of casein, exogenous CCK, and a meal. l-Arginine significantly reversed the NNA-induced inhibition of pancreatic secretion in all experiments. NNA did not alter significantly the plasma levels of secretin, VIP, and CCK. Our results indicated that endogenous NO plays a significant role in the regulation of pancreatic exocrine secretion stimulated by secretin and CCK. However, NO does not influence the release of secretin, VIP, or CCK in the rat.

Role of secretin in negative feedback regulation of postprandial pancreatic secretion in dogs

Abstract Background: A negative feedback regulation of pancreatic exocrine secretion has been observed in fasting rats, pigs, and humans, but this phenomenon could not be found in fasting dogs. The aims of the present study were to investigate whether or not postprandial pancreatic secretion is regulated by a negative feedback mechanism and to determine if the mechanism is mediated by secretion and/or cholecystokinin (CCK) in dogs. Methods: In eight dogs prepared with gastric and Herreras pancreatic cannulas, pancreatic juice was collected for 3 hours after feeding a mixed meal to measure volume, bicarbonate, and trypsin output. Peripheral venous blood was obtained to determine plasma immunoreactive secretin and CCK levels. Four groups of experiments were performed while pancreatic juice was diverted from the duodenum: (1) diversion of pancreatic juice alone, (2) intraduodenal infusion of fresh pancreatic juice while pancreatic juice was diverted, (3) intraduodenal infusion of 150 mg/h of trypsin and 300 mg/h of chymotrypsin, and (4) intraduodenal infusion of 0.1 mol/L NaHCO 3 . Results: Pancreatic secretion during diversion of pancreatic juice was significantly greater than that obtained while pancreatic juice was returned. Diversion-induced pancreatic hypersecretion was significantly suppressed by intraduodenal administration of pancreatic proteases, but it was not influenced significantly by 0.1 mol/L NaHCO 3 . The suppression by either pancreatic juice or proteases paralleled the decrease in plasma secretin response, whereas the CCK response remained unchanged. The inhibitory effect of pancreatic proteases on pancreatic secretion was blocked by a physiological dose of exogenous secretin, 0.06 clinical U · kg −1 · h −1 . Conclusions: In dogs, postprandial pancreatic secretion is controlled by a negative feedback mechanism mediated mainly by the release of secretin, but not by CCK.

Effect of Endogenous Insulin on Pancreatic Exocrine Secretion in Perfused Dog Pancreas

Canine pancreas was perfused with an intraar-terial infusion of Krebs-Ringer bicarbonate solution containing 5% dog red blood cells, 0.1% bovine serum albumin, and 3% dextran at 15 ml/min, while portal effluent was continuously collected. Pancreatic juice was obtained in 15-min samples via main pancreatic duct cannulation. After a 1-h basal period, secretin and cholecys-tokinin-8 (CCK), at doses of 2.5 ng. min−1 each, were simultaneously infused for 10 min, with background infusion of a normal rabbit serum (NRS) or an antiinsulin serum (Anti-I) in 5 ml each via a sidearm of the intraar-terial catheter. The infusion of secretin and CCK resulted in a significant increase in pancreatic bicarbonate and protein secretion during the infusion of NRS, whereas the pancreatic secretory response of bicarbonate and protein was profoundly suppressed by the infusion of Anti-I in six pancreata so studied. This suppression by Anti-I coincided with significant increases in somatostatin and pancreatic polypeptide levels in portal venous effluent. In three additional pancreata, simultaneous infusions of Anti-I with antisomatostatin (5 ml) and antipancreatic polypeptide (5 ml) serum failed to inhibit the pancreatic exocrine secretion. These results indicate that secretin-and CCK-stimulated pancreatic secretion of bicarbonate and protein depends heavily on local action of insulin. The suppression by Anti-I of pancreatic secretion is mediated, in part, by local releases of somatostatin and pancreatic polypeptide. Thus, the insuloacinar axis plays an important regulatory role in pancreatic exocrine secretion in the dog, and it involves at least three islet hormones including insulin, somatostatin, and pancreatic polypeptide.

Role of cholecystokinin in pancreatic bicarbonate secretion in dogs

We investigated the effects of endogenous and exogenous cholecystokinin (CCK) on pancreatic exocrine secretion, in particular that of bicarbonate. In six dogs prepared with gastric cannulas and Thomas duodenal cannulas, intraduodenal administration of corn oil (Lipomul) incubated with hog pancreatic enzymes significantly increased pancreatic secretion of both bicarbonate and protein. Increase in pancreatic secretion of both bicarbonate and protein was accompanied by the increase in plasma CCK concentration. However, the increase in bicarbonate as well as protein secretion was blocked by proglumide, a CCK antagonist, given intravenously. In contrast, intraduodenal infusion of undigested Lipomul failed to stimulate the pancreatic exocrine secretion or release of endogenous CCK. These observations indicate that endogenous CCK plays an important role in secretion of both bicarbonate and protein stimulated by digested corn oil. In a group of four dogs with pancreatic fistulas, intravenous infusion of CCK potentiated the stimulatory effect of secretin on pancreatic bicarbonate secretion. The stimulatory effect as well as potentiating effect of CCK on pancreatic bicarbonate secretion was blocked by infusion of proglumide. We conclude that endogenous CCK plays a significant role in fat-stimulated pancreatic secretion, and it is apparent that both endogenous CCK and secretin are equally important for stimulation of pancreatic bicarbonate secretion, which results from potentiation of the action of the two hormones.