Plasma secretin, plasma cholecystokinin, pancreaticobiliary secretion, and fat absorption: effect of duodenal osmolality and polysorbate 80

Expression analysis of a cholecystokinin system in human and rat white adipose tissue

AIM

Cholecystokinin (CCK) participates in the storage of dietary triglycerides in white adipose tissue (WAT). Our goal was to characterize, both in subcutaneous (Sc-WAT) and visceral WAT (Vis-WAT), the functional expression of the two known CCK receptors, CCK-1 (CCK-1R) and CCK-2 (CCK-2R), as well as of CCK.

MAIN METHODS

Gene and protein expression was assessed in different cell types of rat and human WAT by means of RT-PCR and western-blot, respectively. The functionality of CCK-Rs was tested by quantifying protein kinase B (Akt) phosphorylation after treatment of pre-adipocytes with the bioactive fragment of CCK, CCK-8. The CCK receptor subtype involved in Akt phosphorylation was investigated by using selective CCK-1R (SR-27,897) and CCK-2R antagonists (L-365,260).

KEY FINDINGS

In rats, CCK-1R (Cckar) and CCK-2R (Cckbr) gene expression was detected in the two types of WAT analyzed as well as in isolated adipocytes, mesenchymal stem cells and pre-adipocytes. CCK-1R and CCK-2R proteins were identified in adipocytes and, to a minor extent, in pre-adipocytes. In addition, CCK-2R were detected in subcutaneous mesenchymal stem cells. Gene expression of the CCK precursor preproCCK as well as CCK immunoreactivity were also found in Sc-WAT and Vis-WAT. In human WAT, CCK gene expression as well as CCK-2Rs and CCK were also identified. CCK-8 evoked Akt phosphorylation in rat pre-adipocytes, and this effect was antagonized by SR-27,897 and L-365,260.

SIGNIFICANCE

Our data show that both human and rat WAT express a complete CCK system, and suggest that CCK may have an autocrine/paracrine role in regulating adipose tissue biology.

Adaptation of the exocrine pancreas to dietary fats

This article reviews studies on the adaptation of the exocrine pancreas to dietary fat. We include all the latest information about the mechanisms that underlie the adaptation of the secretory mechanism of the exocrine pancreas to the amount and the type of dietary fat. We review the kinetics of pancreatic adaptation and the mediators of the adaptive response of the pancreas including cellular and molecular mechanisms (modulation of intracellular messengers and gene expression of the different enzymes and secretagogues involved in the adaptation process). At the same time we include our results in this field in dogs and humans.

A new twist in the brain-gut axis

Gastrointestinal functions are precisely regulated by hormonal and neural negative feedback loops. In addition to the classic hormonal and vago-vagal reflex mechanisms, these studies indicate that there are direct actions of gut hormones on the dorsal vagal complex. The current data demonstrate that pancreatic polypeptide is released into the circulation by vagal-cholinergic dependent mechanisms. It travels to the brainstem in the circulation, transverses the blood-brain barrier through "leaky" regions of this barrier in the area postrema and nucleus of the tractus solitarius and binds to specific receptors in the dorsal vagal complex. By binding to these sites, pancreatic polypeptide can directly inhibit vagal input to the pancreas and other gastrointestinal organs. These observations provide an anatomic basis to explain why pancreatic polypeptide is a more potent inhibitor of the action of central stimulants of pancreatic secretion than it is of the response to peripheral secretagogues. They also establish a novel mechanism by which gut peptides can influence brain function directly.

Effects of oleic acid and oleyl alcohol on cholecystokinin and secretin in plasma and pancreatobiliary secretion

To evaluate the importance of the terminal carboxy group of the oleic acid molecule in the release of secretin and cholecystokinin (CCK) to plasma, six normal persons were stimulated twice with duodenal perfusates containing either 20 mM oleic acid or 20 mM oleyl alcohol. Oleic acid increased the pancreatic secretion of bicarbonate and amylase and the concentrations of secretin and CCK in plasma and provoked gallbladder emptying. Oleyl alcohol only increased the amylase output slightly, but significantly. It is concluded that the carboxy group of the fat molecule has an important role in triggering the release of secretin and CCK to plasma.

Fat and pancreatic secretion

In 10 healthy volunteers we investigated the effects of intraduodenal oleic acid at various concentrations (0-40 mM) and at various degrees of emulsification on pancreaticobiliary secretion and the release of secretin and cholecystokinin (CCK) into plasma. We found that the release of the two hormones was directly related to the dose of fat and to the degree of emulsification. The threshold of CCK release (and amylase output) was low in comparison with the threshold for secretin release (and bicarbonate or volume output). When the degree of emulsification of the fat was increased, no simple relation was observed between hormone levels and pancreatic exocrine secretion. The output of bile salts was identical at various fat concentrations. We conclude that both secretin and CCK are dose-dependently released by emulsions of oleic acid in normal man and that the thresholds for release are probably different.

Effect of trypsin on the hormonal regulation of the fat-stimulated human exocrine pancreas

To study the effects of trypsin on the pancreaticobiliary secretion and the release of secretin and cholecystokinin (CCK) to plasma, seven normal subjects were stimulated twice with duodenal perfusates containing 20 mM oleic acid (pH 6.0) with and without 1 g of bovine trypsin added per liter. In addition, six patients with advanced pancreatic insufficiency who received only the oleic acid were compared with eight normal subjects. The concentrations of secretin and CCK in plasma and the pancreatic enzyme and volume secretions were unaffected by the addition of trypsin, but the initial bile acid output and the bicarbonate secretion in the period after gallbladder emptying were reduced during perfusion with trypsin. The severely reduced enzyme secretion in chronic pancreatitis did not influence the basal or oleic acid-stimulated concentrations of the hormones in plasma. The study does not support the hypothesis of a trypsin-mediated negative feedback control of human pancreatic enzyme secretion. Furthermore, the reduced duodenal output of bicarbonate found in response to trypsin is not explained by changes in the release of secretin or CCK.