Evidence for an intestinal mechanism of pancreatic polypeptide release

Temporal relationships of cholecystokinin release, pancreatobiliary secretion, and gastric emptying of a mixed meal

The influence of gastric emptying of nutrients on plasma cholecystokinin and pancreatobiliary functions is poorly understood. We therefore temporally related the emptying of fat, protein, and glucose of a mixed meal to release of the gut hormones cholecystokinin, pancreatic polypeptide, and peptide YY and outputs of trypsin, lipase, bilirubin, and bile salts. Five healthy volunteers with a multilumen duodenal tube ingested a mixed meal with phase-specific markers for the aqueous phase, liquid fat, solid fat, and solid protein phases. Duodenal passage was determined by intraduodenal infusion of a second set of phase-specific nonabsorbable markers. Plasma cholecystokinin levels and pancreatobiliary secretions rose to a maximum at 30-60 min and then gradually declined (p less than 0.01) despite continued entry of protein and fat into the duodenum throughout the whole 4-h experimental period. High levels of both pancreatic polypeptide and peptide YY were observed in the last 2 h of the experiment. Release of factors capable of inhibiting cholecystokinin release and subsequently pancreatobiliary secretion may be responsible for the observed time-course.

Studies of the role of opioids in control of human pancreatic secretion

Opioids, both endogenous and exogenous, have been shown to exert a modulating influence on many gastrointestinal functions. Animal studies suggest an inhibitory role for opioids in the control of pancreatic secretion, which may be exerted through their inhibitory effect on acetylcholine release from intramural neurons. We studied pancreatic secretion in six healthy male volunteers (mean age 42 years) by means of duodenal aspiration using PEG-4000 as a recovery marker. Pancreatic secretion was stimulated with duodenal perfusion of mannitol (510 mosmol/kg) and an equimolar solution (0.3 M/liter) of L-phenylalanine and L-tryptophan and also with an intravenous bolus of secretin and cholecystokinin (each 1 CU/kg). Each subject underwent six studies on separate days with intravenous infusion of saline, naloxone 40 micrograms/kg/hr, morphine sulfate 40 micrograms/kg/hr, morphine and naloxone combined, naloxone and atropine 13 micrograms/kg/hr, and morphine and bethanechol 16 micrograms/kg/hr. Naloxone caused a significant increase in amylase outputs during the basal period and in response to all stimuli, without affecting bicarbonate outputs. This effect was blocked by atropine. Morphine caused significant reductions in both amylase and bicarbonate outputs with all stimuli. This was accompanied by a reduction in amylase concentrations but not in those of bicarbonate. Bethanechol counteracted these effects of morphine. The combination of morphine and naloxone had no significant effect on pancreatic secretion. Changes in pancreatic polypeptide levels did not explain the observations. In conclusion, these studies suggest that endogenous opioids exert a modulating influence on human pancreatic secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of vagal blockade on food- and hormone-stimulated release of pancreatic polypeptide and motilin in dogs

Vagal control of food- and hormone-stimulated release of pancreatic polypeptide (PP) and motilin was investigated in four conscious dogs by examining the effect of cryogenic vagosympathetic blockade. The postprandial PP response of 189 +/- 7 pM was totally, although reversibly, inhibited to 58 +/- 11 pM with the vagi blocked. Similarly, bombesin-, CCK-OP-, or neurotensin-stimulated PP release was abolished. Although the PP response to intraduodenal perfusion of an elemental diet was also reduced by blockade, the 52 +/- 15% inhibition was less than observed with the meal. In contrast to PP, plasma motilin fell after the meal from a fasting level of 128 +/- 16 pM to a nadir of 52 +/- 7 pM. Vagal blockade reversed this decline as plasma motilin rose to a peak of 121 +/- 18 pM with a pattern resembling the motilin response in the interdigestive state. This motilin increment during blockade was inhibited by atropine and by infusion of porcine PP. Plasma motilin also fell with the elemental diet, but this response was not affected by blockade. During infusion of bombesin, plasma motilin rose by 60 +/- 9 pM; vagal blockade augmented this increment twofold. Thus, the PP response to a meal and to hormonal stimulation is controlled by a vagal cholinergic excitatory pathway. However, intestinal release of PP is mediated in part by the vagus and in part by a vagally independent mechanism which may be neural or hormonal. Alternatively, vagal noncholinergic inhibition is a major mechanism modulating the motilin response after oral food but motilin release exclusively from intestinal nutriments is mediated by nonvagal, noncholinergic mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of truncal vagotomy on intestinal phase of pancreatic polypeptide release in dogs

In order to elucidate the role of the vagus nerve in the intestinal phase of pancreatic polypeptide (PP) release, mongrel dogs were given a 4-min intraduodenal infusion of saline, 20% glucose, or 10% soybean oil solution (50 ml each), before and one month after truncal vagotomy including pyloroplasty (TV). The saline infusion did not change the basal PP level, while the glucose infusion elicited a monophasic transient PP release, and the soybean oil infusion elicited a monophasic prolonged PP release in the intact dogs. The PP response following glucose infusion was almost abolished after TV, while the PP response to fat was attenuated, but a significant increase was nevertheless observed after TV. These results suggest that the vagus nerve has an important role in the intestinal phase of PP release and that other factors, e.g. hormonal, might also be involved in the regulatory mechanism, especially after fat loading.

Gastric and enteral pancreatic polypeptide (PP) immunoreactivity in the dog

Extrapancreatic pancreatic polypeptide (PP) was quantified by determination of PP-cell density using antibodies to human PP (hPP) and bovine PP (bPP) in stomach, duodenum, jejunum, ileum, and colon of the dog. Further, these organs of 3 beagles were extracted and subjected to radioimmunoassay again using 2 different antisera (one directed against bPP, the other against hPP). Purification of these extracts were performed by Sephadex chromatography. We found bPP immunoreactive cells in stomach, small and large bowel, but hPP immunoreactive cells, besides in the pancreas, only in stomach and duodenum, thus demonstrating heterogeneity of extrapancreatic PP. Extraction and chromatography confirm the presence of PP immunoreactivity in the pancreas, stomach, small and large intestine. The observation of a postprandial rise of serum PP levels in a man with total duodenopancreatectomy suggests that the extrapancreatic PP cell may release PP.

Effect of proglumide, a cholecystokinin receptor antagonist, on caerulein-stimulated pancreatic enzyme secretion and pancreatic polypeptide release in the dog

In five conscious dogs we studied the effect of proglumide, a cholecystokinin (CCK) antagonist, on caerulein-stimulated pancreatic secretion and release of pancreatic polypeptide (PP). Graded doses of caerulein (15-240 ng/kg per h) were infused intravenously. Experiments were repeated with a fixed infusion of proglumide (40 mg/kg per h). Release of PP following increasing doses of caerulein was significantly inhibited by proglumide (P less than 0.01). However, proglumide did not significantly affect caerulein-stimulated pancreatic protein secretion. Proglumide might be useful in defining the physiological role of CCK.

Cholinergic inhibition of meal stimulated plasma neurotensin like immunoreactivity in man

Meal stimulated plasma neurotensin like immunoreactivity (NTLI) was compared during saline or atropine infusion in six volunteers over six hours. Plasma gastrin and pancreatic polypeptide were also measured to compare the timing of their release to that of NTLI. Like plasma gastrin and PP, plasma NTLI rose rapidly following the meal, rising from 27 +/- 7 pmol/l to a peak of 45 +/- 8 pmol/l at 20 minutes (p less than 0.05). Also, like that of pancreatic polypeptide, the release of NTLI was biphasic. Sixty minutes after the meal, plasma NTLI had returned to basal values, followed by a rise to a prolonged peak of 64 +/- 10 pmol/l between 90-180 minutes (p less than 0.05) returning once more to basal values by 240 minutes. Following atropine, basal plasma NTLI fell from 22 +/- 4 pmol/l to 11 +/- 2 pmol/l (p less than 0.05), but rose to basal levels 30-60 minutes after the meal, where it remained unaltered for the remainder of the study. We conclude that both basal and meal stimulated plasma NTLI are inhibited by cholinergic blockade. Further, the similar temporal relationship between plasma NTLI and pancreatic polypeptide in the late phase of the meal response, suggests that a component of NTLI may mediate part of the intestinal phase of pancreatic polypeptide release.

Cholinergic stimulation of pancreatic polypeptide release by a meal, bombesin, neurotensin, tetragastrin, cholecystokinin, and cerulein

It has been demonstrated that pancreatic polypeptide (PP) release can be markedly impaired by vagotomy or anticholinergic drugs. The current studies examine the role of cholinomimetic stimulation on PP release in dogs. Eight conscious animals underwent a series of tests: (1) a test meal (10 g/kg Alpo); (2) tetragastrin infusion (4 micrograms/kg/hr); (3) bombesin infusion (1.0 microgram/kg/hr); (4) cerulein infusion (100 ng/kg/hr); (5) cholecystokinin octapeptide (CCK-OP) infusion (100 ng/kg/hr); (6) neurotensin infusion (3 ng/kg/hr). All the studies were repeated individually with intravenous bethanecol (100 micrograms/kg/hr) as the background stimulant. The mean increment of PP released by a meal (160 +/- 32 fmol/ml) was significantly increased by bethanecol infusion (316 +/- 49 fmol/ml) (P less than 0.05). Each individual peptide released a significant amount of PP; tetragastrin: 53 +/- 11; neurotensin: 58 +/- 14; CCK-OP: 42 +/- 9; cerulein: 42 +/- 12; bombesin: 118 +/- 24 (P less than 0.05). Bethanecol did not significantly augment PP release by any of the individual peptides (P greater than 0.05). This study indicates that PP release by a meal is sensitive to cholinomimetic stimulation and that the peptide involved is neither gastrin, neurotensin, CCK, bombesin, nor cerulein. These data support the possibility of the existence of a cholinergic stimulatable mechanism, possibly a peptide responsible for the release of PP.