Differential stimulation of pancreatic-polypeptide and gastrin secretion by bombesin in man

Effects of cholecystokinin and glucagon-like peptide 1 on the secretion of pancreatic polypeptide in mice

The gut hormones, cholecystokinin (CCK) and truncated glucagon-like peptide 1 (GLP-1(7-36)amide or GLP-1) both stimulate insulin secretion and affect glucagon secretion in mice, but their effects on the secretion of other islet hormones have not been established in rodents. In the present study, we have examined the influence of the C-terminal octapeptide of CCK, CCK-8, and GLP-1 on the secretion of pancreatic polypeptide (PP) in the mouse by the use of a radioimmunoassay for rodent PP. Mice were injected intravenously with CCK-8 (doses in the range of 0.053-5.3 nmol/kg) or with GLP-1 (doses in the range of 1-32 nmol/kg) and blood was sampled at 2, 6 or 10 min after the injection. Controls were injected with saline. It was found that CCK-8 at 5.3 nmol/kg increases plasma levels of both PP and insulin when the sample was taken at 2 min, but not at 6 or 10 min, after injection. These effects were blocked by the selective CCKA-receptor antagonist, L-364,718 (2.4 micromol/kg). GPL-1 increased plasma insulin levels at 32 nmol/kg at 2 and 6 min after the injection, but plasma PP levels were unaltered. In conclusion, this study, using a newly developed radioimmunoassay for PP in rodents, shows that CCK-8 but not GPL-1 stimulated PP secretion in mice at dose levels where both peptides stimulate insulin secretion. Furthermore, PP secretion in response to CCK-8 showed a similarity with that of insulin in terms of dose- and time-response characteristics as well as sensitivity to CCKA-receptor antagonism.

Mechanism of action of bombesin on amylase secretion. Evidence for a Ca2+-independent pathway

The mode of action of bombesin on amylase secretion was investigated in rat pancreatic acini. Bombesin induced a dose-dependent increase in inositol 1,4,5-trisphosphate and cytosolic free Ca2+. The threshold concentration capable of inducing both effects was 0.1 nM and the half-maximal dose of the peptide for Ca2+ mobilization was approximately 0.6 nM. By contrast, amylase release was approximately 30 times more sensitive than inositol 1,4,5-trisphosphate production and Ca2+ mobilization to bombesin action, with 1 pM being the first stimulatory concentration and a half-maximal effect at approximately 20 pM. The ability of low bombesin doses to trigger enzyme secretion was unaffected by chelation of extracellular Ca2+ with EGTA. In order to test whether the stimulation of amylase release was truly a Ca2+-independent response, the intracellular Ca2+ stores were depleted by pretreating acini with EGTA plus ionomycin, the Ca2+ ionophore. Under these conditions bombesin was still capable of eliciting a significant twofold enhancement of the secretory activity. These results indicate that bombesin, an agonist thought to activate secretion mainly through mobilization of Ca2+ from intracellular stores, elicits amylase release at low concentrations, independently of a concomitant rise in cytosolic free Ca2+. The relevance of these findings to the physiological regulation of pancreatic exocrine secretion is discussed.

Effect of atropine and somatostatin on bombesin-stimulated plasma gastrin, cholecystokinin and pancreatic polypeptide in man

Infusion of the neuropeptide bombesin stimulates the secretion of several gastrointestinal hormones by an unknown mechanism. We have investigated the effects of atropine (15 ng/kg as bolus followed by 2.5 ng/kg X 30 min) and somatostatin (125 micrograms as i.v. bolus followed by 62.5 micrograms/30 min) on the stimulation of 3 hormones (gastrin, cholecystokinin and pancreatic polypeptide) by 60 pmol/kg X 20 min bombesin in 6 healthy volunteers. Plasma samples for measurement of hormones by sensitive and specific radioimmunoassays were obtained at -5, 0, 2.5, 5, 7.5, 10, 15, 20, 25 and 30 min. Bombesin induced significant increases in plasma gastrin (12 +/- 2 to 34 +/- 3 pM; P less than 0.0005), cholecystokinin (1.2 +/- 0.2 to 8.9 +/- 0.7 pM; P less than 0.0001) and pancreatic polypeptide (22 +/- 4 to 72 +/- 19 pM; P less than 0.05). There were great differences between the effects of atropine and somatostatin on the hormonal responses to bombesin. Atropine slightly increased the response of gastrin by 19% and that of cholecystokinin by 15%, but strongly inhibited the bombesin-stimulated pancreatic polypeptide secretion by 97%. On the other hand, somatostatin inhibited the bombesin-induced secretion of gastrin by 48%, cholecystokinin by 82% and pancreatic polypeptide by 107%. These results point to considerable qualitative and quantitative differences in the stimulatory mechanisms of bombesin on the hormones studied.

The effect of gastrin-releasing peptide on the endocrine pancreas

The 27-amino acid peptide gastrin releasing peptide (GRP-(1-27] was infused at 4 dose levels (0.01, 0.1, 1.0, and 10 nM) into the arterial line of the isolated perfused porcine pancreas. Infusions were performed at 3 different perfusate glucose levels (3.5, 5.0, and 8.0 mM) and at two levels of amino acids (5 and 15 mM). GRP-(1-27) stimulated insulin and pancreatic polypeptide secretion and inhibited somatostatin secretion in a dose-dependent manner. Glucagon secretion was unaffected by infusion of GRP under all circumstances. The effect of GRP-(1-27) on insulin secretion was enhanced with increasing perfusate glucose levels, whereas the effects upon somatostatin and pancreatic polypeptide secretion were independent of perfusate glucose levels. The responses to GRP were unaffected by elevation of the concentration of amino acids in the perfusate. The effects of GRP were unaffected by atropine at 10(-6) M. The localization of GRP within the porcine pancreas, its release during electrical stimulation of the vagus nerve, and its potent effects upon pancreatic endocrine secretion make it conceivable that the peptide participates in parasympathetic regulation of pancreatic endocrine secretion.

Influence of cholecystokinin, vasoactive intestinal peptide and secretin on plasma concentration of avian pancreatic polypeptide in laying hens

The influence of saline infusion (i.v.) followed by infusion of either cholecystokinin, vasoactive intestinal peptide, or secretin on plasma concentration of avian pancreatic polypeptide (aPP) was studied in sixteen 18-26-week old Single Comb White Leghorn hens. Three concentrations were used for each hormone. Blood was drawn after both saline and hormone infusions and assayed for aPP content. No significant influence of any of the three hormones on plasma aPP level was found in either fed or fasted hens.

Bombesin effects on human GI functions

In this article some of the actions of amphibian skin peptide Bombesin (BBS) on human gastrointestinal and pancreatic functions are reviewed. BBS causes increases of lower esophageal sphincter pressure, delay of gastric emptying, inhibition of mechanical activity of duodenum and jejunum and gallbladder emptying. BBS also releases in man gastrin and stimulates gastric acid secretion. BBS administration induces release of insulin, glucagon and pancreatic polypeptide from human Islet of Langerhans and causes secretion of pancreatic bicarbonates and enzymes in duodenal juice and release of pancreatic enzymes in blood stream.

Bombesin-like immunoreactivity in human gastrointestinal tract

In the present study the distribution and molecular characteristics of bombesin-like immunoreactivity (BLI) were studied in acid extracts of human gastrointestinal tract. The highest levels were found in the fundus, antrum, pylorus and pancreas with lower levels in the duodenum, jejunum, terminal ileum and colon. BLI was also detected in both the muscle and mucosal layers of the antrum and colon. Sephadex G-50 gel chromatography under acid dissociating conditions revealed two peaks of immunoreactivity, one in the position of synthetic porcine gastrin releasing peptide (GRP) and the second eluting with synthetic amphibian bombesin. Variations in the proportions of the two molecular forms were seen in different regions of the gut. In the stomach and pancreas greater than 70% of the BLI eluted with the GRP marker while in pylorus, jejunum and terminal ileum only 20% was present in this form. Reverse-phase ODS silica HPLC of the major antral BLI peak, utilising a methanol/trifluoroacetic acid gradient indicated that this peptide was similar to porcine GRP. We have therefore (1) demonstrated the presence and heterogeneity of bombesin-like immunoreactivity throughout the human gastrointestinal tract and (2) shown for the first time that a proportion of this BLI closely resembles porcine GRP.

Comparative study of plasma pancreatic polypeptide responses to food, secretin, and bombesin in normal subjects and in patients with chronic pancreatitis

Plasma pancreatic polypeptide (PP) responses to food, secretin (1 CU/kg, intravenous) and bombesin (60 pmol/kg/20 min intravenous) were compared in 14 normal subjects and in 17 patients with chronic pancreatitis and pancreatic insufficiency. Basal plasma PP concentrations were significantly higher in normal subjects than in patients with chronic pancreatitis, but there was considerable overlap between the groups. Ingestion of food induced significant increases in plasma PP both in normal subjects and in patients with chronic pancreatitis, whereas secretin and bombesin significantly increased plasma PP levels in normal subjects only. The PP responses to the three stimuli were significantly larger in normal subjects than in patients with chronic pancreatitis, but there was marked overlap. Calculation of the peak to basal plasma PP ratio did not result in better discrimination between the two groups. In normal subjects there were no significant correlations between the PP responses to the three stimuli, whereas in patients with chronic pancreatitis these responses were significantly correlated. Furthermore, in patients with chronic pancreatitis the PP responses to the various stimuli were significantly correlated with the basal plasma PP level.

Pancreatic polypeptide: a unique model for vagal control of endocrine systems

Pancreatic polypeptide (PP) is a hormone synthesized only in the duodenal pancreas where the PP cell is the dominating endocrine cell type. The secretion of PP is regulated by food-intake and by plasma glucose--in both cases through vagal cholinergic mechanisms. In vitro cholinergic stimulation is 4-10 times as potent as any other stimulatory mechanism, e.g. beta-adrenergic stimulation. Although other agents such as gastrointestinal hormones and neuropeptides are potent stimulators of PP secretion in vivo, their action is totally eliminated by blockade of the muscarinic receptor and in several cases also abolished by vagotomy. Furthermore, these peptides have no or only a weak effect in vitro. The stimulation of PP secretion by hypoglycemia and inhibition by hyperglycemia is mediated also through an efferent vagal mechanism. Under extreme hypoglycemia the stimulation of PP release becomes partially atropine resistant, although still totally dependent on the vagus; conceivably the release of other transmitters, like e.g. VIP, is activated under these circumstances. In the basal state, PP secretion is under oscillating cholinergic tone. Thus, the secretion of PP is unique in the way that the cholinergic, vagal stimulation is not only the most powerful stimulatory mechanism, but also the key through which other mechanisms act. PP secretion can be used, e.g. as a sensitive indicator of autonomic disorders in patients with diabetes.

The effect of atropine on bombesin and gastrin releasing peptide stimulated gastrin, pancreatic polypeptide and neurotensin release in man

The effects of 1-h infusions of bombesin and gastrin releasing peptide (GRP) at 50 pmol/kg per h and neurotensin at 100 pmol/kg per h on gastrin, pancreatic polypeptide (PP) and neurotensin release in man were determined following either saline or atropine infusion (20 micrograms/kg). Bombesin produced a rise in plasma neurotensin from 32 +/- 6 to 61 +/- 19 pmol/l and of PP from 26 +/- 8 to 36 +/- 7 pmol/l. There was a further rise of plasma PP to 50 +/- 13 pmol/l after cessation of the infusion. GRP had no significant effect on plasma neurotensin, but compared to bombesin, produced a significantly greater rise in plasma PP from 34 +/- 6 to 66 +/- 19 pmol/l during infusion. There was no post-infusional increase. At this dose, GRP was as effective as bombesin in releasing gastrin, although unlike bombesin its effect was enhanced by atropine. Neurotensin produced a rise in plasma PP from 17 +/- 4 to 38 +/- 8 pmol/l. Atropine blocked the release of PP during GRP and neurotensin infusion. Atropine had no effect on neurotensin or PP release during bombesin infusion, but did block the rise in plasma PP following bombesin infusion. We conclude that, in contrast to meal-stimulated neurotensin release, bombesin-stimulated neurotensin release is cholinergic independent. Despite structural homology, bombesin and GRP at the dose used are dissimilar in man in their actions and sensitivity to cholinergic blockade.

Secretion of pancreatic polypeptide in man in response to beef ingestion is mediated in part by an extravagal cholinergic mechanism

The effect of atropine, a muscarinic cholinergic blocking agent, on the response of plasma pancreatic polypeptide (hPP) to the ingestion of beef was investigated. Six healthy subjects ingested 250 g broiled ground beef on three occasions. After beef ingestion alone, the expected biphasic plasma hPP response was observed. On the two other occasions atropine (intravenous bolus followed by infusion) was begun either at 4 or 60 min after the beginning of beef ingestion so as to coincide with the early (first) and late (second) phases of hPP response to beef ingestion. On both occasions plasma hPP concentrations returned rapidly to baseline. Mean integrated incremental hPP responses in the absence of atropine were 9.1 +/- 3.4 ng min ml-1 for the first phase (0-40 min) and 29.7 +/- 5.7 ng min ml-1 for the second phase (60-180 min); with atropine at 4 min, respective responses were 0.8 +/- 0.9 and -1.0 +/- 1.3 ng min ml,-1 and with atropine at 60 min they were 10.6 +/- 5.0 and 1.3 +/- 1.6 ng min ml.-1 After atropine administration, the half-time of disappearance of hPP from the circulation was 4-6 min, suggesting the complete cessation of stimulated hPP secretion. We conclude that the mechanisms of both the early and late phases of beef meal-stimulated release of hPP involve muscarinic cholinergic-neural transmission. The portion of the second (late) phase response which has been shown to persist after truncal vagotomy must be mediated by a cholinergic mechanism which is extravagal in character.