Characterization of cholecystokinin receptors on the sphincter of Oddi

To characterize directly the ability of cholecystokinin (CCK) to interact with receptors on the sphincter of Oddi (SO), we measured binding of 125I-labeled Bolton-Hunter-labeled COOH-terminal octapeptide of cholecystokinin (125I-BH-CCK-8) to tissue sections from the guinea pig SO. Autoradiography localized binding of 125I-BH-CCK-8 over the SO smooth muscle layer. Binding was saturable, specific, dependent on time, pH, and temperature, and was reversible. Binding of 125I-BH-CCK-8 was inhibited by various CCK receptor agonists with the following potencies: CCK-8 much greater than des(SO3)CCK-8 much greater than gastrin-17-I and by various CCK receptor antagonists with the following potencies: L-364,718 greater than proglumide analogue 10 much greater than carbobenzoxy-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-NH2 greater than N2,O2' dibutyryl guanosine 3',5'-cyclic monophosphate. The potencies of agonists in stimulating and of antagonists in inhibiting CCK-8-stimulated SO contractions correlated closely with their abilities to inhibit binding of 125I-BH-CCK-8. Analysis of binding of 125I-BH-CCK-8 to SO tissue sections revealed two classes of CCK binding sites: a high-affinity site [dissociation constant (Kd) 0.2 nM] and a low-affinity site (Kd 70 nM). Atropine or tetrodotoxin (TTX) caused a similar rightward shift of the CCK-8 dose-response curve for stimulation of SO contraction. Comparison of receptor occupation to CCK-8-induced contraction suggested that CCK-8 occupation of the high-affinity binding site correlated with contraction in the absence of atropine and the low-affinity CCK binding with contraction in the presence of atropine or TTX.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of the gastrin receptor antagonist proglumide on survival in gastric carcinoma

Gastric cancer remains a disease with a very poor prognosis, and there is no safe and effective form of therapy for advanced disease. Evidence is now abundant to show that gastrin stimulates the growth of both gastric and colorectal cancer cells in vitro and in vivo, and that blockade of gastrin receptors can prolong survival in xenografted nude mice. We have thus performed a randomized, controlled study of the gastrin/cholecystokinin receptor antagonist proglumide as therapy in 110 patients with gastric carcinoma. Proglumide had no overall effect on survival (Mantel-Cox statistic = 0.5, P = 0.48). The 95% confidence interval for the proglumide treated group was 260 to 474 days compared to 230 to 372 days for the control group. No significant difference was seen with proglumide, which has a relatively low affinity with the gastrin receptor and also has partial agonist activity. Drugs that are far more specific and potent gastrin receptor antagonists are becoming available, which may have a greater effect on survival, and further clinical trials of such compounds are clearly indicated to determine the efficacy of hormonal control of gastrointestinal malignancy.

Pharmacological activity of cholecystokinin analogues modified in the Met28-Gly29 region

Analogues of the C-terminal octapeptide of cholecystokinin (CCK) modified in the Met28-Gly29 region, were tested for their ability to interact with peripheral cholecystokinin receptors on rat pancreatic acini and to stimulate amylase secretion. These analogues were further evaluated for their ability to recognize central CCK receptors on guinea pig brain membranes. The behavioral effect of these analogues was also tested after intrastriatal injection into mice. It appeared that these analogues were full CCK agonists in the peripheral system. Although some induced dopaminomimetic effects after intrastriatal injection into mice, being as potent as the C-terminal octapeptide of cholecystokinin (CCK-8), others did not have any effect and were able to antagonize CCK-8 actions in the striatum. The results of this study confirm that one can obtain very potent CCK analogues by modifying the peptide bond between Met28 and Gly29, and that this modification can produce either CCK agonists or antagonists of CCK-induced dopamine transmission in the striatum.

PD134308, a selective antagonist of cholecystokinin type B receptor, enhances the analgesic effect of morphine and synergistically interacts with intrathecal galanin to depress spinal nociceptive reflexes

The effects of systemic PD134308 [0.1-3 mg/kg; an antagonist of the cholecystokinin (CCK) type B receptor], morphine, and intrathecal (i.t.) galanin (GAL) on the excitability of the spinal nociceptive flexor reflex and in the hot plate test were examined in rats. PD134308 caused a weak naloxone-reversible depression of the flexor reflex and a moderate antinociceptive effect in the hot plate test. However, PD134308 significantly potentiated the antinociceptive effect of morphine as well as its depressive effect on the flexor reflex. PD134308 and i.t. GAL synergistically depressed the flexor reflex, an effect that was reversed by naloxone. Finally, the magnitude and duration of the depression of the flexor reflex by morphine were synergistically increased by coadministering PD134308 and GAL i.t. The results demonstrated that a CCK antagonist directed to the central CCK type B receptor potentiates the analgesic effects of opioids and nonopioid drugs at the spinal level, thus supporting the notion that CCK in the central nervous system may be an endogenous, physiological opioid antagonist.

CCK antagonists interact with CCK-B receptors on human small cell lung cancer cells

The ability of cholecystokinin (CCK) receptor antagonists to interact with CCK receptors in small cell lung cancer (SCLC) cells was investigated. L-365,260, CCK-8, L-364,718, CBZ-CCK(27-32)-NH2 and proglumide analogue 10 inhibited specific 125I-CCK-8 binding to SCLC cells with IC50 values of 0.2, 2, 500, 100,000 and 500,000 nM, respectively. Gastrin-I and CCK-8 elevated the cytosolic Ca2+ when SCLC cells were loaded with Fura 2-AM. L-365,260 inhibited the cytosolic Ca2+ increase caused by 10 nM CCK-8 in a dose-dependent manner. The effects of 10 nM L-365,260 were reversed by high concentrations of CCK-8. These data indicate that L-365,260 functions as a reversible CCK-8 antagonist using SCLC cells.

Effects of CCK-receptor antagonists on CCK-stimulated pepsinogen secretion and calcium increase in isolated guinea pig gastric chief cells

The effects of CCK-receptor antagonists such as dibutyryl cyclic GMP (dbcGMP), L-364,718, and CR1409 on COOH-terminal octapeptide of CCK (CCK-8)-stimulated chief cell responses were examined using isolated guinea pig gastric chief cells. L-364,718 and CR1409 inhibited CCK-8-stimulated pepsinogen secretion over the same concentration range as they inhibited CCK-8-stimulated increases in intracellular Ca2+ concentration ([Ca2+]i), respectively. Schild analysis of the CCK dose-response curve indicates that L-364,718 and CR1409 exert their inhibitory effects on CCK-8-stimulated chief cell responses in a competitive manner. By contrast, dbcGMP inhibited not only CCK-8- but also carbachol-stimulated pepsinogen secretion. Furthermore, dbcGMP inhibited CCK-8-stimulated pepsinogen secretion more potently than the increases in [Ca2+]i. These results suggest that L-364,718 and CR1409 act as CCK-receptor antagonists, but dbcGMP has another inhibitory effect on pepsinogen secretion in addition to the effect as a CCK-receptor antagonist in guinea pig gastric chief cells.

Glutamine derivatives as novel anticancer agents acting via gastrin binding sites

With regard to the structural analogies between the C-terminal tetrapeptide of gastrin, gastrin receptor antagonists and anticancer derivatives of glutamine, it is plausible to propose that the mechanism of the anticancer activity may be related to interaction with the gastrin receptor. Dietary glutamine intake and glutamine metabolites may also influence the activity of gastrin receptors.

The effect of a novel CCK-antagonist (lorglumide) on human and guinea pig gallbladder strips: a tensiometric study

The effect of a novel CCK-antagonist (lorglumide, CR 1409) was evaluated by "in vitro" tensiometric studies on 16 human (gallstone patients) and 12 guinea pig gallbladder smooth muscle strips. In the animal experiments, increasing doses of lorglumide (0.2-6.5 uM) caused a rightward shift of the dose-response curves of CCK-OP, with an increase of the ED50 from 8.2 nM +/- 1.62 SEM, n = 12; to 100 nM +/- 12, n = 4) without affecting the maximal effect (Emax). Schild plot gave an affinity constant of 7.19. In human gallbladders, the effect of lorglumide was also present (ED50 increased from 47 nM +/- 8 SEM, n = 16; to 300 nM +/- 10 SEM, n = 4) coexisting with a large inter-sample variation for CCK-OP ED50s and maximal contractions, most likely due to the histological changes of the wall in chronic cholecystitis. The affinity constant was similar to that found in animal experiments. We confirm the studies previously reported in animals on the existence of a competitive antagonism of lorglumide on CCK gallbladder receptors. Moreover, our results on gallbladders from gallstone patients show that lorglumide is a highly effective antagonist of CCK-induced contractions despite the presence of chronic cholecystitis. Our study might help for a better comprehension of the role of these new anti-CCK drugs in the treatment of biliary pain.

Receptor type for cholecystokinin on isolated intestinal muscle cells of the guinea pig

Smooth muscle cells isolated from the longitudinal muscle layer of guinea pig ileum were used to determine the presence and type of cholecystokinin/gastrin receptor mediating contraction. This was accomplished with a series of cholecystokinin and gastrin agonists (CCK-8, des(SO3)CCK-8, gastrin-17, des(SO3)gastrin-17 and pentagastrin) and antagonists (glutaramic acid derivatives CR 1392, CR 1409, CR 1505 and proglumide). The order of potency of agonists based on EC50 values derived from concentration-response curves was: CCK-8 greater than des(SO3)CCK-8 greater than gastrin-17 greater than des(SO3)gastrin-17. The inhibitory dissociation constant (Ki) for the antagonist CR 1505 derived from Schild plots was the same whether sulfated CCK-8 or desulfated gastrin-17 was used as agonist (4.47 +/- 0.76 versus 4.68 +/- 0.78 nM). Pentagastrin acted as a partial agonist and inhibited partially the response to CCK-8. The Ki values determined for all antagonists with pentagastrin as agonist were similar to those with CCK-8 as agonist. The order of potency of agonists and the independence of Ki values from the type of agonist used implied that CCK and gastrin interact with one receptor type; the receptor is more sensitive to CCK-8 but is minimally influenced by sulfation of the tyrosine residue. In this respect, the receptor appears to be distinct from the CCK receptor on gallbladder muscle cells and pancreatic acinar cells.

Long-term diazepam treatment produces changes in cholecystokinin receptor binding in rat brain

This study examined the effect of chronic diazepam administration on central benzodiazepine and CCK-8 receptor binding in rat brain. After a two-week treatment with diazepam (5 mg/kg per day) tolerance developed towards the sedative but not towards the anxiolytic action of this drug as determined using elevated plus-maze and open field tests. The % entries the rats made onto open arms and % time the rats spent in open arms were markedly decreased 24 h after the last dose of diazepam, probably indicating withdrawal anxiety. There were no changes in [3H]flunitrazepam binding either 30 min or 24 h after the last diazepam dose. However, 30 min after the last diazepam administration the apparent number of sulphated [3H]CCK-8 binding sites was significantly increased in the primary olfactory cortex. Acute diazepam treatment (5 mg/kg) had no influence on [3H]flunitrazepam or sulphated [3H]CCK-8 binding in any brain region studied. Cessation of chronic diazepam treatment was followed after 24 h by an increase in the number of CCK-8 receptors in frontal cortex and hippocampus as compared to the vehicle group. These results demonstrate that certain alterations in CCK-8 receptor characteristics may be important in the anti-anxiety effect, tolerance, and withdrawal reaction reaction after benzodiazepine administration.

Effect of a new cholecystokinin receptor antagonist loxiglumide on acute pancreatitis in two experimental animal models

We evaluated the effects of a new cholecystokinin (CCK) receptor antagonist, loxiglumide, in a model of mild pancreatitis induced by repeated injections of cerulein and in a severe necrotizing form of pancreatitis induced by retrograde ductal injection of sodium taurocholate (NaTc) in rats. A single subcutaneous injection or oral administration of 50 mg/kg of body weight of loxiglumide almost completely reduced the increases of serum amylase activity and pancreatic wet weight, and caused histologic improvements of the cerulein-induced acute pancreatitis when given 30 min before the first cerulein injection. Loxiglumide was also effective in reducing the elevated serum amylase activity, pancreatic wet weight, and histologic alterations even when administered after the induction of acute pancreatitis. However, loxiglumide offered no apparent beneficial effects when given 30 min before and 3 h after the induction of acute pancreatitis by NaTc as determined by changes in serum amylase activity, pancreatic wet weight, and histology. These results do not necessarily suggest that CCK is not important in the pathogenesis of pancreatitis, but do suggest that the sole blockade of peripheral CCK receptors is ineffective against NaTc-induced severe necrotizing pancreatitis.

Regulation of pancreatic endocrine function by cholecystokinin: studies with MK-329, a nonpeptide cholecystokinin receptor antagonist

A cholecystokinin (CCK) receptor antagonist, MK-329, was used to explore the physiological role of CCK in regulating pancreatic endocrine function in humans. The ability of CCK to increase plasma pancreatic polypeptide (PP) concentrations and blockade of this effect with MK-329 were evaluated in a double blind, balanced, four-period cross-over study. Eight subjects received single oral doses of 0.5, 2, or 10 mg MK-329 or placebo, followed by an iv infusion of CCK-8 (34 ng/kg.h). In placebo-treated subjects, PP increased from basal levels of 70 +/- 15 (+/- SE) to peak values of 291 +/- 58 pg/mL after CCK infusion (P less than 0.05 compared to basal). This increase in plasma PP concentration was inhibited in a dose-dependent fashion by MK-329, with 10 mg antagonizing the stimulatory effect of CCK infusion by nearly 80%. Second, the effect of MK-329 on meal-stimulated pancreatic endocrine responses was evaluated by giving placebo or 10 mg MK-329 2 h before ingestion of a mixed meal. Eight subjects were treated in a randomized two-period cross-over fashion. With placebo treatment, peak postprandial plasma insulin, glucagon, and glucose concentrations were 101 +/- 8 microU/mL, 195 +/- 15 pg/mL, and 150 +/- 10 mg/dL, respectively (all P less than 0.05). The integrated PP response following the meal was 56.3 +/- 11.1 ng/mL.minute. With MK-329 treatment, the integrated PP concentration was reduced to 33.9 +/- 2.2 ng/mL.min (P less than 0.05 compared to placebo treatment). Mean postprandial insulin, glucagon, and glucose concentrations did not differ between placebo and MK-329 treatments. We conclude that CCK receptor blockade with 10 mg MK-329 does not alter plasma insulin, glucagon, or glucose responses to a mixed meal. However, the observation that physiological concentrations of CCK increase plasma levels of PP, and the finding that CCK receptor blockade selectively attenuates the postprandial increase in plasma PP concentrations support a physiological role for CCK in regulating PP secretion.

Two functionally distinct cholecystokinin receptors show different modes of action on Ca2+ mobilization and phospholipid hydrolysis in isolated rat pancreatic acini. Studies using a new cholecystokinin analog, JMV-180

A new hepatapeptide cholecystokinin (CCK) analog, JMV-180 (Boc-Tyr(SO3-)-Nle-Gly-Trp-Nle-Asp-2-phenylethylester), acts as an agonist at high affinity CCK receptors on rat pancreatic acini to stimulate amylase release but unlike cholecystokinin octapeptide (CCK8) does not act on low affinity CCK receptors to inhibit amylase release (Galas, M. D., Lignon, M. F., Rodriguez, M., Mendre, C., Fulcrand, P., Laur, J., and Martinez, J. (1988) Am. J. Physiol. 254, G176-G188). To investigate the biochemical mechanisms initiated by CCK acting on each class of CCK receptor, the effects of JMV-180 and CCK8 on amylase release, Ca2+ mobilization, and phospholipid hydrolysis were studied in isolated rat pancreatic acini. When acini were loaded with the intracellular Ca2+ chelator BAPTA, amylase release stimulated by both JMV-180 and CCK8 was reduced. Measurement of 45Ca2+ efflux and cytosolic free calcium concentration ([Ca2+]i) by the fluorescence of fura-2-loaded acini in a stirred cuvette showed that JMV-180 induced a concentration-dependent increase but with a maximal response only two-thirds that induced by CCK8. When [Ca2+]i of individual fura-2-loaded acinar cells was measured by microspectrofluorometry, all concentrations of JMV-180 (1 nM-10 microM) induced repetitive transient [Ca2+]i spikes (Ca2+ oscillations). By contrast, stimulation with a high concentration of CCK8 (1 nM) caused a large increase in [CA2+]i followed by a small sustained elevation of [Ca2+]i. The measurement of inositol trisphosphate (IP3) production by both [3H]inositol labeling and 1,4,5-IP3 radioreceptor assay showed that JMV-180 had only minimal effects at 10 microM in contrast to the large increase induced by high concentrations of CCK8 (more than 1 nM). JMV-180 blocked the effect of a high concentration of CCK8 on both [Ca2+]i and 1,4,5-IP3 productions but did not affect the response to carbamylcholine. JMV-180 caused a delayed monophasic stimulation of 1,2-diacylglycerol (DAG) sustained to 60 min without the early increase in DAG observed in response to CCK8. Furthermore, JMV-180 stimulated the release of [3H]choline metabolites, primarily phosphorylated choline, from [3H]choline-labeled acini at low concentrations and to the same extent as CCK8. Since JMV-180 interacts not only with high affinity CCK receptors as an agonist but also with low affinity CCK receptors as a functional antagonist, the present results indicate that the occupancy of high affinity state receptors by CCK induces Ca2+ oscillations, DAG formation from phosphatidylcholine hydrolysis, and amylase release with minimal phosphatidylinositol 4,5-bisphosphate hydrolysis.(ABSTRACT TRUNCATED AT 400 WORDS)

A71378: a CCK agonist with high potency and selectivity for CCK-A receptors

Receptors for the brain and gut peptide cholecystokinin (CCK) have been classified into two classes, CCK-A and CCK-B. To date, peptide analogues with selectivity for the CCK-B receptors have been identified, and selective antagonists for CCK-A and CCK-B receptors have been reported as well; until now, there have been no reports of highly selective CCK-A agonists. Herein we describe the properties of A71378 [desamino-Try(SO3H)-Nle-Gly-Trp-Nle-(N-methyl)Asp-Phe-NH2], a highly selective CCK-A receptor ligand. Characterization of A71378 was carried out in the guinea pig pancreas, cortex, gastric gland, and ileum, as well as in NCI-H345 cells. The IC50 values of A71378 for the pancreatic CCK-A, cortical CCK-B, and gastrin receptor were 0.4 nM, 300 nM, and 1,200 nM, respectively. A71378 proved to be a potent agonist in eliciting pancreatic amylase secretion (EC50 = 0.16 nM) and ileal muscle contraction (EC50 = 3.7 nM). In contrast, A71378 was relatively weak (EC50 = 600 nM) in mobilizing intracellular calcium from NCI-H345 cells, which express CCK-B/gastrin receptors. The high potency and selectivity of A71378 for the CCK-A over CCK-B and gastrin receptors is unprecedented among CCK peptides. Studies on CCK-7 analogues indicate that N-methylation of the Asp residue is responsible for the observed selectivity for CCK-A receptors. This discovery of a selective CCK-A agonist should prove valuable for studies aimed at understanding the physiological roles of CCK-A receptors in the brain and periphery.

Characterization of cholecystokinin octapeptide-stimulated endogenous dopamine release from rat nucleus accumbens in vitro

1. The effect of cholecystokinin sulphated octapeptide (CCK8S) on endogenous dopamine release was examined in rat striatal and nucleus accumbens slices, by high performance liquid chromatography (h.p.l.c.) with electrochemical detection. 2. CCK8S was shown to increase dopamine release from slices of nucleus accumbens but not striatum in a dose-dependent manner between 0.1 and 10 microM. 3. Pentagastrin was without effect on dopamine release at doses up to 10 microM. 4. The dopamine release produced in the presence of CCK8S was abolished by preincubation of the slice with 1.0 or 10.0 nM L-364,718 (the CCKA-selective antagonist) while 1 microM L-365,260 (the CCKB-selective antagonist) had no action. 5. These results suggest that the CCK8S-evoked release of dopamine from the nucleus accumbens is mediated by a CCKA-receptor.

Cholecystokinin causes contraction of the pupillary sphincter in monkeys but not in cats, rabbits, rats and guinea-pigs: antagonism by lorglumide

The effects of intracameral injections of cholecystokinin (CCK) on the pupil size were determined in monkeys, cats, rabbits, rats and guinea-pigs. In animals under muscarinic cholinergic blockade, CCK caused miosis in monkeys but not in the other species investigated. In monkeys CCK-8 was more potent than CCK-33, which was, however, much more potent than non-sulphated CCK-8. These observations indicate that peripheral type A receptors mediated the miotic response. The effect of CCK-8 was not appreciably influenced by pretreatment with indomethacin, 3 mg kg-1 body wt, indicating that prostaglandins were not involved in the response. Nerve blockade with 0.9 micrograms tetrodotoxin intracamerally had no clear effect on the dose-response relationship for CCK-8. The effect of the peptide thus seems to be directly on receptors on the pupillary sphincter muscle. Pretreatment with lorglumide caused a dose-dependent rightward shift of the dose-response curve, indicating competitive antagonism. The results indicate that in monkeys, but not in rabbit, cats, rats and guinea-pigs, CCK is a potent miotic with a direct effect on the pupillary sphincter mediated by type A CCK receptors on the muscle.

Endogenous cholecystokinin reduces feeding in young rats

The hypothesis that endogenous cholecystokinin (CCK) released from the small intestine during feeding causes satiety was tested in rat pups, 9 to 12 days old. Intragastric administration of soybean trypsin inhibitor, a procedure that releases CCK from the small intestine, decreased the subsequent intake of a test meal. This effect was reversed by prior treatment with MK-329, a selective antagonist of CCK at alimentary-type CCK (CCK-A) receptors. Thus, endogenous, small intestinal CCK can cause satiety in the neonatal rat and this effect involves CCK-A receptors.

Characterization of neuronal cholecystokinin receptor by L-364,718 in Auerbach's plexus

The release of [3H]acetylcholine [( 3H]ACh) from Auerbach's plexus and the contraction of longitudinal muscle strips in response to the administration of cholecystokinin (CCK) were measured and recorded simultaneously. The peripheral CCK receptor antagonist, 3S(-)-N-(2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepine-3-yl)-1H- indole-2-carboxamide (L-364,718), antagonized the ACh releasing effect of CCK in a dose-dependent manner. The IC50 value and the dissociation constant (KD) were 41.0 +/- 2.0 pM and 0.06 +/- 0.01 nM, respectively. These results suggest that L-364,718 is a very potent antagonist of the neuronal CCK receptors.

Effects of a cholecystokinin receptor antagonist on intestinal phase of pancreatic and biliary responses in man

The present study was designed (a) to characterize the activity of loxiglumide as a peripheral cholecystokinin (CCK) antagonist in healthy human subjects, and (b) to determine whether CCK is a physiologic regulator of the intestinal phase of meal-stimulated exocrine pancreatic and biliary secretions in man. Intravenous loxiglumide (22 mumol/kg per h) was highly potent in antagonizing CCK8-induced pancreatic enzyme and bile acid secretion as well as pancreatic polypeptide release. The potency and selectivity of loxiglumide as an antagonist of CCK provides the tool for evaluating the role of CCK as a physiological mediator of meal-induced pancreatic and biliary responses in humans. Infusion of a liquid test meal into the duodenum evoked an immediate response of pancreatic enzyme and bilirubin outputs, respectively. Intravenous loxiglumide significantly inhibited the meal-induced pancreatic amylase output by 63% (P less than 0.05), lipase output by 43% (P less than 0.05), and bilirubin output by 59% (P less than 0.05). These data suggest that CCK is a physiological mediator of the intestinal phase of meal-stimulated pancreatic and biliary responses.

Effect of thyroxine on the maturation of cholecystokinin (CCK) receptors in pancreatic acini of neonatal rats

Neonatal rat pancreata are not responsive to stimulation by cholecystokinin (CCK) and this has been shown to be due partly to low binding of CCK to pancreatic acinar cells of rats at this age. The effect of thyroxine on the maturation of CCK receptor binding and enzyme secretion is studied. One-day-old rat pups were injected daily with thyroxine (0.1 microgram/g of body weight) for 3 days and killed on day 5. Control littermates were injected with normal saline at the same volume and schedule as the thyroxine group. The pancreatic weight and amylase activity were significantly higher in pups from the thyroxine group. Amylase release after stimulation with various concentrations of CCK was also higher in the thyroxine group. The maximal binding to [125I]BH-CCK-8 was significantly higher in dispersed acini from the thyroxine group when compared to the control group (5.2 vs. 2.0%). Analysis of binding data showed that the higher binding was due to a higher maximal binding capacity in the thyroxine group (1.1 +/- 0.41 vs. 5.2 +/- 1.4 fmol/mg of protein). Thyroxine, therefore, induces a precocious maturation of the secretory function of the pancreatic acini, specifically by modulating the maximal binding capacity of the high-affinity binding sites.

Control of acid secretion

Receptors for the main neural (acetylcholine), hormonal (gastrin) and paracrine (histamine) secretory stimulants and the signal transduction pathways to which these receptors are coupled have been identified on the parietal cell. The stimulatory effect of histamine is mediated via an increase in adenylate cyclase activity, whereas the effect of acetylcholine and gastrin are mediated via an increase in cytosolic levels of calcium. Strong synergism between histamine and either gastrin or acetylcholine may reflect postreceptor interaction between the distinct pathways. Acetylcholine and gastrin are also capable of releasing histamine from the gastric mucosa, probably from ECL cells. The inhibitory effects of somatostatin and prostaglandin E on acid secretion are mediated by receptors coupled via guanine nucleotide binding proteins to inhibition of adenylate cyclase activity. All the pathways converge on and modulate the activity of the luminal enzyme, H+K(+)-ATPase, ultimately responsible for acid secretion. The intramural neural and paracrine pathways involved in the regulation of gastrin secretion in the antrum and acid secretion in the fundus have also been identified. Of prime importance is the somatostatin cell, which exerts a paracrine restraint on gastrin secretion and acid secretion. Elimination of this restraint or disinhibition is one of the mechanisms by which the stimulatory influence of cholinergic neurons is exerted on gastrin and parietal cells. Gastrin secretion is regulated by a cholinergic neuron that causes inhibition of somatostatin secretion and thus stimulation of gastrin secretion (disinhibition) and a noncholinergic neuron that causes direct stimulation of gastrin secretion by releasing the neurotransmitter, bombesin (or gastrin-releasing peptide). Acid secretion is regulated by a cholinergic neuron that causes direct stimulation of the parietal cell and indirect stimulation by decreasing somatostatin secretion, thus eliminating its inhibitory effect on the parietal cell (disinhibition). In addition, a regulatory feedback mechanism exists whereby intraluminal acidification stimulates somatostatin secretion, which in turn attenuates acid secretion. Gastric acid secretion may also be regulated by one or more intestinal inhibitory hormones, the most likely candidates being secretin, intestinal somatostatin, and neurotensin. Enterogastrone activity probably reflects the combined effect of all these hormones. Precise information on receptors and signal transduction mechanisms as well as on intramural neural and paracrine regulatory pathways has led to the development of new drugs capable of inhibiting acid secretion. These include antagonists that interact with stimulatory receptors (histamine H2-receptor antagonists, muscarinic receptor antagonists, and gastrin receptor antagonists), agonists that interact with inhibitory receptors (somatostatin and prostaglandin E analogues), and irreversible inhibitors of the luminal enzyme, H+K(+)-ATPase.

The role of carbohydrate moieties of cholecystokinin receptors in cholecystokinin octapeptide binding: alteration of binding data by specific lectins

Cholecystokinin (CCK) receptors on rat pancreatic acini have been demonstrated to be glycoproteins. In order to study whether their carbohydrate moieties play a role in ligand binding, membrane preparations (adjusted to 0.2 mg protein me) were incubated with 20 pM 125-I-CCK octapeptide (125I-CCK8) for 4 h at 30 degrees C in the presence of lectins with different sugar specificities. Concanavalin A, soy-bean agglutinin, and peanut agglutinin in concentrations up to 1 mM did not alter specific 125I-CCK8 binding. Ulex europeus lectin I showed a dose-dependent enhancement of CCK binding up to 150% of controls at a concentration of 1 mM. Wheat-germ agglutinin (WGA) was the only lectin found to have an inhibitory effect. Inhibition was dose-dependent, with maximal reduction attained at 42 nM, but CCK binding was only partially inhibited to 66.2 +/- 4.4%. Inhibition by WGA was prevented by the presence of N-acetyl-D-glucosamine or N,N',N"-triacetylchitotriose, sugars that are specific for WGA. The inhibitory effect of WGA was not due to an increase in non-specific binding, increased CCK degradation, or CCK binding to WGA. Binding data indicated that the presence of WGA resulted in a decrease in receptor affinity (Kd = 567 +/- 191 v. 299 +/- 50 pM). No significant change in the number of available binding sites was observed. This suggests that WGA is not binding to the active binding site. It is conceivable that binding of WGA to N-acetyl-D-glucosamine or its polymers can lead to a conformational change in the receptor protein, and that this carbohydrate moiety is essential for optimal receptor-ligand interaction.