A double antibody immunoassay for glucagon

A double antibody adaptation of the glucagon radioimmunoassay is described in detail. Guinea pig antiserum allowing the assay of 2,500 unknowns per milliliter of undiluted serum was induced by biweekly immunization. Glucagon was iodinated with I-125 and purified by elution from a cellulose column, maximal immunoprecipitibility approximating 80 per cent. Standard curves in buffer were sensitive to purified glucagon in amounts of less than 100 μμg.

Rapid degradation of labeled glucagon added to human blood was demonstrated. Trasylol was confirmed as an effective inhibitor of this degradation. Endogenous plasma immunoreactive glucagon (IRG) appeared less labile and may have interfered with the quantitative detection of unlabeled exogenous beef-pork glucagon in plasma recovery studies.

The hypothesis is offered that plasma IRG, herein measured at 400 ± 220 (S.D.) μμg./ml., may be composed of a pancreatic and an extrapancreatic fraction of similar but not identical immunological activity. The cross-reactivity of the two components remains a problem in the interpretation of the glucagon immunoassay as applied to plasma.

Effect of glucose concentration on insulin and glucagon release from isolated islets of Langerhans of the rat

The isolated islets of Langerhans of the rat have been utilized to study simultaneous insulin and glucagon release. There is very little degradation of insulin or glucagon when a proteolytic enzyme inhibitor, Trasylol, is added to the incubation vessels. Glucose exerts an opposite effect on insulin and glucagon release. As the glucose concentration in the incubation media is raised from 30 to 300 mg. per 100 ml., there is a decreased secretion of glucagon and an increased secretion of insulin. The addition of 2-deoxyglucose to the media inhibited insulin release but had no effect on glucagon. The results are discussed in reference to the role of glucagon in insulinogenesis and in regard to the possible mechanisms of insulin and glucagon release.

Secretory granule formation and release in rabbit pancreatic A-cells

A-cells of the islets of Langerhans from normal rabbits as well as rabbits treated with insulin, cortisone, or tolbutamide or subjected to a semistarvation diet were studied by light and electron microscopy. Consistent morphological differences between A-cells of normal and experimentally treated rabbits were not found. Individual A-cells varied considerably in degree of granulation and cytoplasmic electron density, however it was not possible to distinguish between A1- and A2-cells. Evidence is presented demonstrating that A-cell granules are formed in the Golgi complex, migrate to the cell membrane, and are extruded intact to the capillary border of the cell between the plasma membrane and basement membrane. The A-cell granules then appear to undergo dissolution and resorption. Based on the morphologic findings no relationship of glucagon secretion to insulin release could be established.

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.

The effects of secretin, pancreozymin, and gastrin on insulin and glucagon secretion in anesthetized dogs

The effects upon islet hormone secretion of highly purified preparations of secretin and of pancreozymin-cholecystokinin and of a crude gastrin-containing extract of hog antrum have been studied in acutely operated dogs. All three preparations were shown to cause a striking increase in insulin concentration in the pancreaticoduodenal venous plasma after their rapid endoportal injection in anesthetized dogs. With each hormone preparation, the peak in insulin secretion occurred 1 minute after injection, and a rapid decline was observed immediately thereafter. Whereas secretin and gastrin failed to alter significantly the pancreaticoduodenal venous glucagon or arterial glucose concentration, pancreozymin caused a dramatic rise in pancreaticoduodenal venous glucagon concentration, which reached a peak 3 minutes after injection, and hyperglycemia was noted to occur soon thereafter. Endoportal infusion of secretin and pancreozymin for 20 minutes caused responses that were sustained but qualitatively identical to the responses noted after rapid injection of the hormones. The beta-cytotropic effect of secretin was abolished by the infusion of epinephrine. These results could not be attributed to the small degree of contamination of the enteric hormone preparations with insulin or glucagon, and it would appear that secretin, pancreozymin, and probably gastrin have insulin-releasing activity and that pancreozymin has, in addition, glucagon-releasing activity.The demonstration that these three hormones possess insulin-releasing activity suggests that there is in the gastrointestinal tract a chain of betacytotropic hormones from antrum to ileum that is capable of augmenting insulin secretion as required for disposal of substrate loads. It is suggested that the existence of this "entero-insular axis" prevents high substrate concentrations that would otherwise follow ingestion of large meals were the insular response entirely a function of arterial substrate concentration.

Glucoregulatory hormones in health and disease. A teleologic model

A teleologic model, based upon currently available knowledge of the glucoregulatory hormones, insulin, glucagon and growth hormone, has been presented in an effort to simplify understanding of their physiologic importance. The approach employed in this conceptual “repackaging” stresses in evolutionary terms the contribution of these hormones to the solution of critical problems of energy storage and supply, which, in their absence, would block phylogenetic progress.

Clinical disorders of glucoregulatory hormone secretion, both real and theoretical, are classified and briefly discussed.

Metabolism of insulin-I 131 and glucagon-I 131 in the kidney of the rat

Crystalline insulin or glucagon labeled with I131 was injected intravenously in adult male Sprague-Dawley rats under ether anesthesia. The left kidney was removed at intervals ranging from 10 seconds to 6 minutes following the injection, and radioautographs of histological sections were prepared. Radioactivity was found to be concentrated in the cortex of the kidney. At intervals less than 2 minutes after injection, radioactivity was prominent in the lumina of proximal convoluted tubules. At later intervals radioactivity tended to become more concentrated in the cells of the proximal tubules than in the lumina. The observations support the hypothesis that proteins may be filtered through the glomerular membrane and reabsorbed in the tubules.