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Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2006-1515
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The Journal of Clinical Endocrinology & Metabolism Vol. 92, No. 5 1778-1784
Copyright © 2007 by The Endocrine Society

Fasting Hyperglycemia Impairs Glucose- But Not Insulin-Mediated Suppression of Glucagon Secretion

Muhammad Abdul-Ghani and Ralph A. DeFronzo

Diabetes Division, University of Texas Health Science Center, San Antonio, Texas 78229

Address all correspondence and requests for reprints to: Muhammad Abdul-Ghani, M.D., Ph.D., Diabetes Division, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, Texas 78229. E-mail: albarado{at}uthscsa.edu.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Aim: Our aim was to assess the effect of chronic hyperglycemia on glucose- and insulin-mediated suppression of glucagon secretion by the {alpha}-cell.

Methods: Thirty subjects with normal glucose tolerance, 27 with impaired fasting glucose and/or impaired glucose tolerance, and 32 type 2 diabetic subjects were studied with oral glucose tolerance test (OGTT) and euglycemic hyperinsulinemic clamp. Fasting plasma glucagon concentration and plasma glucagon concentration during the OGTT and insulin clamp were measured.

Results: During the OGTT, the decrement in the plasma glucagon concentration (area under the curve) was correlated inversely with the fasting plasma glucose concentration (r = –0.35; P < 0.001). As the fasting glucose level increased, the suppression of plasma glucagon progressively diminished. In contrast, during the euglycemic insulin clamp, the suppression of plasma glucagon was not correlated with the fasting plasma glucose concentration and was similar in subjects with normal glucose tolerance, subjects with impaired fasting glucose/impaired glucose tolerance, and diabetic subjects: 18, 23, and 18%, respectively.

Conclusion: Insulin-mediated suppression of glucagon secretion is unrelated to the fasting plasma glucose concentration and is not impaired by chronic hyperglycemia. Thus, the defect in plasma glucagon suppression during the OGTT most likely results from impaired glucose-mediated glucagon suppression. The close correlation between fasting plasma glucose concentration and reduced glucagon suppression suggests a glucotoxic effect on {alpha}-cell function.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
HYPERGLYCEMIA IS A sine qua non in patients with type 2 diabetes mellitus. A large body of evidence indicates that chronically elevated blood glucose levels impair insulin action and exacerbate insulin resistance (1). Conversely, correction of hyperglycemia ameliorates insulin resistance in both humans (2) and animals (3). High blood glucose levels exert a deleterious effect on ß-cell function both in vivo and in cultured ß-cells (4, 5, 6, 7, 8). Collectively, these deleterious effects of hyperglycemia on insulin secretion and insulin action have been referred to as glucotoxicity and result in impaired glucose disposal in skeletal muscle and enhanced endogenous glucose production.

In contrast to the deleterious effect of hyperglycemia on ß-cell function, which has been extensively studied, much less is known about the impact of chronic hyperglycemia on {alpha}-cell function. Fasting plasma glucagon levels are elevated in type 2 diabetic patients (9), and the insulin/glucagon ratio is an important determinant of basal hepatic glucose production (10, 11). After a meal, plasma glucose and insulin levels rise and combine to suppress glucagon secretion by the {alpha}-cell (12). The resultant decline in plasma glucagon concentration contributes to the reduction in hepatic glucose production and plays an important role in maintenance of normal glucose homeostasis. Patients with type 2 diabetes mellitus are characterized by abnormalities in both ß- and {alpha}-cell function (12, 13). Despite marked hyperglycemia, type 2 diabetic subjects have high fasting plasma glucagon levels (14), increased acute glucagon response to an iv arginine stimulus (15), and impaired glucagon suppression after glucose ingestion (14, 16, 17, 18). In nondiabetic subjects, an increased glucagon response to arginine predicts worsening of glucose tolerance and progression to type 2 diabetes (19). Studies employing the pancreatic clamp technique have demonstrated that lack of glucagon suppression after carbohydrate ingestion causes postprandial hyperglycemia in healthy nondiabetic subjects (20) and contributes to postprandial hyperglycemia in type 2 diabetes subjects (21). Therefore, perturbed glucagon secretion in conjunction with impaired insulin secretion is likely to contribute to the elevated rate of hepatic glucose production (22) and postprandial hyperglycemia (21) in type 2 diabetic patients.

The impairment in glucagon suppression after glucose ingestion could result from refractoriness of the {alpha}-cell to the inhibitory effect of either hyperinsulinemia or hyperglycemia. A study in dogs demonstrated that chronic hyperglycemia impairs glucose-mediated suppression of plasma glucagon level (16) without altering the ability of insulin to inhibit glucagon secretion (23). Correction of the hyperglycemia restored the ability of glucose to suppress glucagon secretion (24). Studies in humans with impaired glucose tolerance (IGT) suggest that both glucose- and insulin-mediated suppression of glucagon secretion are impaired (25). To further examine the regulation of glucagon secretion by insulin and glucose, we studied 89 subjects with a wide range of fasting plasma glucose (FPG) concentrations and examined glucagon suppression after glucose ingestion and euglycemic-hyperinsulinemic clamp.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects

The participants included 89 subjects of Mexican-American descent who were recruited through advertising within the medical center and in local newspapers. All studies were performed at the General Clinical Research Center (GCRC) of the University of Texas Health Science Center at 0730–0800 h after a 10- to 12-h overnight fast. Subjects responding to the advertisement were screened with a 75-g oral glucose tolerance test (OGTT) and within 3–10 d returned to the GCRC for a euglycemic insulin clamp study. Based on the OGTT, subjects were classified as having normal glucose tolerance (NGT) (fasting glucose is <5.6 mmol/liter and 2-h glucose is <7.8 mmol/liter; n = 30), IGT (fasting glucose is <5.6 mmol/liter and 2-h glucose is ≥7.8 but <11.1 mmol/liter), impaired fasting glucose (IFG) (fasting glucose between 5.6–7.0 mmol/liter and 2-h glucose is <7.8 mmol/liter; n = 27), or type 2 diabetes mellitus (fasting glucose is >7.0 mmol/liter or 2-h glucose is ≥11.1 mmol/liter; n = 32) according to the American Diabetes Association criteria (26).

All subjects had normal liver, cardiopulmonary, and kidney function as determined by medical history, physical examination, screening blood tests, electrocardiogram, and urinalysis. No NGT, IFG or IGT subject was taking any medication known to affect glucose tolerance. No diabetic subject had received treatment with oral hypoglycemic agents or insulin. None of the subjects participated in any regular physical activity program. Body weight was stable (±2 kg) for at least 3 months before study in all subjects. The study protocol was approved by the Institutional Review Board of the University of Texas Health Science Center, San Antonio, and informed written consent was obtained from all subjects before their participation.

OGTT

OGTT was performed at the GCRC at 0800 h after a 10- to 12-h overnight fast. Subjects were advised to consume a high-carbohydrate diet for 3 d before the OGTT. Before the start of the OGTT, a small polyethylene catheter was placed into an antecubital vein, and blood samples were collected at –30, –15, 0, 30, 60, 90, and 120 min for the measurement of plasma glucose, insulin, and glucagon concentrations. At time zero, subjects ingested 75 g glucose and received a 100 µCi bolus of 3H2O. Plasma samples for 3H2O radioactivity were obtained at 100, 110, and 120 min for determination of fat-free mass (27). On the day of OGTT, the waist circumference was determined by measurement at the narrowest part of the torso.

Euglycemic insulin clamp

The insulin clamp was performed at 0730 h after a 10- to 12-h overnight fast. Before the start of the insulin clamp, a catheter was placed into an antecubital vein for the infusion of all test substances. A second catheter was inserted retrogradely into a vein on the dorsum of the hand, and the hand was placed into thermoregulated box heated to 60 C. At 0800 h, all subjects received a primed (25 µCi) continuous (0.25 µCi/min) infusion of 3-[3H]glucose (DuPont NEN Life Science Products, Boston, MA) for 2 h (3 h in diabetics) to measure basal endogenous glucose production (28). In diabetic subjects, the tracer prime was increased in proportion to the increase in FPG (25 µCi x FPG/100). After the basal tracer equilibration period, subjects received a primed-continuous insulin infusion at the rate of 240 pmol (40 mU)/min·m2 for 120 min, and the tritiated glucose infusion was continued. During the last 30 min of the basal equilibration period, plasma samples were taken at 5- to 10-min intervals for the determination of plasma glucose and insulin concentrations and tritiated glucose radioactivity. During insulin infusion, plasma glucose concentration was measured every 5 min, and a variable infusion of 20% glucose was adjusted, based on the negative feedback principle, to maintain the plasma glucose concentration at each subject’s FPG level with a coefficient of variation less than 5%. In the diabetic group, the plasma glucose concentration was allowed to decline to 5.6 mmol/liter (100 mg/dl), at which level it was clamped. Plasma samples were collected every 15 min from 0–90 min and every 5–10 min from 90–120 min for the determination of plasma glucose and insulin concentrations and tritiated glucose specific activity.

Calculations

During the postabsorptive state, steady-state conditions prevail and endogenous glucose production (EGP) was calculated as the tritiated glucose infusion rate (DPM/min) divided by the plasma tritiated glucose specific activity (DPM/mg). During the insulin clamp, non-steady-state conditions prevail, and the rate of appearance of glucose was calculated with Steele’s equation. The rate of residual EGP during the insulin clamp was calculated by subtracting the rate of exogenous glucose infusion from the tracer-derived rate of glucose appearance. Total-body insulin-stimulated rate of glucose disposal was calculated by adding the rate of residual EGP to the exogenous glucose infusion rate. Areas under the curve (AUC) were calculated as incremental value above baseline according to the trapezoid rule.

Analytical techniques

Plasma glucose was measured by the glucose oxidase reaction (Glucose Analyzer from Beckman, Fullerton, CA). Plasma insulin and C-peptide concentrations were measured by RIA kits (Linco Research, St. Louis, MO). Plasma 3-[3H]glucose radioactivity was measured on Somogyi precipitates.

Statistical analysis

Data are presented as the mean ± SE. Simple Pearson’s correlation was used to assess the relationship between variables. Fasting plasma glucose concentration, insulin-mediated glucose disposal, and change in plasma glucose, insulin, and glucagon AUC during OGTT were treated as continuous variables. For comparison between groups, Student’s t test was used. To compare the mean of more than two groups, ANOVA was used. Statistical significance was considered at P < 0.05.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
The clinical and metabolic characteristics of the study population are presented in Table 1Go. Subjects with glucose intolerance and diabetes were slightly older and had higher body mass index, waist circumference, and percent body fat. Male/female ratio did not differ significantly between the three groups. FPG and insulin concentrations rose progressively from NGT to IGT to type 2 diabetes mellitus, whereas the plasma insulin response to ingested glucose displayed the typical inverted U-shaped curve. Despite fasting hyperglycemia and hyperinsulinemia, diabetic subjects had a slightly, but not significantly, higher fasting plasma glucagon concentration than NGT and IFG/IGT subjects.


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  		TABLE 1. Clinical and metabolic characteristics of the study population, based upon oral glucose tolerance status

 
Glucagon suppression during OGTT

During the OGTT, the plasma glucagon concentration in the entire cohort (n = 89) declined by 26%, from 89 ± 32 to 71 ± 33 pg/ml (P < 0.0001). The decremental area under the plasma glucagon curve ({Delta}AUC) (Fig. 1Go) was highly correlated with the FPG concentration. As the FPG level increased, the suppression of plasma glucagon became progressively diminished (Fig. 1Go). Similarly, the suppression of plasma glucagon concentration during the OGTT (measured as percent suppression at 90–120 min during the OGTT relative to the fasting plasma glucagon) declined progressively as FPG concentration increased (data not shown). The impaired suppression of plasma glucagon concentration was evident with increase in plasma glucose concentrations within the normal range, i.e. less than 100 mg/dl (Fig. 2Go). In subjects with FPG of 116–125 mg/dl, the decrement in plasma glucagon concentration was 16 ± 8 compared with 43 ± 26 pg/ml·h for subjects with a FPG less than 85 mg/dl, representing a 63% impairment in the suppression of plasma glucagon concentration (P < 0.05).


Figure 1
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  		FIG. 1. The relationship between suppression of plasma glucagon concentration and FPG concentration during the OGTT. The figure depicts the decremental area under the plasma glucagon concentration curve vs. the FPG concentration in 89 subjects. Each point represents the mean ± SEM of 13 consecutive subjects, based upon progressively increasing FPG concentrations. The last point on the curve represents the mean of 11 subjects.

 

Figure 2
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  		FIG. 2. A, Incremental area under the plasma C-peptide concentration curve during 0–120 min of the OGTT; B, decremental area under plasma glucagon concentration curve during 0–120 min of the OGTT; C, ratio between the decremental area under the plasma glucagon curve to the incremental area under the plasma C-peptide curve from 0–120 min. Values represent the mean ± SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

 
Most of the suppression of plasma glucagon concentration occurred during the first hour of the OGTT (Fig. 3Go). The increase in FPG was associated with a decrease in the rate of glucagon suppression during the OGTT. The rate of decrease in plasma glucagon concentration during the OGTT, measured as the slope of plasma glucagon concentration between 0–60 min, decreased as the FPG increased, and it was 3.94 x 10–3, 3.19 x 10–3, 2.43 x 10–3, and 0.77 x 10–3 pg/min in subjects with FPG <90 mg/dl, 90–99 mg/dl, 100–125 mg/dl, and >125 mg/dl, respectively (P < 0.001 by ANOVA).


Figure 3
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  		FIG. 3. Time course of change in plasma glucagon concentration during the OGTT in subjects with FPG less than 90, 90–99, 100–125, and more than 126 mg/dl. *, P < 0.05; **, P < 0.01; ***, P <0.001. T2DM, Type 2 diabetes mellitus.

 
To exclude the possibility that the impaired suppression of plasma glucagon concentration as the FPG increased was due to an impairment in insulin secretion, we examined the ratio between the suppression in plasma glucagon concentration and the increase in plasma C-peptide concentration during the entire OGTT [{Delta}glucagon(AUC0–120)/{Delta}C-peptide(AUC0–120)] (Fig. 2Go) and during the first 30 min of the OGTT [{Delta}glucagon(AUC0–30)/{Delta}C-peptide(AUC0–30)] (Fig. 4Go). Figures 2Go and 4Go demonstrate that as the FPG concentration increased from less than 85 mg/dl to 85–94 mg/dl to 95–100 mg/dl, the decremental area under glucagon curve, both at 0–30 min and 0–120 min, markedly declined despite an increase in insulin secretion as measured by {Delta}C-peptide(AUC0–120) (Fig. 3Go) or {Delta}C-peptide(AUC0–30) (Fig. 4Go). Consequently, {Delta}glucagon(AUC0–120)/{Delta}C-peptide(AUC0–120) (Fig. 3CGo) and {Delta}glucagon(AUC0–30)/{Delta}C-peptide(AUC0–30) (Fig. 3CGo) markedly decreased as the FPG increased from less than 85 mg/dl to 85–94 mg/dl to 95–100 mg/dl.


Figure 4
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  		FIG. 4. A, Incremental area under the plasma C-peptide concentration curve during 0–30 min of the OGTT; B, decremental area under plasma glucagon concentration curve during 0–30 min of the OGTT; C, ratio between the decremental area under the plasma glucagon curve to the incremental area under the plasma C-peptide curve from 0–30 min. Values represent the mean ± SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

 
Glucagon suppression during euglycemic insulin clamp

To examine the effect of physiological hyperinsulinemia, independent of change in plasma glucose concentration, we examined glucagon suppression during the euglycemic hyperinsulinemic clamp. During the last 30 min of the euglycemic insulin clamp, both the absolute and the percent plasma glucagon concentration was similar in the NGT (16 ± 16 pg/ml, or 18 ± 18%), IFG/IGT (18 ± 11 pg/ml, or 23 ± 11%), and diabetic (14 ± 17 pg/ml, or 18 ± 24%) groups (P value was not significant between groups) despite steady-state plasma insulin concentrations that were comparable to those during the OGTT. The percent glucagon suppression during the clamp divided by steady-state plasma insulin concentration during the clamp was similar among the three groups (Table 1Go) and was independent of the FPG concentration. In the NGT and IFG/IGT subjects, in whom the plasma glucose was clamped at their ambient FPG concentration, the suppression of plasma glucagon level during the insulin clamp was independent of FPG concentration (Fig. 5Go), whereas the suppression of plasma glucagon during the OGTT declined progressively as the FPG concentration increased (Fig. 5Go). The magnitude of glucagon suppression during the euglycemic insulin clamp was not correlated with either the FPG concentration (r = 0.06; P > 0.4) or with the rate of insulin-mediated glucose disposal during the euglycemic insulin clamp (r = –0.02; P > 0.4).


Figure 5
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  		FIG. 5. Suppression of plasma glucagon level in nondiabetic subjects. The upper panel shows the FPG concentration and the plasma glucose concentration during the insulin clamp in nondiabetic subjects. The middle panel depicts the decremental area under the plasma glucagon concentration curve during the OGTT. The lower panel displays the percent suppression of plasma glucagon concentration during the insulin clamp.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The main finding of the present study is that the magnitude of plasma glucagon suppression during the OGTT is highly and inversely correlated with the FPG concentration (Fig. 1Go), whereas glucagon suppression during euglycemic-hyperinsulinemic clamp is not related to the FPG concentration despite similar plasma insulin concentration during both studies. Because during the euglycemic-hyperinsulinemic clamp the plasma glucose concentration is clamped at a constant basal value (~100 mg/dl), which is similar in all subjects, it is reasonable to assume that suppression of glucagon secretion reflects the inhibitory action of physiological hyperinsulinemia on glucagon secretion by the {alpha}-cell. Because the suppression of plasma glucagon concentration was similar in NGT, IFG/IGT, and diabetic groups and was not correlated with the FPG concentration, we conclude that the inhibitory action of insulin on glucagon secretion is not adversely affected by the chronically elevated FPG concentrations; i.e. glucose toxicity does not affect the ability of insulin to inhibit glucagon secretion by the {alpha}-cell. However, it could be argued that the acute reduction in plasma glucose concentration during the insulin clamp in diabetic subjects removed the glucotoxic effect on the {alpha}-cell, allowing insulin to elicit a normal suppression of glucagon secretion. However, glucagon suppression during the insulin clamp was independent of the FPG concentration in NGT and IFG/IGT subjects, in whom the plasma glucose was clamped at each subject’s own FPG concentration. Nonetheless, glucagon suppression during the OGTT progressively declined as the FPG concentration increased. These results demonstrate that an increase in FPG concentration, at least in the nondiabetic range, does not interfere with glucagon suppression by insulin. In a previous study, a blunted suppression of plasma glucagon concentration was observed during euglycemic-hyperinsulinemic clamp studies performed in subjects with IGT (25). The difference between this previous study (25) and the present one may be due to the level of insulin achieved and/or the relatively small number of subjects studied in the previous publication (25). It should be noted that in an experimental animal model for diabetes, infusion of insulin in dogs with severe fasting hyperglycemia and hyperglucagonemia resulted in a marked and normal suppression of plasma glucagon concentration (23).

In contrast to the insulin clamp, glucagon suppression during the OGTT was markedly impaired as the FPG concentration increased (Fig. 1Go). It is particularly noteworthy that the impaired suppression of plasma glucagon concentration during the OGTT was observed with increases in plasma glucose concentration within the nondiabetic range, i.e. 85–125 mg/dl (Figs. 1–5GoGoGoGoGo). During the OGTT, plasma glucose and insulin levels rise, and both contribute to the suppression of glucagon secretion. Thus, glucagon suppression during the OGTT can be divided into two components: hyperglycemia and hyperinsulinemia. Because suppression of glucagon secretion during the euglycemic-hyperinsulinemic clamp was similar in all three groups (NGT, IFG/IGT, and diabetics) and was not correlated with the FPG concentration, it can be argued that the component of glucagon suppression that is attributed to hyperinsulinemia during the OGTT is not impaired. Therefore, the impaired suppression of plasma glucagon concentration during the OGTT most likely results from a defect in the glucose suppression component. Previous studies have demonstrated that intra-islet insulin plays an important role in glucose action on the {alpha}-cell (26, 27). The impairment in insulin secretion that accompanies the deterioration in glucose tolerance could, therefore, indirectly contribute to the impairment in glucagon suppression by hyperglycemia. However, our results argue strongly against a role for reduced availability of intra-islet insulin in the impaired glucagon suppression during the OGTT. First, the impairment in glucagon suppression during the OGTT is observed with fasting plasma concentrations well within the normal range (85–100 mg/100 ml) at a time when insulin secretion by the ß-cell (as evidenced by the C-peptide AUC) is progressively increasing with the rise in FPG concentration (Figs. 3Go and 4Go). Second, when glucagon suppression is related to the level of insulin secretion [{Delta}glucagon(AUC)/{Delta}C-peptide(AUC)], glucagon suppression during the OGTT declined markedly and progressively with the increase in the FPG concentration (Figs. 3Go and 4Go). However, a possible role for GLP-1 deficiency in the impairment of glucagon suppression in the diabetic subjects cannot be excluded. Our results suggest that the sensitivity of the {alpha}-cell response to an acute increase in the plasma glucose concentration is highly dependent on the preceding chronic exposure to elevated blood glucose levels. The magnitude of the glucagon secretory response to an acute arginine stimulus also is highly dependent on the preceding plasma glucose level both in normal and diabetic subjects (17). Hamaguchi et al. (28) reported an exaggerated glucagon response to iv arginine in subjects with glucose intolerance and type 2 diabetes compared with NGT subjects. Furthermore, insulin infusion before the arginine stimulus restored the normal glucagon response to arginine. These authors concluded that insulin infusion restored the normal glucagon response by overcoming the insulin resistance that is present in subjects with IGT and diabetes. However, because insulin infusion in subjects with IGT and diabetes corrected the fasting hyperglycemia, it could be argued that the restoration of normal glucagon secretion by insulin was due to correction of the fasting hyperglycemia. In experimental animals, chronic hyperglycemia markedly impairs the suppression of plasma glucagon by glucose (16), and correction of hyperglycemia with phlorizin restores the ability of glucose to suppress glucagon secretion by glucose (24).

It is noteworthy that the impaired suppression of plasma glucagon during the OGTT begins with increased FPG concentrations that are well less than 100 mg/dl, i.e. within the range of fasting glucose values that are considered normal (29). This decline in {alpha}-cell sensitivity to an acute rise in plasma glucose concentration is very similar to that observed for the ß-cell. Thus, in NGT individuals, the insulin secretion/insulin resistance (so-called disposition) index [({Delta}I0–120/{Delta}G0–120) x (1/total glucose disposal)] decreases precipitously as the FPG concentration rises from 100 to 125 mg/dl. In animals, chronic physiological hyperglycemia impairs the ß-cell insulin secretory response to an acute elevation in plasma glucose concentration (4, 6, 7), and this defect can be corrected by normalizing the plasma glucose concentration with phlorizin (6). The dependency of both {alpha}- and ß-cell function on the blood glucose level may explain the significant correlation observed in our study between impaired glucagon suppression during the OGTT and the impaired early (0–30 min) insulin secretion/insulin resistance index. It should be noted that although this index of ß-cell function is impaired, the absolute insulin response (as reflected by the C-peptide AUC) is increased at 30 min during the OGTT.

The cellular mechanism(s) through which a chronic increase in plasma glucose concentration impairs glucagon secretion has yet to be determined. In contrast to the suppressive action of elevated plasma glucose levels on glucagon secretion in human and animal studies in vivo and in the isolated perfused pancreas (30), most (31, 32, 33, 34, 35) but not all (36, 37) studies have shown that, in single {alpha}-cells isolated from rat or mouse pancreatic islets, a rise in extracellular glucose concentration triggers an increase in intracellular calcium concentration, accompanied by an increase in glucagon secretion. Potential explanations for this paradoxical in vitro result include species differences (rat and mouse vs. dog and human), disruption of the normal architecture of the islet, or interruption of the neuronal innervation of the islets. With respect to species differences, increased extracellular glucose concentration in intact human islets has opposite effects on intracellular calcium concentrations in {alpha}-cells and ß-cells (38). Moreover, differences in the {alpha}-cell response to an acute change in extracellular glucose concentration may be different from the {alpha}-cell response to chronic hyperglycemia (39). Thus, as in the ß-cell, although an acute rise in extracellular glucose is a powerful stimulus to insulin secretion, chronic exposure of the ß-cell to hyperglycemia impairs its responsiveness to an acute increase in plasma glucose concentration, i.e. glucotoxicity (1, 4, 6, 7). A similar glucotoxic effect on the {alpha}-cell could explain our results, and this would be consistent with the strong correlation observed between the elevated FPG concentration and impaired suppression of glucagon secretion by hyperglycemia (OGTT) in the present study. Lastly, the paradoxical increase in glucagon secretion in response to an increase in extracellular glucose concentration by isolated {alpha}-cells (31, 32, 33, 34, 35) could be explained by islet denervation or altered spatial relationships between the {alpha}-cell and other cells within the islet. With regard to the latter, there is evidence that somatostatin secretion by the {delta}-cells plays an important role in the regulation of glucagon secretion (33, 40, 41). There also is evidence that ß-cell secretory products including insulin (33, 34, 35, 37), zinc (34, 42), and {gamma}-aminobutyric acid (GABA) (32) have a direct paracrine inhibitory effect on glucagon secretion by the {alpha}-cell.

In summary, the suppression of plasma glucagon concentration by hyperglycemia becomes progressively impaired as one progresses from NGT to IFG/IGT to diabetes, and the severity of the defect in glucagon suppression correlates strongly with the increase in FPG concentration. Whether the association is explained by glucotoxicity remains to be determined. However, we hypothesize that the effect of chronic elevation in plasma glucose concentration on the {alpha}-cell may explain the development of hyperglucagonemia in type 2 diabetic subjects (14). As ß-cell failure progresses, the resultant hyperglycemia leads to impaired suppression of glucagon secretion, thereby creating a negative reverberating cycle wherein hyperglucagonemia leads to increased basal hepatic glucose production, impaired suppression of hepatic glucose production by insulin, and aggravation of both fasting and postprandial hyperglycemia in type 2 diabetic subjects (21).


   Footnotes
 
R.A.D. consults for Bristol Myers Squibb, Takeda, Amylin, and Novartis and has grants from Bristol Myers Squibb, Takeda, Amylin, Novartis, and Pfizer. M.A.-G. has nothing to declare. None of these activities and grant support present a conflict of interest for the present investigation.

First Published Online February 27, 2007

Abbreviations: AUC, Area under the curve; EGP, endogenous glucose production; FPG, fasting plasm glucose; GCRC, General Clinical Research Center; IFG, impaired fasting glucose; IGT, impaired glucose tolerance; OGTT, oral glucose tolerance test; NGT, normal glucose tolerance.

Received July 13, 2006.

Accepted February 16, 2007.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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