Samuel Cataland

Stimulation of Secretion of Gastric Inhibitory Polypeptide and Insulin by Intraduodenal Amino Acid Perfusion

The effect of intraduodenal or intravenous administration of a 30-gm mixed amino acid solution of serum gastric inhibitory polypeptide (GIP), alpha-amino nitrogen (AAN), glucose, and insulin concentrations was studied in 10 normal subjects. Initially, an intraduodenal amino acid perfusion (15 ml per min X 60 min) was performed in each subject and was followed in 1 to 2 weeks by an intravenous infusion. Peak AAN concentrations occurred at 60 min after both routes of administration, but were greater with intravenous infusion, 145 +/- 5.7 mug per ml vs. 89 +/- 4.4 mug per ml (P less than 0.001). Although serum AAN levels were significantly lower after intraduodenal administration, incremental insulin concentrations were greater after intraduodenal perfusion, 77.3 +/- 8.8 muM per ml vs. 43.1 +/- 5.6 muU per ml (P less than 0.005). Total integrated insulin secretion was also greater after intraduodenal amino acids, 5000 vs. 2400 muU-min ml-1 (P less than 0.005). With intravenous amino acid infusion, serum GIP concentrations remained below the assay detection limit. After intraduodenal perfusion, a mean maximum GIP increment of 468 pg per ml occurred at 15 min. In all subjects peak GIP concentrations occurred at 15 min and preceded the maximum insulin rise by 15 to 30 min. Total integrated GIP secretion was significantly greater after intraduodenal amino acid perfusion, 13,000 pg-min ml-1 vs. no measurable response with intravenous infusion. In separate studies performed in 12 subjects, no significant changes in serum GIP concentrations occurred after intraduodenal perfusion of 0.45% saline, 0.9% saline, or 10% mannitol. The results of this study demonstrate that intraduodenal amino acid administration stimulates the secretion of GIP and suggest that endogenously released GIP may be important in the enteric mediated release of insulin.

Gastric inhibitory polypeptide (GIP). Intestinal distribution and stimulation by amino acids and medium-chain triglycerides.

Radioimmunoassayable gastric inhibitory polypeptide was measured in extracts of canine antrum, duodenum, jejunum, and ileum. The highest GIP concentrations were found in the duodenum (347±53 ng/g) and jejunum (300±68 ng/g). An immunochemical similarity was demonstrable between porcine GIP and canine GIP. Dogs prepared with Mann-Bollman fistulae were given an amino acid (AA) mixture or medium-chain triglycerides (MTC) by intraduodenal perfusion. With AA, a peak mean serum concentration of 672±106 pg/ml was reached 15 min after starting the perfusion. MCT resulted in a peak mean serum GIP concentration of 504±55 pg/ml 30 min after beginning the perfusion. When compared to results previously reported from this laboratory, AA and MCT are not as potent as corn oil (long-chain triglyceride) or glucose in stimulating GIP release. We conclude: (1) Immunoassayable GIP concentrations are highest in the canine proximal small intestine. (2) AA and MCT are weak stimulants of GIP release in the dog.

Gastric inhibitory polypeptide (GIP) in maturity-onset diabetes mellitus.

Serum GIP, insulin, and glucose concentrations were determined during a standard oral glucose tolerance test in 80 individuals, 45 of whom were normal and 35 of whom had adult-onset diabetes mellitus according to USPHS criteria. As a group, the diabetics had fasting hyperglycemia (219 ± 17 mg./dl.) and, in response to glucose, displayed a peak serum glucose of 373 ± 23 mg./dl. and sustained hyperglycemia (315 ± 24 mg./dl.) at 180 minutes. There were no statistically significant differences in absolute serum insulin levels between the two groups. However, insulin secretion was delayed, IRI increments were smaller, and the IRI concentrations were inappropriately low for the simultaneous serum glucose concentrations in the diabetics at every time interval tested. Mean fasting serum GIP was 335 ± 30 pg./ml. in the diabetics as against 262 ± 15 pg./ml. in normal individuals (p < 0.025). After the ingestion of glucose, diabetics had significantly higher (p < 0.001) mean serum GIP levels between five and 120 minutes. By 180 minutes, serum GIP levels remained above fasting in both groups, but the diabetics had higher than normal serum concentrations (p < 0.05). Peak serum GIP concentrations, which occurred at 30 minutes in both groups, were 1,376 ± 106 and 806 ± 75 pg./ml. in the diabetics and normals, respectively (p < 0.001). Total integrated serum GIP was also greater in diabetics than normals (140,852 ± 14,208 vs. 64,602 ± 8,719 pg.-min./ml.−1, p < 0.001). The higher serum GIP concentrations observed following glucose ingestion in diabetics could not be attributed to obesity or age. We conclude that both fasting and glucose-stimulated GIP concentrations are higher than normal in obese adult-onset diabetics. The significance of this observation is uncertain. However, since our current understanding suggests the GIP may be an important enteric signal for the release of insulin in man, and because GIP has been shown to stimulate the release of immunoreactive glucagon, GIP may play a role in the pathogenesis of diabetes mellitus.

Diabetes responsive to intravenous but not subcutaneous insulin: effectiveness of aprotinin.

Patients with diabetes that is insensitive to subcutaneous insulin but sensitive to intravenous insulin have recently been described. We have studied this phenomenon is five female diabetics (14 to 31 years of age) who required excessive amounts of insulin (2.5 to 30.0 units per kilogram of body weight per day) to avoid recurrent ketoacidosis. Known causes of insulin resistance were excluded. All patients had normal responses to conventional doses of intravenous insulin (0.35 to 0.9 unit per kilogram per day). Four patients required continuous intravenous infusion of insulin for one to six months. When a mixture of aprotinin (a protease inhibitor) and regular porcine insulin was given subcutaneously, conventional doses (0.7 to 1.4 units per kilogram per day) produced euglycemia; plasma levels of free insulin rose, and ketonuria disappeared. Four patients had episodes of spontaneous, severe hypoglycemia before and during aprotinin therapy, necessitating continuous infusion of glucose for two to 14 days. Although no insulin was administered, hyperinsulinemia (50 to 2000 muU of free insulin per milliliter [359 to 14,350 pmol per liter]) was present. These findings suggest excessive degradation or sequestration of insulin at the site of injection.

Localization of Gastric Inhibitory Polypeptide Release by Intestinal Glucose Perfusion in Man

To determine the site of endogenous release of gastric inhibitory polypeptide (GIP), glucose perfusions (556 mmoles per liter) of duodenum, proximal jejunum, midjejunum, and ileum were performed in human volunteers using an occluding balloon perfusion technique. Preperfusion GIP concentrations were below assay sensitivity (125 pg per ml) in all subjects. After glucose perfusion, maximal serum GIP concentrations for the four groups were: duodenum, 1383 +/- 152 pg per ml; proximal jejunum, 904 +/- 87 pg per ml; midjejunum, 545 +/- 91 pg per ml, and ileum 305 +/- 38 pg per ml. Integrated GIP secretion was significantly greater with duodenal perfusion (111 +/- 21 ng-min ml-1) d proximal jejunal perfusion (69 +/- 5 ng-min ml-1), as compared to either midjejunal (47 +/- 7 ng-min ml-1) or ileal (25 +/- 6 ng-min ml-1) perfusions. Peak serum insulin concentrations and integrated insulin secretion were also significantly greater with perfusion of the duodenum or proximal jejunum. Serum glucose concentrations, integrated serum glucose, and glucose absorption were similar for the four areas perfused. The results of this study indicate that the proximal small intestine is the primary site of endogenous GIP release in man, but that smaller quantities are also released by the distal small bowel.

Effects of pancreatic polypeptide and vasoactive intestinal polypeptide on rat ileal and colonic water and electrolyte transportin vivo

Two gastrointestinal peptides, vasoactive intestinal polypeptide (VIP) and pancreatic polypeptide, suspected of being associated with symptoms of WDHA syndrome (pancreatic cholera) were tested on the rat small and large intestine for their effects on water and electrolyte transport. Intravenous infusion of VIP (14.3 μg/kg/hr) inhibited net absorption of water and electrolytes in the ileum and reversed net absorption to net secretion in the colon. In contrast, bovine pancreatic polypeptide (52 μg/kg/hr) did not inhibit absorption or stimulate secretion. These data indicate VIP causes colonic secretionin vivo, an effect previously shown onlyin vitro, and that bovine pancreatic polypeptide (at this dose) is not a secretagogue in the small or large intestine of the rat. Thus, while consistent with VIP being a contributory agent to the secretion of pancreatic cholera, the data do not support the notion that pancreatic polypeptide might be a causative agent in this syndrome.

The influence of vasoactive intestinal polypeptide and cholecystokinin of prolactin release in rat and human monolayer cultures

We have evaluated the effects of the gut-brain peptides, VIP and CCK, on pituitary PRL secretion in monolayer cultures of normal and tumor bearing rodent and human pituitary tissue. In cultures prepared with normal human pituitary tissue obtained from three patients with metastatic breast cancer, VIP at 10−7M and 10−9M (but not 10−11M) significantly (p<.05) increased PRL secretion in the wells by 6 hrs. Similar concentrations of VIP also significantly (p<.05) promoted PRL release from pituitary tissue obtained by transphenoidal hypophysectomy from one of two prolactinoma patients. Dopamine (10−5M) inhibition of PRL secretion was not affected by 10−11 to 10−7M VIP. In contrast to these findings VIP did not significantly influence 6 hr rat PRL release in monolayer cultures of normal or transformed cells (GH3) with or without the addition of bacitracin (10−5M). CCK33 significantly (p<.01) increased rat PRL release in human pituitary monolayer cultures at 10−5M. The more biologically potent CCK8 significantly (p<.02) increased rat PRL release at a 10-fold lower concentration, 10−6M. In contrast, CCK8 10−8 to 10−6M, did not significantly influence PRL release from normal human pituitary cultures or from tumor bearing human (prolactinoma) and rat (GH3) cultures. We conclude that 1) the gut-brain peptides, VIP and CCK, can directly stimulate pituitary PRL release and 2) VIP may be a physiologic prolactin releasing factor in man.

Effect of Atropine on Glucose-stimulated Gastric Inhibitory Polypeptide

Gastrointestinal augmentation of insulin secretion observed in response to glucose ingestion may be largely mediated by gastric inhibitory polypeptide (GIP). Since the enteric signal potentiating insulin secretion is blunted by atropine, we studied the effects of this drug on glucose-stimulated GIP secretion in normal subjects. Eight individuals, studied on two occasions, were given intraduodenal glucose (75 gm. over 60 minutes) by perfusion tube, on one occasion with intravenous atropine (15 μg. per kilogram bolus followed by 17 μg. per minute for 60 minutes) and on another occasion with intravenous saline (control). Comparing the results of atropine and control studies, mean serum immunoreactive GIP (IRGIP) was significantly lower at 5 and 15 minutes (444 ± 82 vs. 277 ± 44, p < 0.05 and 1,072 ± 151 vs. 427 ± 74 pg./ml., p < 0.02 at 5 and 15 minutes, respectively), and the integrated incremental area under the 120-minute curve was significantly less (62,170 ± 11,880 vs. 91,820 ± 14,200 pg. ·minute·ml.−1 p < 0.05) with atropine. Serum IRI levels were also significantly lower during the atropine studies (28 ± 8 vs. 14 ± 2, p < 0.05, 54 ± 7 vs. 25 ± 5, p < 0.02, 62 ± 8 vs. 36 ± 7 μU. per milliliter, p < 0.05, at 15, 30, and 45 minutes, respectively). The integrated incremental area under the IRGIP curve was reduced about 33 per cent, while the area under the IRI curve was reduced about 45 per cent by atropine. In a second group of similar experiments substituting d-xylose for glucose, the effect of atropine on d-xylose absorption was studied in normal subjects. Atropine did not alter d-xylose absorption during the first hour, but significantly enhanced its absorption during the second hour, as compared with control studies. We conclude that atropine blunts the rise in serum IRGIP normally stimulated by intestinal glucose perfusion, which is probably a direct effect of the drug on the gastrointestinal tract, as opposed to an effect mediated by inhibition of glucose absorption. These observations are consistent with the hypothesis that GIP is the enteric signal potentiating insulin secretion that is attenuated by atropine.

Renal effects on serum gastric inhibitory polypeptide (GIP)

Fasting and meal-stimulated serum immunoreactive gastric inhibitory polypeptide (GIP) concentrations were measured in normal subjects and in uremic patients undergoing chronic hemodialysis. Mean fasting GIP was higher in the uremic patients (1006 +/- 145 (SE) pg/ml) than in the normal control subjects (132 +/- 31 pg/ml, p less than 0.001). Also, postcibal absolute and incremental serum GIP concentrations between 15 and 180 min were greater (p less than 0.05) in the uremic patients than in the control subjects; in the former they failed to return to fasting levels 180 min after the meal. In a second study, using anesthetized normal dogs, simultaneous renal arterial and venous serum GIP concentrations were measured during an intraduodenal perfusion of glucose. The renal arterial-venous (A-V) GIP gradient became greater as serum arterial GIP concentrations increased. The correlation between renal A-V GIP gradient and renal arterial GIP concentration was quite good (r = 0.85), with a 39% maximum mean A-V reduction in serum GIP concentrations observed across the kidney. This large renal A-V GIP gradient observed under nonsteady conditions suggests that the kidney may be an important site for the removal of GIP from the circulation. Thus, the higher than normal fasting and stimulated serum GIP concentrations observed in uremic patients can be attributed, at least in part, to a loss of the renal extraction mechanism for GIP.

Metabolic Effects of Long-Acting Somatostatin Analogue (Sandostatin) in Type I Diabetic Patients on Conventional Therapy

We evaluated the effectiveness of a more potent and longer-acting somatostatin analogue (SMS 201–995) as an adjunct to insulin therapy, in a double-blind placebo-controlled randomized study of 26 C-peptide-negative type I (insulin-dependent) diabetic patients (20 women, 6 men, aged 22–40 yr) on their conventional drug regimens for 12 wk. Eight patients received a low dose (10 μg) of the analogue, 9 received a high dose (50 μg) of the analogue, and 9 received placebo subcutaneously before breakfast and dinner. Twenty-four-hour serum glucose, free insulin, plasma growth hormone (GH), and glucagon profiles were obtained before and during treatment at 4-wk intervals. The mean age, duration of diabetes, daily insulin dose, and body weight were not significantly different among the groups. The mean weekly capillary blood glucose values and exogenous insulin requirements were not changed by the SMS 201–995 therapy. Mean glycosylated hemoglobin A1 levels were unchanged in both the analogue- and placebo-treated groups at wk 12. Basal and postprandial glucose, free insulin, GH, and glucagon profiles were not influenced by the SMS 201–995 therapy throughout the study. Nocturnal glucose turnover rates (D-[3-3H]glucose technique) remained unaltered by the analogue therapy. Dose-dependent gastrointestinal (GI) adverse effects (e.g., diarrhea) were documented in the analogue-treated patients. Visual acuity and fundic photomicrographs of our patients were not changed by the analogue therapy. In conclusion, the prominent adverse GI effects our patients experienced preclude the use of larger doses of the analogue that may be necessary to suppress GH and glucagon and improve glucose control in type I diabetic patients.