Glucose regulation of glucagon secretion independent of B cell activity

Alpha cell dysfunction in type 1 diabetes

Type 1 diabetes is characterized by selective loss of beta cells and insulin secretion, which significantly impact glucose homeostasis. However, this progressive disease is also associated with dysfunction of the alpha cell component of the islet, which can exacerbate hyperglycemia due to paradoxical hyperglucagonemia or lead to severe hypoglycemia as a result of failed counterregulation. In this review, the physiology of alpha cell secretion and the potential mechanisms underlying alpha cell dysfunction in type 1 diabetes will be explored. Because type 1 diabetes is a progressive disease, a synthesized timeline of aberrant alpha cell function will be presented as an attempt to delineate the natural history of type 1 diabetes with respect to the alpha cell.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

A K ATP channel-dependent pathway within alpha cells regulates glucagon release from both rodent and human islets of Langerhans

Glucagon, secreted from pancreatic islet α cells, stimulates gluconeogenesis and liver glycogen breakdown. The mechanism regulating glucagon release is debated, and variously attributed to neuronal control, paracrine control by neighbouring β cells, or to an intrinsic glucose sensing by the α cells themselves. We examined hormone secretion and Ca²⁺ responses of α and β cells within intact rodent and human islets. Glucose-dependent suppression of glucagon release persisted when paracrine GABA or Zn²⁺ signalling was blocked, but was reversed by low concentrations (1–20 μM) of the ATP-sensitive K⁺ (K_ATP) channel opener diazoxide, which had no effect on insulin release or β cell responses. This effect was prevented by the K_ATP channel blocker tolbutamide (100 μM). Higher diazoxide concentrations (≥30 μM) decreased glucagon and insulin secretion, and α- and β-cell Ca²⁺ responses, in parallel. In the absence of glucose, tolbutamide at low concentrations (<1 μM) stimulated glucagon secretion, whereas high concentrations (>10 μM) were inhibitory. In the presence of a maximally inhibitory concentration of tolbutamide (0.5 mM), glucose had no additional suppressive effect. Downstream of the K_ATP channel, inhibition of voltage-gated Na⁺ (TTX) and N-type Ca²⁺ channels (ω-conotoxin), but not L-type Ca²⁺ channels (nifedipine), prevented glucagon secretion. Both the N-type Ca²⁺ channels and α-cell exocytosis were inactivated at depolarised membrane potentials. Rodent and human glucagon secretion is regulated by an α-cell K_ATP channel-dependent mechanism. We propose that elevated glucose reduces electrical activity and exocytosis via depolarisation-induced inactivation of ion channels involved in action potential firing and secretion.

Glucagon is a critical regulator of glucose homeostasis. Its major action is to mobilize glucose from the liver. Glucagon secretion from α cells of the pancreatic islets of Langerhans is suppressed by elevated blood sugar, a response that is often perturbed in diabetes. Much work has focused on the regulation of α-cell glucagon secretion by neuronal factors and by paracrine factors from neighbouring cells, including the important islet hormone insulin. In contrast, we provide evidence in support of a direct effect of glucose on α cells within intact rodent and human islets. Notably, our work implicates an α-cell glucose-sensing pathway similar to that found in insulin-secreting β cells, involving closure of ATP-dependent K⁺ channels in the presence of glucose. Furthermore, we find that membrane depolarisation results in inhibition of Na⁺ and Ca²⁺ channel activity and α-cell exocytosis. Thus, we propose that elevated blood glucose reduces α-cell electrical activity and glucagon secretion by inactivating the ion channels involved in action potential firing and secretion.

Elevated glucose levels reduce electrical activity and the release of glucagon via inactivation of ion channels in pancreatic islet cells.

Impaired glucagon response to sympathetic nerve stimulation in the BB diabetic rat: effect of early sympathetic islet neuropathy

We investigated the functional impact of a recently described islet-specific loss of sympathetic nerves that occurs soon after the autoimmune destruction of beta-cells in the BB diabetic rat (35). We found that the portal venous (PV) glucagon response to sympathetic nerve stimulation (SNS) was markedly impaired in newly diabetic BB rats (BB D). We next found a normal glucagon response to intravenous epinephrine in BB D, eliminating the possibility of a generalized secretory defect of the BB D alpha-cell as the mediator of the impaired glucagon response to SNS. We then sought to determine whether the glucagon impairment to SNS in BB D was due solely to their loss of islet sympathetic nerve terminals or whether other effects of autoimmune diabetes contributed. We therefore reproduced, in nondiabetic Wistar rats, an islet nerve terminal loss similar to that in BB D with systemic administration of the sympathetic neurotoxin 6-hydroxydopamine. The impairment of the glucagon response to SNS in these chemically denervated, nondiabetic rats was similar to that in the spontaneously denervated BB D. We conclude that the early sympathetic islet neuropathy of BB D causes a functional defect of the sympathetic pathway to the alpha-cell that can, by itself, account for the impaired glucagon response to postganglionic SNS.

Plasma concentrations of glucagon during hyperglycemic clamp with or without somatostatin infusion in obese subjects

The aim of the present study was to evaluate whether the inhibitory effect on pancreatic A-cell exerted by hyperglycemic hyperinsulinemia and/or by somatostatin administration is impaired in human obesity. For this purpose plasma glucagon concentrations were measured in 8 obese and 8 nonobese nondiabetic subjects during a 4-h hyperglycemic clamp. Synthetic cyclic somatostatin-14 was infused at the rate of 2.5 nmol/min during the third hour of the study. Fasting plasma glucagon was higher in obese than in nonobese subjects (242 +/- 32 vs 163 +/- 15 pg/ml, p less than 0.05) (mean +/- SEM). In the last 20 min of the glucose infusion period preceding somatostatin administration (100-120 min of the study) plasma glucagon averaged 195 +/- 26 pg/ml in obese and 122 +/- 13 pg/ml in nonobese subjects (p less than 0.05), with a reduction of 19 +/- 3% in the former and 28 +/- 4% in the latter (p = n.s.). In both groups somatostatin infusion did not result in a further decrease in plasma glucagon, which averaged 192 +/- 27 pg/ml in obese and 123 +/- 16 pg/ml in nonobese subjects (p less than 0.05) in the 160-180 min period of the study. Also after discontinuing somatostatin infusion plasma glucagon levels did not change. These results suggest that in human obesity hyperglycemic hyperinsulinemia has a normal inhibitory effect on pancreatic A-cell and that somatostatin administration has no additive effect on hyperglycemia and hyperinsulinemia in either obese or nonobese nondiabetic subjects.

Responses of the pancreatic A cell during hypoglycemia and hyperglycemia are dependent on the B cell

It is controversial whether stimulation of glucagon secretion by hypoglycemia or suppression by hyperglycemia is a direct effect of glucose on the A cell or whether it is mediated indirectly through the B cell. To determine the role of the B cell in the mediation of glucagon secretion adolescent male baboons were studied before and after successive injections of streptozocin designed to cause B-cell destruction in a series of stages. Following one dose of streptozocin, B-cell function was impaired but fasting blood glucose remained normal. B-cell function declined further with additional doses of streptozocin. As B-cell function declined, there was a corresponding reduction in the glucagon response to hypoglycemia (Ghyp). There were significant correlations between the percentage reduction in Ghyp and the percentage reduction in B-cell function (acute insulin response to arginine, r = .47; acute insulin response to glucose, r = .69; Kg, r = .79). In a second study the ability of hyperglycemia to suppress the acute glucagon response to arginine (AGRarg) was studied. Before streptozocin AGRarg was 128 +/- 26 pg/mL at a glucose level of 80 +/- 4 mg/dL and was suppressed to 74 +/- 20 pg/mL when glucose was raised and clamped at 209 +/- 14 mg/dL. After the initial dose of streptozocin, with mild B cell damage and fasting normoglycemia, AGRarg was not suppressed by hyperglycemia. With severe B cell dysfunction and fasting hyperglycemia, there was paradoxical enhancement of AGRarg by additional hyperglycemia. In conclusion, the ability of the A cell to respond appropriately to hypoglycemia and to arginine during hyperglycemia is dependent on normal B-cell function.

Retrograde perfusion as a model for testing the relative effects of glucose versus insulin on the A cell

In order to determine whether the A cell may be directly suppressed by glucose in the absence of insulin, canine pancreata were perfused in vitro, both antegrade, through the arterial system and retrograde, through the venous system. Studies of the islet microvasculature have suggested that blood flows from the B cell core to the mantle; thus, the A cell may be tonically inhibited by intra-islet insulin. Retrograde perfusion may then be expected to prevent insulin from reaching the A cell, releasing it from inhibition. Retrograde perfusion with 88 mg/dl glucose markedly increased both insulin and glucagon secretion relative to antegrade levels. In a series of experiments, glucose concentrations were changed from 88 to 200 mg/dl. An antegrade glucose change resulted in increased insulin (134+/-21%; P less than 0.0025) and decreased glucagon (-26+/-9%, P less than 0.025) secretion. A retrograde glucose increase resulted in increased secretion of both insulin (91+/-15%; P less than 0.0005) and glucagon (23+/-9%; P less than 0.0125). To confirm that retrograde perfusion deprived the A cell of endogenous core derived, vascularly delivered insulin, possibly resulting in increased insulin sensitivity, 0.3 mU/ml exogenous porcine insulin was infused. Antegrade, 0.3 mU/ml insulin, had no effect on glucagon secretion (P less than 0.250), while retrograde infusion of 0.3 mU/ml insulin significantly inhibited glucagon secretion (-31 + 8%; P less than 0.0005). The results of our study support the concept that the direction of blood flow and of flow-dependent intra-islet hormone interactions are from the islet B cell core to the mantle. It was further concluded that the normal A cell may not be suppressed by glucose in the absence of insulin.

Mechanisms of glucagon secretion during insulin-induced hypoglycemia in man. Role of the beta cell and arterial hyperinsulinemia

To elucidate the mechanisms controlling the response of glucagon to hypoglycemia, a vital component of the counterregulatory hormonal response, the role of intraislet insulin was studied in seven normal subjects and five subjects with insulin-dependent diabetes mellitus (IDDM) (of less than 15-mo duration). In the normal subjects, hypoglycemia (arterial plasma glucose [PG] 53 +/- 3 mg/dl) induced by an intravenous insulin infusion (30 mU/m2 X min for 1 h, free immunoreactive insulin [FIRI] 58 +/- 2 microU/ml) elicited a 100% fall in insulin secretion and an integrated rise in glucagon of 7.5 ng/ml per 120 min. When endogenous insulin secretion was suppressed by congruent to 50 or congruent to 85% by a hyperinsulinemic-euglycemic clamp (FIRI 63 +/- 1.5 or 147 +/- 0.3 microU/ml, respectively) before hypoglycemia, the alpha cell responses to hypoglycemia were identical to those of the control study. When the endogenous insulin secretion was stimulated by congruent to 100% (hyperinsulinemic-hyperglycemic clamp, FIRI 145 +/- 1.5 microU/ml, PG 132 +/- 2 mg/dl) before hypoglycemia, the alpha cell responses to the hypoglycemia were also superimposable on those of the control study. Finally, in C-peptide negative diabetic subjects made euglycemic by a continuous overnight intravenous insulin infusion, the alpha cell responses to hypoglycemia were comparable to those of normal subjects despite absent beta cell secretion, and were not affected by antecedent hyperinsulinemia (hyperinsulinemic-euglycemic clamp for 2 h, FIRI 61 +/- 2 microU/ml). These results indicate that the glucagon response to insulin-induced hypoglycemia is independent of the level of both endogenous intraislet and exogenous arterial insulin concentration in normal man, and that this response may be normal in the absence of endogenous insulin secretion, in contrast to earlier reports. Thus, loss of beta cell function is not responsible for alpha cell failure during insulin-induced hypoglycemia in IDDM.