Characterization of the responses of circulating glucagon-like immunoreactivity to intraduodenal and intravenous administration of glucose

The effects of ingested and infused glucose upon circulating glucagon-like immunoreactivity (GLI) were compared in 14 triply catheterized conscious dogs. Within 60 min after the intraduodenal administration of 2 g/kg of glucose, the mean level of glucagon-like immunoreactivity in the vena caval plasma more than doubled, whereas after intravenous infusion of the same dose over a 90 min period no change in the mean vena caval level was observed; during glucose infusion mean glucagon-like immunoreactivity in the pancreatic venous effluent declined, suggesting that hyperglycemia suppresses rather than stimulates pancreatic glucagon secretion. To determine if the rise in glucagon-like immunoreactivity that occurs during glucose absorption was of pancreatic origin, the effect of pancreatectomy performed 1 hr after the intraduodenal administration of glucose was determined. Although circulating insulin disappeared after resection of the pancreas, the level of glucagon-like immunoreactivity continued to rise, establishing its extrapancreatic origin. In other experiments, measurements of Glucagon-like immunoreactivity in plasma obtained simultaneously from pancreaticoduodenal and mesenteric veins and from the vena cava revealed the increment after intraduodenal glucose loading to be greatest in the mesenteric vein in 8 of 12 experiments, favoring the gut as the likely source of the rise. To characterize gut glucagon-like immunoreactivity, acid-alcohol extracts of canine jejunum were compared with similar glucagon-containing extracts of canine pancreas with respect to certain physical and biological properties. On a G-25 Sephadex column the elution volume of the jejunal immunoreactivity was found to be smaller than that of glucagon, which suggested a molecular size at least twice that of pancreatic glucagon. Furthermore, the in vivo and in vitro biological activities of the eluates containing jejunal glucagon-like immunoreactivity appeared to differ from those of eluates containing pancreatic glucagon. The jejunal material lacked hyperglycemic activity when injected endoportally into dogs, was devoid of glycogenolytic activity in the isolated perfused rat liver, and did not increase hepatic 3',5' cyclic adenylate in the perfused liver; however, like glucagon it appeared to stimulate insulin release. It seems quite clear the material in intestinal extracts either is a different substance or a different form from that of true pancreatic glucagon, although it crossreacts in the radioimmunoassay with antibodies to glucagon. It is concluded, (a) that hyperglycemia does not stimulate and probably suppresses the secretion of pancreatic glucagon; (b) that during intestinal absorption of glucose, a rise in glucagon-like immunoreactivity occurs; (c) this immunoreactivity is derived from an extrapancreatic site, probably the gut; (d) that the glucagon-like immunoreactivity extractable from jejunum is not the same as pancreatic glucagon but is a larger molecule devoid of hyperglycemic and glycogenolytic activity, a cross-reactant in radioimmunoassay for glucagon; and (e) that the eluate in which jejunal immunoreactivity is contained can stimulate insulin release in conscious dogs.

Studies of human insulin from nondiabetic and diabetic pancreas

Insulin has been isolated from single human pancreases from diabetic and nondiabetic sources. Purification was accomplished primarily by gel filtration. The average yield of insulin from the nondiabetic pancreas was 8.6 mg. per 100 gm. of tissue, and from the diabetic pancreas 2.9 mg. per 100 gm. The amino acid composition of nondiabetic insulin was in agreement with the known structure of human insulin. The insulin isolated from the diabetic pancreas had the same amino acid composition as normal human insulin in all cases but one. Based upon the amino acid analysis, it is possible that a different amino acid sequence occurs in the insulin from this diabetic source.

Biosynthesis of insulin by the isolated perfused dog pancreas

Evidence is presented to show that the isolated, perfused dog pancreas is capable of incorporating C-14 labeled amino acids into fractions that have immunologically reactive and biologically effective insulin activity. The perfusion medium was recirculated through the pancreas from a reservoir for four hours. At the end of the perfusion, the pancreas, gut segment and medium were individually extracted and the active fractions separated on a Sephadex column. Each fraction was tested using the two antibody assay of Hales and Handle and the hemidiaphragm and fat pad assays. Peak 2 of the pancreas consistently had the greatest amount of IRI, ILA, and C-14 radioactivity. The in-corporation of C-14 amino acids into the fraction with ILA and IRI appeared to be an energy requiring process since 2,4-dinitrophenol completely blocked the appearance of C- 14 into the active fractions. It is concluded that the feasibility of the use of this preparation for studying of pancreatic hormones is demonstrated.