help button home button Endocrine Society JCEM
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2005-0225
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow A correction has been published
Right arrow All Versions of this Article:
90/11/6244    most recent
Author Manuscript (PDF)
Right arrow Submit a related Letter to the Editor
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Meyer, C.
Right arrow Articles by Linn, T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Meyer, C.
Right arrow Articles by Linn, T.
Related Collections
Right arrow Diabetes and Insulin
Right arrow Metabolism
The Journal of Clinical Endocrinology & Metabolism Vol. 90, No. 11 6244-6250
Copyright © 2005 by The Endocrine Society

A Potential Important Role of Skeletal Muscle in Human Counterregulation of Hypoglycemia

Christian Meyer, Petra Saar, Nedim Soydan, Michael Eckhard, Reinhard G. Bretzel, John Gerich and Thomas Linn

Carl T. Hayden Veterans Affairs Medical Center (C.M.), Department of Endocrinology and Metabolism, Phoenix, Arizona 85012; University of Giessen, Third Medical Department (C.M., P.S., N.S., M.E., R.G.B., T.L.), 35390 Giessen, Germany; and University of Rochester (J.G.), Department of Medicine, Rochester, New York 14627

Address all correspondence and requests for reprints to: Christian Meyer, M.D., Carl T. Hayden Veteran Affairs Medical Center, Department of Endocrinology, 650 East Indian School Road, Phoenix, Arizona 85012. E-mail: christian.meyer{at}med.va.gov.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Context: During hypoglycemia, systemic glucose uptake (SGU) decreases and endogenous glucose release (EGR) increases. Skeletal muscle appears to be primarily responsible for the reduced SGU and may be important for the increased EGR by providing lactate for gluconeogenesis (GN).

Objective: The objective of the study was to test the hypothesis that reduced muscle glucose uptake and increased muscle lactate release both make major contributions to glucose counterregulation using systemic isotopic techniques in combination with forearm net balance measurements.

Setting: The study was conducted at the University of Giessen Clinical Research Center.

Participants: Nine healthy volunteers participated in the study.

Intervention: A 2-h hyperinsulinemic euglycemic clamp (blood glucose ~ 4.4 mM) was followed by a 90-min hypoglycemic clamp (blood glucose ~ 2.6 mM).

Results: Compared with the euglycemic clamp, SGU decreased (21.0 ± 2.0 vs. 29.6 ± 1.8 µmol·kg body weight–1·min–1; P < 0.001), whereas EGR (11.2 ± 1.7 vs. 4.9 ± 1.3 µmol·kg body weight–1 ·min–1; P < 0.003), arterial lactate concentrations (1051 ± 162 vs. 907 ± 115 µM; P < 0.02), systemic lactate release (23.5 ± 0.9 vs. 17.1 ± 0.9 µmol·kg body weight–1·min–1; P < 0.001), and lactate GN (4.50 ± 0.60 vs. 2.74 ± 0.30 µmol·kg body weight–1·min–1; P < 0.02) increased during hypoglycemia; the proportion of lactate used for GN remained unchanged (38 ± 4 vs. 32 ± 3%; P = 0.27). Whole-body muscle glucose uptake decreased approximately 50% during hypoglycemia (6.4 ± 1.9 vs. 13.6 ± 2.9 µmol·kg body weight–1·min–1; P < 0.001), which accounted for approximately 85% of the reduction of SGU. Whole-body muscle lactate release increased 6.6 ± 1.6 µmol·kg body weight–1· min–1 (P < 0.01), which could have accounted for all the increase in systemic lactate release and, considering the proportion of lactate used for GN, contributed 1.4 ± 0.4 µmol·kg body weight–1·min–1 (~25%) to the increase in EGR.

Conclusions: Reduced muscle glucose uptake and increased muscle lactate release both make major contributions to glucose counterregulation in humans.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
IN HUMANS, BOTH increased release of glucose into the circulation and reduced glucose removal from the circulation are involved in counterregulation of hypoglycemia. The increased glucose release is largely due to increased gluconeogenesis by liver and kidney (1, 2), which causes a greater demand for gluconeogenic substrates. Among these, lactate is considered most important (3), but the tissue(s) responsible for making lactate available for gluconeogenesis are unclear.

In studies using the microdialysis technique, lactate concentrations were approximately two to three times greater in muscle extracellular fluid than in plasma during hypoglycemia in humans, suggesting that skeletal muscle is a major source of lactate under these conditions (4, 5). These findings contrast to those of three other studies using net balance measurements, which showed that lactate release by the forearm increased very little during hypoglycemia (6, 7, 8). However, interpretation of the data of at least one of these studies is limited because arterialized venous blood as opposed to arterial blood was used for forearm net balance measurements (6). Because lactate is released by adipose tissue and skin (9, 10), venous contamination of arterialized blood might have overestimated true arterial lactate concentrations resulting in an underestimation of lactate release. Moreover, none of the studies determined the contribution of muscle lactate release to glucose counterregulation because lactate gluconeogenesis was not measured.

In addition to its possible role in providing lactate, skeletal muscle is believed to be primarily responsible for the reduced glucose uptake during hypoglycemia (11). This view is consistent with the general finding that muscle glucose uptake decreases markedly under these conditions in humans (6, 7, 8). However, to our knowledge, muscle and systemic glucose uptake have not yet been simultaneously measured during hypoglycemia, and the decrease in glucose uptake may involve the whole body due to reduced mass action effects of glycemia. The contribution of reduced muscle glucose uptake to the reduction in systemic glucose uptake during hypoglycemia is therefore unclear.

The present studies were therefore undertaken to investigate in more detail the contribution of skeletal muscle to glucose counterregulation in humans. To this end, we simultaneously measured systemic and forearm glucose uptake, forearm lactate release, and lactate gluconeogenesis during hypoglycemia using isotopic techniques in combination with arteriovenous net balance measurements across the forearm.


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

Informed written consent was obtained from nine healthy volunteers after the study had been approved by the University of Giessen Institutional Review Board. Subjects (five men, four women) were 38 ± 2 yr of age and had a body mass index of 23.8 ± 0.3 kg/m2. All subjects had normal glucose tolerance according to World Health Organization criteria and no family history of diabetes mellitus.

Protocol

Subjects were admitted to the University of Giessen Clinical Research Unit between 1800 and 1900 h in the evening before experiments; they consumed a standard meal (10 kcal/kg, 50% carbohydrate, 35% fat, and 15% protein) between 1830 and 2000 h and were fasted overnight until experiments were completed.

At approximately 0530 h the next morning, an antecubital vein was cannulated and primed-continuous infusions of [1-13C] lactate (~20 µmol, 0.20 µmol·kg–1·min–1) and [6,6-2H2] glucose (~24 µmol, 0.24 µmol·kg–1·min–1) were started (both from Isotec Inc., Miamisburg, OH). At approximately 0900 h, a radial artery and a deep antecubital vein were cannulated for blood sampling. About 1 h later, simultaneous arterial and venous blood samples were obtained at 15-min intervals for baseline measurements of insulin, glucagon, epinephrine, norepinephrine, cortisol, GH, glucose, lactate, and alanine concentrations, glucose and lactate enrichments, and blood gas analyses. Forearm blood flow (FBF) was measured after each blood sampling using capacitance plethysmography. Whole-body CO2 production and O2 consumption were determined over 20-min intervals two times during the baseline period using a canopy indirect calorimetry system (Delta Trac II, Anaheim, CA). To allow for equilibration, only data during the last 10 min were used for analyses. After the baseline period, a 31/2-h infusion of regular insulin (0.8 mU·kg–1·min–1) was started. The first 120 min consisted of a euglycemic clamp. Over the next approximately 45 min, plasma glucose was allowed to decrease to 50 mg/dl and was maintained at that level until 210 min. During the insulin infusion, plasma glucose was measured at 5-min intervals and adjusted by a concomitant infusion of 20% dextrose, which was enriched with [6,6-2H2] glucose to minimize changes in plasma glucose enrichments; blood samples and FBF measurements were obtained at 30-min intervals, and indirect calorimetry measurements were obtained during the last 30 min of both clamp periods as described above. Protein oxidation was estimated from urinary urea production measured over the duration of the experiment.

Analytical procedures

Plasma glucose was immediately determined in triplicate with a glucose analyzer (Beckman Instruments Inc., Fullerton, CA). Plasma [13C] and [2H2] glucose enrichments and plasma [13C] lactate enrichments were determined in duplicate by selected ion monitoring gas chromatography-mass spectroscopy of the acetylbutylboronate ester derivative and the bis-trimethylsilyl derivative, respectively (12). Plasma lactate, alanine, insulin, glucagon, epinephrine, norepinephrine, cortisol, and GH were determined as previously described (12, 13). Samples for blood gas analyses were drawn into heparinized syringes, immersed in ice, and analyzed in duplicate within 5 min (Ciba Corning 280, Ciba Corning Diagnostics, Fernwald, Germany).

Calculations

Plasma concentrations of glucose and lactate were converted to whole blood values by multiplying with 1 – (0.3 x hematocrit) and 0.86, respectively (14, 15). Systemic uptake and release (RA) of glucose and lactate were determined with steady-state equations under basal conditions (16) and subsequently during insulin infusion with non-steady-state equations of DeBodo et al. (17). The pool fraction was set at 0.65 for glucose and lactate; the volume of distribution was set at 200 ml/kg for glucose and 320 ml/kg for lactate (18, 19).

In the steady state, the proportion of glucose RA due to lactate gluconeogenesis was calculated as arterial [13C] glucose enrichment/(arterial [13C] lactate enrichment x 2). Systemic lactate gluconeogenesis was calculated as glucose RA multiplied by the proportion of glucose RA from lactate gluconeogenesis. During the insulin infusion, systemic lactate gluconeogenesis was calculated using the equation of Chiasson et al. (20).

Forearm net balances of glucose, lactate, and alanine were calculated by multiplying arteriovenous differences in substrate concentrations by FBF. Because lactate exchanges rapidly with pyruvate (21), conversion of lactate to pyruvate and conversion of pyruvate to lactate by the forearm were estimated by the dilution of [13C] lactate across the forearm. The former was calculated as arterial lactate concentration x [(arterial [13C] lactate enrichment x arterial lactate concentration) – (deep venous [13C] lactate enrichment x deep venous lactate concentration)]/(arterial [13C] lactate enrichment x arterial lactate concentration) x FBF; the latter was calculated as forearm conversion of lactate to pyruvate – forearm lactate net balance. However, the lactate isotopic technique used has no role in assessing the contribution of skeletal muscle to circulating lactate and hence to lactate available for gluconeogenesis. This contribution is represented by muscle net lactate release (21).

Forearm glucose uptake and net lactate release per 100 ml of tissue were converted to values per kilogram forearm muscle by multiplying the data by 13.3, assuming that 80% of the blood flow to the forearm was directed to muscle tissue and that muscle comprised 60% of the forearm volume (22). Assuming that forearm muscle was representative of muscle elsewhere in the body, these values were multiplied by total body skeletal muscle mass, which was calculated from midarm circumference and triceps skinfold thickness using the equation of Heymsfield et al. (23). Whole-body muscle lactate release was multiplied by the proportion of systemic lactate release that was converted to glucose to calculate its contribution to lactate gluconeogenesis.

Rates of systemic and forearm carbohydrate oxidation were calculated using the equations of Frayn (24), whereby forearm protein oxidation was assumed to be 85 nmol·100 ml–1·min–1 (25).

Statistical analysis

Data are expressed as mean ± SEM. Average data obtained at baseline, and average data obtained during the final 30 min of both clamp periods, during which near steady-states had been achieved, were compared using one-way ANOVA followed by post hoc analyses of parameters that were found to be significantly different. P < 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Plasma hormone and blood glucose concentrations

Infusion of insulin increased plasma insulin to physiologic concentrations (~250 pM) (Fig. 1Go). Arterial blood glucose was maintained at baseline levels during the euglycemic clamp (4.4 ± 0.2 vs. 4.5 ± 0.1 mM at baseline, P = 0.49) and decreased to 2.6 ± 0.1 mM during the hypoglycemic clamp (Fig. 2Go). In the euglycemic clamp, plasma epinephrine, norepinephrine, GH, and cortisol remained unchanged from baseline, whereas plasma glucagon decreased (P < 0.01). Subsequently during the hypoglycemic clamp, all counterregulatory hormones increased significantly (all P < 0.05 vs. euglycemia) (Fig. 1Go).



View larger version (21K):
[in this window]
[in a new window]
  		FIG. 1. Arterial plasma insulin and counterregulatory hormone concentrations during the hyperinsulinemic euglycemic (0–120 min) and hypoglycemic (120–210 min) clamp.

 


View larger version (25K):
[in this window]
[in a new window]
  		FIG. 2. Arterial blood glucose concentrations, endogenous glucose release, and systemic glucose uptake during the hyperinsulinemic euglycemic (0–120 min) and hypoglycemic (120–210 min) clamp.

 
Systemic glucose and lactate kinetics

Endogenous glucose release decreased approximately 60% during the euglycemic clamp and subsequently returned to rates comparable with baseline during hypoglycemia (P = 0.56). Systemic glucose uptake (µmol·kg body weight–1·min–1) increased to 29.6 ± 1.8 during the euglycemic clamp (P < 0.001) and subsequently decreased to 21.0 ± 2.0 during hypoglycemia (P < 0.001 vs. euglycemia) (Fig. 2Go).

Arterial lactate concentrations increased from 544 ± 62 to 907 ± 115 µM during the euglycemic clamp (P < 0.003) and slightly further to 1051 ± 162 µM during the hypoglycemic clamp (P < 0.02 vs. euglycemia). Systemic lactate release (µmol·kg body weight–1·min–1) increased from 13.2 ± 0.8 to 17.1 ± 0.9 during the euglycemic clamp and to 23.5 ± 0.9 during the hypoglycemic clamp (P < 0.001 vs. euglycemia). Systemic lactate uptake increased similarly but slightly later than systemic lactate release so that it was transiently exceeded by lactate release during both clamp periods (Fig. 3Go).



View larger version (25K):
[in this window]
[in a new window]
  		FIG. 3. Arterial blood lactate concentrations, and systemic lactate uptake and release during the hyperinsulinemic euglycemic (0–120 min) and hypoglycemic (120–210 min) clamp.

 
Systemic gluconeogenesis from lactate (µmol·kg body weight–1·min–1) increased from 1.56 ± 0.31 at baseline to 2.74 ± 0.30 during the euglycemic clamp (P < 0.01) and to 4.50 ± 0.60 during the hypoglycemic clamp (P < 0.02 vs. euglycemia) (Fig. 4Go); this accounted for approximately 55 and 40% of endogenous glucose release during eu- and hypoglycemia, respectively. The proportion of lactate released into the circulation that was converted to glucose averaged 32 ± 3% in the euglycemic clamp and 38 ± 4% in the hypoglycemic clamp (P = 0.27 vs. euglycemia).



View larger version (19K):
[in this window]
[in a new window]
  		FIG. 4. Systemic gluconeogenesis from lactate during the hyperinsulinemic euglycemic (0–120 min) and hypoglycemic (120–210 min) clamp.

 
FBF, forearm glucose, and lactate metabolism and extrapolation of forearm data to whole-body skeletal muscle

During the euglycemic clamp, FBF and forearm glucose fractional extraction increased approximately 25% (P < 0.03) and approximately 350% (P < 0.001), respectively, so that forearm glucose uptake was increased more than 5-fold (P < 0.001). Subsequently during the hypoglycemic clamp FBF increased additional approximately 50%, whereas forearm glucose fractional extraction decreased approximately 50% (both P < 0.001 vs. euglycemia). This, in concert with the approximately 40% lower blood glucose concentrations, resulted in approximately 50% reduced forearm glucose uptake, compared with the euglycemic clamp (P < 0.001) (Table 1Go). When these forearm data were extrapolated to whole-body skeletal muscle, the decrement in muscle glucose uptake (7.2 ± 1.2 µmol·kg body weight–1·min–1) accounted for approximately 85% of the decrement in systemic glucose uptake.


View this table:
[in this window]
[in a new window]
  		TABLE 1. FBF, forearm glucose metabolism, forearm net release of gluconeogenic substrates, and forearm glycogen breakdown during the hyperinsulinemic euglycemic-hypoglycemic clamp

 
Forearm lactate release remained unchanged from baseline during the euglycemic clamp (P = 0.75) but increased more than 4-fold (P < 0.01 vs. euglycemia) during the hypoglycemic clamp (Table 1Go). This increase was solely due to increased conversion of pyruvate to lactate (2.46 ± 0.44 vs. 1.14 ± 0.17 µmol per 100 ml–1·min–1, P < 0.005) because the conversion of lactate to pyruvate remained unchanged (0.73 ± 0.10 vs. 0.77 ± 0.08 µmol per 100 ml–1·min–1, P = 0.55).

When forearm lactate data were extrapolated to total body skeletal muscle, whole-body muscle lactate release increased 6.6 ± 1.6 µmol·kg body weight–1·min–1 during the hypoglycemic clamp, compared with the euglycemic clamp, which could have accounted for all of the increase in systemic lactate release. Considering the proportion of lactate used for gluconeogenesis, the increase in lactate release by whole-body skeletal muscle during hypoglycemia contributed 1.4 ± 0.4 µmol·kg body weight–1·min–1 to the increase in endogenous glucose release.

Systemic and forearm carbohydrate oxidation, arterial concentrations and forearm net balance of alanine, and forearm glycogenolysis

Whole-body carbohydrate oxidation (µmol·kg body weight–1·min–1) increased from 7.9 ± 0.7 at baseline to 17.3 ± 0.7 during the euglycemic clamp (P < 0.002) and subsequently decreased to 15.3 ± 1.6 during the hypoglycemic clamp (P < 0.02). Similarly, forearm carbohydrate oxidation increased markedly during the euglycemic clamp (P < 0.01) and subsequently tended to decrease during the hypoglycemic clamp (P = 0.07) (Table 1Go).

Arterial alanine concentrations increased approximately 15% during the euglycemic clamp (P < 0.02 vs. baseline) and subsequently decreased to levels comparable with baseline during the hypoglycemic clamp (P = 0.40). Forearm alanine net balance remained unchanged during the euglycemic clamp but increased subsequently approximately 50% during the hypoglycemic clamp (P < 0.01 vs. euglycemia).

Forearm net glycogen breakdown, calculated as forearm glucose uptake minus the sum of forearm carbohydrate oxidation and net release of lactate and alanine, decreased to negative values during the euglycemic clamp (P < 0.01), indicating net glucose storage. Subsequently during the hypoglycemic clamp, forearm net glycogen breakdown increased (P < 0.001 vs. euglycemia) to rates that were similar to those at baseline (Table 1Go).


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Using a combination of forearm net balance and systemic isotopic techniques, the present study indicates that both reduced glucose uptake and increased lactate release by skeletal muscle are important for glucose counterregulation in humans. Compared with euglycemia, approximately 7.2 µmol·kg body weight–1·min–1, less glucose was taken up by skeletal muscle during hypoglycemia, which accounted for approximately 85% of the reduction in systemic glucose uptake. Approximately 55% of the reduced muscle glucose uptake was due to decreased muscle glucose fractional extraction, whereas the remaining approximately 45% was due to decreased blood glucose concentrations because muscle blood flow actually increased.

In addition, approximately 6.6 µmol·kg body weight–1·min–1 more lactate was released by skeletal muscle during hypoglycemia, compared with euglycemia. Considering the proportion of lactate used for gluconeogenesis, the increase in muscle lactate release contributed approximately 1.4 µmol·kg body weight–1·min–1 or approximately 25% to the increased endogenous glucose release during hypoglycemia. Accordingly, the sum of the reduced muscle glucose uptake and the increased muscle lactate release was responsible for approximately 8.6 µmol·kg body weight–1·min–1 glucose trafficking involved in glucose counterregulation. This value represents a minimum estimate because lactate gluconeogenesis was not corrected for dilution of the lactate label due to Krebs cycle carbon exchange, which has been reported from 15 to 50% (16, 26). If this dilution were taken into consideration, skeletal muscle would have been responsible for approximately 8.9–10.0 µmol·kg body weight–1·min–1 glucose trafficking involved in glucose counterregulation.

Regarding the source of the increased muscle lactate release during hypoglycemia, we found that this was solely due to increased conversion of pyruvate to lactate because conversion of lactate to pyruvate remained unchanged, indicating an increased turnover of the intracellular pyruvate pool. Muscle carbohydrate oxidation was less extensively inhibited than muscle glucose uptake during hypoglycemia, which is in agreement with the corresponding systemic data of the present and previous studies (27). Assuming that proteolysis remained unaltered (28), we calculated that net muscle glucose storage of approximately 1.3 µmol per 100 ml–1 ·min–1 during euglycemia switched to net muscle glycogen breakdown of approximately 0.6 µmol per 100 ml–1·min–1 during hypoglycemia to account for the increased release of lactate and alanine. These findings are consistent with a reduced activity of glycogen synthase and an increased activity of glycogen phosphorylase in human skeletal muscle during glucose counterregulation (27, 29). Animal studies indicate that there is greater lactate release by glycolytic muscle (mainly type IIb fibers) than mixed muscle and the lowest lactate release by oxidative muscle (mainly type 1 fibers) under resting conditions (30), suggesting that type IIb muscle fibers may be most important for lactate release during hypoglycemia.

Our finding that muscle lactate release increased appreciably during hypoglycemia seems to be at variance to previous studies (6, 7, 8), for which there are, however, several possible explanations. First, differences in the experimental design could be involved. For example, as mentioned in the introduction, Abildgaard et al. (6) used arterialized venous blood as opposed to arterial blood for measurements of muscle lactate release, which might have led to an underestimation of muscle lactate release. Moreover, in contrast to the two other previous studies (7, 8), we performed a hyperinsulinemic euglycemic clamp before hypoglycemia, which resulted in net glucose storage by skeletal muscle. Consequently, muscle glycogen content might have been greater immediately before hypoglycemia in the present than in the previous studies, which might have caused increased glycogenolysis with subsequent lactate formation during glucose counterregulation. The conditions of the present study may therefore be more reflective of the postprandial state in which muscle glycogen is increased, whereas those of other studies reflect the postabsorptive state. Second, perhaps due to slightly lower glucose concentrations during the hypoglycemic clamp in the present study, our subjects’ epinephrine responses were approximately 2-fold greater than those in the previous studies (6, 7, 8), which would be expected to cause increased formation of lactate by skeletal muscle (31). Third, it is noteworthy that in the study by Capaldo et al. (7), muscle lactate release during hypoglycemia was approximately 8-fold greater than baseline and nearly 2-fold greater than during control euglycemia, whereas in the study by Abildgaard et al. (6) muscle lactate release was increased more than 10-fold during hypoglycemia, compared with euglycemia. These findings are similar to those of the present study. However, in both of these studies (6, 7), baseline muscle lactate release was substantially lower than has generally been found (8, 22, 25, 32) [which in the case of the study by Abildgaard et al. (6) might have been due to the use of arterialized blood] so that during hypoglycemia, despite the increases of great proportion, amounts of muscle lactate release remained relatively little.

Our finding that gluconeogenesis from lactate increased approximately 75% during the hyperinsulinemic euglycemic clamp may be surprising because insulin would be expected to reduce the expression of key gluconeogenic enzymes. However, whereas the transcriptional effects are rapid, the half-life of the enzyme proteins is quite long, i.e. approximately 6 h for phosphoenolpyruvate carboxykinase (33). Accordingly, the 2-h hyperinsulinemia of the present study might have been too short to noticeably reduce the efficiency of lactate conversion to glucose so that the rate of lactate gluconeogenesis was largely dependent on its availability to liver and kidney. In fact, arterial lactate concentrations increased approximately 70% during the present euglycemic clamp, which is similar to the increase in lactate gluconeogenesis. Moreover, it is possible that Krebs cycle carbon exchange decreased during hyperinsulinemia so that lactate gluconeogenesis was less underestimated during the euglycemic clamp than at baseline. Accordingly, the increase in lactate gluconeogenesis during the euglycemic clamp would have been overestimated.

In postabsorptive humans, gluconeogenesis from lactate and the sum of gluconeogenesis from glutamine, alanine, and glycerol each account for approximately 50% of total systemic gluconeogenesis (12), and hyperinsulinemia less or similar to the present study has been found to suppress gluconeogenesis from the latter substrates 50–80% (34, 35, 36). An increase in lactate gluconeogenesis during short-term hyperinsulinemia, as suggested by the present 2-h euglycemic clamp data, would therefore provide a possible explanation for the general finding that total systemic gluconeogenesis is suppressed very little, if at all, by physiological hyperinsulinemia of similar duration in humans (37, 38). Differential effects of insulin on gluconeogenesis from the different substrates may not be unexpected because, in contrast to lactate, hyperinsulinemia decreases plasma concentrations of glutamine and glycerol and reduces the transport of alanine and glutamine into liver and kidney, respectively (34, 35, 39).

Certain limitations of the methods used need to be taken into consideration besides those mentioned above. First, extrapolation of forearm data to whole body skeletal muscle relies on several assumptions. Errors in these assumptions may therefore change the contribution of skeletal muscle to systemic data and glucose counterregulation. Second, as mentioned above, Krebs cycle carbon exchange might have decreased in the euglycemic clamp so that the increase in lactate gluconeogenesis might have been overestimated; on the other hand, studies in rat hepatocytes indicate that counterregulatory hormones (40) may stimulate Krebs cycle activity. If this were the case in the present study, the increase in lactate gluconeogenesis during hypoglycemia, compared with the euglycemic clamp, would have been underestimated. Third, some imprecision might have resulted from the fact that we assumed similar volumes of distribution and pool fractions for calculations of glucose and lactate turnover during the euglycemic and hypoglycemic clamp. This imprecision can, however, be considered minor because plasma glucose and lactate had reached near steady states at the end of both clamp periods. For example, if the true pool fractions or volumes of distribution were 50% lower during the hypoglycemia clamp than assumed, glucose and lactate turnover would have been underestimated 1.9 and 2.1%, respectively. Lastly, we would like to emphasize that the findings of the present study may not be applicable to other hypoglycemic conditions.

Because of these limitations, our data shall not be considered strictly quantitatively and interpreted as skeletal muscle being the primary tissue responsible for glucose counterregulation in humans. Nevertheless, they do indicate that skeletal muscle plays an important role in these processes. The clinical implication of these findings is that in patients with type 1 diabetes, who generally lack glucagon responses (11), skeletal muscle would be expected to play a disproportionately greater role in glucose counterregulation because stimulation of hepatic glucose production is markedly diminished (11).

In conclusion, the present study provides evidence that: 1) skeletal muscle is largely responsible for the decreased systemic glucose uptake and the increased systemic lactate release during hypoglycemia; 2) both of these processes make considerable contributions to glucose counterregulation; and 3) acutely, physiological hyperinsulinemia paradoxically increases gluconeogenesis from lactate which may provide an explanation for the relatively inefficient suppression of total systemic gluconeogenesis by insulin in humans.


   Acknowledgments
 
We thank Becky Miller for her excellent editorial assistance and the nursing and laboratory staff of the Clinical Research Unit for their superb help.


   Footnotes
 
This work was supported by National Institutes of Health Grant R01-DK56962-01, General Clinical Research Center Grant 5 MO1 RR00044, and an American Diabetes Association Career Development Award.

First Published Online September 6, 2005

Abbreviations: FBF, Forearm blood flow; RA, release of glucose.

Received February 2, 2005.

Accepted August 29, 2005.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Lecavalier L, Bolli G, Cryer P, Gerich J 1989 Contributions of gluconeogenesis and glycogenolysis during glucose counterregulation in normal humans. Am J Physiol 256:E844–E851
  2. Meyer C, Dostou J, Gerich J 1999 Role of the human kidney in glucose counterregulation. Diabetes 48:943–948[Abstract]
  3. Gerich J, Campbell P 1988 Overview of counterregulation and its abnormalities in diabetes mellitus and other conditions. Diabetes Metab Rev 4:93–111[Medline]
  4. Maggs DG, Jacob R, Rife F, Caprio S, Tamborlane WV, Sherwin RS 1997 Counterregulation in peripheral tissues: effect of systemic hypoglycemia on levels of substrates and catecholamines in human skeletal muscle and adipose tissue. Diabetes 46:70–76[Abstract]
  5. Hagstrom-Toft E, Enoksson S, Moberg E, Bolinder J, Arner P 1997 Absolute concentrations of glycerol and lactate in human skeletal muscle, adipose tissue, and blood. Am J Physiol 273:E584–E592
  6. Abildgaard N, Orskov L, Peterson J, Alberti K, Schmitz O, Moller N 1995 Forearm substrate exchange during hyperinsulinemic hypoglycemia in normal man. Diabet Med 12:218–222[Medline]
  7. Capaldo B, Napoli R, Guida R, Di Bonito P, Antoniello S, Auletta M, Pardo F, Rendina V, Sacca L 1995 Forearm muscle insulin resistance during hypoglycemia: role of adrenergic mechanisms and hypoglycemia per se. Am J Physiol 268:E248–E254
  8. Hoffman RP, Sinkey CA, Tsalikian E 1999 Effect of local sympathetic blockade on forearm blood flow and glucose uptake during hypoglycemia. Metabolism 48:1575–1583[CrossRef][Medline]
  9. Jansson PA, Krogstad AL, Lonnroth P 1996 Microdialysis measurements in skin: evidence for significant lactate release in healthy humans. Am J Physiol 271:E138–E142
  10. Jansson PA, Smith U, Lonnroth P 1990 Evidence for lactate production by human adipose tissue in vivo. Diabetologia 33:253–256[CrossRef][Medline]
  11. Cryer PE 2001 Hypoglycemia-associated autonomic failure in diabetes. Am J Physiol 281:E1115–E1121
  12. Meyer C, Stumvoll M, Dostou J, Welle S, Haymond M, Gerich J 2002 Renal substrate exchange and gluconeogenesis in normal postabsorptive humans. Am J Physiol 282:E428–E434
  13. Meyer C, Grossmann R, Mitrakou A, Mahler R, Veneman T, Gerich J, Bretzel R 1998 Effects of autonomic neuropathy on counterregulation and awareness of hypoglycemia in type 1 diabetic patients. Diabetes Care 21:1960–1966[Abstract]
  14. Dillon RS 1965 Importance of the hematocrit in interpretation of blood sugar. Diabetes 14:672–674[Medline]
  15. 1962 Scientific tables. 6th ed. Ardsley, NY: Geigy Pharmaceuticals
  16. Wolfe R 1992 Radioactive and stable isotope tracers in biomedicine: principles and practice of kinetic analysis. New York: Wiley-Liss
  17. DeBodo R, Steele R, Dunn A, Bishop J 1963 On the hormonal regulation of carbohydrate metabolism: studies with 14C glucose. Rec Prog Horm Res 19:445–448
  18. Issekutz B, Shaw W, Issekutz A 1976 Lactate metabolism in resting and exercising dogs. J Appl Physiol 40:312–319[Abstract/Free Full Text]
  19. Radziuk J, Norwich K, Vranic M 1974 Measurement and validation of nonsteady-state turnover rates with applications to insulin and glucose systems. Fed Proc 33:1855–1864[Medline]
  20. Chiasson J, Liljenquist J, Lacy W, Jennings A, Cherrington A 1977 Gluconeogenesis: methodological approaches in vivo. Fed Proc 36:229–235[Medline]
  21. Landau B, Wahren J 1992 Nonproductive exchanges: the use of isotopes gone astray. Metabolism 41:457–459[CrossRef][Medline]
  22. Consoli A, Nurjhan N, Gerich J, Mandarino L 1992 Skeletal muscle is a major site of lactate uptake and release during hyperinsulinemia. Metabolism 41:176–179[CrossRef][Medline]
  23. Heymsfield S, McManus C, Stevens V, Smith J 1982 Muscle mass: reliable indicator of protein-energy malnutrition seventy and outcome. Am J Clin Nutr 35:1192–1199[Free Full Text]
  24. Frayn K 1983 Calculation of substrate oxidation rates in vivo from gaseous exchange. J Appl Physiol 55:628–634[Abstract/Free Full Text]
  25. Kelley DE, Reilly JP, Veneman T, Mandarino LJ 1990 Effects of insulin on skeletal muscle glucose storage, oxidation, and glycolysis in humans. Am J Physiol 258:E923–E929
  26. Magnusson I, Schumann W, Bartsch G, Chandramouli V, Kumaran K, Wahren J, Landau B 1991 Noninvasive tracing of Krebs cycle metabolism in liver. J Biol Chem 266:6975–6984[Abstract/Free Full Text]
  27. Orskov L, Bak JF, Abildgard, Schmitz O, Andreasen F, Richter EA, Skjaerbaek C, Moller N 1996 Inhibition of muscle glycogen synthase activity and non-oxidative glucose disposal during hypoglycaemia in normal man. Diabetologia 39:226–234[Medline]
  28. Battezzati A, Benedini S, Fattorini A, Piceni Sereni L, Luzi L 2000 Effect of hypoglycemia on amino acid and protein metabolism in healthy humans. Diabetes 49:1543–1551[Abstract]
  29. Cohen N, Rossetti L, Shlimovich P, Halberstam M, Hu M, Shamoon H 1995 Counterregulation of hypoglycemia: skeletal muscle glycogen metabolism during three hours of physiologic hyperinsulinemia in humans. Diabetes 44:423–430[Abstract]
  30. Pagliassotti MJ, Donovan CM 1990 Influence of cell heterogeneity on skeletal muscle lactate kinetics. Am J Physiol 258:E625–E634
  31. Raz I, Katz A, Spencer MK 1991 Epinephrine inhibits insulin-mediated glycogenesis but enhances glycolysis in human skeletal muscle. Am J Physiol 260:E430–E435
  32. Stanley W, Gertz E, Wisneski J, Neese R, Morris D, Brooks G 1986 Lactate extraction during net lactate release in legs of humans during exercise. J Appl Physiol 60:1116–1120[Abstract/Free Full Text]
  33. Boden G 2003 Effects of free fatty acids on gluconeogenesis and glycogenolysis. Life Sci 72:977–988[CrossRef][Medline]
  34. Cersosimo E, Garlick P, Ferretti J 1999 Insulin regulation of renal glucose metabolism in humans. Am J Physiol 276:E78–E84
  35. Meyer C, Dostou J, Nadkarni V, Gerich J 1998 Effects of physiological hyperinsulinemia on systemic, renal and hepatic substrate metabolism. Am J Physiol 275:F915–F921
  36. Chiasson J, Atkinson R, Cherrington A, Keller U, Sinclair-Smith B, Lacy W, Liljenquist J 1980 Effects of insulin at two dose level on gluconeogenesis from alanine in fasting man. Metabolism 29:810–818[CrossRef][Medline]
  37. Adkins A, Basu R, Persson M, Dicke B, Shah P, Vella A, Schwenk WF, Rizza R 2003 Higher insulin concentrations are required to suppress gluconeogenesis than glycogenolysis in nondiabetic humans. Diabetes 52:2213–2220[Abstract/Free Full Text]
  38. Gastaldelli A, Toschi E, Pettiti M, Frascerra S, Quinones-Galvan A, Sironi A, Natali A, Ferrannini E 2001 Effect of physiological hyperinsulinemia on gluconeogenesis in nondiabetic subjects and in type 2 diabetic patients. Diabetes 50:1807–1812[Abstract/Free Full Text]
  39. Felig P 1975 Amino acid metabolism in man. Ann Rev Biochem 44:933–955[CrossRef][Medline]
  40. Oberhaensli RD, Schwendimann R, Keller U 1985 Effect of norepinephrine on ketogenesis, fatty acid oxidation, and esterification in isolated rat hepatocytes. Diabetes 34:774–779[Abstract]



This article has been cited by other articles:


Home page
 J. Clin. Endocrinol. Metab.Home page
T. Linn, B. Fischer, N. Soydan, M. Eckhard, J. Ehl, C. Kunz, and R. G. Bretzel
Nocturnal Glucose Metabolism after Bedtime Injection of Insulin Glargine or Neutral Protamine Hagedorn Insulin in Patients with Type 2 Diabetes
J. Clin. Endocrinol. Metab., October 1, 2008; 93(10): 3839 - 3846.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow A correction has been published
Right arrow All Versions of this Article:
90/11/6244    most recent
Author Manuscript (PDF)
Right arrow Submit a related Letter to the Editor
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Meyer, C.
Right arrow Articles by Linn, T.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Meyer, C.
Right arrow Articles by Linn, T.
Related Collections
Right arrow Diabetes and Insulin
Right arrow Metabolism

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Endocrinology Endocrine Reviews J. Clin. End. & Metab.
Molecular Endocrinology Recent Prog. Horm. Res. All Endocrine Journals