CCK receptors in release of pancreatic polypeptide (PP) in dogs

Pancreatic polypeptide (PP) is released after ingestion of protein-fat meals and following administration of some gut hormones (CCK and bombesin), but the hormonal contribution to the physiological release of PP has not been elucidated. We used specific and potent CCK-receptor antagonist, L-364,718, administered intravenously in a dose of 0.5 mumol/kg or intraduodenally in a dose of 2 mumol/kg to assess the role of CCK in the release of PP. Exogenous CCK-8 infused intravenously in gradually increasing doses (12.5-400 pmol/kg/hr) caused a dose-dependent increase in plasma PP from basal 28 +/- 4 pM to 136 +/- 18 pM, and this PP increase was completely suppressed by both intravenous and intraduodenal administration of L-364,718. Meat feeding caused a dramatic increase in plasma PP from a basal level of 26 +/- 4 pM to a peak of about 190 +/- 32 pM, and the pretreatment with intravenous or intraduodenal L-364,718 reduced this PP increase by about 60%. Duodenal perfusion with oleate (0.12-4.0 mmol/hr) or L-Trp (0.12-4.0 mmol/hr) also increased plasma PP, reaching, respectively, 180 +/- 28 pM and 76 +/- 6 pM. Pretreatment with intravenous or intraduodenal L-364,718 completely abolished the plasma PP responses to oleate and L-Trp. Bombesin (100 pmol/kg/hr) raised plasma PP to the level similar to that achieved by meat feeding and L-364,718 given intravenously or intraduodenally blocked completely these plasma PP increments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diurnal profile of pancreatic exocrine secretion and plasma levels of gut hormones (cholecystokinin and pancreatic polypeptide)

We examined diurnal changes of pancreatic exocrine secretion and plasma levels of cholecystokinin (CCK) and human pancreatic polypeptide (hPP) in men with postoperative transient external pancreatic drainage (3 with pancreatico-jejunostomy: PJ, and 5 with pancreato-duodenectomy:PD). Plasma CCK levels increased and the pancreatic exocrine secretion showed a corresponding increase after meals in both groups. In the PJ group hPP levels increased after meals and there was a significant correlation between the plasma levels of CCK and hPP. In the PD group the plasma hPP levels were not detectable throughout the study and there was a better correlation between the exocrine secretion and the plasma CCK levels than in the PJ group. Plasma CCK levels showed rhythmic changes associated with pancreatic exocrine secretion, with no significant changes in plasma glucose and insulin in the night (24:00-07:00). These results suggested that (1) the resection of the pancreatic head which is rich in hPP contents led to a complete loss of anti-CCK effects on CCK, and that (2) fluctuation in plasma CCK levels, in the fasting state, might initiate pancreatic exocrine secretion.     

Effects of cholecystokinin (CCK)-8, CCK-33, and gastric inhibitory polypeptide (GIP) on basal and meal-stimulated pancreatic hormone secretion in man.

Gastrointestinal hormones with insulinotropic effects, like cholecystokinin (CCK) and gastric inhibitory polypeptide (GIP) might tentatively be used in the treatment of non-insulin-dependent diabetes mellitus. We therefore examined the effects of intravenous injection of pharmacological dose levels of CCK-8 (100 and 300 pmol/kg), CCK-33 (100 pmol/kg), GIP (100 pmol/kg), and CCK-8 plus GIP (100 pmol/kg of each) on plasma levels of glucose, insulin, somatostatin, glucagon, and pancreatic polypeptide (PP) in healthy human volunteers. The peptides were given under basal conditions or in combination with a mixed meal. CCK-8, CCK-33, and GIP were all found to increase the basal plasma levels of insulin, somatostatin, and PP; the increases were observed already in samples taken at 2 min after the injection. In contrast, the plasma glucagon levels were unaffected by the peptides. CCK-8, CCK-33, and GIP (100 pmol/kg) all potentiated the meal-induced plasma responses of insulin and PP, whereas plasma levels of glucagon after the meal were not affected. Plasma somatostatin levels after the meal were increased by GIP but not affected by CCK-8 or CCK-33. CCK-8 and GIP together (100 pmol/kg for both) increased plasma levels of insulin, PP and somatostatin as much as each of the peptides given alone, both under basal conditions and after the meal intake. Plasma levels of glucagon were not affected by CCK-8 and GIP together. We conclude that in man, both CCK-8, CCK-33, and GIP moderately stimulate basal and meal related insulin release without any synergistic effects and that the peptides do not inhibit the secretion of glucagon.     

L-364,718, a new CCK antagonist,inhibits postprandial pancreatic secretion and PP release in dogs

The effects of L-364,718, a new CCK receptor antagonist, on food-stimulated exocrine pancreatic secretion and plasma levels of PP, insulin, CCK, and gastrin were examined in four conscious dogs with pancreatic fistulas. Intravenous injections of L-364,718 (20 nmol/kg) significantly inhibited pancreatic protein and enzyme responses by food (33% inhibition) but not juice volume output. Both rapid and secondary prolonged postprandial rises of plasma PP were also significantly suppressed by L-364,718 (50% inhibition); however, plasma levels of insulin were not altered. Postprandial levels of gastrin were not affected by L-364,718 administration, whereas 3-hr integrated CCK response was significantly enhanced by L-364,718. This study indicates that L-364,718 inhibits pancreatic protein and enzyme secretion and the release of pancreatic polypeptide stimulated by food in conscious dogs. This inhibition might be due to the selective blockage of receptor binding of circulating CCK molecules. The results suggest that L-364,718 may be useful for the physiological and pathophysiological studies associated with CCK.     

Fasting and postprandial plasma pancreatic polypeptide (PP) levels in obesity

Twenty-three morbidly obese and 17 control subjects were studied with a breakfast meal. Neither fasting nor postprandial plasma pancreatic polypeptide (PP) levels differed significantly between the two groups, whereas postprandial serum insulin and blood glucose were significantly higher in the obese subjects. Our results do not support the suggestion that PP participates in the appetite regulation or the development of obesity.     

Effect of hydrocortisone on the maturation of cholecystokinin (CCK) binding and CCK stimulated amylase release 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 primarily to low binding of CCK to pancreatic acinar cells of rats of this age group. To see if hydrocortisone has any effect on the maturation of CCK binding and enzyme secretion, day-old rat pups were injected three times intraperintoneally with hydrocortisone at a dose of 5 mg/100 g body weight per dose and sacrificed 48 h after the first injection. Control age-matched pups were injected with 0.9% saline at the same volume and schedule as the hydrocortisone injected pups. The pancreatic weight, protein, and DNA contents were found to be significantly lower in the pups from the hydrocortisone-treated group than in the pups from the control group. The protein content per unit weight of DNA, however, was not different between the two. The maximal output of amylase under stimulation by 3 X 10(-10) M CCK was significantly higher in the dispersed acini prepared from the hydrocortisone-treated group as compared to dispersed acini prepared from the control (575 +/- 50 vs. 390 +/- 40% when expressed as a percentage of basal release). The maximal binding to 125I-BH-CCK was also significantly higher in the dispersed acini from the hydrocortisone group when compared to the dispersed acini from the control group (2.6 +/- 0.5 vs. 1.4 +/- 0.4%). Hydrocortisone, therefore, induces the precocious maturation of the secretory apparatus of the pancreatic acini, specifically the increase in capacity to bind and the greater responsiveness of the acini to CCK.     

Effect of loxiglumide, a cholecystokinin antagonist, on pancreatic polypeptide release in humans

The purpose of this study was to determine the role of cholecystokinin in the regulation of postprandial pancreatic polypeptide secretion in humans. The pancreatic polypeptide responses to modified sham feeding and gastric instillation of a test meal were first compared with the response to oral ingestion of the same meal. The experiments were repeated under cholinergic (atropine) and cholecystokinin (loxiglumide) blockade. Atropine completely abolished the pancreatic polypeptide response to sham feeding and caused significant reductions after gastric and oral food intake. Loxiglumide, on the other hand, significantly reduced pancreatic polypeptide release to oral food (51% inhibition) without affecting the response to sham feeding. In separate experiments using a duodenal perfusion system, the effects of atropine and loxiglumide on intestinal phase-stimulated pancreatic polypeptide release were examined, and both cholinergic and cholecystokinin blockade induced complete suppression. It was concluded (a) that cholecystokinin is involved in postprandial pancreatic polypeptide response, especially during the intestinal phase stimulation, and (b) that the cholinergic system is crucial and superimposed on cholecystokinin in stimulating pancreatic polypeptide release.     

Effect of pancreatic proteases on plasma cholecystokinin, secretin, and pancreatic exocrine secretion in response to sodium oleate

The effect of pancreatic proteases or juice on the sodium oleate-stimulated pancreatic secretion and plasma concentrations of secretin and cholecystokinin in anesthetized rats was investigated. Each rat received sodium oleate in a dose of 0.12 mmol.h-1 via a duodenal tube. Sodium oleate infusion significantly increased pancreatic secretion (volume and protein output) compared with the saline given the control group. The increase in pancreatic secretion paralleled significant elevations of plasma concentrations of secretin and cholecystokinin. To determine a possible role of pancreatic proteases on the responses induced by sodium oleate, saline, chymotrypsin, and trypsin, a combination of chymotrypsin and trypsin or pancreatic juice was infused into the duodenum. The pancreatic secretion was significantly reduced by pancreatic proteases or pancreatic juice, and the reduction paralleled the decreases in plasma concentrations of the two hormones. These agents suppressed both pancreatic secretion and plasma hormone levels in the following order of magnitude: (pancreatic juice or chymotrypsin + trypsin) greater than (trypsin) greater than (chymotrypsin). The reduction of pancreatic secretion by pancreatic proteases was reversed by intravenous administration of secretin and cholecystokinin in physiological doses. It is concluded that negative-feedback regulation of pancreatic secretion is operative in the intestinal phase in rats and that both secretin and cholecystokinin are involved in the regulation.     

A reduced pancreatic protein secretion in response to cholecystokinin (CCK) in the obese Zucker rat correlates with a reduced receptor capacity for CCK

Pancreatic membrane receptors for cholecystokinin (CCK) in obese and nonobese Zucker rats were compared with the use of a biologically active [125I]iodo-CCK-8 radioprobe. Membrane homogenates from obese rats bound half the amount of radioligand in 2 h as did membranes from lean rats (specifically bound, 7.0% vs. 14.0%; P less than 0.001). The reduced binding in membranes from obese rats did not result from kinetic effects or radioligand degradation; similar rates of association and dissociation of [125I]iodo-CCK-8 were obtained in membrane preparations from both, and no differences were found in the extent of radioligand degradation in the two membrane preparations. These differences also did not reflect an effect of cell size, as pancreatic acinar cells from obese and nonobese rats had about the same perimeters (24.6 and 26.3 micron, respectively) and areas (30.1 and 34.2 micron 2, respectively). Scatchard-type plots of competitive displacement data for CCK-binding sites on pancreatic membranes from both genotypes were curvilinear and were analyzed by a two-site binding model. The Kd values for both the high (0.56 vs. 0.45 nM) and low (9.0 vs. 14 nM) affinity sites on membranes from nonobese and obese rats, respectively, were the same (P greater than 0.1), whereas the capacities for CCK in the high (365 vs. 165 fmol/mg protein) and low (1020 vs. 360 fmol/mg protein) affinity regions were significantly different (P less than 0.025). This difference in CCK receptor capacity was reflected by a reduced pancreatic protein secretory response in the obese rat. After injections of 40, 80, 160, and 320 ng CCK/kg BW, total pancreatic protein secretion in nonobese rats increased 5, 12, 19, and 21 times above basal levels, whereas the same doses caused 2-, 6-, 12-, and 13-fold increases in obese rats. Whereas the reduced secretion in the obese rat may reflect a difference in the intracellular machinery leading to protein secretion between the two genotypes, these data are more consistent with a direct mechanism, whereby reduced numbers of pancreatic receptors for CCK are responsible for reduced protein secretion.     

Effects of intraduodenal administration of a low dose of cholecystokinin (CCK) antagonist (CR-1505) on plasma CCK concentration,intestinal CCK content,and levels of CCK mRNA

The effects of the intraduodenal administration of a low dose of CR-1505 for 3–7 days on the gene expression of cholecystokinin (CCK), plasma CCK concentration, and CCK content in the intestinal mucosa were examined in rats. The simultaneous changes of protein and enzyme content in the pancreas were also determined. CR-1505 was infused continuously into the duodenum at a dose of 3 mg/kg per day, calculated to correspond to a dose of 150–200 mg/day in humans. Seven days after the administration of CR-1505, a liquid meal (4.5 kcal/3 ml) was introduced into the stomach and changes in the intestinal CCK content and plasma CCK concentration were examined. The level of CCK mRNA in the intestine was significantly higher in rats treated with CR-1505 than in control rats. The plasma CCK concentration, the CCK content of the intestinal mucosa, and the composition of pancreatic enzymes did not significantly differ in rats treated with CR-1505 and the untreated controls. In control rats, the administration of the liquid meal increased the plasma CCK concentration and significantly decreased the intestinal CCK content in water extracts, but did not affect the amount extracts in acid whereas the ingestion of the meal did not cause any significant changes in rats treated with CR-1505. These findings indicate that a low dose of CR-1505 stimulates the gene expression of CCK without enhancing CCK release or exerting an effect on the pancreas.     

Role of cholecystokinin in bombesin- and meal-stimulated pancreatic polypeptide secretion in dogs

This study was undertaken to delineate the role of cholecystokinin (CCK) in bombesin- and meal-stimulated pancreatic polypeptide (PP) secretion in seven conscious dogs by studying: (1) the stimulatory effect of similar plasma levels of CCK induced by a meal and infusions of bombesin and the synthetic CCK analog cerulein on plasma PP, and (2) the inhibition of PP secretion by the CCK-receptor antagonist CR-1409 during these three stimuli. The stimulation of PP secretion during bombesin (11.0±1.6 nM/hr) and after the meal (8.9±2.0 nM/hr) were significantly (P<0.05) greater than during infusion of the CCK analog cerulein (2.7±0.4 nM/hr). CR-1409 significantly inhibited the bombesin- and meal-stimulated PP secretion to 2.0±0.4 nM/hr (81%; P<0.05) and 3.1±1.2 nM/hr (47%; P<0.05), respectively, while the cerulein-stimulated PP release was almost abolished to 0.2±0.1 nM/hr (93%; P<0.05) by the drug. These findings point to an important role for CCK in the regulation of bombesin- and meal-stimulated PP secretion.Supported by grant 13-37-43 from the Netherlands Foundation for Medical Research MEDIGON and grant Si 228/7-1 from the Deutsche Forschungsgemeinschaft.     

Increased plasma pancreatic polypeptide (PP) in diabetic ketoacidosis; normalization following treatment

As both hypoglycaemia and hyperglycaemia may modify plasma levels pancreatic polypeptide (PP), we measured plasma PP by a homologous radioimmunoassay in seven diabetics who were admitted to the hospital in overt diabetic ketoacidosis (initial blood glucose 27.1-55.0 mmol/l). None had circulating PP-antibodies. All were treated with continuous infusion of insulin and fluids. Before starting treatment, plasma PP ranged from 2585-136 mmol/l (mean 629 pmol/l). Following treatment plasma PP decreased gradually in all patients. The following morning mean plasma PP was 242 pmol/l, 67 pmol/l when one ureamic patient was excluded. The normal value is less than 100 pmol/l. This study shows that plasma PP is clearly elevated in diabetic ketoacidosis and decreases following treatment.     

Effects of cisapride on gallbladder emptying and pancreatic polypeptide and cholecystokinin release in humans

We investigated the effects of cisapride on gallbladder motility and on the release of pancreatic polypeptide and cholecystokinin in the fasting and postprandial states. Cisapride (7.5mg) and/or a test meal was administered intraduodenally to seven healthy volunteers with or without atropine pretreatment (0.5mg, i.m.). In the fasting state, cisapride increased gallbladder volume to 154% of the basal level, and significantly elevated plasma pancreatic polypeptide levels. The effects of cisapride were inhibited by atropine. In the postprandial state, integrated pancreatic polypeptide and cholecystokinin responses were increased by cisapride to 180% and 192%, respectively, of control values. Atropine inhibited the integrated gallbladder and pancreatic polypeptide response to about 60% of the control value but did not affect the cholecystokinin response. These observations suggest that: (1) fasting gallbladder tone is influenced by cholinergic inhibitory mechanisms, (2) acetylcholine (ACh) is the final mediator for about 40% of the postprandial gallbladder emptying and pancreatic polypeptide response, and (3) coordination between the ACh-independent cholecystokinin response and ACh-dependent pancreatic polypeptide response may be important in the regulation of postprandial gallbladder emptying.     

Effect of a cholecystokinin antagonist on meal-stimulated insulin and pancreatic polypeptide release in humans

A cholecystokinin (CCK) receptor antagonist, loxiglumide, was used to investigate the potential regulating role of CCK in the entero-insular axis in humans. Ingestion of a mixed liquid meal stimulated plasma CCK, insulin, and pancreatic polypeptide (PP) release in the control experiment. With iv loxiglumide (22 mumol/kg.h), mean plasma insulin and glucose levels did not differ between placebo and loxiglumide treatment. The area under the plasma concentration for PP was reduced to 6,060 +/- 1,706 (P less than 0.05) compared to that during placebo treatment (12,266 +/- 4,748). Administration of loxiglumide failed to change insulin secretion in response to perfusion of the same meal or perfusion of a 10-amino acid solution into the duodenum. However, PP secretion in response to the intraduodenal meal or amino acid mixture was abolished after loxiglumide (P less than 0.05). Intravenous administration of the 10-amino acid mixture stimulated insulin from a mean basal level of 7 +/- 3 microU/mL to a peak level of 16 +/- 4 microU/mL. Infusion of a CCK octapeptide (CCK-8) at 8.6 pmol/kg.h, which produced a plasma concentration of 3.3 pmol/L, which is within the postprandial range, augmented amino acid-stimulated insulin and PP output (P less than 0.05). When CCK-8 was infused with loxiglumide, the insulin and PP responses were similar to the values found with loxiglumide alone. We conclude that CCK receptor blockade with iv loxiglumide does not affect postprandial insulin secretion. CCK is, therefore, not a major incretin. However, it is involved in the postprandial PP response, especially during the intestinal phase stimulation. These data suggest that CCK has a role in the human enteroinsular axis.     

Effect of pancreatico-biliary diversion on endogenous CCK secretion, pancreatic enzyme synthesis, and amylase release]

Pancreatico-biliary diversion (PBD) by jejunal transposition in rats caused pronounced hyperplasia of pancreas. The increase of pancreatic trypsinogen contents exceeded the pancreatic growth, while pancreatic lipase and amylase contents relatively reduced. The high level of plasma CCK at the early period of PBD gradually decreased. The plasma secretin levels remained unchanged for 14 days after PBD. The plasma CCK levels were not elevated in fasting, but increased after intrajejunal infusion of 0.1 N HCl or Clinimeal. In acini prepared from PBD rats, the responsiveness to CCK8 was decreased when amylase release was expressed relative to DNA. The dose-response curve for CCK8 was shifted 3 fold toward higher concentrations of CCK8 4 days after PBD, but on 7 and 14 days after PBD returned to the same curve as the transected rats (control).     

Elemental diet stimulates gallbaldder contraction and secretion of cholecystokinin and pancreatic polypeptide in man

This study was undertaken to investigate the effect of ingestion of 80 g Vivonex on gallbladder volume, plasma cholecystokinin (CCK), and pancreatic polypeptide (PP) in eight healthy volunteers and to compare the results with those obtained after ingestion of 60 ml corn oil. Gallbladder volumes were measured by ultrasonography. Plasma CCK was determined by radioimmunoassay using region-specific antibodies; antibody 1703 binds to COOH-terminal CCK-peptides containing at least 14 amino acid residues, while antibody T204 binds to COOH-terminal CCK-peptides containing the sulfated tyrosine region. Plasma PP was also measured by radioimmunoassay. Ingestion of Vivonex induced significant increases in plasma CCK (0.6±0.1 to 4.6±0.6 pM, antibody 1703; 1.8±0.3 to 5.9±0.5 pM, antibody (T204;P<-0.0005) and decreases in gallbladder volume (21.4±2.8 to 11.2±2.3 cm³;P=0.0001). Integrated plasma CCK secretion and gallbladder contraction after Vivonex were not significantly different from the results found after corn oil. Both Vivonex and corn oil-induced small increases in plasma PP. We conclude that Vivonex is a potent stimulus for the secretion of CCK and contraction of the gallbladder.Supported by the Netherlands Foundation for Medical Research FUNCO (grants 13-37-32 and 13-37-43).     

Comparative study of pancreatic polypeptide (PP) secretion, endocrine and exocrine function, and structural damage in chronic alcohol induced pancreatitis (CAIP)

The serum pancreatic polypeptide response to intravenous Boots secretin (1.5 U/kg), glucose tolerance, and insulin responses have been studied in 25 patients with chronic alcohol induced pancreatitis of varying severity, and these results compared with a secretin-pancreozymin test, and the structural damage noted on pancreatography. For the pancreatic polypeptide response 16 healthy subjects acted as controls. There was a marked reduction in pancreatic polypeptide response in patients with advanced structural changes of chronic alcohol induced pancreatitis compared with patients with minimal/moderate changes (p less than 0.01) and with healthy controls (p less than 0.05) although there was no difference between the latter two groups. Similarly, while the ratio of peak to mean basal pancreatic polypeptide concentration was also significantly reduced in patients with advanced changes compared with healthy controls (p less than 0.05) there was a marked degree of overlap in patients with lesser degrees of structural damage and control subjects. For all patients with chronic alcohol induced pancreatitis, however, there was a significant correlation between the pancreatic polypeptide response and each parameter of the standard secretin-pancreozymin test and with glucose tolerance and the integrated insulin response. We conclude therefore that while the secretin stimulated pancreatic polypeptide response correlates significantly with accepted tests of pancreatic structure and function, there is a significant degree of overlap in the response obtained in patients who have minimal/moderate damage and healthy controls making the test insufficiently sensitive for routine diagnostic use.