Role of calcium in monitor peptide-stimulated cholecystokinin release from perifused intestinal cells

The extracellular calcium-sensing receptor is required for cholecystokinin secretion in response to L-phenylalanine in acutely isolated intestinal I cells

The extracellular calcium-sensing receptor (CaSR) has recently been recognized as an L-amino acid sensor and has been implicated in mediating cholecystokinin (CCK) secretion in response to aromatic amino acids. We investigated whether direct detection of L-phenylalanine (L-Phe) by CaSR results in CCK secretion in the native I cell. Fluorescence-activated cell sorting of duodenal I cells from CCK-enhanced green fluorescent protein (eGFP) transgenic mice demonstrated CaSR gene expression. Immunostaining of fixed and fresh duodenal tissue sections confirmed CaSR protein expression. Intracellular calcium fluxes were CaSR dependent, stereoselective for L-Phe over D-Phe, and responsive to type II calcimimetic cinacalcet in CCK-eGFP cells. Additionally, CCK secretion by an isolated I cell population was increased by 30 and 62% in response to L-Phe in the presence of physiological (1.26 mM) and superphysiological (2.5 mM) extracellular calcium concentrations, respectively. While the deletion of CaSR from CCK-eGFP cells did not affect basal CCK secretion, the effect of L-Phe or cinacalcet on intracellular calcium flux was lost. In fact, both secretagogues, as well as superphysiological Ca(2+), evoked an unexpected 20-30% decrease in CCK secretion compared with basal secretion in CaSR(-/-) CCK-eGFP cells. CCK secretion in response to KCl or tryptone was unaffected by the absence of CaSR. The present data suggest that CaSR is required for hormone secretion in the specific response to L-Phe by the native I cell, and that a receptor-mediated mechanism may inhibit hormone secretion in the absence of a fully functional CaSR.

Amino acids stimulate cholecystokinin release through the Ca2+-sensing receptor

Cholecystokinin (CCK) is produced by discrete endocrine cells in the proximal small intestine and is released following the ingestion of food. CCK is the primary hormone responsible for gallbladder contraction and has potent effects on pancreatic secretion, gastric emptying, and satiety. In addition to fats, digested proteins and aromatic amino acids are major stimulants of CCK release. However, the cellular mechanism by which amino acids affect CCK secretion is unknown. The Ca(2+)-sensing receptor (CaSR) that was originally identified on parathyroid cells is not only sensitive to extracellular Ca(2+) but is activated by extracellular aromatic amino acids. It has been postulated that this receptor may be involved in gastrointestinal hormone secretion. Using transgenic mice expressing a CCK promoter driven/enhanced green fluorescent protein (GFP) transgene, we have been able to identify and purify viable intestinal CCK cells. Intestinal mucosal CCK cells were enriched >200-fold by fluorescence-activated cell sorting. These cells were then used for real-time PCR identification of CaSR. Immunohistochemical staining with an antibody specific for CaSR confirmed colocalization of CaSR to CCK cells. In isolated CCK cells loaded with a Ca(2+)-sensitive dye, the amino acids phenylalanine and tryptophan, but not nonaromatic amino acids, caused an increase in intracellular Ca(2+) ([Ca(2+)](i)). The increase in [Ca(2+)](i) was blocked by the CaSR inhibitor Calhex 231. Phenylalanine and tryptophan stimulated CCK release from intestinal CCK cells, and this stimulation was also blocked by CaSR inhibition. Electrophysiological recordings from isolated CCK-GFP cells revealed these cells to possess a predominant outwardly rectifying potassium current. Administration of phenylalanine inhibited basal K(+) channel activity and caused CCK cell depolarization, consistent with changes necessary for hormone secretion. These findings indicate that amino acids have a direct effect on CCK cells to stimulate CCK release by activating CaSR and suggest that CaSR is the physiological mechanism through which amino acids regulate CCK secretion.

Transgenic expression of pancreatic secretory trypsin inhibitor-1 rescues SPINK3-deficient mice and restores a normal pancreatic phenotype

Endogenous trypsin inhibitors are synthesized, stored, and secreted by pancreatic acinar cells. It is believed that they play a protective role in the pancreas by inhibiting trypsin within the cell should trypsinogen become prematurely activated. Rodent trypsin inhibitors are highly homologous to human serine protease inhibitor Kazal-type 1 (SPINK1). The mouse has one pancreatic trypsin inhibitor known as SPINK3, and the rat has two trypsin inhibitors commonly known as pancreatic secretory trypsin inhibitors I and II (PSTI-I and -II). Rat PSTI-I is a 61-amino acid protein that shares 65% sequence identity with mouse SPINK3. It was recently demonstrated that mice with genetic deletion of the Spink3 gene (Spink3(-/-)) do not survive beyond 15 days and lack normal pancreata because of pancreatic autophagy. We have shown that targeted transgenic expression of the rat Psti1 gene to acinar cells in mice [TgN(Psti1)] protects mice against caerulein-induced pancreatitis. To determine whether the autophagic phenotype and lethality in Spink3(-/-) mice were due to lack of pancreatic trypsin inhibitor, we conducted breeding studies with Spink3(+/-) heterozygous mice and TgN(Psti1) mice. We observed that, whereas Spink3(+/+), Spink3(+/-), and Spink3(-/-)/TgN(Psti1) mice had similar survival rates, no Spink3(-/-) mice survived longer than 1 wk. The level of expression of SPINK3 protein in acini was reduced in heterozygote mice compared with wild-type mice. Furthermore, endogenous trypsin inhibitor capacity was reduced in the pancreas of heterozygote mice compared with wild-type or knockout mice rescued with the rat Psti1 gene. Surprisingly, the lesser amount of SPINK3 present in the pancreata of heterozygote mice did not predispose animals to increased susceptibility to caerulein-induced acute pancreatitis. We propose that a threshold level of expression is sufficient to protect against pancreatitis.

Pancreatic phospholipase A2 from the small intestine is a secretin-releasing factor in rats

A secretin-releasing activity exists in the upper small intestine and pancreatic juice in the rat and the dog. Group I pancreatic phospholipase A2 (PLA2) in canine pancreatic juice and porcine pancreatic PLA2 stimulate the release of secretin from both STC-1 cells and a secretin-producing cell (S cell)-enriched preparation isolated from rat duodenal mucosa. We investigated the distribution and release of pancreatic PLA2-like immunoreactivity in the gastrointestinal tract and the role of PLA2 on the release of secretin and pancreatic exocrine secretion in response to duodenal acidification in anesthetized rats. PLA2-like immunoreactivity was detected in the mucosa throughout the gastrointestinal tract. High concentrations of PLA2 were found in both the small intestine and the pancreas. Duodenal acidification significantly increased the release of PLA2 from the upper small intestine (385% over basal secretion). Intravenous infusion of an anti-PLA2 serum (anti-PLA2) dose-dependently inhibited the release of secretin and pancreatic exocrine secretion in response to duodenal acid perfusion. Preincubation of the concentrate of intestinal acid perfusate (10-fold) from donor rats with the anti-PLA2 significantly suppressed its stimulation of secretin release and pancreatic exocrine secretion in recipient rats. We conclude that pancreatic PLA2 also functions as a secretin-releasing factor in the small intestine that mediates acid-stimulated release of secretin in rats.

Cefaclor, a cephalosporin antibiotic, delays gastric emptying rate by a CCK-A receptor-mediated mechanism in the rat

Studies in vitro suggest that cephalosporin antibiotics release the gut hormone cholecystokinin. Cholecystokinin is known to inhibit gastric emptying. Here we examine the effects of cefaclor on gastric emptying and intestinal motility. Male Sprague-Dawley rats were fitted with gastric cannulas. Following a 3-week recovery, the rate of gastric emptying of saline, peptone (4.5%) or cefaclor was determined after instillation into the gastric cannula, while intestinal transit was measured by using the propagation of arabic gum + charcoal mixture given intraduodenally. Gastric emptying of saline was significantly delayed by the addition of cefaclor (3, 10, 30 or 100 mM). The CCK-A antagonist SR-27897B (1 mg kg(-1), i.p.) reversed the delay induced by 10 mM cefaclor, whereas the CCK-B antagonist CI-988 (1 mg kg(-1), i.p.) had no significant effect. In capsaicin-treated rats, 10 mM cefaclor emptied more rapidly than in vehicle-treated animals. Thirty-minute intestinal transit was increased at 30 and 100 mM of cefaclor, while the gastric acid secretion following cefaclor instillation was no different than the group which received saline. The cephalosporin antibiotic cefaclor appears to be a potent stimulant of CCK release from gut endocrine cells, resembling the effects of peptone. Cefaclor delays gastric emptying via capsaicin-sensitive afferent pathways, which involve CCK-A receptor interaction.

Fatty acid-induced cholecystokinin secretion and changes in intracellular Ca2+ in two enteroendocrine cell lines, STC-1 and GLUTag

1. Fatty acid-induced cholecystokinin (CCK) secretion in humans and from the enteroendocrine cell line STC-1 depends critically on acyl chain length. 2. Therefore we have characterized the relationship between acyl chain length and the potency of the fatty acid to induce CCK secretion and changes in intracellular Ca2+ concentration ([Ca2+]i) in two enteroendocrine cell lines (STC-1 and GLUTag). We found that the potency of the fatty acid was directly proportional to its chain length and therefore inversely proportional to its solubility. 3. In both cell types, the fatty acid-induced rise in [Ca2+]i in response to decanoic acid (C10), dodecanoic acid (C12) and tetradecanoic acid (C14) was significantly reduced in Ca2+-free medium and largely blocked by nicardipine. Intracellular stores also contributed to the overall shape of the [Ca2+]i peak. Thus all the fatty acids tested caused the release of Ca2+ from stores and influx of extracellular Ca2+, presumably through L-type calcium channels. 4. To probe the site of fatty acid action, we studied the distribution of 14C-labelled dodecanoic acid. This label was rapidly and irreversibly accumulated by both cell types, where it became concentrated about 20-fold. Confocal microscopy of a fluorescent analogue of dodecanoic acid clearly demonstrated that it entered the cytosol and was not merely partitioning in the cell membrane. These data indicate that an intracellular action for fatty acid-induced CCK secretion cannot be eliminated. 5. Dodecanoic acid itself, and not a metabolite, is the agent responsible for triggering Ca2+ entry since a non-metabolizable form of dodecanoic acid (2-bromododecanoic acid) was also capable of inducing a rise in [Ca2+]i in both cell types. 6. In conclusion, the rise in [Ca2+]i in STC-1 and GLUTag cells evoked by medium- to long-chain fatty acids results from the triggering of a specific signalling pathway. Whether triggering occurs through activation of a membrane-bound receptor or at an intracellular site remains to be clarified.

Diazepam binding inhibitor is a potent cholecystokinin-releasing peptide in the intestine

Pancreatic proteases in the duodenum inhibit the release of cholecystokinin (CCK) and thus exert feedback control of pancreatic exocrine secretion. Exclusion of proteases from the duodenum either by the diversion of bile-pancreatic juice or by the addition of protease inhibitors stimulates exocrine pancreatic secretion. The mechanism by which pancreatic proteases in the duodenum regulate CCK secretion is unknown. In this study, we isolated a trypsin-sensitive peptide that is secreted intraduodenally, releases CCK, and stimulates pancreatic enzyme secretion in rats. This peptide was found to be identical to the porcine diazepam binding inhibitor by peptide sequencing and mass spectrometry analysis. Intraduodenal infusion of 200 ng of synthetic porcine diazepam binding inhibitor1-86 in rats significantly stimulated pancreatic amylase output. Infusion of the CCK antagonist MK-329 completely blocked the diazepam binding inhibitor-stimulated amylase secretion. Similarly, diazepam binding inhibitor33-52 [corrected] also stimulated CCK release and pancreatic secretion in a dose-dependent manner although it was 100 times less potent than the whole peptide. Using a perfusion system containing isolated mucosal cells from the proximal intestine of rats, porcine diazepam binding inhibitor 10(-12) M) dose dependently stimulated CCK secretion. In separate studies, it was demonstrated that luminal secretion of the diazepam binding inhibitor immunoreactivity (7.5 X 10(11) M) could be detected in rat's intestinal washing following the diversion of bile-pancreatic juice. The secretion of this peptide was inhibited by atropine. In conclusion, we have isolated and characterized a CCK-releasing peptide that has a sequence identical to the porcine diazepam binding inhibitor from pig intestinal mucosa and that stimulates CCK release when administered intraduodenally in rat. This peptide may mediate feedback regulation of pancreatic enzyme secretion.

A specific binding of the cholecystokinin-releasing peptide (monitor peptide) to isolated rat small-intestinal cells

A specific binding of the cholecystokinin (CCK)-releasing peptide (monitor peptide) to isolated rat jejunal mucosal cells was investigated. The 125I-labelled purified monitor peptide bound to the rat jejunal cells, and a large excess amount of the non-labelled monitor peptide inhibited the binding. The binding was completed within 60 min at 37 degrees C. The optimum pH for the binding was 8-9. A Scatchard plot of the specific binding was linear, and the dissociation constant was 50 nM. The density of the monitor-peptide-binding sites was high in duodenum but low in ileal and absent in colonic mucosa. A recombinant monitor peptide and four kinds of point mutants of it were prepared. The binding of the mutant monitor peptides to the cells indicated that only a trypsin inhibitor of the mutants could bind to the mucosal cells. Human pancreatic secretory trypsin inhibitor inhibited the specific binding, but other trypsin inhibitors, i.e. bovine basic pancreatic trypsin inhibitor, soybean trypsin inhibitor, egg-white trypsin inhibitor, leupeptin, antipain and FOY-305, did not affect the specific binding at all. These findings suggested that the specific binding site for the monitor peptide on the jejunal mucosal cells has a trypsin-like specificity, exhibiting a special specificity for the pancreatic-secretory-trypsin-inhibitor family. Autoradiography of an affinity-cross-linked complex of the 125I-labelled intact monitor peptide and the binding site suggested that its molecular mass was 33 kDa or 53 kDa in the presence or absence of 2-mercaptoethanol respectively.