Glucose and ATP levels in pancreatic islet tissue of normal and diabetic rats.

It has been suggested that the hyperglucagonemia observed in diabetic animals and man may be due to an impairment of glucose uptake and metabolism by the alpha-cells resulting in a decreased production of ATP. To test this hypothesis glucose, ATP, glucagon, and insulin were measured in pancreatic islets of normal and alloxan or streptozotocin diabetic rats. Two experimental approaches were used. In the first, the pancreas was perfused in vitro for assessing insulin and glucagon release due to 10 mM amino acids with and without 5 mM glucose. These perfusions were performed in the presence and absence of insulin. After perfusion, the pancreas was frozen and processed for analysis of islet glucose, ATP, insulin, and glucagon content. The second approach was to investigate the islet sucrose, urea, and glucose spaces together with ATP, insulin, and glucagon content in vivo in normal and in insulin-treated and untreated streptozotocin diabetic rats. Perfusion of the pancreas in vitro with 5 mM glucose resulted in higher glucose content of normal islets than in alloxan and streptozotocin diabetic islets. Similarly in the in vivo studies, the intracellular glucose space of the streptozotocin diabetic islets was 30% the value found in normals. In the in vivo experiments, despite the relatively small intracellular glucose space of alpha-cell islets, the ATP content of these islets was only 15-20% lower than the ATP content of normal islets. In the in vitro experiments, perfusion with glucose resulted in ATP contents of alpha-cell islets and of normal mixed alpha-beta-cell islets which were indistinguishable. However, the ATP content of alpha-cell islets was maintained for prolonged periods in the absence of glucose in contrast to mixed islets, composed primarily of beta-cells, in which the ATP level decreased by 45% when glucose-free medium was perfused for sustained periods. Finally, insulin infused in high concentrations or administered to the diabetic animal had no effect on the glucose spaces or the ATP contents of normal or alpha-cell islets. It can be calculated that in vivo the intracellular glucose level of islets from streptozotocin treated rats is approximately 15 mM. Since in normals an extracellular glucose concentration of this magnitude inhibits stimulated glucagon release completely, it would seem unlikely that a lack of intracellular glucose is the cause of the apparent glucose "blindness" of the alpha-cells in diabetes. In fact, in perfusion studies as little as 2.5 mM free intracellular glucose was sufficient to suppress glucagon secretion from diabetic alpha-cells. The results of the ATP measurements clearly eliminate a possible energy deficit of diabetic alpha-cells as cause of the apparent glucose resistance of alpha-cells.     

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.     

Decreased Glucagon Receptors in Diabetic Rat Hepatocytes: EVIDENCE FOR REGULATION OF GLUCAGON RECEPTORS BY HYPERGLUCAGONEMIA

The effects of endogenous and exogenous hyperglucagonemia on the specific binding of glucagon to hepatocyte receptors was studied, as was the response of cAMP to glucagon. In streptozotocin diabetic rats, blood glucose and plasma glucagon increased and plasma insulin decreased as compared with controls. Insulin treatment in diabetic rats restored blood glucose and plasma glucagon toward normal and elevated plasma insulin. Specific binding of (125)I-glucagon to isolated hepatocytes (10(6) cells) decreased in diabetic rats (8.17+/-0.38%) compared to controls (14.05+/-0.87%) and was restored by insulin treatment (12.25+/-0.93%). Specific binding of (125)I-insulin in controls was 7.30+/-10.16%; it increased in diabetic rats to 12.50+/-0.86%, and decreased in diabetic rats after insulin treatment (9.08+/-0.87%). Scatchard analysis and the competition plots of the data indicate that decreased glucagon binding and increased insulin binding in diabetes were due to change in the number of receptors rather than a change in their affinity. Hepatocyte cAMP response to glucagon (0.25-5.0 ng/ml) was almost abolished in diabetic rats and was restored with insulin treatment.Specific glucagon binding by hepatocytes from chronically hyperglucagonemic (glucagon injected) rats was decreased (P < 0.005) to 8.76+/-0.61% compared with controls (13.20+/-0.74%) and acutely hyperglucagonemic animals (13.53+/-1.33%). The decreased binding was associated with a 70% decrease in hepatocyte cAMP response to glucagon compared with a normal response in acutely hyperglucagonemic rats.These data appear to support the concept of receptor regulation by ambient hormone level. In both endogenous and exogenous hyperglucagonemia, however, there was a disproportionately large decrease in cAMP response to glucagon compared to the decrease in glucagon binding.     

Somatostatin-Induced Changes in Insulin and Glucagon Secretion in Normal and Diabetic Dogs

In conscious dogs intravenously infused somatostatin (3.3 mug per min for 1 h) caused prompt and sustained declines in mean plasma insulin and glucagon, even during alanine infusion and intraduodenal casein hydrolysate feeding; plasma glucose declined, but not significantly. 6.7 mug per min of somatostatin significantly lowered pancreatoduodenal vein glucagon and insulin within 2.5 min and profoundly suppressed their secretion throughout the infusion. Consistent bihormonal suppression occurred at rates as low as 24 ng per kg per min, but was variable at 12 and 2.4 ng per kg per min. When somatostatin-induced (3.3 mug per min) hypoglucagonemia was corrected by exogenous glucagon, hyperglycemia occurred. In dogs with long-standing insulin-requiring alloxan diabetes 3.3 mug per min of somatostatin suppressed glucagon to 55 pg per ml throughout the 30-min infusion and lowered glucose by 36.4+/-6.1 mg per dl, about 1 mg per dl per min. Glucagon suppression was maintained despite alanine infusion, and glucose, which rose 29 mg per dl during alanine infusion without somatostatin, declined 58 mg per dl in the somatostatin-treated diabetic dogs despite alanine. Continuous infusion of somatostatin for 24 h in five insulin-requiring alloxan-diabetic dogs suppressed glucagon and lowered glucose significantly, usually to below normal.It is concluded that in normal dogs pharmacologic doses of somatostatin virtually abolish insulin and glucagon secretion in the basal state and during hyperaminoacidemia. Hyperglycemia occurs during somatostatin-induced insulin lack only if hypoglucagonemia is corrected. Somatostatin suppresses glucagon in diabetic dogs and lowers their plasma glucose approximately 1 mg per dl per min, even when the gluconeogenic substrate alanine is abundant. Glucagon suppression can be maintained for several hours in such dogs and hyperglycemia is thereby reduced.     

Comparison of the suppressive effects of elevated plasma glucose and free fatty acid levels on glucagon secretion in normal and insulin-dependent diabetic subjects. Evidence for selective alpha-cell insensitivity to glucose in diabetes mellitus.

To examine whether abnormal pancreatic alpha-cell function found in human diabetes mellitus may represent a selective insensitivity to glucose, plasma glucagon responses to hyperglycemia and elevation of plasma free fatty acid levels (both known suppressors of glucagon secretion) were compared in juvenile-onset, insulin-requiring diabetic subjects, and in normal nondiabetic subjects. In the latter, both elevation of plasma free fatty acid levels induced by heparin administration of hyperglycemia produced by intravenous infusion of glucose resulted in a comparable 30--40% suppression of circulating glucagon levels (P less than 0.01). In the diabetic subjects, glucagon suppression by hyperglycemia (less than 20%) was less than that occurring in normal subjects (P less than 0.01), even when accompanied by infusion of supraphysiologic amounts of insulin. However, suppression of glucagon levels by elevation of plasma free fatty acids in the diabetic group was similar to that found in normal subjects and of comparable magnitude to that due to hyperglycemia in the normal subjects. These results thus demonstrate a selective impairment of the diabetic alpha-cell response to glucose and provide further evidence for the presence of an abnormal alpha-cell glucoreceptor in human diabetes mellitus.     

Glucose Modulation of Amino Acid-Induced Glucagon and Insulin Release in the Isolated Perfused Rat Pancreas

Interactions between glucose and arginine and a mixture of 20 amino acids found in normal rat serum were studied in the isolated perfused rat pancreas of normal rats, with release of immunoreactive glucagon and insulin as parameters. Secretion of both pancreatic hormones was low during the steady state, whether glucose (5 mM) was included in the perfusion medium or not. This glucose concentration significantly stimulated insulin release twofold and resulted in an 80% inhibition of basal glucagon release. Arginine and the amino acid mixture were potent stimulants of both hormones. Secretion of both hormones followed identical biphasic response patterns after addition of arginine or the amino acid mixture. However, stimulation of insulin release occurred only when glucose was included, whereas both phases of glucagon release were elicited in the absence of glucose and markedly reduced in its presence. The dose-dependency curves of hormone release due to arginine on one hand and the amino acid mixture on the other differed substantially: with arginine, release of insulin and glucagon was linear between a concentration of 0.3 and 20 mM. In contrast, the amino acid mixture resulted in half-maximal release for both hormones between a concentration of 3 and 4.5 mM, and maximal release between 6 and 8 mM. The dose-dependencies of glucose modulation of alpha- and beta-cell activity were also different: when the amino acid mixture was maintained at 15 mM and glucose varied (0-6.25 nM), no insulin release occurred until glucose was above 2.5 mM, whereas incremental inhibition of glucagon occurred through the complete dose range. It was also observed that glucose inhibition of amino acid-stimulated glucagon release was dissociated from glucose-dependent increase of insulin release.THESE STUDIES INDICATE THAT: (a) the alpha-cell, like the beta-cell, secretes at a low basal rate; (b) hypoglycemia per se is a weak stimulus for glucagon secretion compared to the high efficacy of a physiologic amino acid mixture; (c) glucose plays opposite roles in the mechanisms leading to amino acid-induced hormone release from the alpha- and beta-cells, functioning as an inhibitor in the first case and a permissive agent in the second, and (d) the data are compatible with the postulated existence of glucose and amino acid receptors in both the alpha- and beta-cells.     

The effect of experimental insulin deficiency on glucagon secretion

Suppression of pancreatic glucagon secretion by hyperglycemia is a characteristic of normal alpha cell function. However, in diabetic subjects, plasma glucagon is normal or high despite hyperglycemia. It seemed possible that the presence of glucose or its metabolites within the alpha cell might be essential for suppression of glucagon secretion, and that in diabetes an intracellular deficiency of glucose secondary to insulin lack might be responsible for the nonsuppressibility. The present study was designed to determine the effect upon glucagon secretion of blockade of glucose metabolism and of experimental insulin deficiency.Blockade of glucose metabolism was induced in dogs by administration of 2-deoxyglucose or mannoheptulose. A striking rise in glucagon was observed despite accompanying hyperglycemia and hyperinsulinemia, which, in the case of mannoheptulose, was induced by infusing crystalline insulin.To determine if insulin lack also causes paradoxical hyperglucagonemia, dogs were made severely diabetic by alloxan. Fasting glucagon levels ranged from 3 to 22 times normal despite severe hyperglycemia, and were quickly restored to normal by infusing insulin. Diabetes induced in rats by anti-insulin serum was also associated with significant elevation in plasma glucagon. However, diazoxide-induced insulin lack did not increase glucagon in dogs.It is concluded that normal suppression of glucagon secretion by hyperglycemia does not occur when glucose metabolism is blocked or when severe insulin deficiency is produced. It is suggested that normal glucose metabolism within the alpha cell may be an insulin-requiring process without which hyperglycemic suppression of glucagon release cannot occur.     

Hepatic Autophagy in Uncontrolled Experimental Diabetes and Its Relationships to Insulin and Glucagon

Exogenous glucagon is known to increase hepatic lysosomes, but the relationships between endogenous glucagon and insulin levels and hepatic lysosomes have not been examined. To determine if the hormones of the pancreatic islets influence the development of these organelles glycogenosomes, dense bodies, and autophagosomes were morphometrically quantitated in normal rats, in rats with mild streptozotocin diabetes with normal hormone levels, and in rats with severe streptozotocin diabetes with hyperglucagonemia, hypo-insulinemia, and clinical evidence of uncontrolled diabetes and ketoacidosis. In the latter volume density of lysosomes averaged 222.8x10(-4) (SEM +/-19.8x10(-4)), significantly above the control value of 75x10(-4) (SEM +/-7.0x10(-4)) (P<0.0005); glycogenosomes were absent in the diabetics, the increase being largely the result of increased autophagosomes. Insulin treatment corrected the hyperglucagonemia, hypoinsulinemia, and other manifestations of uncontrolled diabetes and reduced the volume density of lysosomes to 37.4x10(-4) (SEM +/-2.0x10(-4)), significantly below both the untreated diabetic rats and the nondiabetic controls (P<0.0025). In mild streptozotocin diabetes, in which hyperglucagonemia, hypoinsulinemia, and other evidence of uncontrolled diabetes were absent, lysosomes averaged 77.6x10(-4) (SEM +/-5.5x10(-4)), not different from the controls. A statistically significant correlation between all measurements of lysosomal volume density and plasma glucagon was observed (r=0.79; P<0.001). It is concluded that uncontrolled streptozotocin diabetes in rats is accompanied by hepatic autophagy which may be related to the increased plasma glucagon level and/or the decreased insulin and which is corrected by insulin therapy.     

Role of insulin and glucagon in the response of glucose and alanine kinetics in burn-injured patients.

We investigated the roles of insulin and glucagon as mediators of changes in glucose and alanine kinetics during the hypermetabolic response to injury in 10 burn patients by infusing somatostatin with and without insulin replacement. Glucose and alanine kinetics were measured by primed-constant infusions of 6,6-d2-glucose and [3-13C]alanine. The basal rate of glucose production and alanine flux were significantly elevated in all patients. Lowering both hormones simultaneously caused an insignificant reduction in glucose production, but plasma glucose rose significantly (P less than 0.01), because of reduced clearance. Alanine flux and total plasma amino nitrogen increased significantly (P less than 0.05) above basal. Selectively lowering glucagon concentration decreased glucose production (P less than 0.05), and exogenous glucose was infused to maintain euglycemia. Alanine flux and total plasma amino nitrogen remained unchanged. In severely burned patients hyperglucagonemia stimulates increased glucose production, basal insulin suppression glucose production, stimulates basal glucose clearance, and is important for regulation of plasma amino acid concentrations, and the selective lowering of glucagon while maintaining basal insulin constant normalized glucose kinetics.     

The Effect of Insulin on the Alpha-Cell Response to Hyperglycemia in Long-Standing Alloxan Diabetes

In acute experimental diabetes in animals, alpha-cell unresponsiveness to hyperglycemia can be promptly corrected by insulin, but in human diabetes, even massive doses of insulin have little effect. To determine if this inability of insulin to correct the alpha-cell abnormality in man is merely the consequence of the long duration of the diabetic state (rather than of a difference in mechanism), the effect of insulin was studied in alloxan diabetes of long duration. Alloxan-diabetic dogs were maintained for 7-18 mo and treated daily with insulin. When glucose was infused without insulin, glucagon did not decline but rose paradoxically. However, when insulin was infused at a rate of 9 mU/kg/min together with glucose, a prompt decline in glucagon from a base-line average of 171 pg/ml SEM+/-34 to a nadir of 41 pg/ml SEM+/-9 was observed. This decline indicated that alpha-cell responsiveness to hyperglycemia is completely restored by large quantities of insulin. To determine if small amounts of insulin would similarly restore alpha-cell responsiveness in long-standing experimental diabetes, 1.4 mU/kg/min was infused. By the time the mean insulin level had risen 43 muU/ml, glucagon had declined significantly and ultimately fell to a nadir of 44 pg/ml. It is concluded from these studies that alpha-cell responsiveness to hyperglycemia can be fully restored in long-standing alloxandiabetic dogs as readily as in acutely diabetic dogs. Its ineffectiveness in restoring alpha-cell responsiveness to hyperglycemia in human diabetes may not, therefore, be related to duration of the diabetic state, and may reflect a primary alpha-cell defect.     

Effect of Intermittent Endogenous Hyperglucagonemia on Glucose Homeostasis in Normal and Diabetic Man

Infusion of glucagon causes only a transient increase in glucose production in normal and diabetic man. To assess the effect of intermittent endogenous hyperglucagonemia that might more closely reflect physiologic conditions, arginine (10 g over 30 min) was infused four times to 8 normal subjects and 13 insulin-dependent diabetic subjects (4 of whom were infused concomitantly with somatostatin to examine effects of arginine during prevention of hyperglucagonemia). Each arginine infusion was separated by 60 min. Diabetic subjects were infused throughout the experiments with insulin at rates (0.07-0.48 mU/kg per min) that had normalized base-line plasma glucose and rates of glucose appearance (Ra) and disappearance (Rd). Basal plasma glucagon and arginine-induced hyperglucagonemia were similar in both groups; basal serum insulin in the diabetics (16+/-1 muU/ml, P < 0.05) exceeded those of the normal subjects (10+/-1 muU/ml, P < 0.05) but did not increase with arginine. Serum insulin in normal subjects increased 15-20 muU/ml with each arginine infusion. In both groups each arginine infusion increased plasma glucose and Ra. Increments of Ra in the diabetics exceeded those of normal subjects, (P < 0.02); Rd was similar in both groups. In normal subjects, plasma glucose returned to basal levels after each arginine infusion, whereas in the diabetics hyperglycemia persisted reaching 151+/-15 mg/dl after the last arginine infusion. When glucagon responses were prevented by somatostatin, arginine infusions did not alter plasma glucose or Ra.CONCLUSIONS: Infusion of arginine acutely increases plasma glucose and glucose production in man solely by stimulating glucagon secretion; physiologic increments in plasma glucagon (100-150 pg/ml) can result in sustained hyperglycemia when pancreatic beta cell function is limited.     

Insulin and Glucagon Secretion by Pancreatic Islets from Nondiabetic and Diabetic Sand Rats (Psammomys obesus)

Abstract. The secretion of insulin and glucagon was investigated in pancreatic islets from diabetic and nondiabetic sand rats of similar age and weight. The metabolic characterization was based on an intraperitoneal glucose tolerance test. Compared to nondiabetic animals diabetic sarid rats had a diminished insulin content in their islets and a decreased insulin secretory response to glucose, glyceraldehyde and theophylline. Diazoxide inhibited insulin release in diabetic as well as in nondiabetic sand rats whereas mannoheptulose was effective only in the nondiabetic rats. There was no significant difference in glucagon content between the two groups. The glucagon secretion by pancreatic islets of diabetic animals was not suppressed by glucose, as in nondiabetic sand rats islets, but by glyceraldehyde. This indicates that the sensitivity to glucose rather than the suppressibility of glucagon release was altered.     

Relationship of glucagon suppression by insulin and somatostatin to the ambient glucose concentration.

The glucagon-suppressing activity of insulin and somatostatin were compared at high and low glucose concentrations. In normal dogs made hyperglucagonemic by phloridzin pretreatment, insulin and somatostatin suppressed glucagon at rates of 47 +/- 8 and 35 +/- 8%/h (NS), respectively, despite profound hypoglycemia. In severely hyperglycemic alloxan-diabetic dogs, insulin and somatostatin suppressed glucagon at rates of 48 +/- 13 and 54 +/- 6%/h, respectively, not different from the nondiabetic dogs. After phloridzin pretreatment to eliminate hyperglycemia in the diabetic dogs, insulin and somatostatin suppressed 51 +/- 8 and 31 +/- 10%/h (NS), respectively. Glucose infused in the phloridzin-pretreated insulin-deprived group suppressed glucagon only partially; insulin was required to reduce it further. We conclude that insulin and somatostatin suppress glucagon at similar rates irrespective of ambient glucose levels, and that diabetic hyperglucagonemia represents the summation of stimulation by insulin lack minus suppression by the associated hyperglycemia.     

Orthotopic pancreas transplantation with portal venous drainage in rats Surgical technique and metabolic effects

Heterotopic pancreas transplantation in type I diabetic patients does not correct hyperglucagonemia, which is thought to be due to insufficiently suppressed glucagon release by the host pancreas. The diabetogenic effects of glucagon then have to be corrected by higher than normal insulin secretion from the transplant, with the attendant risk of earlier loss of islet cell function, and development of atherosclerosis. To establish whether this situation can be prevented, we investigated glucose homeostasis and blood lipids, as well as fecal fat and chymotrypsin as indicators for pancreatic exocrine function 14 weeks after orthotopic pancreas transplantation in inbred rats. The pancreas was resected before orthotopic transplantation of the donor pancreas with portal venous drainage (n=8). Laparotomized animals served as controls (n=8). Basal plasma glucagon, basal plasma insulin to glucagon molar ratio, and basal and integrated incremental responses of plasma glucose, insulin, and C-peptide after an oral glucose load (2 g/kg body weight) were similar in both groups. However, hepatic insulin clearance was slightly but significantly lower in the transplanted group (1.1± 0.1 vs 1.6±0.2; P<0.05). Basal plasma levels of free fatty acids, phospholipids, triglycerides, cholesterol, low-density lipoproteins, and high-density lipoproteins were unchanged after transplantation. Also unchanged were fecal fat and chymotrypsin levels, thus indicating preserved pancreatic exocrine function. We concluded that orthotopic pancreas transplantation with portal venous drainage achieves almost optimal metabolic control with respect to endocrine and exocrine pancreatic function as well as blood lipids. This technique could therefore be used to treat combined endocrine and exocrine     

Comparative study of early metabolic events resulting from the administration of the two diabetogenic agents alloxan and streptozotocin

Abstract In this study it was demonstrated that comparable doses of streptozotocin and alloxan produced similar degrees of pancreatic insulin depletion 48 hours after intravenous injection into rats. The metabolic status of animals injected with either streptozotocin (65 mg/kg) or alloxan (60 mg/kg) was assessed prior to and 24 and 48 hours after injection. Metabolic changes consistent with the induction of acute diabetes were observed after both agents. No qualitative differences were revealed between the responses to the two drugs as reflected by plasma glucose, FFA, glycerol and triglyceride responses and by changes in FFA and glycerol release in vitro. This applied whether the animals were fed ad libitum or were restricted to glucose as the sole caloric source. These results indicate that potential extrapancreatic toxic effects of alloxan and streptozotocin do not significantly interfere with the acute diabetic metabolic changes induced by these agents and that these changes, over the first 48 hours, were primarily a consequence of β-cell destruction and insulin deficiency. —- Comparison of the effects of the differing dietary regimes indicates that both endogenous and exogenous triglyceride contribute to the hypertriglyceridaemia of the acute diabetes induced by both agents. Higher plasma triglyceride concentrations were observed in streptozotocin than in alloxan treated animals fed a free diet but this difference was not evident when the diet was replaced by glucose. Such quantitative differences in the metabolic responses to the two agents could be explained on the basis of the observed variation in the rate of pancreatic insulin depletion and/or variations in food intake.     

Effects of type 2 diabetes on the regulation of hepatic glucose metabolism.

Glucose production is inappropriately increased in people with type 2 diabetes both before and after food ingestion. Excessive postprandial glucose production occurs in the presence of decreased and delayed insulin secretion and lack of suppression of glucagon release. These abnormalities in hormone secretion, coupled with impaired insulin-induced suppression of glucose production and stimulation of splanchnic glucose uptake, likely account in large part for the excessive amounts of glucose that reach the systemic circulation for disposal by peripheral tissues following food ingestion. In contrast, when adequate basal insulin concentrations are present, neither glucagon-induced stimulation of glucose production nor glucose-induced suppression of glucose production differs in diabetic and nondiabetic subjects matched for gender, age, and degree of obesity. However, when insulin secretion is defective, lack of suppression of glucagon can cause substantial hyperglycemia by enhancing rates of glucose production. Therefore, normalization of hepatic glucose metabolism in people with type 2 diabetes mellitus likely will require normalization of insulin and glucagon secretion as well as hepatic insulin action.     

Hyperglucagonemia and insulin-mediated glucose metabolism.

The effect of chronic physiologic hyperglucagonemia on basal and insulin-mediated glucose metabolism was evaluated in normal subjects, using the euglycemic insulin clamp technique (+50, +100, and +500 microU/ml). After glucagon infusion fasting glucose increased from 76 +/- 4 to 93 +/- 2 mg/dl and hepatic glucose production (HGP) rose from 1.96 +/- 0.08 to 2.25 +/- 0.08 mg/kg X min (P less than 0.001). Basal glucose oxidation after glucagon increased (P less than 0.05) and correlated inversely with decreased free fatty acid concentrations (r = -0.94; P less than 0.01) and decreased lipid oxidation (r = -0.75; P less than 0.01). Suppression of HGP and stimulation of total glucose disposal were impaired at each insulin step after glucagon (P less than 0.05-0.01). The reduction in insulin-mediated glucose uptake was entirely due to diminished non-oxidative glucose utilization. Glucagon infusion also caused a decrease in basal lipid oxidation and an enhanced ability of insulin to inhibit lipid oxidation and augment lipid synthesis. These results suggest that hyperglucagonemia may contribute to the disturbances in glucose and lipid metabolism in some diabetic patients.     

Glucagon Binding and Adenylate Cyclase Activity in Liver Membranes from Untreated and Insulin-Treated Diabetic Rats

To investigate the role of hepatic glucagon receptors in the hypersensitivity to glucagon observed in insulin-deprived diabetics, liver plasma membranes were prepared from control rats and from streptozotocin-induced diabetic rats some of whom were treated with high-dose and low-dose insulin. The untreated diabetic animals exhibited hyperglycemia, weight loss, hypoinsulinemia, and hyperglucagonemia. High-dose insulin treatment (2 U Protamine-zinc-insulin/100 g per day) resulted in normoglycemia, normal weight gain, mild hyperinsulinemia, and return of glucagon levels toward base line. The low-dose (1 U protamine-zinc-insulin/100 g per day) insulin-treated diabetic group demonstrated chemical changes intermediate between the untreated and the high-dose insulin-treated animals.In liver plasma membranes from the untreated diabetic rats, specific binding of (125)I-glucagon was increased by 95%. Analysis of binding data suggested that the changes in glucagon binding were a consequence of alterations in binding capacity rather than changes in binding affinity. Furthermore, in the untreated diabetic rats, both basal and glucagon (2 muM)-stimulated adenylate cyclase activity were twofold higher than in controls. In the high-dose insulin-treated diabetic rats, glucagon binding and basal and glucagon-stimulated adenylate cyclase activity were normalized to control values, whereas low-dose insulin treatment resulted in changes intermediate between control and untreated diabetic rats. In contrast to glucagon-stimulated adenylate cyclase activity, fluoride-stimulated adenylate cyclase activity was similar in all groups of rats. Liver plasma membranes from untreated and insulin-treated diabetic animals degraded (125)I-glucagon to the same extent as control rats.The specific binding of (125)I-insulin in the untreated diabetic animals was 40% higher than in control rats. In low-dose insulin-treated diabetic rats, insulin binding was not significantly different from that of control rats, whereas in the high-dose insulin-treated group in whom plasma insulin was 70% above control levels, insulin binding was 30% lower than in control rats.These findings suggest that alterations in glucagon receptors may contribute to the augmented glycemic and ketonemic response to glucagon observed in insulin-deprived diabetics.     

Influence of uremia and hemodialysis on the turnover and metabolic effects of glucagon.

To evaluate the mechanism and role of hyperglucagonemia in the carbohydrate intolerance of uremia, 19 patients with chronic renal failure (12 of whom had undergone chronic hemodialysis for at least 11 mo) and 35 healthy control subjects were studied. Plasma glucagon, glucose, and insulin were measured in the basal state, after glucose ingestion (100 g), after intravenous alanine (0.15 g/kg), and during a 3-h continuous infusion of glucagon (3 ng/kg per min) which in normal subjects, raised plasma glucagon levels into the upper physiological range. Basal concentrations of plasma glucagon, the increment in glucagon after infusion of alanine, and post-glucose glucagon levels were three- to fourfold greater in uremic patients than in controls. The plasma glucagon increments after the infusion of exogenous glucagon were also two- to threefold greater in the uremics. The metabolic clearance rate (MCR) of glucagon in uremics was reduced by 58% as compared to controls. In contrast, the basal systemic delivery rate (BSDR) of glucagon in uremics was not significantly different from controls. Comparison of dialyzed and undialyzed uremics showed no differences with respect to plasma concentrations, MCR, or BSDR of glucagon. However, during the infusion of glucagon, the increments in plasma glucose in undialyzed uremics were three- to fourfold greater than in dialyzed uremics or controls. When the glucagon infusion rate was increased in controls to 6 ng/kg per min to produce increments in plasma glucagon comparable to uremics, the glycemic response remained approximately twofold greater in the undialyzed uremics. The plasma glucose response to glucagon in the uremics showed a direct linear correlation with oral glucose tolerance which was also improved with dialysis. The glucagon infusion resulted in 24% reduction in plasma alanine in uremics but had no effect on alanine levels in controls. It is concluded that (a) hyperglucagonemia in uremia is primarily a result of decreased catabolism rather than hypersecretion of this hormone; (b) sensitivity to the hyperglycemic effect of physiological increments in glucagon is increased in undialyzed uremic patients; and (c) dialysis normalizes the glycemic response to glucagon, possibly accounting thereby for improved glucose tolerance despite persistent hyperglucagonemia. These findings thus provide evidence of decreased hormonal catabolism contributing to a hyperglucagonemic state, and of altered tissue sensitivity contributing to the pathophysiological action of this hormone.