The effect of duodenal infusion of bile on plasma VIP, GIP, and secretin and on duodenal bicarbonate secretion

Nutrient-Induced Cellular Mechanisms of Gut Hormone Secretion

The gastrointestinal tract can assess the nutrient composition of ingested food. The nutrient-sensing mechanisms in specialised epithelial cells lining the gastrointestinal tract, the enteroendocrine cells, trigger the release of gut hormones that provide important local and central feedback signals to regulate nutrient utilisation and feeding behaviour. The evidence for nutrient-stimulated secretion of two of the most studied gut hormones, glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), along with the known cellular mechanisms in enteroendocrine cells recruited by nutrients, will be the focus of this review. The mechanisms involved range from electrogenic transporters, ion channel modulation and nutrient-activated G-protein coupled receptors that converge on the release machinery controlling hormone secretion. Elucidation of these mechanisms will provide much needed insight into postprandial physiology and identify tractable dietary approaches to potentially manage nutrition and satiety by altering the secreted gut hormone profile.

Bile acid sequestrants in type 2 diabetes: potential effects on GLP1 secretion

Bile acid sequestrants have been used for decades for the treatment of hypercholesterolaemia. Sequestering of bile acids in the intestinal lumen interrupts enterohepatic recirculation of bile acids, which initiate feedback mechanisms on the conversion of cholesterol into bile acids in the liver, thereby lowering cholesterol concentrations in the circulation. In the early 1990s, it was observed that bile acid sequestrants improved glycaemic control in patients with type 2 diabetes. Subsequently, several studies confirmed the finding and recently - despite elusive mechanisms of action - bile acid sequestrants have been approved in the USA for the treatment of type 2 diabetes. Nowadays, bile acids are no longer labelled as simple detergents necessary for lipid digestion and absorption, but are increasingly recognised as metabolic regulators. They are potent hormones, work as signalling molecules on nuclear receptors and G protein-coupled receptors and trigger a myriad of signalling pathways in many target organs. The most described and well-known receptors activated by bile acids are the farnesoid X receptor (nuclear receptor) and the G protein-coupled cell membrane receptor TGR5. Besides controlling bile acid metabolism, these receptors are implicated in lipid, glucose and energy metabolism. Interestingly, activation of TGR5 on enteroendocrine L cells has been suggested to affect secretion of incretin hormones, particularly glucagon-like peptide 1 (GLP1 (GCG)). This review discusses the role of bile acid sequestrants in the treatment of type 2 diabetes, the possible mechanism of action and the role of bile acid-induced secretion of GLP1 via activation of TGR5.

Prolonged stimulation of pancreatic serous secretions by bile and sodium taurocholate in anaesthetized rats

There have been numerous reports that infusion of either natural bile or bile salts into the duodenum evokes a rapid increase in pancreatic secretion through the release of the hormone secretin from the duodenal mucosa. We have extended this observation by the demonstration of an additional late increase in secretion which persisted for many hours and have sought to identify the processes underlying this increase. In anaesthetised rats, infusion of 20 mM taurocholate into the duodenum caused a staircase-like increase in the weight of pancreatic secretion which extended over many hours during which, the HCO[Formula: see text] and protein output of the secretion showed only minimal changes. This effect was also reproduced with intra-duodenal infusion of natural bile which was inferred to act though its taurocholate content. Since the stimulatory action was also obtained with superfusion of taurocholate or natural bile onto the small intestine and by intravenous injection of taurocholate, it was concluded that taurocholate acted by being absorbed into the bloodstream and then by exerting a stimulatory action on the exocrine pancreas. This action was inhibited by puromycin (a protein synthesis inhibitor), by furosemide (a Na( + )/K( + )/2Cl(-) cotransporter inhibitor), though not by SITS (an inhibitor of Cl(-)/HCO[Formula: see text] exchange). The long lasting increase in pancreatic serous secretion would be consistent with the possible activation of gene transcription by taurocholate leading to increased activity of the Na( + )/K( + )/2Cl(-) cotransporter through which the acinar cells increased their secretions.

Negative control of human pancreatic secretion: physiological mechanisms and factors

The negative control of pancreatic exocrine secretion in man occurs during the interdigestive and postprandial periods of the digestive cycle. The physiological mechanisms involved include negative feedback mechanisms, well described and accepted in animals, and controlled by the cholecystokinin- and secretin-releasing factors of pancreatic and duodenal origin, along with the active pancreatic proteases present in the upper gut. The presence of these factors and their efficacy in humans, however, have their supporters and detractors, with a possibility for reconciliation among opponents. Besides these releasing factors, hormones, mostly from the intestine, are also involved in this inhibitory process of pancreatic secretion. Somatostatin, peptide YY, pancreatic polypeptide, glucagon, ghrelin, and leptin were described as potentially involved from studies mostly performed on animals. Finally, bile and bile salts have mixed responses on this inhibition, and their effects seem to be at the intestine level with gastrointestinal hormones involved. Future studies will have to be performed in humans to determine the presence of cholecystokinin- and secretin-releasing factors and their role. Finally, the demonstrated modulatory action of hormones and bile acids in other species needs to be confirmed in humans.

Long-term bile diversion enhances basal and duodenal oleate-stimulated pancreatic exocrine secretion in dogs

There have been no previous reports whether long-term bile diversion enhances pancreatic exocrine secretion. The aim of this study was to elucidate the effect of long-term bile diversion on pancreatic exocrine secretion. Four mongrel dogs were prepared for chronic gastric and pancreatic fistulas and received intraduodenal sodium oleate infusion (controls). These dogs, then underwent diversion of bile from the intestines by ligating the common bile duct and interposing a segment of jejunum between the gallbladder and the urinary bladder (total biliary diversion [TBD]). After three weeks, the dogs received an identical sodium oleate infusion. TBD augmented basal pancreatic exocrine secretion compared with controls (4.4-fold increase in basal flow volume; 9.0-fold increase in bicarbonate output; and 3.3-fold increase in protein output). Likewise, TBD augmented oleate-stimulated exocrine secretion (2.0-fold increase in cumulative flow volume; 2.6-fold increase in bicarbonate output; and 1.4-fold increase in protein output). TBD also augmented basal and oleate-stimulated plasma cholecystokinin levels. Administration of a Cholecystokinin-A receptor antagonist (loxiglumide) after TBD reduced the flow volume and bicarbonate output to the control levels, and the protein output to less than a half of the control level. Long-term bile diversion enhances basal and oleate-stimulated pancreatic exocrine secretion, at least partly via increased cholecystokinin secretion.

Secretion of gastric inhibitory polypeptide in patients with bile duct obstruction

BACKGROUND

The direct contribution of bile to gastric inhibitory polypeptide (GIP) release and the role of bile in regulating GIP secretion in response to fat ingestion are still obscure. The present study was aimed to clarify the influence of bile on GIP release.

METHODS

Seven patients with obstruction of the common bile duct and nine volunteers participated in the study. Fifty milliliters of Lipomul was ingested, and GIP was measured serially for 180 min. After intraduodenal instillation of pooled autologous bile for 2 days, the same study was carried out.

RESULTS

The fat-stimulated GIP response was significantly lower in the patients than in the controls. The basal GIP level did not change on bile instillation, but the GIP response to fat ingestion was significantly increased on bile instillation compared with that in the absence of bile.

CONCLUSIONS

Intraluminal bile alone does not stimulate the secretion of GIP, but it promotes GIP secretion in response to fat ingestion.

Enteric hyperoxaluria associated with external biliary drainage

Several chronic disorders of small bowel function have been associated with enteric hyperoxaluria. We report on a patient in whom enteric hyperoxaluria and recurrent calcium oxalate calculi developed during treatment with temporary external biliary drainage for sclerosing cholangitis. The hyperoxaluria subsequently resolved with reestablishment of intraluminal biliary flow at successful liver transplantation. We propose a mechanism for this previously unrecognized cause of hyperoxaluria.

The mediators of bile action on the exocrine pancreas

Under basal conditions, bile and bile salts applied intraduodenally influence plasma levels of several gastroenteropancreatic peptides. Besides those with stimulatory effects on exocrine pancreatic secretion, others with inhibitory or no effects are released as well. Furthermore, cholinergic and peptidergic neural mechanisms may also be activated. Secretin seems to be the most important mediator of bile- or bile salt-induced water and bicarbonate secretion. In addition, VIP released from peptidergic nerve endings in the pancreas may also be involved in the mediation of the hydrokinetic effect. With regard to water and bicarbonate secretion, cholinergic mechanisms probably are of minor importance. Cholinergic mechanisms, however, seem to be the most important mediator of bile- or bile salt-induced pancreatic enzyme secretion. CCK may act as an additional mediator of the ecbolic effect. This statement, however, is based on few results only and has to be confirmed by further studies. Gastroenteropancreatic peptides with an inhibitory action on the exocrine pancreas were also released by intraduodenal bile or bile salts. Somatostatin is released in physiologically relevant amounts to bring about a counter-regulation. Plasma PP levels are also enhanced by bile and bile salts. The amounts of PP released, however, are below those observed postprandially. In contrast to their stimulatory action on basal pancreatic secretion, bile and bile salts have no or even an inhibitory effect on pancreatic secretion stimulated by intraluminal nutrients. Accordingly, the release of gastroenteropancreatic peptides is not influenced (for example, secretin) or even reduced (for example, CCK) when bile or bile salts are added to intraluminal nutrients.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of vasoactive intestinal polypeptide (VIP), secretin and gastrin in the genesis of the late exocrine pancreatic hypersecretion, food intake dependent in conscious dogs

Plasma levels of vasoactive intestinal polypeptide (VIP), secretin and gastrin were studied in four saphenous vein-catheterized dogs during the first 12 h after ingestion of a standard solid meal. Under these conditions we found significant postprandial increases in secretion only, which rose from a basal value of 219 +/- 27 pg/ml to 449 +/- 66 pg/ml 60 min postprandial (p < 0.001), and remained elevated until 4 h after food intake. However, no increase was seen from 8 to 12 h in any of the hormones studied, indicating that they are not directly involved in the enhancement of exocrine pancreatic secretion during this period.

Effect of intraduodenal bile and Na- taurodeoxycholate on exocrine pancreatic secretion and on plasma levels of secretin, pancreatic polypeptide, and gastrin in man

The effect of intraduodenally administered cattle bile (CB) and Na-taurodeoxycholate (TDC) on basal pancreatic secretion and plasma levels of secretin, pancreatic polypeptide (PP), and gastrin were investigated on two separate days in 10 fasting volunteers. Doses of 2-6 g CB and 200-600 mg TDC were given intraduodenally at 65-min intervals. Volume, bicarbonate, lipase, trypsin, amylase, and bilirubin were measured in 10-min fractions of duodenal juice, and GI peptides determined by radioimmunoassay. CB and TDC enhanced significantly and dose-dependently volume, bicarbonate and enzyme secretion, and plasma secretin and PP levels. In contrast, plasma gastrin showed only a marginal increase. We conclude that the hydrokinetic effect of intraduodenal CB and TDC is at least partially mediated by secretin. Gastrin could be ruled out as a mediator of the ecbolic effect, whereas other GI peptides, primarily CCK, and/or neural mechanisms must be considered possible mediators. Both pathways may also play a role in the PP release observed.

Effects of bile salts on plasma concentration of beta-endorphin-like immunoreactivity in men

The effects of bile salts on the release of beta-endorphin-like immunoreactivity (beta-END-LI) were investigated in men using a specific radioimmunoassay developed by the authors. Plasma beta-END-LI was determined after extraction by the acid-acetone method (recovery: 73 +/- 5%). Oral administration of 400 mg of sodium taurocholate caused a rise in plasma beta-END-LI from 9.9 +/- 0.5 pmol/liter to 21.3 +/- 1.2 pmol/liter after 30 min and 18.1 +/- 0.5 pmol/liter after 60 min, with return to the initial value after 90 min. Oral administration of chenodeoxycholic acid (CDCA) also increased plasma beta-END-LI from a basal level of 8.4 +/- 0.7 pmol/liter to 18.7 +/- 0.8 pmol/liter after 30 min. Oral administration of ursodeoxycholic acid (UDCA) increased plasma beta-END-LI from 7.3 +/- 0.3 pmol/liter to 30.6 +/- 0.2 pmol/liter after 30 min. In gel chromatography, the beta-END-LI released after UDCA administration separated into two components, which eluted in the same positions as human beta-lipotropin and human beta-endorphin, respectively. These results suggested that bile salts may participate the release of beta-END-LI.

Pancreatic secretory response to intestinal stimulants: a review

In humans and many laboratory animals, protein digestion products such as peptides and amino acids and fat digestion products such as fatty acids and monoglycerides are potent intestinal stimulants of pancreatic enzyme secretion. The pancreatic enzyme response to these intestinal stimulants is related to the perfused load (amount per unit time) rather than to concentration. Both neural and hormonal pathways mediate the enzyme response to these intestinal stimulants. Enteropancreatic, cholinergic, vago-vagal reflexes are probably the most important mediators of the enzyme response to low loads of amino acids and fatty acids; hormones, such as cholecystokinin, seem to be the major mediators of the response to high loads of amino acids and fatty acids. Under physiological conditions it is probably the interplay of neural and hormonal mechanisms which regulates the pancreatic response to these stimulants. Gastric acid is the major regulator of postprandial pancreatic bicarbonate secretion. Secretion released by HCl is probably the most important physiological hormonal mediator of postprandial pancreatic bicarbonate secretion; its effect being potentiated by extrinsic (vagal) and intrinsic (intrapancreatic) cholinergic nerves and release of other hormones, such as cholecystokinin.

Na-taurodeoxycholate acts as a specific intestinal stimulus of exocrine pancreatic secretion in man

The effect of intraduodenally administered cattle bile, Na-taurodeoxycholate, and Na-taurocholate on secretin-stimulated exocrine pancreatic secretion was investigated on 40 fasting young healthy volunteers. Intraduodenal bile stimulated significantly and dose-dependently hydrokinetic and ecbolic pancreatic secretion. Only bile, but not secretin intravenously, both applied in a dosage equivalent with respect to their hydrokinetic action, caused a significant increase of enzyme output and enzyme concentration as well. Intraduodenal Na-taurodeoxycholate enhanced also dose-dependently secretin-stimulated volume, bicarbonate, and enzyme secretion. The effect was related to the load, not to the concentration of this bile salt. On the other side, Na-taurocholate had only a weak and not dose-dependent hydrokinetic and no ecbolic effect. It is concluded that not bile salts in general, but only certain of them--like Na-taurodeoxycholate--are the effective constituents of bile, acting as specific intraduodenal stimulants of hydrokinetic and ecbolic pancreatic secretion.

Plasma secretin in response to pure bile salts and endogenous bile in man

The effect of exogenous bile salts on plasma concentrations of secretin was studied by infusion of chenodeoxycholate, cholate, glycocholate, and taurocholate into the duodenum of normal subjects. The effect of endogenous bile on plasma secretin was studied by ingestion of a liquid test meal, by reinfusion of postprandial duodenal aspirates with known contents of bile salts, and by stimulation of gallbladder contraction by cholecystokinin. Each experiment was performed in groups of seven subjects. The relative secretin-releasing potencies of glycocholate, cholate, taurocholate and chenodeoxycholate (2.25 mmol) were 1.0:1.3:1.9:3.2. Hydrochloric acid (0.5 mmol) was, on a molar basis, approximately ten times more potent than sodium cholate. The effect of taurocholate was diminished when a liquid meal was used as vehicle instead of saline. Endogenous bile did, in no circumstance, elicit release of secretin. It is concluded that although bile salts have the ability to stimulate secretin release, endogenous secretin release is of minor, if any, importance for secretin release when physiological conditions are approached.

Release of secretin immunoreactivity from the ileum

In the rat, about 80% of the intestinal content of secretin immunoreactivity is found distal to the duodenum with a peak in the ileum. This work therefore studied whether the secretin immunoreactivity in distal ileum could be released by substances normally found in the intestinal lumen. An isolated loop of ileum or duodenum was perfused with 100 mM HCl, 3 mM HCl, 50 mM taurocholate, 50 mM oleate, 308 mM glucose, 273 mM amino acids, or 308 mM NaCl in anesthetized rats. Release of secretin immunoreactivity was estimated by the integrated response above basal in portal blood. Substantial release of secretin immunoreactivity was seen after both ileal and duodenal perfusion with concentrated HCl and taurocholate, whereas perfusion with oleate was followed by a lesser response. The response to taurocholate was slower than that to HCl, but more likely represents a physiologic mechanism. The results show that the distal small intestine is capable of contributing to secretin immunoreactivity in the circulation and it is speculated that bile salts and secretin constitute parts of a negative feedback loop.

The effect of insulin-induced hypoglycemia with and without atropine on plasma vasoactive intestinal polypeptide in man

Plasma vasoactive intestinal polypeptide (VIP) was measured in six healthy male students on 2 separate days after insulin-induced hypoglycemia with and without atropine and on a 3rd day in five of the students after atropine alone. A significant increase in peripheral plasma VIP was observed when atropine was given together with insulin, whereas insulin or atropine alone had no effect on plasma VIP. It is suggested that cholinergic nicotinic receptors may be involved in the increase of VIP after insulin-induced hypoglycemia and that the lack of VIP increase seen after insulin alone may be caused by an inhibitory effect of other gastrointestinal hormones.