Physiological significance of secretin in the pancreatic bicarbonate secretion. II. Pancreatic bicarbonate response to a physiological increase in plasma secretin concentration

Etiology of pancreatic cancer, with a hypothesis concerning the role of N-nitroso compounds and excess gastric acidity

In the United States, pancreatic cancer is the fourth most frequent cause of cancer death in males as well as females, after lung, prostate or breast, and colorectal cancer. Each year, approximately 30 000 Americans are diagnosed with pancreatic cancer and about the same number die of it. Germline mutations in a few genes including p16 and BRCA2 have been implicated in a small fraction of cases, as has chronic pancreatitis. The one established risk factor for pancreatic cancer is cigarette smoking: current smokers have two to three times the risk of nonsmokers. Studies of dietary factors have not been entirely consistent but do suggest associations of higher risk with consumption of smoked or processed meats or with animal foods in general and lower risk with consumption of fruits and vegetables. Colonization by Helicobacter pylori appears to increase risk, and a history of diabetes mellitus may also increase risk. The purpose of this epidemiologic review is to consider the possibility that risk of pancreatic cancer is increased by factors associated with pancreatic N-nitrosamine or N-nitrosamide exposures and with chronic excess gastric or duodenal acidity. Host genetic variation in inflammatory cytokine mechanisms may also be involved in this process. Many features of the evidence bearing on the pathophysiology of pancreatic cancer appear to support connections with N-nitroso compounds and with gastric acidity.

Gastrointestinal endocrinology

During the last quarter of this century gastrointestinal endocrinology has grown explosively. In 1970, three hormones (secretin, gastrin, and cholecystokinin) were identified and by authorities in the field considered sufficient to explain the entire hormonal regulation of digestion. That was some underestimation. Today the gut is known to express more than 20 different hormonal/regulatory peptide systems. Their widespread cellular occurrence, gene expression cascades, secretory mechanisms, receptors and receptor binding, as well as normal and pathophysiological effects are now also fairly well known owing to the marked progress in basic sciences and biochemical technologies (immuno and peptides chemistry, molecular and cell biology). Thus, the gut is now recognized as the largest endocrine organ of the body; and a substantial part of the gastroenterologic research over the latest decades has been devoted to gut hormones. The following review describes the recent development, with emphasis on gastrointestinal peptide systems that have been studied and even discovered in Denmark. Hence, as reflected by the number of doctoral theses and PhD studies (> 50 since 1974), gastrointestinal endocrinology has been a major research area in this country in the past 25 years.

Duodenal mucosal bicarbonate secretion in pigs is accompanied by compensatory changes in pancreatic and biliary HCO3- secretion

BACKGROUND

The purpose of the study was to examine the effect of stimulation and inhibition of duodenal mucosal bicarbonate secretion on pancreatic and hepatic bicarbonate secretion in response to acid.

METHODS

The effect of inhibition (indomethacin) or stimulation (misoprostol) of duodenal mucosal bicarbonate secretion on pancreatic and biliary bicarbonate secretion in response to intraduodenal infusion of HCl or intravenous infusion of secretin was studied in anaesthetized pigs.

RESULTS

The hepatic and pancreatic response to exogenous secretin was not significantly altered by stimulation/inhibition of duodenal bicarbonate secretion. However, pancreatic and biliary bicarbonate secretion in response to duodenal acidification was significantly augmented by inhibition of duodenal mucosal bicarbonate secretion; conversely, it was reduced by stimulation of duodenal bicarbonate secretion. The increase in plasma secretin levels in response to duodenal acidification was reduced by stimulation and augmented by inhibition of duodenal mucosal bicarbonate secretion.

CONCLUSIONS

Duodenal mucosal bicarbonate secretion can serve as a modulator of both pancreatic and biliary bicarbonate secretion in response to luminal acidification, possibly through regulation of the release of secretin.

Pancreatic, hepatic, and duodenal mucosal bicarbonate secretion during infusion of secretin and cholecystokinin. Evidence of the importance of hepatic bicarbonate in the neutralization of acid in the duodenum of anaesthetized pigs

The effect of infusion of secretin alone or in combination with cholecystokinin (CCK) on pancreatic, hepatic, and duodenal mucosal bicarbonate secretion was studied in anaesthetized pigs. After laparotomy, catheters were inserted into the common bile duct, the pancreatic duct, and both ends of the duodenum. Pancreatic, hepatic, and duodenal mucosal secretions were collected during intraportal infusion of increasing doses of secretin, either alone or in combination with CCK. During infusion of secretin in doses that caused physiologic increases in plasma secretin concentrations the liver produced significantly more bicarbonate than the pancreas. A physiologic dose of CCK augmented the effect of secretin on both hepatic and pancreatic bicarbonate secretion, but the hepatic production of bicarbonate was still larger than the pancreatic production. Neither secretin alone nor secretin combined with CCK caused any changes in duodenal mucosal bicarbonate secretion. These results suggest that the liver plays an important role in the neutralization of acid in the duodenum.

Secretin receptors in the neuroglioma hybrid cell line NG108-15. Characterization and regulation of their expression

Secretin receptors in membranes from the neuroblastoma-glioma hybrid cell line NG108-15 were investigated by 125I-secretin binding and adenylate cyclase activation. On both parameters the corresponding relative potencies of parent peptides were, respectively: secretin greater than helodermin greater than peptide histidine isoleucinamide = vasoactive intestinal peptide. With secretin analogs and secretin fragments, the order of potency for binding was: secretin = [Val5]secretin greater than [Ala2]secretin = [Ala11]secretin greater than [Ala4, Val5] secretin greater than [Ala4]secretin greater than [D-Phe4] secretin greater than [D-Phe2]secretin = secretin (2-27) greater than secretin (3-27) greater than secretin (7-27). Also, on adenylate cyclase, [D-Phe4]secretin, [D-Phe2]secretin, secretin (2-27) and secretin (3-27) were partial agonists while secretin (7-27) was ineffective. The differentiating agent N6,2'-O-dibutyryladenosine 3',5'-monophosphate (1 mM) increased the density of secretin receptors and secretin-stimulated adenylate cyclase activity after a lag period of 4 h. After incubation for 24 h, receptor number and enzyme activity were increased 4- and 3-fold, respectively. These effects were inhibited totally by 1 microgram/ml cycloheximide and halved by 5 micrograms/ml actinomycin D. They were mimicked by 1 mM sodium butyrate but were not reproduced by either 8-bromoadenosine 3',5'-monophosphate or the phosphodiesterase inhibitor rac-4-(3-Butoxy-4-methoxybenzyl)-2-imidazolidinone.

Secretin release in coeliac disease. Plasma secretin concentration and bicarbonate output to the duodenum after intraduodenal acid infusion in coeliac patients before and after treatment

Infusion of 40 ml 0.1 mol/l HCl into the duodenum in eight untreated coeliac patients was followed by an increase of the plasma immunoreactive secretin (IRS) concentration from 1.6 +/- 0.2 pmol/l to a peak level of 2.4 +/- 0.3 pmol/l (p less than 0.05). After treatment with a gluten-free diet, the same patients showed an increase from 1.4 +/- 0.3 pmol/l to a peak level of 5.5 +/- 0.9 pmol/l after intraduodenal acid infusion, which was significantly higher than before treatment (p less than 0.01). In control subjects, intraduodenal acid infusion was followed by an increase from 1.4 +/- 0.2 pmol/l to 6.7 +/- 1.1 pmol/l, which was significantly higher than in untreated coeliac disease (p less than 0.01) but did not differ from what was found in treated coeliac patients. Significant differences in pH, volume, or bicarbonate content of the duodenal aspirates or the basal IRS levels were not found.

Secretin release from the isolated, vascularly perfused pig duodenum

1. A method for the isolation and vascular perfusion of the porcine pancreas and duodenum was developed. 2. The oxygen consumption of the whole preparation was similar to that of the pancreas alone, and since the duodenal arteriovenous oxygen deficit was similar to that of the total preparation, it was concluded that the duodenum respired adequately. 3. The duodenum rapidly absorbed luminally administered radioactive glucose, and this absorption was strongly inhibited by ouabain and phloridzin. 4. The duodenum secreted secretin rapidly in response to hydrochloric acid, but did not respond to any other luminal stimuli, including lipids, proteins, carbohydrates and bile. Neither was secretin release stimulated by intra-arterially injected acetylcholine. 5. By gel permeation chromatography the release immunoreactive secretin behaved identically to pure natural secretin, indicating that the tissue form and the circulating form have identical molecular size. 6. It is concluded that this model offers an unique opportunity to study the endocrine secretion of the duodenum.

The effect of secretin on gastric H+ and pepsin secretion and on urinary electrolyte excretion in man

After an initial 40-min period of gastric suction, the gastric juice was collected for 12 30-min periods from 8 healthy young male students. During the first hour 30-min periods saline alone was infused, whereafter secretin in a dose of 0.05 CU/kg and h was given in the middle four 30-min periods and in a dose of 0.25 CU/kg and h for the final four 30-min periods. Urine was collected in three 2-h portions corresponding to each change of infusion procedure, and blood for determination of plasma secretin by a radioimmunoassay procedure was drawn at hourly intervals. Plasma secretin was unaltered both during the saline infusion and during the infusion of secretin in a dose of 0.05 CU/kg and h but rose during the infusion of 0.25 CU/kg and h. Compared with the mean 30-min output in the initial saline control period, gastrin pepsin secretion rose significantly in the third 30-min period, and gastric H+ output fell significantly in the fourth 30-min period of infusion of secretin in a dose of 0.05 CU/kg and h. During the higher secretin dose gastric output could not be evaluated owing to duodenogastric reflux. Urinary sodium excretion increased only during the infusion of secretin in a dose of 0.25 CU/kg and h, whereas secretin showed no effect on the urinary outputs of water, potassium, and solutes. The results of the present study indicate that secretin may play a physiological role in the regulation of gastric H+ and pepsin secretions, whereas its diuretic effect probably is a pharmacological effect only.

Meal-stimulated secretin release in man: effect of acid and bile

Nine normal subjects were studied before and after intragastric instillation of a liquid meal. Gastric emptying rates of acid and pancreaticobiliary secretions were quantitated by means of a dilution indicator technique. A significant, positive correlation was found between load of acid to the duodenum and the concentration of secretin in plasma. No correlation was found between load of the bile acids and plasma secretin. The buffering capacity of gastric contents should be taken into account when fasting and postprandial periods are compared. Plasma secretin concentration remained low during the first postprandial hour. Maximum secretin concentrations and duodenal disappearance rate of acid was observed 1 1/2-2 h after instillation of the meal. In contrast, trypsin output was maximum within 10 min. The data support the concept that in man release of secretin is governed principally by the amount of hydrogen ions emptied into duodenum and indicate the importance of secretin in the late postprandial period, when the acidity of the gastric contents is high.

Radioimmunoassay of secretin. A critical review and current status

The radioimmunoassay methods of secretin are reviewed with respect to production of antibody, preparation of radioactive tracers, and effect of plasma interference. The major difference in the secretin assay methods resides in handling plasma interference. Thus the assay sensitivity decreased markedly when the assay was conducted by diluting plasma samples. When the assay was conducted by compensating for plasma interference with homologous hormone-free plasma, the effect of plasma interference was greatly reduced, leading to a more sensitive assay. However, this method probably can not obtain consistent results with plasma samples collected under various experimental conditions. The method is still subject to considerable desensitization and assay variation. On the other hand, the elimination of plasma interference before assay results in the most sensitive secretin assays capable of detecting consistently a significant postprandial rise in plasma secretin level. It is concluded that a sensitive, validated secretin radioimmunoassay should be one that is capable of detecting increments of plasma secretin in response to doses of intraduodenal acid at 0.055 mEq/min or lower and intravenous administration of exogenous secretin at 0.03 CU/kg/hr with concomitant stimulation of pancreatic bicarbonate and water secretion. With a sensitive and accurate radioimmunoassay for secretin, it is now possible to further investigate the physiology and pathophysiology of secretin.

Vasoactive intestinal polypeptide in vagally mediated pancreatic secretion of fluid and HCO3

The role of nerves that liberate vasoactive intestinal polypeptide (VIP) in the porcine pancrease as mediators of the atropine-resistant action of the vagus on flow and bicarbonate (HCO3) secretion was examined. Efferent electrical stimulation of the vagus in atropinized pigs produced a profuse flow of pancreatic juice with high HCO3 content concomitantly with a significant increase in pancreatic VIP output from 13 to 113 fmol/min. Intravenous administration of somatostatin (SRIF) during continuous electrical vagal stimulation caused a parallel suppression of the VIP release and the pancreatic fluid and HCO3 secretion to prestimulatory values. The SRIF-induced reduction in fluid and HCO3 secretion seemed to be mediated via an inhibition of the VIP release rather than through a direct effect on the exocrine cells, inasmuch as SRIF did not influence the VIP-provoked exocrine response from the in vitro isolated perfused porcine pancreas. The results support the view that VIP is transmitter in the vagally induced atropine-resistant water and HCO3 secretion from the porcine pancreas.