A pH-sensitive, neurogenic pathway mediates disease severity in a model of post-ERCP pancreatitis

BACKGROUND

Endoscopic retrograde cholangiopancreatography (ERCP) has a high risk of pancreatitis although the underlying mechanisms are unclear. Transient receptor potential vanilloid 1 (TRPV1) is a cation channel expressed on C and Adelta fibres of primary sensory neurons and is activated by low pH. TRPV1 activation causes release of inflammatory mediators that produce oedema and neutrophil infiltration. We previously demonstrated that neurogenic factors contribute to the pathogenesis of pancreatitis. Resiniferatoxin (RTX) is a TRPV1 agonist that, in high doses, defunctionalises C and Adelta fibres. When we discovered that the pH of radio-opaque contrast solutions used for ERCP was 6.9, we hypothesised that low pH may contribute to the development of contrast-induced pancreatitis via activation of TRPV1.

METHODS

Rats underwent equal pressure pancreatic ductal injection of contrast solutions at varying pH with or without RTX.

RESULTS

Contrast solution (pH 6.9) injected into the pancreatic duct caused a significant increase in pancreatic oedema, serum amylase, neutrophil infiltration, and histological damage. Solutions of pH 7.3 injected at equal pressure caused little damage. The severity of the pancreatitis was significantly increased by injection of solutions at pH 6.0. To determine if the effects of low pH were mediated by TRPV1, RTX was added to the contrast solutions. At pH levels of 6.0 and 6.9, RTX significantly reduced the severity of pancreatitis.

CONCLUSIONS

Contrast solutions with low pH contribute to the development of pancreatitis through a TRPV1-dependent mechanism. It is possible that increasing the pH of contrast solution and/or adding an agent that inhibits primary sensory nerve activation may reduce the risk of post-ERCP pancreatitis.

Capsaicin vanilloid receptor-1 mediates substance P release in experimental pancreatitis

We examined whether the capsaicin vanilloid receptor-1 (VR1) mediates substance P (SP) release from primary sensory neurons in experimental pancreatitis. Pancreatitis was achieved by 12 hourly injections of caerulein (50 microg/kg ip) in mice. One group received capsazepine (100 micromol/kg sc), a competitive VR1 antagonist, at 4-h intervals. Neurokinin-1 receptor (NK1R) internalization in acinar cells, used as an index of endogenous SP release, was assessed by immunocytochemical quantification of NK1R endocytosis. The severity of pancreatitis was assessed by measurements of serum amylase, pancreatic myeloperoxidase (MPO) activity, and histological grading. Caerulein administration caused significant elevations in serum amylase and pancreatic MPO activity, produced histological evidence of pancreatitis, and caused a dramatic increase in NK1R endocytosis. Capsazepine treatment significantly reduced the level of NK1R endocytosis, and this was associated with similar reductions in pancreatic MPO activity and histological severity of pancreatitis. These results demonstrate that repeated caerulein stimulation causes experimental pancreatitis that is mediated in part by stimulation of VR1 on primary sensory neurons, resulting in endogenous SP release.

Neurohumoral control of the exocrine pancreas

Recent advances in the study of pancreatic exocrine secretion are reviewed, with an emphasis on neurohumoral mechanisms. In the past year, cDNA for the human pancreatic sodium-bicarbonate cotransporter was cloned, and the expressed protein was localized to pancreatic acini and ductal cells. Recent information suggests that the cholecystokinin B receptor has a role in pancreatic amylase release. Further evidence supports the concept of a protease-sensitive negative feedback mechanism regulating pancreatic exocrine secretion. Study of the expression of the receptors responsible for the regulation of pancreatic function has proven fruitful in the determination of the molecular mechanisms of hormone signal transduction and desensitization. Studies of peptide 1, pituitary adenylate cyclase-activating peptide, and gastrin-releasing peptide have shown how these peptides participate in the regulation of pancreatic secretion and have provided information on intracellular signaling pathways obtained using rat pancreatic tumor cells. Neural regulation via cholinergic receptors in isolated pancreatic acini and the mechanisms responsible for other neurotransmitters, such as calcitonin gene-related peptide, histamine, and dopamine, are reviewed. This review highlights recent discoveries in the neurohumoral regulation of pancreatic exocrine secretion.

Activation of calcium channels by cAMP in STC-1 cells is dependent upon Ca2+ calmodulin-dependent protein kinase II

Activation of L-type calcium channels in the neuroendocrine, cholecytstokinin-secreting cell line, STC-1, is vital for secretion of CCK. In the present study, the regulation of L-type Ca2+ channels by cAMP and Ca2+ calmodulin dependent protein kinase II (CaM-KII) in STC-1 cells was investigated. Exposure to 3-isobutyl-1-methylxanthine (IBMX) increased intracellular cAMP levels, whole cell Ca2+ currents and activated Ca2+ channels in cell-attached membrane patches. Furthermore, in Fura-2AM loaded cells, cytosolic Ca2+ levels increased upon exposure to IBMX. By contrast, pretreatment of cells with the CaM-KII inhibitor KN-62, prevented IBMX activation of Ca2+ channels in cell-attached patches or increases in cytosolic Ca2+ levels. Inclusion of the synthetic peptide fragment 290-309 of CaM-KII, a CaM-KII antagonist, in the pipette solution, blocked the activation of whole cell Ca2+ currents upon addition of IBMX. These results indicate a unique mechanism of L-type Ca2+ channel activation involving two phosphorylation events.

Monitor peptide binding sites are expressed in the rat liver and small intestine

125I-monitor peptide binding was performed using frozen sections of the rat liver and gut and visualized using autoradiography. Saturable binding was observed in unidentified single cells in the liver and in the mucosa of the small intestine. Epidermal growth factor (EGF) and GTPgammaS did not inhibit 125I-monitor peptide binding indicating that the binding sites are not EGF receptors or G protein-coupled receptors. The liver binding site exhibited an affinity 3.7-4.4-fold higher than those in the small intestine. It has been established that intraluminal monitor peptide releases cholecystokinin from the small intestine. The present results indicate that monitor peptide may also have liver associated functions.

Inhibition of Na+/H+ exchange stimulates CCK secretion in STC-1 cells

It has been demonstrated that K+ channel regulation of membrane potential is critical for control of CCK secretion. Because certain K+ channels are pH sensitive, it was postulated that pH affects K+ channel activity in the CCK-secreting cell line STC-1 and may participate in regulating CCK secretion. The present study examines the role of electroneutral Na+/H+ exchange on extracellular acidification and hormone secretion. Treatment of STC-1 cells with the amiloride analog ethylisopropyl amiloride (EIPA) to inhibit Na+/H+ exchange inhibited Na+-dependent H+ efflux and increased basal CCK secretion. Substituting choline for NaCl in the extracellular medium elevated basal intracellular Ca2+ concentration and stimulated CCK release. Stimulatory effects on hormone secretion were blocked by the L-type Ca2+ channel blocker diltiazem, indicating that secretion was dependent on the influx of extracellular Ca2+. To determine whether the effects of EIPA and Na+ depletion were due to membrane depolarization, we tested graded KCl concentrations. The ability of EIPA to increase CCK secretion was inhibited by depolarization induced by 10-50 mM KCl in the bath. Maneuvers to lower intracellular pH (pHi), including reducing extracellular pH (pHo) to 7.0 or treatment with sodium butyrate, significantly increased CCK secretion. To examine whether pH directly affects membrane K+ permeability, we measured outward currents carried by K+, using whole cell patch techniques. K+ current was significantly inhibited by lowering pHo to 7.0. These effects appear to be mediated through changes in pHi, because intracellular dialysis with acidic solutions nearly eliminated current activity. These results suggest that Na+/H+ exchange and membrane potential may be functionally linked, where inhibition of Na+/H+ exchange lowers pHi and depolarizes the membrane, perhaps through inhibition of pH-sensitive K+ channels. In turn, K+ channel closure and membrane depolarization open voltage-dependent Ca2+ channels, leading to an increase in cytosolic Ca2+ and CCK release. The effects of pHi on K+ channels may serve as a potent stimulus for hormone secretion, linking cell metabolism and secretory functions.

Inhibition of gastric emptying in response to intestinal lipid is dependent on chylomicron formation

Lipid in the intestine initiates feedback inhibition of proximal gastrointestinal function and food intake. In rats and humans, inhibition of gastric emptying is mediated, at least in part, by cholecystokinin (CCK)-A receptors, and in rats there is evidence for involvement of an intestinal vagal afferent pathway. The mechanism by which luminal lipid acts to release CCK or activate vagal afferent nerve terminals is unclear. The role of chylomicron formation in this sensory transduction pathway has been investigated using the hydrophobic surfactant Pluronic L-81 that inhibits chylomicron formation. Gastric emptying of liquids was measured in awake rats fitted with a Thomas gastric fistula and a duodenal cannula. Intestinal perfusion of lipid induced a dose-dependent inhibition of gastric emptying (6, 12, and 39% inhibition for 25, 50, and 100 mg lipid, respectively). Perfusion of lipid with Pluronic L-81 (2.8% wt/vol) reversed the lipid-induced inhibition of gastric emptying. Pluronic L-63, a chemically similar surfactant that has no effect on chylomicron formation, had no effect on lipid-induced inhibition of gastric emptying. Perfusion of the intestine with lipid (100 mg) increased plasma levels of CCK from 1.9 +/- 0.8 to 6. 5 +/- 1 pM. This increase was blocked by Pluronic L-81 but unaffected by L-63. These results provide evidence that chylomicron formation is important in the signaling of lipid in the intestinal lumen to CCK endocrine cells and to producing feedback inhibition of gastric emptying.

Bioactivity of intraduodenally and intravenously infused fragments of luminal cholecystokinin releasing factor (LCRF)

A luminal cholecystokinin releasing factor (LCRF), has been purified from intestinal secretion and found to have a mass of 8136 daltons. The amino-terminal 41 residues have been sequenced. Previous studies showed that intraduodenal infusion of the synthetic amino-terminal 35 amino acid peptide, LCRF1-35 significantly stimulated pancreatic protein and fluid secretion in conscious rats, but the peptide did not stimulate amylase release from isolated, dispersed pancreatic acini. In the present study, several fragments of LCRF were synthesized and tested for CCK-releasing activity (pancreatic protein secretion) to determine whether shorter fragments of LCRF exhibit the characteristic biological activity of native LCRF and synthetic LCRF1-35. Compounds tested were LCRF1-41, LCRF1-35, LCRF1-65 and LCRF11-25. Of the fragments shorter than LCRF1-35, only LCRF11-25 but not LCRF1-6 had significant CCK releasing activity. LCRF1-41 was equivalent to LCRF1-35 in potency and efficacy. Intravenous and intraduodenal infusion of LCRF1-35 elicited nearly identical dose-response curves.

Distribution and localization of a novel cholecystokinin-releasing factor in the rat gastrointestinal tract

The purpose of this study was to examine the distribution and localization of an intestinal cholecystokinin (CCK)-releasing factor, called luminal CCK-releasing factor (LCRF), in the gastrointestinal tract and pancreas of the rat. RIA analysis indicates that LCRF immunoreactivity is found throughout the gut including the pancreas, stomach, duodenum, jejunum, ileum, and colon with the highest levels in the small intestine. Immunohistochemistry analysis shows LCRF immunoreactivity staining in intestinal villi, Brunner's glands of the duodenum, the duodenal myenteric plexus, gastric pits, pancreatic ductules, and pancreatic islets. These results indicate potential sources for secretagogue-stimulated release of luminal LCRF and support the hypothesis that LCRF is secreted into the intestinal lumen to stimulate CCK release from mucosal CCK cells.

Regulation of biliary secretion through apical purinergic receptors in cultured rat cholangiocytes

To evaluate whether ATP in bile serves as a signaling factor regulating ductular secretion, voltage-clamp studies were performed using a novel normal rat cholangiocyte (NRC) model. In the presence of amiloride (100 microM) to block Na+ channels, exposure of the apical membrane to ATP significantly increased the short-circuit current (Isc) from 18.2 +/- 5.9 to 52.8 +/- 12.7 microA (n = 18). The response to ATP is mediated by basolateral-to-apical Cl- transport because it is inhibited by 1) the Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (1 mM), diphenylanthranilic acid (1.5 mM), or 5-nitro-2-(3-phenylpropylamino)benzoic acid (50 or 100 microM) in the apical chamber, 2) the K+ channel blocker Ba2+ (5 mM), or 3) the Na(+)-K(+)-2Cl- cotransport inhibitor bumetanide (200 microM) in the basolateral chamber. Other nucleotides stimulated an increase in Isc with a rank order potency of UTP = ATP = adenosine 5'-O-(3)-thiotriphosphate, consistent with P2u purinergic receptors. ADP, AMP, 2-methylthioadenosine 5'-triphosphate, and adenosine had no effect. A cDNA encoding a rat P2u receptor (rP2uR) was isolated from a liver cDNA library, and functional expression of the corresponding mRNA in Xenopus laevis oocytes resulted in the appearance of ATP-stimulated currents with a similar pharmacological profile. Northern analysis identified hybridizing mRNA transcripts in NRC as well as other cell types in rat liver. These findings indicate that exposure of polarized cholangiocytes to ATP results in luminal Cl- secretion through activation of P2u receptors in the apical membrane. Release of ATP into bile may serve as an autocrine or paracrine signal regulating cholangiocyte secretory function.

An amino-terminal fragment of LCRF, LCRF-(1-35), has the same activity as the natural peptide

A cholecystokinin (CCK)-releasing peptide, luminal CCK-releasing factor (LCRF), has been purified from rat jejunal secretion. Amino acid analysis and mass spectral analysis showed that the purified peptide is composed of 70-75 amino acid residues and has a mass of 8,136 Da. Microsequence analysis of LCRF yielded an amino acid sequence for the amino-terminal 41 residues. To determine the biologically active region of the molecule, a peptide was synthesized consisting of the amino-terminal 35 amino acids of LCRF. In this study, intraduodenal infusion of LCRF-(1-35) significantly stimulated pancreatic secretion in conscious rats. The dose-response curves to LCRF-(1-35) and to monitor peptide were similar and biphasic, with higher doses producing submaximal pancreatic secretory responses. The CCK-A receptor antagonist MK-329 abolished the pancreatic secretory response to intraduodenally infused LCRF-(1-35). These results demonstrate that LCRF biological activity is contained within the amino-terminal 35-amino acid portion of LCRF, and this fragment may be useful for investigating the role of LCRF in gastrointestinal function.

Postprandial cholecystokinin release and gastric emptying in patients with bulimia nervosa

This study was designed to investigate the biological underpinnings of the observed deficit in satiety in patients with bulimia nervosa. Eight women with bulimia nervosa and 10 age- and weight-matched control subjects consumed three laboratory meals consisting of 200, 400, and 600 g of a radiolabeled liquid meal. For 1 h after each meal, blood samples were obtained at 10-min intervals for measurement of cholecystokinin concentration and gastric emptying was measured. Subjects also completed perceptual rating scales at 10-min intervals. Compared with control subjects, patients with bulimia nervosa showed a blunting of postprandial cholecystokinin release, particularly with larger meal sizes, as well as delayed gastric emptying. Increasing meal size was associated with increased desire to binge eat in patients but not in control subjects. These data lend support to a model in which increased gastric capacity, perhaps resulting from repeated binge eating, gives rise to delayed gastric emptying and blunted postprandial cholecystokinin release, leading to an impaired satiety response, which tends to perpetuate the illness.

Cholecystokinin cells

Cholecystokinin (CCK) is an important hormonal regulator of the digestive process. CCK cells are concentrated in the proximal small intestine, and hormone is secreted into the blood upon the ingestion of food. The physiological actions of CCK include stimulation of pancreatic secretion and gallbladder contraction, regulation of gastric emptying, and induction of satiety. Therefore, in a highly coordinated manner, CCK regulates the ingestion, digestion, and absorption of nutrients. CCK is produced by two separate cell types: endocrine cells of the small intestine and various neurons in the gastrointestinal tract and central nervous system. Accordingly, CCK can function as either a hormone or a neuropeptide. This review focuses on the physiology of the CCK cell in the intestine and, in particular, on how the CCK cell is regulated to secrete its hormone product. The effects of ingested nutrients on the CCK cell and the intracellular messenger systems involved in controlling secretion are reviewed. A summary is provided of recent studies examining the electrophysiological properties of CCK cells and newly discovered proteins that act as releasing factors for CCK, which mediate feedback pathways critical for regulated secretion in the intact organism.

Regulation of cholecystokinin secretion in STC-1 cells by nitric oxide

In the present study we evaluated the effects of agents anticipated to change NO levels on the secretion of cholecystokinin (CCK) from STC-1 cells. After a 15-min treatment with the nitric oxide (NO) generating agent sodium nitroprusside (SNP; 10 microM), a 24% inhibition in basal CCK release and an increase in cellular guanosine 3',5'-cyclic monophosphate (cGMP) levels were noted. By contrast, SNP (10 microM) had no effect on CCK release stimulated by L-phenylalanine (20 mM). Inhibition of NO synthase (NOS) with NG-nitro-L-arginine methyl ester (L-NAME) produced dose-dependent stimulation in CCK release. L-NAME (100-400 microM) also inhibited ATP-sensitive potassium (KATP) channels in cell-attached patches. Pretreatment of cells with disopyramide (200 microM), a KATP channel blocker, blocked L-NAME stimulation of CCK release. After inhibition of potassium channel activity by L-NAME, addition of the nonhydrolyzable cGMP analogue 8-bromo-cGMP (1-2 mM) reactivated potassium channels. NO-generating agents had no effect on channel activity in inside-out membrane patches. It is concluded that NO may serve as an important regulator of basal CCK release.

Evidence that CCK-58 has structure that influences its biological activity

Many biologically active peptides exist in multiple molecular forms, but the functional significance of regions outside the region of bioactivity is unknown. The biological and immunological data presented in this study indicate that cholecystokinin-58 (CCK-58), unlike other forms of cholecystokinin, has structure that influences its bioactivity. CCK-58 was purified from acid extracts of canine intestinal mucosa until a single absorbance peak was obtained during reverse-phase chromatography. Amino acid analysis precisely determined the peptide concentrations of purified CCK-58 and synthetic CCK-8. Our hypothesis was that if the amino terminus of CCK-58 influences its bioactivity then its activity would be modified when this region was removed from the peptide. To evaluate the importance of the amino terminus of CCK-58 to influence its biological activity, the abilities of CCK-58 and CCK-8 to release amylase from pancreatic acini were compared before and after tryptic digestion. Tryptic digestion of CCK-58 decreased the half-maximal stimulation (EC50) for amylase release from 96 to 28 pM. The EC50 for digested CCK-58 was similar to that for CCK-8 (17 pM). These results suggest that CCK-58 has a structure that shields its bioactive carboxyl terminus. This is further supported by the finding that carboxyl fragments generated from CCK-58 by trypsin or by partial acid hydrolysis were greater than twofold more immunoreactive than the intact CCK-58. The diminished activity of CCK-58 SK shields the carboxyl terminus, which is important to its biological and immunological activities.

Purification and characterization of a luminal cholecystokinin-releasing factor from rat intestinal secretion

Cholecystokinin (CCK) secretion in rats and humans is inhibited by pancreatic proteases and bile acids in the intestine. It has been hypothesized that the inhibition of CCK release caused by pancreatic proteases is due to proteolytic inactivation of a CCK-releasing peptide present in intestinal secretion. To purify the putative luminal CCK-releasing factor (LCRF), intestinal secretions were collected by perfusing a modified Thiry-Vella fistula of jejunum in conscious rats. From these secretions, the peptide was concentrated by ultrafiltration followed by low-pressure reverse-phase chromatography and purified by reverse-phase high-pressure liquid chromatography. Purity was confirmed by high-performance capillary electrophoresis. Fractions were assayed for CCK-releasing activity by their ability to stimulate pancreatic protein secretion when infused into the proximal small intestine of conscious rats. Partially purified fractions strongly stimulated both pancreatic secretion and CCK release while CCK receptor blockade abolished the pancreatic response. Amino acid analysis and mass spectral analysis showed that the purified peptide is composed of 70-75 amino acid residues and has a mass of 8136 Da. Microsequence analysis of LCRF yielded an amino acid sequence for 41 residues as follows: STFWAYQPDGDNDPTDYQKYEHTSSPSQLLAPGDYPCVIEV. When infused intraduodenally, the purified peptide stimulated pancreatic protein and fluid secretion in a dose-related manner in conscious rats and significantly elevated plasma CCK levels. Immunoaffinity chromatography using antisera raised to synthetic LCRF-(1-6) abolished the CCK releasing activity of intestinal secretions. These studies demonstrate, to our knowledge, the first chemical characterization of a luminally secreted enteric peptide functioning as an intraluminal regulator of intestinal hormone release.

Depolarization-stimulated cholecystokinin secretion is mediated by L-type calcium channels in STC-1 cells

To examine the role of calcium channels in depolarization-activated cholecystokinin (CCK) release, studies were performed in an intestinal CCK-secreting cell line, STC-1. Blockade of potassium channels with barium chloride (5 mM) increased the release of CCK by 374.6 +/- 46.6% of control levels. Barium-induced secretion was inhibited by the L-type calcium-channel blocker, nicardipine. Nicardipine (10(-9)-10(-5) M) produced a dose-dependent inhibition in barium-stimulated secretion with a half-maximal inhibition (IC50) value of 0.1 microM. A second L-type calcium-channel blocker, diltiazem (10(-9)-10(-4) M), also inhibited barium-induced CCK secretion with an IC50 value of 5.1 microM. By contrast, the T-type calcium-channel blocker, nickel chloride (10(-7)-10(-8) M), failed to significantly inhibit barium-induced CCK secretion. To further evaluate a role for L-type calcium channels in the secretion of CCK, the effects of the L-type calcium channel opener, BAY K 8644, were examined. BAY K 8644 (10(-8)-10(-4) M) produced a dose-dependent stimulation in CCK release with a mean effective concentration value of 0.2 microM. Recordings of single-channel currents from inside-out membrane patches showed activation of calcium channels by BAY K 8644 (1 microM), with a primary channel conductance of 26.0 +/- 1.2 pS. It is concluded that inhibition of potassium channel activity depolarizes the plasma membrane, thereby activating L-type, but not T-type, calcium channels. The corresponding influx of calcium serves to trigger secretion of CCK.

Beta-adrenergic regulation of cholecystokinin secretion in STC-1 cells

Previously, it has been shown that an increase in adenosine 3',5'-cyclic monophosphate (cAMP) levels stimulates intestinal secretion of cholecystokinin (CCK); however, the mechanisms for increasing intracellular cAMP levels are not known. Using the CCK-secreting intestinal cell line, STC-1, we evaluated whether beta-adrenergic receptors (beta-ARs) might be present on STC-1 cells and whether they stimulated CCK release through increases in cAMP. Photoaffinity labeling of beta-ARs from solubilized STC-1 cell membranes revealed photoincorporation of the agonist [125I]iodocyanopindolol into an approximately 75-kDa band. Addition of the beta-AR agonist, isoproterenol, in the presence of 3-isobutyl-1-methylxanthine, produced a concentration-dependent increase in both cAMP levels and CCK release. Blockade of beta 1- and/or beta 2-ARs significantly inhibited isoproterenol-stimulated increases in cAMP production and CCK release. With the use of fura 2-loaded cells to measure changes in intracellular Ca2+ concentration ([Ca2+]i), isoproterenol stimulation was found to increase cytosolic Ca2+ levels. To evaluate whether this increase in [Ca2+]i was due to release of Ca2+ or influx of Ca2+, cells were treated with the L-type calcium channel blocker, diltiazem, which inhibited isoproterenol-stimulated CCK secretion. Furthermore, in patch-clamp studies with inside-out membrane patches, addition of the catalytic subunit of protein kinase A activated diltiazem-sensitive Ca2+ channels. It is concluded that beta-ARs are present on STC-1 cells and are coupled to the production of cAMP, which may increase CCK release through a calcium-dependent process.

Adaptation to fat markedly increases pancreatic secretory response to intraduodenal fat in rats

Exposure to higher levels of fat in the diet increases the secretion of fat-digesting enzymes in pancreatic juice. This study examines the functional consequences of this phenomenon and demonstrates that adapting rats to high fat (triglyceride) loads increases the release of cholecystokinin (CCK) and the pancreatic secretory response to intraduodenal fat. Lipolytic activity in the small intestine was also higher in adapted rats. Exchanging pancreatic juice from unadapted rats with pancreatic juice from adapted rats decreased the response to fat in adapted rats and increased the response to fat in unadapted rats. Infusing oleic acid into unadapted rats stimulated CCK secretion and pancreatic exocrine secretion to levels observed with triglycerides in adapted rats. Pancreatic exocrine secretion in response to intraduodenal fat in rats adapted to a high-fat (20%) diet were significantly higher than the responses seen in rats fed a low-fat (5%) diet. Adaptation to fat increases the pancreatic secretory and plasma CCK responses to fat, apparently by increasing the efficiency of triglyceride digestion and thereby increasing CCK release.

Regulation of cholecystokinin secretion by intraluminal releasing factors

Ingested nutrients stimulate secretion of gastrointestinal hormones that are necessary for the coordinated processes of digestion and absorption of food. One of the most important hormonal regulators of the digestive process is cholecystokinin (CCK). This hormone is concentrated in the proximal small intestine and is secreted into the blood on the ingestion of proteins and fats. The physiological actions of CCK include stimulation of pancreatic secretion and gallbladder contraction, regulation of gastric emptying, and induction of satiety. Therefore, in a highly coordinated manner CCK regulates the ingestion, digestion, and absorption of nutrients. The manner by which foods affect enteric hormone secretion is largely unknown. However, it has recently become apparent that two CCK-releasing factors are present in the lumen of the proximal small intestine. One of these factors, known as monitor peptide, has been chemically characterized. Monitor peptide is produced by pancreatic acinar cells and is secreted by way of the pancreatic duct into the duodenum. On reaching the small intestine, monitor peptide interacts with CCK cells to induce hormone secretion. A CCK-releasing factor of intestinal origin has been partially characterized and is responsible for stimulation of CCK secretion after 1) ingestion of protein or fats, 2) instillation of protease inhibitors into the duodenum, or 3) diversion of bile-pancreatic juice from the upper small intestine. Together, these releasing factors provide positive and negative feedback mechanisms for regulation of CCK secretion. This review discusses the physiological observations that have led to the chemical characterization of the CCK-releasing factors and the potential implications of this work to other hormones of the gastrointestinal tract.

Dietary regulation of glucose-dependent insulinotropic peptide (GIP) gene expression in rat small intestine

The hormone, glucose-dependent insulinotropic peptide (GIP), is an important incretin regulator of the gastrointestinal tract. To investigate whether diet is important for the control of GIP gene expression in the small intestine, GIP messenger RNA (mRNA) levels were measured in rats during fasting and after glucose or fat administration. Ribonuclease protection analyses revealed that glucose and fat administration increased GIP mRNA levels by 4-fold and 2.5-fold, respectively, compared with the control, and that prolonged fasting decreased GIP mRNA levels to 44% of those of control animals. Glucose infusion increased plasma GIP levels and tended to stimulate an increase in the GIP hormone concentration in the mucosa of the small intestine. Administration of fat also stimulated an increase of plasma GIP levels but did not modify tissue GIP concentrations. Prolonged fasting tended to decrease plasma GIP levels, although GIP tissue concentrations did not change. These data suggest that dietary glucose or fat stimulates GIP synthesis and secretion, and that food deprivation causes a decrease in GIP synthesis and secretion. This regulation involves changes at the pretranslational level and is reflected by modifications of GIP mRNA expression.

Regulation of cholecystokinin secretion by bombesin in STC-1 cells

Bombesin stimulates cholecystokinin (CCK) secretion, presumably by a direct effect on the intestinal CCK cell. The present objectives were to characterize bombesin-stimulated CCK release and to investigate the role of calcium in CCK secretion in an intestinal CCK-producing cell line (STC-1). Bombesin caused a dose-dependent release of CCK, which was reduced either in the absence of extracellular calcium or by calcium channel blockade, suggesting that influx of calcium is necessary for CCK secretion. Bombesin caused an increase in intracellular calcium concentration ([Ca2+]i) and increased efflux of 45Ca2+ from 45Ca(2+)-loaded cells. Radioligand binding studies and Northern analysis were consistent with the expression of a bombesin receptor. Thus bombesin stimulation of CCK release occurs via binding to a receptor and is dependent on increased [Ca2+]i. We propose that the STC-1 cell line may provide a useful model for studying the regulation of intestinal CCK secretion.

Characterization of ATP-sensitive potassium channels in intestinal, cholecystokinin-secreting cells

In the present study, the electrophysiologic properties of ATP-sensitive potassium channels were evaluated in an intestinal, cholecystokinin-secreting cell line (STC-1). Channels were operative under basal conditions and, in cell-attached membrane patches, channel activity was decreased by glucose or disopyramide, agents which classically inhibit ATP-sensitive potassium channels. Channel activity was increased by the KATP channel opener, diazoxide. Intestinal ATP-sensitive potassium channels appear to behave in a similar manner to those found in cardiac and pancreatic beta cells.

Regulation of cholecystokinin secretion by ATP-sensitive potassium channels

The relationship of potassium channel activity to the secretion of cholecystokinin (CCK) was evaluated in STC-1 cells, an intestinal CCK-secreting cell line. Patch-clamp and 86Rb efflux studies showed that an ATP-sensitive potassium channel was endogenously expressed in STC-1 cells. Furthermore, channels are present in sufficient number to significantly modulate whole cell potassium permeability after either channel activation or closure with diazoxide (100 microM) or disopyramide (200 microM), respectively. Inhibition of channel activity with glucose (5-20 mM) was found to depolarize the plasma membrane, increase cytosolic calcium levels, and stimulate CCK release. Glucose-mediated release of CCK, as well as the increase in cytosolic calcium, was inhibited by the calcium channel blocker diltiazem (10 microM). It is concluded that intestinal secretion of CCK may be tonically controlled by activity of basally active ATP-sensitive potassium channels, and after inhibition of channel activity, calcium-dependent CCK secretion is stimulated.

Regulation of cholecystokinin synthesis and secretion in rat intestine

Cholecystokinin is a classical gastrointestinal hormone that is produced by discrete endocrine cells of the upper small intestine. Cholecystokinin is produced in various molecular forms that result from differences in posttranslation processing of a single gene product. Cholecystokinin is secreted from the intestine in response to the ingestion of food. We observed that specific dietary substances increase the rate of transcription of the cholecystokinin gene and stimulate cholecystokinin release in rats. In contrast the paracrine transmitter, somatostatin, inhibits dietary-stimulated cholecystokinin secretion and lowers intestinal mRNA levels. Evidence that cholecystokinin gene expression is not necessarily linked to hormone secretion is supported by the observation that the neuropeptide, bombesin, stimulates cholecystokinin release but does not modify intestinal cholecystokinin mRNA levels. To examine the intracellular messengers that might regulate the cholecystokinin cell directly, we developed an in vitro method for studying cholecystokinin release from isolated intestinal mucosal cells. In this perifusion system, cholecystokinin release was stimulated by membrane depolarizing concentrations of KCl (50 mmol/L), the calcium ionophore A23187 (1 mumol/L), and the cAMP analogue dibutyryl cAMP (1 mumol/L). Biologically active cholecystokinin was also released in a dose-dependent manner by the peptide transmitters, bombesin and monitor peptide. These findings indicate that neurotransmitters and hormones may directly regulate the cholecystokinin cell and suggest that the phosphoinositide and adenylate cyclase cascades mediate stimulated-cholecystokinin secretion.

Regulation of cholecystokinin secretion by calcium-dependent calmodulin kinase II: differential effects of phenylalanine and cAMP

The release of cholecystokinin was investigated in STC-1 cells, an intestinal cholecystokinin-secreting cell line. Fifteen minute incubation of cells with the amino acid, L-phenylalanine (20 mM), or the phosphodiesterase inhibitor, IBMX (100 microM), stimulated cholecystokinin secretion. Stimulation of secretion by both agents was associated with an increase in cytosolic calcium and was inhibited by the calcium channel blocker, diltiazem (10 microM). The calcium-calmodulin kinase II inhibitor, KN-65 (1.4 microM), markedly reduced IBMX-stimulated secretion, but had no effect on phenylalanine-mediated activity. KN-62 also inhibited IBMX-induced increases in cytosolic calcium, suggesting that cAMP may activate diltiazem-sensitive calcium channels by a calmodulin-dependent process.

Developmental expression of the glucose-dependent insulinotropic polypeptide gene in rat intestine

The developmental expression of the glucose-dependent insulinotropic polypeptide (GIP) gene was investigated in rat intestine. Steady state levels of GIP mRNA were determined in the intestine during fetal and postnatal development by double ribonuclease protection assays. GIP mRNA could be detected as early as day 20 of embryonic development and very low levels remained until postnatal day 3. The GIP mRNA levels increased markedly in the period between days 3 and 5 of postnatal life and then gradually increased toward adult levels. Since intron 1 of the GIP gene contains putative TATA and CCAAT boxes, and some potential cis-acting promoter elements, we examined whether or not another transcript starting from exon 2 of the GIP gene is expressed during development of rat intestine. Ribonuclease protection assays suggested that although an abbreviated transcript might exist starting from exon 2, it appears to be minor and its relative abundance is unchanged during development or following intraduodenal glucose stimulation. These observations suggest that GIP may play an important role in early postnatal development probably associated with suckling.

Natural and synthetic CCK-58. Novel reagents for studying cholecystokinin physiology

CCK-58 is a unique reagent for testing how segments of a peptide far removed from its active site can influence the expression of its biological activity. Indications of tertiary structure have come from studies with natural peptide purified from canine small intestine. These studies gave clear indications that tertiary structure affects CCK-58 bioactivity, but the small quantities of CCK-58 available made it impossible to characterize completely how tertiary structure influenced bioactivity. Canine CCK-58 was synthesized manually using a solid support and was purified by reverse phase high pressure liquid chromatography (HPLC). Synthetic CCK-58 was characterized by isocratic reverse phase and gradient HPLC, amino acid analysis, mass spectral analysis, sequence analysis, and three bioassays. Synthetic and natural canine CCK-58 had the same elution profiles, amino acid composition, sequence, and mass. The two peptides were equipotent for the stimulation of pancreatic secretion. Natural canine CCK-58 was equipotent to CCK-8 for CCK "B" receptor binding, a further indication of the purity of the natural peptide. However, natural CCK-58 was more potent than CCK-8 for CCK "A" receptor binding and less potent than CCK-8 for stimulation of pancreatic secretion. These data support the concept that CCK-58 has a stable tertiary structure. This structure does not affect its binding to CCK "B" receptors, enhances its binding to low affinity CCK "A" receptors, and decreases its activity expressed through binding to high affinity CCK "A" receptors. The concept of a stable tertiary structure is also supported by the fact that many antibodies directed towards the carboxyl terminus of cholecystokinin react better with CCK-8 than CCK-58. The availability of synthetic CCK-58 will allow analysis of its tertiary structure by physical and chemical methods as well as studies on how peptide tertiary structure can affect receptor binding, receptor activation, metabolism in blood, degradation in interstitial fluid, and inactivation at the receptor. Evaluating all of these systems will help investigators understand the regulation of cholecystokinin activity by its major endocrine form, CCK-58.

Inhibitors of ATP-sensitive potassium channels stimulate intestinal cholecystokinin secretion

Recently, a role for adenosine 5'-triphosphate(ATP)-sensitive potassium channels in the regulation of cholecystokinin (CCK) secretion has been described in STC-1 cells, an intestinal CCK-secreting cell line. To examine whether a similar mechanism might participate in the regulation of hormone secretion from native CCK cells, the effects of two established inhibitors of ATP-sensitive potassium channels (e.g. glucose, disopyramide) were examined on CCK release from dispersed murine intestinal cells. Both glucose and disopyramide were found to stimulate CCK secretion. Furthermore, CCK release induced by glucose was inhibited by the calcium channel blocker diltiazem. It is concluded that, ATP-sensitive potassium channels may play a role in the regulation of intestinal CCK secretion.

Total synthesis, purification, and characterization of human [Phe(p-CH2SO 3Na)52, Nle32,53,56, Nal55]-CCK20-58, [Tyr52, Nle32,53,56, Nal55]-CCK-58, and [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58

The synthesis of [Phe(p-CH2SO3Na)52, Nle32,53,56 Nal55]-CCK20-58, [Tyr52, Nle32,53,56, Nal55]-CCK-58 and of [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58 using the (9-fluorenylmethyloxy)-carbonyl (Fmoc) strategy on a 2,4-DMBHA resin is described. The crude peptide preparations were extremely complex when analyzed by RP-HPLC, capillary zone electrophoresis (CZE), and ion-exchange chromatography (IE-FPLC). We found that the most effective strategy for purification included cation-exchange chromatography followed by a RP-HPLC desalting step. The highly purified peptides (purity greater than 90%) were characterized by RP-HPLC, size exclusion HPLC (SEC), IE-FPLC, CZE, mass spectrometry, amino acid analysis, and Edman sequence analysis (for [Tyr52, Nle32,53,56, Nal55]-CCK-58). The results demonstrate the applicability of the 2,4-DMBHA resin for Fmoc solid-phase synthesis of long peptides amides (58 residues in length in this case) as well as the efficacy of an FPLC/RP-HPLC approach for the purification of very long, heterogeneous crude peptides, allowing a true assessment of the biological properties of these analogs to be carried out. [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK20-58 was less than 1% as potent as CCK-8 while [Tyr52, Nle32,53,56, Nal55]-CCK-58 and [Phe(p-CH2SO3Na)52, Nle32,53,56, Nal55]-CCK-58 were inactive at the doses tested (< 0.01%).

Potassium channels regulate cholecystokinin secretion in STC-1 cells

Following blockade of plasma membrane potassium channels with barium or tetraethylammonium chloride, release of cholecystokinin was increased in an intestinal cell line (STC-1). Treatment with calcium channel blockers inhibited barium- or TEA-induced secretion. Barium chloride also stimulated 45Ca efflux from STC-1 cells. Whole cell patch clamp recordings revealed a voltage-activated, L-type calcium current. We conclude that, inhibition of basally active potassium channels may depolarize STC-1 cells, producing activation of voltage-gated calcium influx pathways. Influx of calcium may lead to a release of intracellular calcium which stimulates cholecystokinin secretion.

Evidence for indirect dietary regulation of cholecystokinin release in rats

Food ingestion stimulates cholecystokinin (CCK) release from the proximal intestine, but the mechanisms involved are not well understood. To investigate this effect in vivo in intact rats, plasma CCK was measured after orogastric feeding of proteins, protein hydrolysates, amino acids, glucose, and starch. Intact proteins were the only nutrients to stimulate CCK release. The possibility of direct interaction between different dietary constituents and intestinal CCK-secreting endocrine cells was then examined using a perfusion system containing isolated mucosal cells from the rat duodenojejunum. The functional validity of this system was established by demonstrating that monitor peptide and bombesin both stimulated CCK release in a dose-dependent manner. The stimulatory effect of bombesin required extracellular calcium and was not inhibited by addition of tetrodotoxin. Perifusion of proteins, protein digests, and carbohydrates did not stimulate CCK release. These results indicate that proteins stimulate CCK release postprandially via an indirect mechanism, most likely related to inhibition of intraluminal trypsin. Perifusion of dispersed mucosal cells constitutes a reproducible model to investigate hormonal and peptidergic regulation of CCK release in vitro.

Synthesis of human CCK26-33 and CCK-33 related analogues on 2,4-DMBHA and TMBHA

New analogues of human cholecystokinin in which the Tyr(SO3H) has been replaced by Phe(p-CH2SO3Na), methionines by norleucines, and tryptophan by 2-naphthylalanine([Phe(p-CH2- SO3Na)27,Nle28,31,Nal30]-CCK26-33 and [Phe(p-CH2SO3Na)27,Nle7,28,31,Nal30]-CCK-33) were synthesized by Fmoc solid phase methodology on two different resins (2,4- dimethoxybenzhydrylamine- and 4-(benzyloxy)-2',4'-dimethoxybenzhydrylamine resins, 2,4-DMBHA and TMBHA resins, respectively). While the syntheses on the TMBHA appeared to be more sluggish than those carried out on the 2,4-DMBHA, both final crude products were of equivalent relative purity and after purification gave approximately the same final yields of analogues estimated to have a purity greater than 93% using RPHPLC and CZE. The peptides were further characterized by amino acid analysis and LSIMS. Phe(p-CH2SO3Na)27,Nle7,28,31,Nal30]-CCK-33 was submitted to 33 Edman cycles and shown to be the desired product with less than 3% preview. Both analogues were tested for their ability to stimulate amylase release from isolated rat pancreatic acini. In this assay, [Phe(p-CH2SO3Na)27,Nle28,31,Nal30]-CCK26-33 and Phe(p-CH2SO3Na)27,Nle7,28,31,Nal30]-CCK-33 were 10 and 30 times less potent than CCK-8, respectively.

Calcium-dependent regulation of cholecystokinin secretion and potassium currents in STC-1 cells

Secretory and electrophysiological properties of STC-1 cells, a cholecystokinin-secreting cell line, were examined with a radioimmunoassay and patch-clamp recording techniques. Stimulation of cholecystokinin secretion was seen after exposure to agents anticipated to increase the level of intracellular calcium, including thapsigargin (8 microM), bombesin (50 nM), potassium-induced depolarization (50 mM), or after blockade of potassium channels with barium chloride (2 mM). The secretory effects of these agents were blocked by pretreatment with the calcium channel blocker diltiazem (1 microM). Whole cell patch-clamp recordings showed a hyperpolarizing shift in reversal potential after exposure to either thapsigargin (8 microM) or bombesin (50 nM) from a control value of -27 +/- 3 to -57 +/- 7 or -48 +/- 6 mV, respectively. This shift was in the direction of the reversal potential for potassium and was blocked by barium chloride (5 mM). Single-channel recordings from cell-attached membrane patches showed an inwardly rectifying potassium channel with channel open probability modulated by bombesin. These results indicate that in STC-1 cells a potassium current is increased by agents that stimulate CCK secretion, presumably by increasing the level of cytosolic calcium. STC-1 cells may serve as a model system to study the electrophysiological and secretory mechanisms involved in the release of cholecystokinin.

Isolation and characterization of the gene encoding rat glucose-dependent insulinotropic peptide

The rat glucose-dependent insulinotropic peptide (GIP) gene has been isolated and characterized. The gene spans approximately 8.2 kilobase pairs (kb) and the GIP mRNA (0.8 kb) is encoded by six exons. The 42 amino acid hormone is encoded by exons 3 and 4. The exon-intron organization of the rat GIP gene revealed that the splice acceptor site for intron 2 is 24 nucleotides downstream compared to the comparable splice acceptor site in the human gene. This intron sliding results in an 8 amino acid deletion in the amino terminal extension of the prepropeptide. Primer extension analysis and RNase protection assay demonstrated the existence of multiple closely spaced sites for transcriptional initiation. Both the 5'-flanking region and intron 1 contain TATA and CCAAT boxes consistent with initiation of gene transcription, although a TATA box in intron 1 is functionally inactive in adult rats in spite of its reasonable location.

Reduced postprandial cholecystokinin (CCK) secretion in patients with noninsulin-dependent diabetes mellitus: evidence for a role for CCK in regulating postprandial hyperglycemia

The plasma cholecystokinin (CCK) response to a test meal was studied in 16 control subjects and 15 patients with noninsulin-dependent diabetes mellitus (NIDDM). Basal CCK levels were approximately 1 pmol in both groups. However, after the test meal, plasma CCK levels were 2-fold greater in the controls when compared to the diabetics. In controls, CCK levels maximally increased by 5.6 +/- 0.8 pmol (mean +/- SEM) 10 min after feeding, whereas in the NIDDM patients this value was 1.9 +/- 0.6 pmol (P < 0.001). After the test meal, the normal subjects showed no postprandial rise in blood glucose, whereas the diabetic patient showed a rise of 2.6 +/- 0.7 mmol. To determine whether the decreased CCK levels may have been related to the postprandial hyperglycemia, 7 diabetic subjects were infused with CCK. With this CCK infusion, postprandial glucose levels did not rise. These data suggest, therefore: 1) a role for cholecystokinin in regulating postprandial hyperglycemia in man, 2) abnormalities in CCK secretion occur in NIDDM and may contribute to the hyperglycemia seen in this disease.

Intraventricular CCK-8 reduces single meal size in the baboon by interaction with type-A CCK receptors

Intraventricular cholecystokinin COOH-terminal octapeptide (CCK-8) decreases meal size in the meal-trained baboon. In the present study, we tested whether this action is mediated by CCK-A receptors, CCK-B receptors, or both. Intraventricular administration of the selective CCK-A receptor agonist A71623 at 1 and 10 nmol/kg suppressed 30-min meal size 69 +/- 22% and 75 +/- 7%, respectively. Additionally, intraventricular A71623 was equipotent to CCK-8 at 1 nmol/kg (% suppression of meal by CCK = 59 +/- 17). However, intraventricular administration of the CCK-B receptor agonist A63387 at 10 nmol/kg had no effect on 30-min meal size (% suppression = 18 +/- 29). Intravenous administration of 10 nmol/kg A71623 did not result in an alteration of meal size, but prandial plasma insulin and glucose responses were delayed and blunted. Basal plasma insulin levels doubled after intravenous administration of A71623. Both behavioral and metabolic responses to A71623 in the baboon are virtually identical to those we have previously observed after CCK-8 treatment. Thus we conclude that the predominant receptor population with which intraventricular CCK-8 interacts are type-A CCK receptors that are accessible to the ventricular system of the baboon.

Molecular cloning of rat glucose-dependent insulinotropic peptide (GIP)

A cDNA clone encoding glucose-dependent insulinotropic peptide (GIP) was identified that consisted of 34 bp of 5' untranslated sequence, an open reading frame of 432 bp and 115 bp in the 3' untranslated region. The deduced amino acid sequence revealed a 144 amino acid preprohormone consisting of a 43 amino acid N-terminal extension including a signal peptide, a 42 amino acid hormone, and a 59 amino acid C-terminal extension. Rat GIP differs from the human hormone by two amino acid substitutions: arginine for histidine at position 18 and leucine for isoleucine at position 40. A single mRNA from small intestine of approximately 800 bases was identified on Northern blot analysis in equivalent amounts in proximal and distal small intestine.

Regulation of appetite and cholecystokinin secretion in anorexia nervosa

Six patients with anorexia nervosa, the same patients after weight normalization, and six healthy control subjects had similar fasting and postprandial plasma cholecystokinin concentrations. These data do not support the hypothesis that low levels of hunger and food intake in anorexic patients reflect hypersecretion of this endogenous hormone, which is thought to inhibit hunger, promote satiety, and reduce feeding.

Cholecystokinin cells purified by fluorescence-activated cell sorting respond to monitor peptide with an increase in intracellular calcium

Cholecystokinin (CCK) is secreted from specific enteroendocrine cells of the upper small intestine upon ingestion of a meal. In addition to nutrients, endogenously produced factors appear to act within the gut lumen to stimulate CCK release. One such factor is a trypsin-sensitive CCK-releasing peptide found in pancreatic juice, known as monitor peptide. This peptide is active within the intestinal lumen and is hypothesized to stimulate CCK secretion by interacting directly with the CCK cell. We have found that monitor peptide releases CCK from isolated rat intestinal mucosal cells and that this effect is dependent upon extracellular calcium. In the present study, we used monitor peptide as a tool for isolating CCK cells from a population of small intestinal mucosal cells. Dispersed rat intestinal mucosal cells were loaded with the calcium-sensitive fluorochrome Indo-1, and CCK secretory cells were identified spectrofluorometrically by their change in fluorescence when stimulated with monitor peptide. Cells demonstrating a change in their emission fluorescence ratio were sorted using a fluorescence-activated cell sorter. More than 90% of the sorted cells stained positively for CCK with immunohistochemical staining. Furthermore, sorted cells secreted CCK when stimulated with membrane-depolarizing concentrations of potassium chloride, dibutyryl cAMP, calcium ionophore, and monitor peptide. These findings indicate that functional intestinal CCK cells can be highly enriched using fluorescence-activated cell sorting. Furthermore, monitor peptide appears to interact directly with CCK cells to signal CCK release through an increase in intracellular calcium.

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

Monitor peptide stimulates cholecystokinin (CCK) release from the intestine, but the cellular mechanisms responsible for this effect are uncertain. In the present study, the roles of membrane potential difference and calcium influx in monitor peptide-mediated CCK release were examined in a perifusion system containing isolated mucosal cells from the rat duodenum. This method represents an in vitro system in which CCK-releasing cells can be challenged with secretagogues or other maneuvers to study the dynamics of hormone secretion. High concentrations of KCl (50 mM), which reduce electrical potential difference across the cell membrane, caused the release of CCK. This effect was inhibited by the calcium channel blocker MnCl2. Monitor peptide stimulated CCK release in a dose-dependent manner at concentrations from 3 x 10(-12) to 3 x 10(-8) M. The requirement for extracellular calcium in secretagogue-stimulated release of CCK was investigated using ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), a calcium chelator, and MnCl2. A calcium-free environment supplemented with 2 mM EGTA completely inhibited CCK secretion in response to stimulatory doses of monitor peptide. CCK secretion was restored when calcium was reintroduced into the system. Similarly, MnCl2 completely blocked monitor peptide-stimulated CCK release. These data indicate that membrane depolarization and monitor peptide stimulate the release of CCK through calcium-dependent mechanisms, suggesting that increases in intracellular calcium within CCK cells are likely to be important in CCK release.

Characterization of canine intestinal cholecystokinin-58 lacking its carboxyl-terminal nonapeptide. Evidence for similar post-translational processing in brain and gut

An antibody raised against a synthetic cholecystokinin (CCK) analog, (1-27)-(CCK)-33, corresponding to the midregion of CCK-58, detected immunoreactivity in intestinal extracts which eluted between the positions of CCK-33/39 and CCK-58 on high performance liquid chromatography. This peak, lacking carboxyl-terminal cholecystokinin immunoreactivity, was purified by reverse phase and cation-exchange chromatographies. Amino acid, mass spectral, and microsequence analysis established that it was the amino-terminal desnonapeptide fragment of cholecystokinin-58, (1-49)-CCK-58. It was demonstrated further that CCK-58 has less biological activity than CCK-8, suggesting that the amino terminus either sterically hindered the ability of CCK-58 to exert its biological activity or that its amino terminus acted at another site to inhibit release of amylase from rat pancreatic acini. The desnonapeptide of CCK-58 by itself had no biological activity, nor did it affect CCK-8-stimulated amylase release from isolated rat pancreatic acini, suggesting that the amino terminus shields the carboxyl terminus from expressing its biological activity. Its presence in intestine suggests that it is released into the circulation where it could be detected by midregion antibodies. The presence of high proportions of (1-49)-CCK-58 indicates that most CCK-8 is directly derived from CCK-58. Its occurrence in brain and intestine indicates similar processing for procholecystokinin in both tissues.

Regulation of intestinal cholecystokinin and somatostatin mRNA by bombesin in rats

The neuropeptide bombesin has been shown to stimulate secretion of several gastrointestinal hormones, including cholecystokinin (CCK). We have previously demonstrated that stimulation of CCK release by feeding is associated with an increase in steady-state intestinal CCK mRNA levels. The purpose of the present study was to determine whether bombesin stimulates CCK release in rats and, if so, to determine whether bombesin regulates CCK mRNA levels in a manner similar to that of feeding. To establish a proper dose of bombesin for stimulating CCK release, rats received 1-h intravenous infusions of 0.25, 1, 4, or 16 micrograms.kg-1.h-1 bombesin. Basal plasma CCK levels averaged 1.8 +/- 0.4 pM and increased to peak levels of 2.9 +/- 0.6 pM within 15 min of infusion with 4 micrograms.kg-1.h-1 bombesin (the maximally effective dose). With the use of this dose, rats then received infusions of bombesin or saline lasting up to 24 h. At 1, 2, 4, and 24 h, animals were killed for collection of plasma for CCK measurements and of intestine for measurements of intestinal CCK and somatostatin mRNA levels. Bombesin treatment stimulated an increase in plasma CCK levels at 1 h, but levels declined to basal by 4 h, where they remained at 24 h. Despite increasing plasma CCK levels, bombesin infusion, unlike dietary stimulation, had no effect on duodenal CCK mRNA levels. Finally, to determine whether the decrease in plasma CCK levels after prolonged bombesin treatment was due to tachyphylaxis, rats treated with bombesin for 4 h were also fed soybean trypsin inhibitor (a known stimulus of CCK secretion).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of food deprivation on intestinal cholecystokinin and somatostatin

Dietary stimulation has trophic effects on the gastrointestinal tract, whereas prolonged fasting causes mucosal atrophy. Whether gastrointestinal endocrine cells within the mucosa are similarly affected is unknown. The present study was designed to determine the effects of food deprivation and refeeding on cholecystokinin (CCK) and somatostatin in the rat small intestine. RNA was prepared from the duodenum, and peptide and messenger RNA (mRNA) levels of CCK, somatostatin, and beta-actin were analyzed by hybridization with complementary DNA probes. During food deprivation for up to 5 days, plasma CCK levels decreased rapidly, followed by a decline in duodenal CCK mRNA levels and a more gradual decrease in mucosal CCK peptide concentrations. After 3 days of fasting, one group of rats was refed. After only 1 day of refeeding, all parameters (levels of plasma CCK, duodenal CCK mRNA, and duodenal CCK peptide) were restored to control levels. The reduction in CCK mRNA levels seen with fasting was specific, because food deprivation and refeeding produced no changes in either duodenal somatostatin concentrations or mRNA levels of somatostatin and beta-actin. These findings provide initial evidence that food deprivation inhibits duodenal CCK mRNA levels but does not affect duodenal somatostatin.

CCK-releasing activity of rat intestinal secretion: effect of atropine and comparison with monitor peptide

A bioassay for studying the cholecystokinin (CCK)-releasing activity of intraluminal protease-sensitive bioactive peptides was developed. In conscious rats, bile and pancreatic juice were chronically diverted from the proximal intestine to the ileum to cause chronic stimulation of CCK release and pancreatic protein secretion. CCK-releasing activity of test substances was assayed during transient inhibition of CCK release by intraduodenal sodium taurocholate (78 mumols/h). Intestinal secretion as a source of the putative trypsin-sensitive intestinal CCK-releasing peptide was obtained by rapid intestinal perfusion of isolated Thiry-Vella fistulae of jejunum in conscious rats, collected with or without atropine pretreatment. Partially purified rat pancreatic secretory trypsin inhibitor (PSTI, or "monitor peptide") was compared with ovomucoid trypsin inhibitor (OMTI) and with concentrated jejunal secretions for CCK-releasing activity and trypsin inhibitor activity. Concentrated, heat-treated jejunal secretions were the strongest stimulants of CCK release and pancreatic protein secretion in this model. OMTI had no CCK-releasing activity in this model, whereas a larger amount (approximately 5x, based on trypsin inhibitor activity) of PSTI weakly but significantly stimulated CCK release. CCK-releasing activity manifested by pancreatic protein secretion was equivalent in intestinal washes from atropine-treated and control Thiry-Vella fistula donor rats. Concentrated jejunal secretions had no trypsin inhibitory activity, indicating that the putative intestinal CCK-releasing peptide and "monitor peptide" are different substances.