The mechanism of exaggerated glucagon response to arginine in diabetes mellitus

The Impact of Amino Acids on Postprandial Glucose and Insulin Kinetics in Humans: A Quantitative Overview

Different amino acids (AAs) may exert distinct effects on postprandial glucose and insulin concentrations. A quantitative comparison of the effects of AAs on glucose and insulin kinetics in humans is currently lacking. PubMed was queried to identify intervention studies reporting glucose and insulin concentrations after acute ingestion and/or intravenous infusion of AAs in healthy adults and those living with obesity and/or type 2 diabetes (T2DM). The systematic literature search identified 55 studies that examined the effects of l-leucine, l-isoleucine, l-alanine, l-glutamine, l-arginine, l-lysine, glycine, l-proline, l-phenylalanine, l-glutamate, branched-chain AAs (i.e., l-leucine, l-isoleucine, and l-valine), and multiple individual l-AAs on glucose and insulin concentrations. Oral ingestion of most individual AAs induced an insulin response, but did not alter glucose concentrations in healthy participants. Specific AAs (i.e., leucine and isoleucine) co-ingested with glucose exerted a synergistic effect on the postprandial insulin response and attenuated the glucose response compared to glucose intake alone in healthy participants. Oral AA ingestion as well as intravenous AA infusion was able to stimulate an insulin response and decrease glucose concentrations in T2DM and obese individuals. The extracted information is publicly available and can serve multiple purposes such as computational modeling.

Metabolic function of a suboptimal transplanted islet mass in nonhuman primates on rapamycin monotherapy

Although islet transplantation may restore insulin independence to individuals with type 1 diabetes mellitus, most have abnormal glucose tolerance. We asked whether the defective glucose tolerance is due to inadequate β-cell mass or to impaired insulin sensitivity. We performed metabolic studies on four cynomolgus primates before inducing diabetes with streptozotocin (STZ), then again 2-3 weeks after restoring insulin independence via intrahepatic islet transplantation utilizing a calcineurin inhibitor-free immunosuppressive regimen (induction with rabbit antithymocyte globulin and maintenance therapy with rapamycin). Engrafted β-cell mass was assessed by acute insulin and C-peptide responses to glucose (AIR(glu) and ACR(glu)) and arginine (AIR(arg) and ACR(arg)). Insulin sensitivity (S(I)) was determined in naive and transplanted primates from an intravenous glucose tolerance test using the minimal model. α-Cell function was determined by the acute glucagon response to arginine (AGR(arg)). Glucose tolerance (K(g)) decreased from 4.1 ± 0.5%/min in naive primates to 1.8 ± 0.3%/min in transplanted primates (p < 0.01). Following transplantation, AIR(glu) was 28.7 ± 13.1 μU/ml compared to 169.9 ± 43.1 μU/ml (p < 0.03) in the naive condition, ACR(glu) was 14.5 ± 6.0 ng/ml compared to 96.5 ± 17.0 ng/ml naive (p < 0.01), AIR(arg) was 29.1 ± 13.1 μU/ml compared to 91.4 ± 28.2 μU/ml naive (p < 0.05), and ACR(arg) was 1.11 ± 0.51 ng/ml compared to 2.79 ± 0.77 ng/ml naive (p < 0.05). S(I) did not differ from naive to posttransplant states. AGR(arg) was reduced in transplanted primates (349 ± 118 pg/ml) when compared to both naive (827 ± 354 pg/ml) and post-STZ diabetic primates (1020 ± 440 pg/ml) (p < 0.01 for both comparisons). These data suggest that impaired glucose tolerance observed in islet transplant recipients is secondary to low functional β-cell mass and not to insulin resistance shortly after transplant. Furthermore, improved glycemic control achieved via islet transplantation over the diabetic state might be attained, in part, via reduced glucagon secretion.

Effect of the amino acid alanine on glucagon secretion in non-diabetic and type 1 diabetic subjects during hyperinsulinaemic euglycaemia, hypoglycaemia and post-hypoglycaemic hyperglycaemia

AIMS/HYPOTHESIS

The aim of our study was to establish whether the well-known defective or absent secretion of glucagon in type 1 diabetes in response to hypoglycaemia is selective or includes lack of responses to other stimuli, such as amino acids.

MATERIALS AND METHODS

Responses of glucagon to hypoglycaemia were measured in eight patients with type 1 diabetes and six non-diabetic subjects during hyperinsulinaemic (insulin infusion 0.5 mU kg(-1) min(-1)) and eu-, hypo- and hyperglycaemic clamp studies (sequential steps of plasma glucose 5.0, 2.9, 5.0, 10 mmol/l). Subjects were studied on three randomised occasions with infusion of low- or high-dose alanine, or saline.

RESULTS

With saline, glucagon increased in hypoglycaemia in non-diabetic subjects but not in diabetic subjects. Glucagon increased further with low-dose (181 +/- 16 ng l(-1) min(-1)) and high-dose alanine (238 +/- 20 ng l(-1) min(-1)) in non-diabetic subjects, but only with high-dose alanine in diabetic subjects (area under curve 112 +/- 5 ng l(-1) min(-1)). The alanine-induced glucagon increase in diabetic subjects paralleled the spontaneous glucagon response to hypoglycaemia in non-diabetic subjects not receiving alanine. The greater responses of glucagon to hypoglycaemia with alanine infusion were offset by recovery of eu- or hyperglycaemia.

CONCLUSIONS/INTERPRETATION

In type 1 diabetes, the usually deficient responses of glucagon to hypoglycaemia may improve after increasing the concentration of plasma amino acids. Amino acid-enhanced secretion of glucagon in response to hypoglycaemia remains under physiological control since it is regulated primarily by the ambient plasma glucose concentration. These findings might be relevant to improving counter-regulatory defences against insulin-induced hypoglycaemia in type 1 diabetes.

Inhibition of glucagon secretion

This chapter describes a physiological and profound effect of amylin to inhibit meal-related glucagon secretion. Glucagon is processed from a large precursor, proglucagon, in a tissue-specific manner in pancreatic alpha-cells. In addition to amino acid nutrient stimuli, glucagon is also secreted in response to stressful stimuli, such as hypoglycemia and hypovolemia. Glucagon primarily acts on liver to initiate glycogenolysis and gluconeogenesis, resulting in a rapid increase in endogenous production of glucose. With longer stimulation, glucagon action at the liver results in a glucose-sparing activation of free fatty acid oxidation and production of ketones. During hypoglycemia, glucagon secretion is clearly a protective feed-back, defending the organism against damaging effects of low glucose in brain and nerves (neuroglycopenia). Amino acid-stimulated glucagon secretion during meals has a different purpose: amino acids stimulate insulin secretion, which mobilizes amino acid transporters and effects their storage in peripheral tissues. At the same time, insulin obligatorily recruits GLUT4 glucose transporters in muscle and fat. The hypoglycemic potential of such GLUT4 mobilization is averted only by the simultaneous liberation of endogenous glucose in response to feedforward (anticipatory) glucagon secretion. The effect of amylin and its agonists to inhibit amino acid-stimulated glucagon secretion is both potent (EC50 = 18 pM) and profound (approximately 70% inhibition). This glucagonostatic action appears to be extrinsic to the pancreatic islet, occurring in intact animals and in patients, but not in isolated islets or isolated perfused pancreas preparations. On the other hand, the effect of hypoglycemia to stimulate glucagon secretion, which is intrinsic to the islet and occurs in isolated preparations, is not affected by amylin or its agonists. The physiological interpretation of these actions fits with the general concept, illustrated in Fig. 1, that amylin and insulin secreted in response to meals shut down endogenous production as a source of glucose, in favor of that derived from the meal. Amylin and insulin secreted in response to nutrients already absorbed act as a feedback switch for glucose sourcing. The insulinotropic (incretin) gut peptides, GLP-1 and GIP, secreted in response to yet-to-be-absorbed intraluminal nutrients, amplify beta-cell secretion and thereby activate the glucose sourcing switch in a feedforward manner. Hypoglycemia-stimulated glucagon secretion and nutrient (amino acid)-stimulated glucagon secretion are two clearly different processes, differently affected by amylin. The balance of glucose fluxes is disturbed in diabetic states, partly as a result of inappropriate glucagon secretion. Although glucose production due to glucagon secreted in response to hypoglycemia is normal or even reduced in diabetic patients, the secretion of glucagon (and production of endogenous glucose) in response to protein meals is typically exaggerated. Absence of appropriate beta-cell suppression of alpha-cell secretion has been invoked as a mechanism that explains exaggerated glucagon responses, especially prevalent in patients with deficient beta-cell secretion (type 1 diabetes and insulinopenic type 2 diabetes). A proposed benefit of insulin replacement therapy is the reduction of absolute or relative hyperglucagonemia. High glucagon is said to be necessary for ketosis in severe forms of diabetes. A further benefit of reversing hyperglucagonemia is reduction of the excessive endogenous glucose production that contributes to fasting and postprandial hyperglycemia in diabetes. The idea that amylin is a part of the beta-cell drive that normally limits glucagon secretion after meals fits with the observation that glucagon secretion is exaggerated in amylin-deficient states (diabetes characterized by beta-cell failure). This proposal is further supported by the observation that postprandial glucagon suppression is restored following amylin replacement therapy in such states. These observations argue for a therapeutic case for amylin replacement in patients in whom excess glucagon action contributes to fasting and postprandial hyperglycemia and ketosis. The selectivity of amylin's glucagonostatic effect (wherein it is restricted to meal-related glucagon secretion, while preserving glucagon secretion and glucagon action during hypoglycemia) may confer additional benefits; the patient population amenable to amylin replacement therapy is likely to also be receiving insulin replacement therapy, and is thereby susceptible to insulin-induced hypoglycemia. Most explorations of the biology of amylin have used the endogenous hormone in the cognate species (typically rat amylin in rat studies). Clinical studies have typically employed the amylinomimetic agent pramlintide. Studies of amylinomimetic effects on glucagon secretion include effects of rat amylin in anesthetized non-diabetic rats (Jodka et al., 2000; Parkes et al., 1999; Young et al., 1995), effects of rat amylin in isolated perfused rat pancreas (Silvestre et al., 1999), effects of pramlintide in anesthetized non-diabetic rats (Gedulin et al., 1997b,c,d, 1998), effects of pramlintide in patients with type l diabetes (Fineman et al., 1997a,b,c,d, 1998a; Holst, 1997; Nyholm et al., 1996, 1997a,b,c; Orskov et al., 1999; Thompson and Kolterman, 1997), and effects in patients with type 2 diabetes (Fineman et al., 1998b). In addition, effects of amylin antagonists have been observed in isolated preparations (Silvestre et al., 1996), and effects of antagonists or neutralizing antibody have been determined in whole-animal preparations (Gedulin et al., 1997a,e,f).

Clinical studies

Recognizing that type 1 diabetes was characterized not only by insulin deficiency, but also by amylin deficiency, Cooper (Cooper, 1991) predicted that certain features of the disease could be related thereto, and he proposed amylin/insulin co-replacement therapy. Although the early physiological rationale was flawed, the idea that glucose control could be improved over that attainable with insulin alone without invoking the ravages of worsening insulin-induced hypoglycemia was vindicated. The proposal spawned a first-in-class drug development program that ultimately led to marketing approval by the U.S. Food and Drug Administration of the amylinomimetic pramlintide acetate in March 2005. The prescribers' package insert (Amylin Pharmaceuticals Inc., 2005), which includes a synopsis of safety and efficacy of pramlintide, is included as Appendix 1. Pramlintide exhibited a terminal t1/2, in humans of 25-49 min and, like amylin, was cleared mainly by the kidney. The dose-limiting side effect was nausea and, at some doses, vomiting. These side effects usually subsided within the first days to weeks of administration. The principal risk of pramlintide co-therapy was an increased probability of insulin-induced hypoglycemia, especially at the initiation of therapy. This risk could be mitigated by pre-emptive reduction in insulin dose. Pramlintide dosed at 30-60 microg three to four times daily in patients with type 1 diabetes, and at doses of 120 microg twice daily in patients with type 2 diabetes, invoked a glycemic improvement, typically a decrease in HbA1c of 0.4-0.5% relative to placebo, that was sustained for at least 1 year. This change relative to control subjects treated with insulin alone typically was associated with a reduction in body weight and insulin use, and was not associated with an increase in rate of severe hypoglycemia other than at the initiation of therapy. Effects observed in animals, such as slowing of gastric emptying, inhibition of nutrient-stimulated glucagon secretion, and inhibition of food intake, generally have been replicated in humans. A notable exception appears to be induction of muscle glycogenolysis and increase in plasma lactate.

Improvement in blunted glucagon response to insulin-induced hypoglycemia by strict glycemic control in diabetics

To elucidate the mechanism of impaired pancreatic A cell function in hypoglycemia in diabetics, the effect of long-term strict glycemic regulations on hypoglycemia-induced glucagon secretion was studied. Firstly, the effect of plasma insulin concentrations on suppressing A cell was studied in healthy volunteers by injecting insulin at doses of 0.1 U/kg and 0.3 U/kg. With 0.3 U/kg of insulin, the rate of fall in glycemia and the nadir of blood glucose were made similar to those with 0.1 U/kg of insulin by glucose infusion with artificial endocrine pancreas. Plasma glucagon response after 0.3 U/kg of insulin was significantly suppressed as compared to that after 0.1 U/kg of insulin, demonstrating that not only hypoglycemic stimulus but also plasma insulin concentration were important determinants responsible for glucagon secretion in insulin-induced hypoglycemia. Secondly, effect of strict glycemic control was studied. Short-acting insulin at a dose of 0.1 U/kg was injected in an intravenous bolus form into 12 insulin-dependent (IDDM) and 9 non-insulin-dependent (NIDDM) diabetics before and 1-3 months after strict glycemic control with multiple insulin injections therapy. Before strict glycemic regulations in IDDM, no significant rise in plasma glucagon concentrations was observed during the insulin-induced hypoglycemia. In NIDDM, a rise in plasma glucagon concentrations was observed, though the response was delayed. After strict glycemic regulations, in patients with residual endogenous insulin secretion, the glucagon response to hypoglycemia improved considerably in IDDM and normalized in NIDDM. In IDDM and NIDDM, improvement in glucagon response to hypoglycemia related positively to daily urinary secretion rate of C-peptide.(ABSTRACT TRUNCATED AT 250 WORDS)

Normoglycemia per se but not normoinsulinemia is responsible for suppressing endogenous insulin secretion after oral glucose load in NIDDM

It is well known that intensive insulin treatment of non-insulin-dependent diabetics (NIDDM) suppresses endogenous insulin secretion and thereafter improves it. To determine whether 'peripheral normo-insulinemia' or 'normoglycemia' established by the treatment is responsible for this suppression, the following five experiments were conducted on 15 well-controlled non-obese NIDDM patients. Experiment 1: a 100 g oral glucose load (OGL) was performed and blood glucose was monitored by an artificial endocrine pancreas (AP). Experiment 2: a 100 g OGL was done and blood glucose was normalized by AP-controlled insulin infusion. Experiments 3 and 4: a 100 g OGL was conducted while 'hyperglycemia' seen in experiment 1 was mimicked by AP-controlled glucose infusion with pre-programmed insulin infusion at the same rates as those in experiment 2 ('normoinsulinemia') or at rates 1.5 times higher than those in experiment 2 ('relative hyperinsulinemia'), respectively. Experiment 5: a 40 g OGL was conducted while AP-controlled insulin and glucose infusions were administered to make the plasma insulin level lower than in experiment 2 ('hypoinsulinemia') and to mimic the normoglycemic profile observed in experiment 2, respectively. In experiments 3 and 4, neither 'normoinsulinemia' nor 'relative hyperinsulinemia' suppressed the increase in plasma C-peptide after a 100 g OGL. In experiment 5, where the plasma insulin level showed a significantly (P less than 0.05) lower level than in experiment 2 and glycemia was normalized, C-peptide did not show a significant rise after OGL. These results indicate that 'normoglycemia' rather than 'normoinsulinemia' attained during exogenous insulin therapy, is responsible for suppressing endogenous insulin secretion against orally administered glucose.

Abnormal glucagon response to arginine and its normalization in obese hyperinsulinaemic patients with glucose intolerance: importance of insulin action on pancreatic alpha cells

An excessive glucagon secretion to intravenous arginine infusion was found in obese hyperinsulinaemic patients with glucose intolerance. This study was designed to determine whether the glucagon hyperresponsiveness to arginine in these patients would improve by insulin infused at a high enough dose to overcome insulin resistance. By infusing high dose insulin during arginine infusion, the previously exaggerated glucagon response to arginine could be normalized. To normalize the abnormal glucagon response, insulin doses of 4.2 +/- 0.7 and 3.8 +/- 0.5 IU were required during arginine infusion in obese hyperinsulinaemic patients with impaired glucose tolerance and Type 2 (non-insulin-dependent) diabetes mellitus, respectively. This achieved plasma peak insulin levels 3 to 4 times higher than those observed in non-obese healthy subjects. Furthermore, we clarified whether or not the effect of normalizing insulin action and/or glycaemic excursions contributed to normalizing the exaggerated glucagon response to arginine in these patients. Blood glucose was clamped while high dose insulin was infused at the same levels as observed during the arginine infusion test with no insulin infusion. As a result, normalization of the exaggerated plasma glucagon response was achieved, whether hyperglycaemia existed or not. These results clearly demonstrate that, similar to non-obese hypoinsulinaemic Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetic patients, the exaggerated Alpha-cell response to arginine infusion in obese hyperinsulinaemic patients with glucose intolerance is secondary to the reduction of insulin action on the pancreatic Alpha cell, and that the expression of insulin action plays an important part in normalizing these abnormalities.