Plasma Immunoreactive Secretin Research Articles

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

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 Herrera's 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 NaHCO3. 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 NaHCO3. 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.

Effects of MCI-727, a New Anti-Ulcer Agent, on Plasma Secretin Concentration in Rats and Dogs and Pancreatic Exocrine Secretion in Rats.

In the present study, we investigated the effects of MCI-727, a new anti-ulcer agent, on plasma immunoreactive secretin concentration in rats and dogs using secretin specific RIA with the ethanol extraction method. Plasma secretin levels were increased dose-dependently 10 min after oral administration of MCI-727 in rats (control: 5.5 +/- 0.6; MCI-727, 10 mg/kg: 10.4 +/- 2.6; 30 mg/kg: 15.3 +/- 1.5; 100 mg/kg: 20.7 +/- 2.6 pg/ml, n = 6). Teprenone also caused a significant increase of plasma secretin at 10 min after oral administration at the doses of 30, 100 and 300 mg/kg. Under the same conditions, MCI-727 and teprenone did not alter the plasma immunoreactive gastrin concentration in rats. From the results of the time course study, the increasing effect of MCI-727 (30 mg/kg, p.o.) on plasma secretin remained for at least 240 min after administration. On the other hand, the increasing effect of teprenone (200 mg/kg, p.o.) was only observed at 30 and 60 min after administration. Furthermore, MCI-727 had increasing effects on plasma secretin concentration in dogs, but teprenone had no effects in this species. The volume of pancreatic secretion and the pancreatic bicarbonate output increased after intra-duodenal administration of MCI-727 at 30 and 100 mg/kg in rats. Similar effects were also observed with teprenone (30-300 mg/kg, i.d.) or secretin (Secrepan, 0.1-1.0 unit/kg, i.v.).

Bile-stimulated secretin release in cats.

Duodenal perfusion with sodium taurocholate (TC), pH 10.5 (60 mM, made isotonic with NaCl, 30 ml/h for 90 min), increased the plasma immunoreactive secretin level (IRS) from 2.1 +/- 1.0 to 19.1 +/- 6.0 pM (p less than 0.025; n = 6) in anaesthetized cats. Dose-response studies with TC, pH 7.2 (0-60 mM, made isotonic with NaCl, 45 ml/h for 20 min), demonstrated a threshold concentration of TC for IRS release between 10 and 15 mM (p = 0.05; n = 6). Instillation of gallbladder bile from the same animal (1.5 +/- 0.1 ml, pH 7.0 +/- 0.1, over 12 min) increased the IRS concentration from 1.5 +/- 0.4 to 8.1 +/- 3.4 pM (p less than 0.025; n = 6). IRS concentrations also increased when gallbladder bile was mixed with either saline (from 3.4 +/- 1.7 to 11.6 +/- 2.5 pM; p less than 0.05; n = 5) or feline pure pancreatic juice (PPJ) (from 3.1 +/- 0.7 to 5.7 +/- 1.1 pM; p less than 0.025; n = 6). The peak value was significantly lower when bile was mixed with PPJ than with saline (p less than 0.05). We conclude that TC, the dominating bile salt in feline bile, can release IRS also above pH 7, that physiological concentrations of TC can elicit this response, that the response is also produced by gallbladder bile from the same animal, and that PPJ inhibits the bile-induced IRS release. Thus secretin release by bile is likely to be a physiological principle in the regulation of the pancreatic secretion.

EARLYLIFE PATTERN OF PLASMA SECRETIN LEVEL IN CALVES

Newborn calves given colostrum showed an increase of plasma immunoreactive secretin (IRS), that did not occur in fasted animals. Thereafter, fasting plasma IRS levels were high in preruminant calves, with a clear fall 1 h after the morning meal. IRS levels were lower in animals given fish instead of milk protein. The postprandial decrease was no longer observed after weaning. Key words: Calf, secretin, preruminant, ruminant

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.

Effect of acid perfusion of the terminal ileum on plasma immunoreactive secretin and pancreatic secretion in the rat.

The distribution of immunoreactive secretin in acid extracts of the alimentary tract of the rat was determined. It was found to be contained in higher concentrations in the duodenum and in the terminal ileum than in other parts of the intestine. To elucidate the possible role of secretin contained in the terminal ileum, this part of the intestine was perfused with hydrochloric acid, a potent secretin secretagogue, and changes in plasma immunoreactive secretin levels and in pancreatic secretion were investigated in anaesthetized rats. Plasma immunoreactive secretin was increased from 65 +/- 5 pg/ml to 108 +/- 13 pg/ml during a 10 minutes perfusion with hydrochloric acid. This was significantly higher than that of saline perfusion but significantly lower than that of duodenal perfusion with hydrochloric acid. Acid perfusion of the terminal ileum also caused a three-fold increase in pancreatic flow rate and a twofold increase in bicarbonate concentration. These results suggest that the terminal ileum, through release of secretin, might play a role in regulating the pancreatic secretion of water and bicarbonate in response to changes in intraluminal fluids in the distal intestine in the rat.

Recovery of secretin in acid small intestinal lumen perfusates in the rabbit.

Immunoreactive secretin concentrations were measured in femoral venous plasma and in intestinal perfusates during acid and neutral perfusion of small intestinal segments in anesthetized rabbits. Steady-state plasma and luminal secretin concentrations were less than 20 pg/ml. Duodenal perfusion with with 0.1 N hydrochloric acid stimulated peak plasma immunoreactive secretin concentrations of 200 - 300 pg/ml within five minutes. Acid stimulated plasma concentrations remained constant during 30 minutes of perfusion. Secretin output into the acid perfusate (2000 - 4000 pg/min) accompanied the release into plasma. Subsequent perfusion with a neutral solution resulted in a rapid return to steady-state plasma and luminal secretin concentrations. Sequential acid perfusion of the ileum, jejunum, and duodenum stimulated similar concentrations of secretin in plasma and luminal perfusates.

Effect of sodium oleate on plasma secretin concentration and pancreatic secretion in dog

The effects of sodium oleate and triglyceride infused into the duodenum of dogs with chronic pancreatic fistulas on pancreatic secretion and plasma immunoreactive secretin were investigated. The responses were compared with those following intravenous secretin infusions. Oleate markedly increased pancreatic water and bicarbonate secretion in a dose-related fashion, and these increases were accompanied by a small, but significant rise in the level of circulating immunoreactive secretin. No change in pancreatic secretion or in plasma secretin level was observed by the administration of triglyceride. Comparison of the bicarbonate response after oleate infusion with that after secretin administration suggests that the pancreatic stimulatory effect of intraduodenal sodium oleate is mediated to a major extent by the release of endogenous secretin.

Pancreatic exocrine secretion and immunoreactive secretin release after intraduodenal instillation of 1-phenyl-1-hydroxy-n-pentane and HCl in rats.

Portal plasma immunoreactive secretin (IRS) concentrations, pancreatic juice flow, and amylase output were simultaneously measured in response to intraduodenal infusion of 1-phenyl-1-hydroxy-n-pentane (PHP), as well as infusion of hydrochloric acid (HCl). These data were compared with those obtained from intravenous bolus injections of synthetic porcine secretin in anesthetized rats. The intraduodenal infusion of PHP or HCl at a rate of 2 ml/min for 2 min produced a dose-related increase in portal plasma secretin concentrations, pancreatic juice flow, and amylase output. However, the mechanism of secretin release by PHP seems to differ from that of HCl. The secretin response to 0.1 N HCl infused at a rate of 0.1 ml/min for 30 min was complete after 10 min, despite continued infusion, while PHP stimulated a secretin release which persisted for 10 min after cessation of infusion. The pH in the second portion of the duodenum, following PHP infusion, remained consistently greater than 6.3. PHP-stimulated pancreatic exocrine secretions were only partially suppressed by somatostatin, while secretin release was almost completely inhibited. However, intraduodenal PHP may stimulate the release of secretin along with other gastrointestinal hormones, and the endogenous release of these hormones may not be inhibited by somatostatin.

The effect of glucose on plasma immunoreactive secretin in normal man and after gastrectomy or pancreatectomy.

Oral intake of glucose increased the peripheral plasma immunoreactive secretin (IRS) concentration in patients after total pancreatectomy with duodenectomy (p < 0.05) and in patients after total gastrectomy (p < 0.01). In young healthy volunteers and in patients after partial gastrectomy, no effect on IRS was observed. Intravenous glucose reduced the IRS concentration in pancreatectomized patients (p < 0.05), whereas IRS was only reduced in young healthy volunteers when intravenous glucose was followed by oral glucose after an interval of 10 min (p < 0.025). The abnormal IRS release in pancreatectomized patients and in patients after total gastrectomy might be due to rapid intestinal passage and/or increased splanchnic blood flow induced by hypertonic glucose. A third possibility is that a glucose-induced increase in bile flow might release IRS.

Pure synthetic bile salts release immunoreactive secretin in man.

When 20 ml of 50 mmol/l pure synthetic bile salts (pH 6.5), of approximately the same composition as human bile, were infused into the duodenum of six healthy volunteers over 5 min during constant gastric aspiration, plasma immunoreactive secretin increased from 2.5 +/- 0.4 pmol/l to 3.6 +/- 0.5 pmol/l at 10 min (p < 0.025). Gastric acid secretion was not increased. No effect was seen when the same amount of physiological saline was infused under the same conditions in another group. It is concluded that bile salts in an amount and composition corresponding to that in the human gallbladder is able to release immunoreactive secretin from the human intestine.

The effect of atropine on secretin release and aspirated bicarbonate secretion after duodenal acidification in man.

The effect of duodenal instillation of hydrochloric acid on peripheral plasma immunoreactive secretin (IRS) concentration--without or with a constant intravenous infusion of atropine--was studied in eight healthy volunteers. Without atropine infusion, IRS increased from 1.2 +/- 0.3 pmol/l to a peak value of 17.1 +/- 4.4 pmol/l. The volume of duodenal aspirates increased from 0.74 +/- 0.12 ml/min to 5.03 +/- 0.46 ml/min, and the bicarbonate output from 7.6 +/- 1.7 mumol/min to 349 +/- 39 mumol/min. With atropine infusion (bolus of 500 micrograms + 750 micrograms/h of atropine sulphate), IRS increased from 1.4 +/- 0.3 pmol/l to 14.8 +/- 3.6 pmol/l. The volume of the duodenal aspirates increased from 0.71 +/- 0.08 ml/min to 2.76 +/- 0.37 ml/min and the bicarbonate output from 4.0 +/- 1.6 mumol/min to 142 +/- 33 mumol/min. The integrated IRS response to acidification was 18% lower during atropine infusion (p < 0.05). Both the aspirated flow rate from the duodenum and the bicarbonate output were markedly reduced by atropine infusion, 45% and 59%, respectively (p < 0.005).

Physiological doses of secretin do not stimulate acute insulin release.

To study the effect of physiological increments in plasma secretin concentrations on basal and glucose-stimulated insulin release, bolus iv glucose injections (5 g) were given to normal weight volunteers (less than 108% ideal BW) before, during, and 30 min after a secretin infusion at a rate of 0.125 U/kg.h, raising mean plasma immunoreactive secretin to 35.5 +/- 8.3 pg/ml. Acute insulin responses to glucose were unaffected during or after the secretin infusion. Furthermore, when plasma glucose was elevated to postprandial levels (128--165 mg/dl), a similar secretin infusion also failed to alter acute insulin responses. In addition, no changes in basal glucose or insulin levels were found when endogenous secretin concentrations were increased by intraduodenal acid infusion. Thus, increases in plasma secretin to concentrations seen in the postprandial state fail to alter acute insulin secretion. It is unlikely that secretin plays any role in the intestinal stimulation of insulin secretion.

Plasma secretin level and exocrine pancreatic secretion in response to intraduodenal infusion of 1-phenyl-1-hydroxy-n-pentane and HCl by using a new four lumen tube.

We have designed a four lumen tube which has enabled us to aspirate duodenal juice while infusing endogenous stimuli into the lower part of duodenum. Exocrine pancreatic secretion and plasma immunoreactive secretin (IRS) levels have been simultaneously observed in response to intraduodenal infusion of 1-phenyl-1-hydroxy-n-pentane (PHP) and 0.1 N HCl in normal subjects, patients with diabetes mellitus and patients with chronic pancreatitis by use of this tube. (1) Pancreatic flow rate and bicarbonate concentration were found to increase to twice those of basal levels and plasma IRS levels rose significantly after infusion of PHP or HCl in normal subjects. The same tendency was observed in patients with diabetes mellitus. (2) Exocrine pancreatic secretion did not change after infusion of PHP or HCl in patients with chronic pancreatitis. Pancreatic flow rate, bicarbonate concentration and amylase output were significantly low in patients with diabetes mellitus. (3) Pancreatic flow rate and bicarbonate concentration incresed to about twice those of basal levels after both PHP and HCl infusion. On the other hand, although pancreatic amylase output increased significantly after infusion of HCl as compared with basal secretion, it did not change after infusion of PHP. It appeared reasonable to conclude that PHP is a morespecific substance for releasing secretin than HCl.

Enkephalin inhibits the release and action of secretin on pancreatic secretion in the dog.

1. Pancreatic bicarbonate and protein secretion as well as immuno-reactive plasma secretin concentration in response to a meal, duodenal acidification and exogenous secretin or octapeptide of cholecystokinin (OP-CCK) have been measured following administration of methionine-enkephalin in chronic pancreatic fistula dogs. 2. Methionine-enkephalin inhibited pancreatic responses to both exogenous hormones (secretin and OP-CCK) and to endogenous hormones released from the gut by food or duodenal acidification. 3. Naloxone, a potent opiate receptor antagonist, partly prevents this methionine-enkephalin-induced inhibition of pancreatic secretion suggesting that this effect might be mediated by opiate receptors. 4. The inhibitory effect of methionine-enkephalin on pancreatic response to endogenous stimulants was more pronounced than that to exogenous hormones and was accompanied by a significant reduction in plasma immuno-reactive secretin concentration. 5. This study indicates that methionine-enkephalin inhibits pancreatic secretion, at least in part, by suppressing the release of the intestinal hormones stimulating the exocrine pancreas.

Effect of rabbit antisecretin serum on postprandial pancreatic secretion in dogs

To determine the contribution made by endogenous secretin to pancreatic bicarbonate secretion in the postprandial state, normal rabbit serum or rabbit antisecretin serum was administered intravenously to dogs with a pancreatic fistula and gastric cannula. In the four dogs tested, plasma immunoreactive secretin concentration increased significantly after ingestion of a standard meal containing meat. In nine dogs, the administration of rabbit antisecretin serum resulted in a profound decrease in pancreatic flow and the concentration and output of pancreatic bicarbonate after the meal. The total bicarbonate output decreased to < 20% of that produced during the control experiment using normal rabbit serum. Protein output decreased significantly also, although its concentration was unchanged. The antisecretin serum also abolished the pancreatic secretion of bicarbonate and protein stimulated by exogenous secretin (GIH) in doses of 0.125 and 0.25 clinical unit/kg-hr. The amount of antisecretin antibody employed in the study produced antisecretin titers in the plasma sufficiently high to bind all of the endogenous secretin present in the plasma. These studies indicate that endogenous secretin plays an important role on the regulation of exocrine pancreatic secretion in the postprandial state.

Plasma secretin concentrations in fasting and postprandial states in dog.

In four dogs with a modified Herrara pancreatic fistula and gastric cannula and three dogs with two duodenal cannulas, ingestion of a meat meal resulted in a significant and sustained increase in the mean plasma immunoreactive secretin concentrations, from mean fasting levels of less than 10 pg/ml to 25--55 pg/ml. This increase in the plasma secretin concentration coincided with a marked increase in pancreatic bicarbonate output and frequent decreases in the mean proximal duodenal pH to less than 4.5 from the range of 6.5 in the fasting state. Intravenous administration of cimetidine, 150 mg, produced a marked suppression of postprandial increases in both pancreatic bicarbonate output and plasma secretin concentration. Moreover, the postprandial duodenal pH rarely reached below 5.0 after cimetidine administration. These studies indicate that plasma secretin concentration does increase significantly after a meal. The postprandial increase in plasma secretin concentration appears to depend on the gastric acid delivered in the proximal duodenum. A possible physiological role of secretin in the pancreatic secretion after a meal is indicated by these findings.

The Effect of Secretin on the Exocrine and Endocrine Pancreas

Previous animal experiments have given contradictory results concerning the effect of secretin on insulin release. The conflicting results are probably due to the fact that the injected hormone was administered in pharmacological amounts not usually attained under physiological conditions in vivo and that the preparation of the secretin used was an extraction which might have contained some other hormones. Moreover, small changes in insulin secretion are reported to remain undetected as the result of hepatic insulin clearance and of the dilution in the peripheral circulation. We have, therefore, investigated the effect of endogenous secretin released by the intraduodenal instillation of HC1 and 1-phenyl-1- hydroxy-n-pentane (PHP), and intravenously infused synthetic secretin in doses which raised portal plasma immunoreactive secretin (IRS) concentrations to those obtained after HC1 and PHP administration on portal plasma immunoreactive insulin (IRI), glucagon (IRG) and glucose concentration. In addition, the effect of somatostatin on IRI and IRG concentrations following acid and PHP administration was investigated by infusing synthetic cyclic somatostatin at the dose of 5 μg/kg/h.Male Wistar rats weighing 240-270g and fasted overnight were used in all experiments. The rats were anesthetized with sodium pentobarbital by a subcutaneous injection. The body temperature was held at 37°C by a heating pad. One of the following solutions was infused into the duodenum at a rate of 2 ml/min for 2 min by means of a micro-tubing pump : 0.1 mo1/1 HC1,200 mg/kg/2 ml PHP and 2.5% arabic gum solution.Blood samples were collected through an indwelling needle inserted into the portal vein in the direction of the venous blood flow at 0, 5, 10, 15, 20 and 30 min after the initiation of the infusion. The samples were centrifuged at 4°C, and plasma was stored at - 20°C until assayed.IRI was measured by polyethylene glycol radioimmunoassay. IRG was determined by radioimmunoassay by means of a talc absorption technique using antiserum 30K. Rat insulin and porcine glucagon were used as standards in the IRI and IRG assay, respectively. Glucose concentrations were measured by the glucose oxidase method with a Toshiba LAC-02A glucose analyzer.Although there was a rapid rise in serum IRS concentrations at 5 min after the administration of HCl and PHP, no significant changes in IRI concentrations were elicited by these stimuli. By contrast, IRG concentrations were significantly increased at 30 min after the intraduodenal infusion of HC1.Constant infusion of somatostatin did not influence either IRI or IRG levels during intraduodenal stimulation with HCl and PHP, but it profoundly inhibited the HC1-induced IRG increase.When synthetic secretin was infused to elevate the portal plasma IRS levels up to the same levels as those after HCl or PHP, no changes in the levels of IRI and IRG in the portal venous blood were observed.The present observations with direct intraduodenal instillation of acid and PHP in quantitites sufficient to produce significant elevations of portal venous IRS concentrations did not result in an elevation of portal plasma IRI levels. In addition, the administration of physiological quantities of synthetic secretin failed to elicite any effect on portal IRI levels. It is therefore unlikely that secretin is responsible for entero-insular axis.

Plasma secretin concentrations in fasting and postprandial state in man.

Plasma immunoreactive secretin concentrations were determined in both healthy subjects and patients with duodenal ulcer. The modified radioimmunoassay method could detect significant increases in the plasma secretin concentrations when 0.05 N HCl was infused intraduodenally at a rate of 1.1 and 2.2 ml/min. The mean fasting plasma secretin concentration of 13 normal healthy subjects was 4.4 +/- 0.38 pg/ml which was significantly less (P less than 0.01) than that of 13 duodenal ulcer patients, 6.9 +/- 0.64 pg/ml. In both groups ingestion of a meat-containing meal resulted in significant increase in the plasma secretin concentrations. Recording of pH from proximal duodenum indicated that pH fell periodically below 4.5 during the postprandial period, indicating that only a short segment of proximal duodenum was exposed to acid after meal. The postprandial rise in plasma secretin levels was abolished when antral pH was raised 5.5 by intragastric infusion of 0.3 N NaHCO3 solution. These observations indicate that although fasting plasma secretin levels are low, the plasma secretin levels increase significantly after ingestion of a meal. This increase appears to be attributable to an increased amount of acid delivered to the proximal duodenum, and patients with duodenal ulcer were found to release more secretin during the postprandial period than normal subjects.

Effect of secretin on basal- and glucose-stimulated insulin secretion in man.

Plasma immunoreactive secretin and insulin concentrations were measured in fasting normal humans after intraduodenal infusions of hydrochloric acid, isotonic or hypertonic glucose. The effect of intraduodenal acidification or intravenous bolus injections of secretin on plasma insulin concentrations during infusions of glucose was also examined. The intraduodenal glucose load did not cause an increase in plasma secretin concentrations. Secretin concentrations rose after acid both in the fasting state and during infusions of glucose. A concomitant rise in insulin levels was however only observed during infusions of glucose. Intravenous injection of secretin in a dose which mimicked the response to intraduodenal acidification was without effect on the glucose-stimulated insulin release, while a 30 times higher dose caused a highly significant augmentation of the insulin release. The insulin response pattern to this high dose of secretin differed completely from that observed after intraduodenal infusion of acid. It is concluded and confirmed that the stimulating effect of secretin on insulin secretion is pharmacological and that secretin plays no significant role in the entero-insular axis.