Studies of pancreatic alpha cell function in normal and diabetic subjects.

The development of a glucagon radioimmunoassay with a relatively high degree of specificity for pancreatic glucagon made possible studies of alpha cell function in healthy nondiabetic subjects and in patients with diabetes mellitus. In the former group mean fasting plasma glucagon averaged 108 mumug/ml (SEM +/-10). In 12 juvenile-type diabetics fasting glucagon averaged 110 (+/-9) and in 33 adult-type diabetics the average was 114 (+/-8). The diabetic averages did not differ significantly from the nondiabetic subjects; however, when hyperglycemia was induced by glucose infusion in the nondiabetic subjects so as to simulate the fasting hyperglycemia of the diabetics, mean glucagon fell to 57 mumug (+/-8), which was significantly below the diabetic mean. In 28 healthy subjects the infusion of arginine elicited a rise in glucagon of at least 100 mumug/ml with a peak level averaging 331 mumug/ml (+/-22) at 40 min. This response to arginine was diminished but not abolished during hyperglycemia induced by simultaneous glucose infusion. In everyone of 45 diabetic subjects tested the infusion of arginine elicited a rise in glucagon of at least 140 mumug/ml to levels significantly greater than in nondiabetics. The peak glucagon level in juvenile-type diabetics averaged 458 mumug/ml (SEM +/-36) and in adult-type diabetics averaged 452 mumug/ml (SEM +/-38). The glucagon response to arginine was unrelated to duration of diabetes, to body weight, type of diabetic treatment, or to other known factors. Marked hyperresponsiveness of glucagon to arginine infusion was observed in two patients with advanced Kimmelsteil-Wilson disease. Glucagon levels were markedly elevated in certain patients with severe diabetic ketoacidosis before treatment with insulin. The findings suggest that alpha cell function is inappropriately increased in diabetes mellitus and could play a significant role in the diabetic syndrome.

The role of aminogenic glucagon secretion in blood glucose homeostasis.

Hyperaminoacidemia is a powerful stimulus of pancreatic glucagon secretion. These studies were designed to elucidate the role of aminogenic hyperglucagonemia in glucoregulation. Conscious dogs with previously implanted indwelling venous catheters were employed. The results support the view that a role of glucagon is to limit blood glucose decline during hyperaminoacidemia.First, a significant negative correlation between the area of glucagon increment during the 1st 20 min of a 10 amino acid infusion and the maximum fall in glucose concentration was observed. Second, when endogenous glucagon secretion was suppressed by means of a continuous glucose infusion, hyperaminoacidemia induced a maximal glucose decline which averaged 35 mg/100 ml, differing significantly from mean maximal fall of 3 mg/100 ml, which normally occurs in the presence of endogenous hyperglucagonemia. Third, when, during hyperglycemic suppression of endogenous glucagon secretion, 50 mmug of exogenous glucagon/min was infused via the mesenteric vein with the amino acids, the fall in glucose was reduced to an average of 5 mg/100 ml. Similarly when pancreozymin, administered during the combined infusion of glucose and amino acids, overcame glucose suppression of endogenous glucagon secretion, plasma glucose did not fall. Similar results were obtained when aminogenic hyperglucagonemia was prevented by other means. Hyperlipacidemia, induced by infusing a triglyceride emulsion and giving heparin injections, also suppressed aminogenic hyperglucagonemia in two of four experiments; in these two dogs glucose fell 15 and 11 mg/100 ml. In a final group of experiments, the canine pancreas was resected except for the uncinate process, which is virtually devoid of alpha-cells. In two dogs, in which this procedure resulted in zero portal venous glucagon levels, the administration of amino acids and/or pancreozymin resulted in a glucose decline of 14 and 16 mg/100 ml, despite the reduced beta-cell population resulting from the subtotal pancreotectomy. It thus appears that the secretion of pancreatic glucagon during hyperaminoacidemia in association with insulin secretion, serves to limit the decline of glucose concentration.

Characterization of response of circulating glucagon to intraduodenal and intravenous administration of amino acids.

Studies were carried out to determine if hyperaminoacidemia stimulates the secretion of pancreatic glucagon, and, if so, to evaluate the effect of endogenous and exogenous pancreozymin and of hyperglycemia upon this response. The intravenous administration to 16 dogs of 1 g/kg of a 10 amino acid mixture over a 60 min period raised amino nitrogen to a mean level of 13.5 mg/100 ml; mean pancreaticoduodenal vein insulin rose from 84 to 459 muU/ml and glucagon from 1.1 to 2.7 mmug/ml. Further augmentation of both insulin and glucagon secretion was achieved during hyperaminoacidemia by infusing pancreozymin. Since endogenous pancreozymin is known to be stimulated by amino acids in the gut, it seemed possible that intraduodenal loading of amino acids would elicit a greater insulin and glucagon response than could be explained by the accompanying hyperaminoacidemia. The intraduodenal administration of 1 g/kg of the amino acid mixture was followed by substantial hyperinsulinemia and hyperglucagonemia, which frequently anticipated the hyperaminoacidemia, and in many of the dogs the ratio of hormone rise to amino nitrogen rise was greater after intraduodenal than after the intravenous route of amino acid administration in the same animal. Intraduodenal administration of amino acids did not cause measurable release of intestinal glucagon-like immunoreactivity into the mesenteric vein plasma. Hyperglycemia induced by constant glucose infusion prevented aminogenic hyperglucagonemia and even suppressed the augmenting action of pancreozymin; sudden termination of the infusion with continued amino acid infusion was associated with a striking rise in glucagon. It is concluded (a) that hyperaminoacidemia stimulates pancreatic glucagon secretion, (b) that aminogenic hyperglucagonemia is augmented by the infusion of pancreozymin, (c) that intraduodenal administration of amino acids stimulates pancreatic glucagon secretion without measurable release of glucagon-like immunoreactivity into the mesenteric vein, and (d) that hyperglycemia prevents aminogenic hyperglucagonemia even during augmentation with pancreozymin. This conclusion suggests that the prevention of hypoglycemia during amino acid-induced insulin secretion may be an important function of glucagon.

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.