Per Cantor

Plasma concentrations of regulatory peptides in obseity following modified sham feeding (MSF) and a liquid test meal

Plasma concentrations of regulatory peptides were monitored in groups of obese and normal-weight subjects following modified sham feeding and a liquid fatty meal. Following modified sham feeding a significant increase in immunoreactive cholecystokinin (CCK) in plasma was recorded in both groups. In the obese subjects, however, the concentrations following sham feeding were significantly lower than in normal-weight subjects, and the initial part of the response was negative. Basal and modified sham feeding stimulated immunoreactive pancreatic polypeptide (PP) concentrations in plasma did not differ between the groups. After the liquid fatty meal plasma CCK concentrations increased similarly in both groups. In contrast immunoreactive neurotensin and somatostatin concentrations following the meal were lower in the obese group, and a changed concentration-time pattern for somatostatin was observed in the obese group. Postprandial concentrations of PP and immunoreactive gastrin were not different in the groups. The results indicate that the plasma concentration patterns of CCK, somatostatin and NT are disarranged in obesity. The changes may promote rapid propulsion and absorption of ingested food, and facilitate deposition of fat in adipose tissue in obesity and thus may be of pathophysiological importance.

The effect of the cholecystokinin receptor antagonist MK-329 on meal-stimulated pancreaticobiliary output in humans

To determine the physiological role of circulating cholecystokinin (CCK), the effect of the CCK receptor antagonist MK-329 on upper digestive processes was investigated in six normal volunteers after a mixed meal. In a double-blind, two-period, randomized crossover design, the subjects received either 10 mg MK-329 or placebo orally 3 hours 15 minutes before the meal, which contained 51CrCl3 as food marker. A five-lumen tube with the tip in the distal duodenum allowed continuous marker infusion (57Co-B12) and duodenal aspiration as well as recordings of antral and duodenal motility patterns via three pressure sensors. Postprandially, MK-329 caused a significant reduction of 30%-60% (P less than 0.05) in pancreatic trypsin output during the initial three 15-minute periods; thereafter, the output was virtually the same than after placebo. Thus, the integrated enzyme response was only reduced by 15% (NS) during the 3-hour period beginning 15 minutes after the meal. In contrast, gallbladder contraction, determined by total bile acid excretion, was inhibited by 77% (P less than 0.05), indicating a crucial role of CCK in regulating gallbladder motility. Except for the initial 30 minutes postprandially, MK-329 also induced a significant reduction in duodenal pH with mean values ranging from 3.5 +/- 0.2 to 4.1 +/- 0.3 compared with 4.5 +/- 0.3 to 5.0 +/- 0.4 after placebo (P less than 0.05), probably because of lowered secretion of pancreatic bicarbonate. Gastric emptying rate was significantly accelerated by MK-329 during the initial 75 minutes after the meal, but the time for 50% emptying did not differ from placebo [127.5 +/- 7.7 vs. 140.0 +/- 9.0 minutes (NS)]. No changes were observed in the motility pattern of the proximal duodenum after feeding. Whereas MK-329 only caused a slight increase of the basal plasma CCK concentrations, the postprandial levels were markedly enhanced. Peak concentrations were 10.0 +/- 1.3 vs. 4.0 +/- 0.5 pmol/L after placebo (P less than 0.001), and the integrated response exceeded the control value by 175% (P less than 0.01). The results suggest that circulating CCK is not an essential mediator of the postprandial pancreatic enzyme secretion in humans, whereas it plays a critical role in gallbladder emptying.

Pituitary Tumors Containing Cholecystokinin

We found small amounts of cholecystokinin in the normal human adenohypophysis and therefore examined pituitary tumors from 87 patients with acromegaly, Cushings disease, Nelsons syndrome, prolactinoma, or inactive pituitary adenomas. Five adenomas associated with Nelsons syndrome contained increased amounts of cholecystokinin, the concentrations being extremely high in two: 8281 and 13,453 pmol per gram as compared with less than 30 pmol per gram in normal pituitary glands. The cholecystokinin concentrations were moderately increased in adenomas from another 12 patients, of whom 5 had Cushings disease and 7 acromegaly with adenomas containing ACTH. The cholecystokinin peptides from the tumors were smaller and less sulfated than cholecystokinin from normal pituitary glands. We conclude that ACTH-producing pituitary cells may also produce an altered form of cholecystokinin.

Cholecystokinetic and Pancreozymic Effect of O-Sulfated Gastrin Compared With Nonsulfated Gastrin and Cholecystokinin

To assess whether sulfated gastrin contributes to the cholecystokinetic and pancreozymic activity of plasma in humans, 8 healthy subjects on separate days received a mixed meal, graded i.v. infusions of synthetic human tyrosine-O-sulfated gastrin 17 (10.9, 32.7, and 98.1 pmol/kg X h), which was compared with nonsulfated gastrin 17 (12.2, 36.6, and 109.8 pmol/kg X h) and O-sulfated cholecystokinin-octapeptide (5.5, 16.5, and 49.5 pmol/kg X h). Gallbladder volumes were measured by ultrasonography, and the concentrations of gastrin and cholecystokinin in the circulation were determined by specific radioimmunoassays. Neither of the gastrins induced changes in gallbladder volume at serum concentrations occurring postprandially, whereas cholecystokinin-octapeptide produced a significant reduction in gallbladder volume even at a plasma cholecystokinin concentration lower than observed postprandially. Another 8 subjects received the same infusions in combination with a background infusion of synthetic secretin (0.3 CU/kg X h). Gastric and duodenal juice was continuously aspirated using a double-marker perfusion technique. Neither of the gastrins caused an increase in the output of amylase, lipase, trypsin, chymotrypsin, or bilirubin, but both induced a modest increase in the output of bicarbonate and duodenal juice, the former only significantly during infusion of sulfated gastrin 17. The output of all parameters was significantly elevated during all doses of cholecystokinin-octapeptide. The results indicate that neither of the gastrins stimulates gallbladder contraction and pancreatic enzyme secretion in humans under physiologic conditions. However, gastrin may, like cholecystokinin, potentiate the effect of secretin on pancreatic secretion of juice and bicarbonate.

Plasma cholecystokinin concentrations in patients with advanced chronic pancreatitis

Plasma concentrations of cholecystokinin (CCK) have been reported to be elevated in patients with chronic pancreatitis. The elevations are suggested to be due to increased release of CCK from the upper small intestine secondary to the absence of protease activity (trypsin and chymotrypsin) in the intestinal lumen. We have studied plasma CCK levels before and after liquid as well as solid meals in eight patients with pancreatic insufficiency due to advanced chronic pancreatitis and in eight healthy controls. CCK concentrations were measured with a sensitive and specific radioimmunoassay using an antibody directed against the sulfated tyrosyl region of CCK. No differences in basal or maximal postprandiel plasma CCK levels between patients and controls were observed. In the liquid meal study, basal CCK concentrations in patients and controls were 2.2 ± 0.7 and 2.5 ± 0.4 pM, respectively, with maximal postprandial concentrations of 9.6 ± 2.2 and 11.2± 1.4 pM. In the solid meal study, basal CCK concentrations in patients and controls were 2.5 ± 0.6 and 2.6 ± 0.4 pM, respectively, with maximal postprandial concentrations of 9.4 ± 1.6 and 8.6 ± 1.4 pM. The only difference observed was a significantly longer time interval to maximal plasma CCK levels in patients as compared with controls after the liquid meal. Two patients with no detectable trypsin activity in the small intestinal lumen during a Lundh test meal had basal CCK levels of 1.3 and 1.8 pM. Thus, the present study does not support the hypothesis that trypsin is involved in the regulation of CCK release. It is concluded that both basal and postprandial plasma CCK concentrations are normal in patients with pancreatitis insufficiency due to advanced chronic pancreatitis.

Cell-specific processing of pro-cholecystokinin and pro-gastrin

The present review argues that the gastrin-cholecystokinin family is a suitable model for the study of cell-specific processing of pro-hormones. First, the homologous active site of the hormones is a precisely defined tetrapeptide amide, which is well preserved during evolution. Second, the genes of both hormones are translated in a variety of cells (neurons, endocrine cells, paracrine cells, lymphocytes, etc,), but to a varying degree during ontogenesis and pathogenesis of various diseases. Third, each pro-hormone contains multiple processing sites (mono- and dibasic cleavage sites, amidation sites and consensus sequences for seryl phosphorylation and tyrosyl sulfation) leaving ample room for variations in the post-translational processing. The review discusses examples of cell-specific processing that appears to be functionally expedient.

The molecular nature of cholecystokinin in human plasma

Using a radioimmunoassay specific for the bioactive, tyrosine-sulfated sequence of cholecystokinin (CCK) we have studied the molecular nature of CCK in plasma from normal human subjects. CCK was extracted from postprandial plasma by Sep-Pak cartridges prior to Sephadex G-50 gel chromatography. Four CCK components eluting like CCK-58, CCK-33, CCK-22, and CCK-8, were identified in all samples. Of these, CCK-33- and CCK-8-like peptides predominated. The heterogeneity of circulating CCK emphasizes the importance of plasma assays that measure all bioactive forms of CCK.

Role of Endogenously Released Cholecystokinin in Determining Postprandial Insulin Levels in Man: Effects of Loxiglumide, a Specific Cholecystokinin Receptor Antagonist

To estimate the contribution of postprandial cholecystokinin (CCK) responses to circulating insulin concentrations and insulin secretion, a specific CCK receptor antagonist (loxiglumide; 10 mg/kg body weight/h) or saline were infused intravenously in normal volunteers, beginning 90 min before insulin secretion was stimulated on separate occasions by the intraduodenal administrations of glucose, glucose and protein, and glucose plus protein with the admixture of pancreatin. The release of CCK (radioimmunoassay) was stimulated by the protein component of the nutrients from basal 2.4 +/- 0.4 to 8.0 +/- 1.2 pmol/l. CCK plasma levels were significantly higher with loxiglumide (p < 0.05). Glucose-dependent insulinotropic polypeptide (GIP) was also released by all nutrient mixtures. Loxiglumide significantly inhibited the amount of bilirubin and pancreatic enzymes recovered from duodenal aspirates. In contrast, in none of the experiments, C-peptide increments and hence insulin secretion rates were altered by loxiglumide. With glucose and protein as intraduodenal stimulus (no pancreatin added), the plasma amino acids rose significantly less (by approximately 50% of the control experiment) and the increment in insulin (but not C-peptide) concentrations was significantly reduced by loxiglumide. This is most likely explained by a change in insulin metabolic clearance. This effect cannot be a primary action of CCK because there was no similar effect of loxiglumide with the same intraduodenal stimulus plus added pancreatin. Pancreatic enzymes reduced maldigestion secondary to loxiglumide effects on pancreatic exocrine secretion: The increment in circulating amino acid concentrations was similar with and without loxiglumide. In conclusion, CCK does not alter insulin secretion and, therefore, is not an incretin hormone in man. Blocking CCK actions on the exocrine pancreas by loxiglumide, however, can secondarily cause reductions in postprandial insulin profiles by altering insulin clearance. These changes are possibly related to reductions in circulating amino acid concentrations.

Pancreatic polyamine concentrations and cholecystokinin plasma levels in rats after feeding raw or heat-inactivated soybean flour.

We investigated the trophic effect on the pancreas of male Wistar rats fed up to 20 days with either raw soybean flour (RSF) containing an active trypsin inhibitor or heat-inactivated soybean flour (HSF). The concentrations of the polyamines putrescine, spermidine, and spermine in the pancreas as well as cholecystokinin (CCK) concentrations in arterial and portal vein plasma were measured. Plasma CCK concentrations were measured by a sensitive radioimmunoassay specific for the sulfated region of CCK, whereas polyamine concentrations are determined by reversed phase high-performance liquid chromatography. The levels of CCK in both arterial and-portal vein plasma were significantly higher in RSF- compared with HSF-fed rats, the concentration in the portal vein being twice as high compared with the aorta. A significant increase in pancreatic weight and protein content was positively correlated to an increase in putrescine and spermidine in the pancreas of RSF-fed rats compared with HSF-fed controls, whereas the spermine content did not differ between the two groups. The pancreatic DNA content in RSF-fed rats was significantly above control values of day 20 only. These data support the hypothesis that the trophic effect of soybean trypsin inhibitor on the pancreas is mediated by CCK and that polyamines might play an important role in CCK-induced pancreatic growth.

Peptide hormone expression and precursor processing

Insight in the mechanisms of peptide hormone expression has grown explosively by elucidation of gene, mRNA and preprohormone structures for most hormone systems during the 1980s. The preprohormones vary considerably in size and organization from poly- to mono-protein structures. According to the structural organization and sequence homology the hormones are grouped in families. The prohormones are processed to bioactive peptides by multiple enzymatic modifications during the intracellular transport from the rough endoplasmatic reticulum to the mature secretory granules. The modifications comprise different proteolytic cleavages and amino acid derivatizations. The same prohormone may be expressed in several different cell-types that process the precursor in entirely different ways. Awareness of such cell-specific processing patterns is important for the understanding of ectopic synthesis in neuroendocrine tumours.