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The Journal of Clinical Endocrinology & Metabolism Vol. 83, No. 7 2461-2469
Copyright © 1998 by The Endocrine Society

From the Clinical Research Centers

Overnight Normalization of Glucose Concentrations Improves Hepatic But Not Extrahepatic Insulin Action in Subjects with Type 2 Diabetes Mellitus1

Steven D. Wise, Michael F. Nielsen, Philip E. Cryer and R. A. Rizza

Division of Endocrinology, Mayo Clinic and Foundation, Rochester, Minnesota 55905; and the Division of Endocrinology, Diabetes, and Metabolism, Washington University (P.E.C.), St. Louis, Missouri 63130

Address all correspondence and requests for reprints to: Dr. Robert A. Rizza, Endocrine Research Unit, Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905. E-mail: rizza.robert{at}mayo.edu


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Subjects with poorly controlled type 2 diabetes are both hyperglycemic and insulin resistant. To determine whether short term restoration of normoglycemia improves insulin action, hyperinsulinemic (~300 pmol/L) euglycemic clamps were performed in diabetic subjects after either overnight infusion of saline or overnight infusion of insulin in amounts sufficient to maintain euglycemia throughout the night. Fasting glucose concentrations (5.2 ± 0.2 vs. 11.9 ± 1.4 mmol/L; P < 0.01) and rates of endogenous glucose production (13.0 ± 1.1 vs. 18.6 ± 1.6 µmol/kg·min; P < 0.05) were both lower after overnight insulin than overnight saline. Insulin-induced stimulation of glucose uptake (to 34.9 ± 6.8 vs. 28.8 ± 3.4 µmol/kg·min; P = 0.2) and inhibition of free fatty acids (to 0.13 ± 0.03 vs. 0.12 ± 0.04 mmol/L; P = 0.6) did not differ after overnight saline and overnight insulin. In contrast, endogenous glucose production during the final hour of the hyperinsulinemic clamps (i.e. when glucose concentrations were the same) remained higher (P = 0.05) after overnight saline than after overnight insulin (5.5 ± 1.5 vs. 0.02 ± 1.4 µmol/kg·min). Thus, acute restoration of euglycemia by means of an overnight insulin infusion improves hepatic (and perhaps renal) but not extrahepatic insulin action.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
SUBJECTS with type 2 diabetes commonly have elevated glucose concentrations. Although there is mounting evidence that insulin action is influenced by genetic factors, hyperglycemia per se can cause insulin resistance (1, 2, 3, 4, 5). This phenomenon has been referred to as glucose toxicity. We as well as others have shown that chronic (weeks to months) improvement in glycemic control is associated with an improvement in insulin action (6, 7, 8, 9). On the other hand, it is currently not known whether shorter term (e.g. overnight) changes in glucose concentration also can alter insulin action in type 2 diabetes. Several studies have suggested that this might be the case. Fasching et al. have shown that overnight normalization of glucose enhances insulin-induced stimulation of glucose uptake in subjects with type 1 diabetes (10). However, the effects were modest and only statistically significant in individuals who had excellent antecedent chronic glycemic control. Perhaps more intriguing is the recent report of Zierath et al. Those researchers demonstrated that incubation of muscle obtained by biopsy from subjects with type 2 diabetes with a glucose concentration of 4 mmol/L for only 2 h virtually normalized insulin-stimulated glucose transport (11). This observation, albeit from in vitro studies, implies that hyperglycemia may cause much, if not all, of the peripheral insulin resistance that is associated with poorly controlled type 2 diabetes. Even less is known regarding the effects of short term changes in glucose concentration on hepatic insulin action. This is probably due at least in part to past difficulties in accurately measuring endogenous glucose production during hyperinsulinemic clamps (12, 13, 14).

If insulin action in subjects with type 2 diabetes is influenced by short term changes in glucose concentration, this may account for the subtle (and at times not so subtle) differences in experimental outcomes observed in various clinical studies depending on whether glucose concentrations were lowered the night before insulin action was assessed (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30). In addition, if normalization of glucose during the night improves insulin action on the subsequent day, then this might also help explain the success of therapeutic programs that use evening injections of intermediate or long acting insulin preparations to maintain nocturnal euglycemia (31, 32, 33). On the other hand, nocturnal insulin administration may further elevate plasma insulin concentrations, which, in turn, may impair rather than improve insulin action (34, 35, 36, 37).

The present studies, therefore, were undertaken to determine whether short term (i.e. overnight) restoration of eu-glycemia alters insulin action in subjects with type 2 diabetes. On one occasion, glucose concentrations were permitted to remain at ambient hyperglycemic levels throughout the night, whereas on the other occasion, euglycemia was achieved by means of an overnight insulin infusion. Insulin action was assessed the following morning on each occasion using the hyperinsulinemic euglycemic clamp technique. We report that overnight insulin infusion, and its resultant euglycemia, enhances insulin-induced suppression of glucose release but does not alter insulin-induced stimulation of glucose uptake, implying an improvement in hepatic but not extrahepatic insulin action.


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

After approval from the Mayo Institutional Review Board, nine healthy obese diabetic volunteers [five men and four women; height, 1.71 ± 0.05 m; weight, 79 ± 6 kg; lean body mass (lbm), 51 ± 5 kg; glycosylated hemoglobin, 10.3 ± 1.7%] gave informed written consent to participate in the study. Treatment of diabetes before the study consisted of diet (n = 1), oral hypoglycemic agents (n = 5), or insulin (n = 3). Oral agents were discontinued at least 2 weeks before each study, and long-acting insulin (NPH) was discontinued at least 48 h before each study. All other over the counter medications (e.g. acetaminophen or cold preparations) were discontinued at least 3 weeks before each study.

Experimental design

All subjects were studied on two occasions separated by at least 10 days. On each occasion, subjects were admitted to the General Clinical Research Center the evening before the study. After placement of an 18-gauge catheter in a forearm vein, each subject ingested a standard mixed meal (15.2 Cal/kg lbm; 55% carbohydrate, 30% fat, and 15% protein) between 1800–1830 h. Additional carbohydrate-containing snacks (4.18 Cal/kg each) were ingested at 2200 and 2400 h. An infusion of either insulin (100 U/liter) or saline was started at the beginning of supper and continued throughout the night. Insulin was adjusted so as to maintain euglycemia throughout the night (38). The order of study was random.

An 18-gauge catheter was placed in a retrograde manner in a dorsal hand vein at 0600 h the following morning, and the hand was placed in a heated unit to provide arterialized venous blood samples. Immediately thereafter, primed continuous infusions of NaH14CO3 (100 µCi; 1 µCi/min; New England Nuclear, Boston, MA) and [6,6-2H2]glucose (3 mg/kg lbm and 0.03 mg/kg lbm·min in four subjects; 6 mg/kg lbm·min and 0.06 mg/kg lbm·min in five subjects; Cambridge Isotope Laboratories, Andover, MA) were started and continued for the remainder of each experiment. The overnight infusions of insulin or saline were discontinued at 1000 h, when an unprimed, continuous infusion of insulin (Humulin R, Eli Lilly Co., Indianapolis, IN) was initiated at a rate of 0.8 mU/kg lbm·min and continued for the next 300 min. Insulin was mixed in 0.9% saline with 0.1% human serum albumin (Miles, Elkhart, IN). Euglycemia was maintained by means of a variable glucose infusion (39). To maintain plasma enrichment constant, all infused glucose was enriched with [6,6-2H2]glucose, and the continuous infusion of [6,6-2H2] glucose was decreased so as to approximate the anticipated rate of fall in endogenous glucose production (29). Blood and expired air were collected at regular intervals as previously described (40). Urine was collected during the hyperinsulinemic clamp for measurement of urinary nitrogen excretion.

Analytical techniques

Blood was placed on ice, centrifuged at 4 C, separated, and stored at -20 C until assay. The glucose concentration was measured using the glucose oxidase method (Yellow Springs Instrument Co., Yellow Springs, OH). Plasma insulin, C peptide, and glucagon concentrations were measured by RIA (Linco Research, St. Louis, MO). The GH concentration also was measured by RIA using a kit from ICN Biomedicals (Costa Mesa, CA). Plasma free fatty acid concentrations were measured using a colorimetric assay (Wako Pure Chemical Industries, Osaka, Japan). Indirect calorimetry was performed with a Deltatrac Metabolic Monitor (SensorMedics Corp., Yorba Linda, CA), and rates of carbohydrate and fat oxidation were calculated using the equations of Frayn (41). The lbm and percent body fat were measured by dual photon absorptiometry (Hologic, Waltham, MA). Plasma [6,6-2H2]glucose enrichment was measured using mass spectrometry (42). Breath 14CO2 and plasma [14C]glucose specific activity were measured using liquid scintillation counting.

Calculations

The plasma atom percent enrichment of [6,62H2] glucose was smoothed using the optimal segments program of Bradley et al. (43). Glucose appearance and disappearance were calculated using the equations of Steele (44). The volume of distribution of glucose was assumed to equal 200 mL/km, with a pool correction factor of 0.65. The percentage of glucose derived from 14CO2 was calculated by dividing the specific activity of [14C]glucose in plasma by the specific activity of 14CO2 in breath as previously described (40). The advantages and limitations of this method in the estimation of gluconeogenesis have been discussed in detail previously (40, 45, 46).

Statistical analysis

All data are expressed as the mean ± SEM. Rates of turnover are expressed per kg lbm/min. Values during the 30 min before the start of the hyperinsulinemic glucose clamp (i.e. -30 to 0 min) were meaned and considered as basal values. Statistical analysis was performed using Student’s two-tailed paired t test. P < 0.05 was considered statistically significant.


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

Glucose concentrations (Fig. 1Go) before supper did not differ in the diabetic subjects on the overnight saline and insulin study days (13.4 ± 3.3 vs. 12.2 ± 2.3 mmol/L; P = 0.45). Glucose concentrations remained elevated throughout the night during the saline infusion, but fell during the insulin infusion to 8.1 ± 1.2 mmol/L by 0030 h and to 4.9 ± 0.3 mmol/L by 0430 h. Glucose concentrations immediately before initiation of the hyperinsulinemic clamps (i.e. -30 to 0 min) were higher (P < 0.01) after overnight saline than after overnight insulin (11.9 ± 1.4 vs. 5.2 ± 0.2 mmol/L). Glucose concentrations fell during the first portion of the hyperinsulinemic clamp after the overnight saline infusion, reaching values that no longer differed from those observed after overnight insulin by 180 min. In contrast, glucose concentrations were maintained at about 5.5 mmol/L throughout the hyperinsulinemic clamp on the overnight insulin study day. Glucose concentrations during the final hour of the hyperinsulinemic clamps (i.e. 240–300 min) did not differ on the overnight saline and overnight insulin study days (5.7 ± 0.2 vs. 5.5 ± 0.1 mmol/L; P = 0.38).



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  		Figure 1. Glucose concentrations in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at 1000 h (time zero).

 
Insulin, C peptide, glucagon, and GH concentrations

Plasma insulin concentrations were comparable both before and during the hyperinsulinemic clamps on the 2 study days (Fig. 2Go, upper panel). Fasting C peptide concentrations were lower (P < 0.001) after overnight insulin than after overnight saline infusion (0.17 ± 0.04 vs. 0.39 ± 0.06 mmol/L). C Peptide concentrations fell during the first part of the hyperinsulinemic clamp on the saline study day in parallel with the fall in plasma glucose concentrations to values that no longer differed from those observed after overnight insulin (Fig. 2Go, middle panel). Glucagon concentrations were lower (P < 0.05) after overnight insulin than after overnight saline (57.7 ± 3.1 vs. 73.1 ± 5.8). Glucagon concentrations fell during the first portion of hyperinsulinemic clamp on the overnight saline study day to levels that although no longer statistically different (P = 0.06), still tended to be higher than those observed on the overnight insulin study day (Fig. 2Go, bottom panel). GH concentrations did not differ after overnight saline and insulin infusions either before (3.7 ± 1.0 vs. 5.9 ± 1.4 pg/mL) or during (8.9 ± 2.0 vs. 9.1 ± 2.5 pg/mL) the hyperinsulinemic clamps (data not shown).



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  		Figure 2. Insulin, C peptide, and glucagon concentrations in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 
Plasma free fatty acid, epinephrine, and norepinephrine concentrations

Plasma free fatty acid concentrations were lower (P < 0.01) after overnight insulin than after overnight saline (0.51 ± 0.08 vs. 0.74 ± 0.04). Fatty acid concentrations were promptly suppressed during the hyperinsulinemic clamps to comparable levels on both study days (Fig. 3Go). Plasma epinephrine and norepinephrine concentrations did not differ before or during the hyperinsulinemic clamps on either study day (Fig. 4Go).



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  		Figure 3. Free fatty acid concentrations in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 


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  		Figure 4. Plasma, norepinephrine, and epinephrine concentrations in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 
[6,6-2H]Glucose atom percent enrichment

The plasma [6,6-2H2]glucose atom percent enrichment was higher (P < 0.001) after overnight insulin than after overnight saline, consistent with lower rates of endogenous glucose production. As all infused glucose contained [6,6-2H2]-glucose, plasma enrichment remained constant during the hyperinsulinemic clamps on both study days (Fig. 5Go).



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  		Figure 5. Atom percent enrichment of [6,6-2H2]glucose in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 
Glucose disappearance, endogenous glucose production, and percentage of plasma glucose derived from 14CO2

Glucose disappearance was higher (P < 0.01) after overnight infusion of saline than after overnight infusion of insulin (21.0 ± 2.1 vs. 13.5 ± 1.3 µmol/kg·min). Glucose disappearance increased during the hyperinsulinemic clamps on both study days (Fig. 6Go, upper panel). Glucose disappearance during the final hour of the clamps (i.e. when glucose concentrations were the same) did not differ on the overnight saline and overnight insulin study days (34.9 ± 6.8 vs. 28.8 ± 3.4 µmol/kg·min; P = 0.19).



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  		Figure 6. Rates of glucose disappearance and endogenous glucose production in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 
Despite the presence of higher plasma glucose concentrations, endogenous glucose production was higher (P < 0.05) after overnight saline than after overnight insulin (18.6 ± 1.6 vs. 13.0 ± 1.1 µmol/kg·min). Endogenous glucose production fell during the hyperinsulinemic clamps on both study days (Fig. 6Go, lower panel). However, endogenous glucose production during the final hour of the hyperinsulinemic clamps (i.e. when glucose concentrations were the same) remained higher (P < 0.05) after overnight saline than after overnight insulin (5.5 ± 1.5 vs. 0.02 ± 1.4 µmol/kg·min), indicating an impairment in hepatic (and perhaps renal) insulin action.

The rate of incorporation of 14CO2 into glucose was used as an index of gluconeogenesis. The percentage of plasma glucose derived from 14CO2 did not differ after overnight saline or overnight insulin infusion (Fig. 7Go). The percentage of plasma glucose derived from 14CO2 decreased (P < 0.01) during the hyperinsulinemic clamp on the overnight insulin study day, but remained essentially unchanged (P = 0.2) on the overnight saline study day. The percentage of plasma glucose derived from 14CO2 during the final hour of the clamp tended (P = 0.09) to be slightly greater on the overnight saline than on the overnight insulin study day.



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  		Figure 7. Percent incorporation of 14CO2 into glucose in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 
Glucose oxidation and storage

Glucose oxidation did not differ before or during the clamps on either study day (Fig. 8Go). As basal rates of glucose disappearance were higher after overnight saline, this resulted in basal rates of nonoxidative glucose storage (calculated by subtracting glucose oxidation from glucose disappearance) that also were higher (P < 0.05) after overnight saline than after overnight insulin (11.7 ± 3.1 vs. 3.3 ± 3.0 µmol/kg·min). Rates of nonoxidative storage no longer differed during the clamps on the 2 study days, presumably due to a decrease in the mass action effect of glucose on glycogen synthesis and/or glycolysis (the components of nonoxidative storage) as the glucose concentration fell to the euglycemic range on the saline day. Rates of lipid oxidation also did not differ either before (10.1 ± 0.8 vs. 9.3 ± 1.0 µmol/kg·min) or during the final hour of the clamps (5.6 ± 1.1 vs. 4.7 ± 0.9 µmol/kg·min) on the saline and insulin infusion study days.



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  		Figure 8. Rates of glucose oxidation and storage in diabetic subjects after an overnight infusion of either saline ({circ}) or insulin (•). A hyperinsulinemic clamp was initiated at time zero.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Both hyperglycemia and hyperinsulinemia can cause insulin resistance (2, 3, 35, 37, 47). The present studies demonstrate that restoration of euglycemia in subjects with type 2 diabetes by means of an overnight insulin infusion not only lowers fasting glucose production, but also results in greater insulin-induced suppression of glucose production. In contrast, maintenance of euglycemia during the night did not alter the ability of insulin to stimulate glucose uptake. Thus, short term normalization of glucose concentration during the night results in an improvement in hepatic (and perhaps renal) but not extrahepatic insulin action.

Both in vitro and in vivo studies have shown that chronic hyperglycemia impairs insulin action (2, 3, 4, 5). This has been referred to as glucose toxicity. Most studies have focused on the effects of hyperglycemia on glucose uptake (2, 3, 4, 5, 48). Lowering of glucose concentrations using phlorizin, metformin, sulfonylureas, or insulin all have been associated with an improvement in insulin-induced stimulation of glucose uptake (4, 5, 7, 8, 49, 50, 51). However, these studies have examined insulin action after weeks to months of therapy. The present studies only lowered glucose concentration during the night. Based on the previous report by Zierath et. al. (11), we anticipated that this duration would be sufficient to improve extrahepatic insulin action. Those investigators demonstrated that insulin action was restored to normal within 2 h when muscle obtained from people with type 2 diabetes was incubated with glucose at a concentration of approximately 4 mmol/L (11). In the present studies, glucose did not reach euglycemia until about 0430 h. However, as the clamps were not started until 5.5 h later, the duration of euglycemia should have been adequate to improve insulin action if results from in vitro incubations truly reflect in vivo responses. However, the same studies showed that only a minimal elevation of glucose (~8 mmol) caused insulin resistance in muscle obtained from diabetic individuals, whereas it had no effect on insulin action in muscle from nondiabetic subjects (11). Thus, it is possible that glucose concentrations lower then the approximately 5 mmol/L achieved in the present experiments are required to rapidly improve insulin action in muscle. If so, it will be difficult to consistently achieve this goal in most patients with type 2 diabetes.

Although overnight insulin infusion did not enhance insulin-induced stimulation of glucose uptake, it did improve insulin-induced suppression of glucose production. Endogenous glucose production was lower after overnight insulin than overnight saline infusion. As glucose concentrations also were lower, this implied an enhanced response to insulin. This conclusion was confirmed by the observation that glucose production was lower during the final hour of the hyperinsulinemic clamp (i.e. when glucose concentrations were equal) on the overnight insulin study day. Whereas glucose uptake in peripheral tissues appears to be regulated primarily by glucose transporter number or function, intrahepatic enzyme activity is the major determinant of glucose production (52, 53). Poorly controlled diabetes is associated with an increase in flux through the gluconeogenic and glucose-6-phosphatase pathways (54, 55, 56, 57). Both processes are regulated by insulin and glucose concentration (58, 59). Rossetti et al. have shown that reversal of hyperglycemia in rats by administration of phlorizin in itself rapidly lowers glucose-6-phosphatase activity (56). In the present studies, insulin administration resulted in a fall in both glucose and glucagon concentrations. We, therefore, cannot determine whether the improvement in hepatic insulin action was due to a direct effect of insulin on the liver or was secondary to a lower glucose concentration per se alone or in combination with a lower glucagon concentration.

Elevated free fatty acids can stimulate gluconeogenesis and can impair insulin-induced suppression of endogenous glucose production (60, 61). Free fatty acid concentrations were lower after overnight insulin than after overnight saline infusion. This potentially could have contributed to the reduction in fasting endogenous glucose production. However, the comparable rates of incorporation of 14CO2 into glucose would argue against a major difference in the contribution of gluconeogenesis to glucose production. In addition, differences in free fatty acid concentrations are unlikely to explain differences in hepatic response to insulin, as free fatty acid levels were virtually identical during the hyperinsulinemic clamps on the 2 study days. Regardless of the mechanism, lowering of fasting endogenous glucose production and improving hepatic insulin action have important clinical implications, as excessive glucose release contributes to both postabsorptive and postprandial hyperglycemia in patients with type 2 diabetes.

Hyperinsulinemic euglycemic clamps are widely used to assess insulin action and are considered by many investigators as the "gold standard" against which other methods are compared. However, euglycemic clamps are not always performed in the same manner (15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30). In many instances, glucose concentrations in diabetic subjects are elevated at the time of the initiation of the clamp. In this circumstance, glucose concentrations are permitted to fall until they reach the euglycemic range, where they are then maintained by means of an exogenous glucose infusion. This was the approach used after the overnight saline infusion. Although insulin action can be measured during the final euglycemic portion of the clamp, it cannot be assessed during the first several hours of the clamp because the effects of insulin cannot be distinguished from those of the falling glucose concentration. Although epinephrine and norepinephrine concentrations remained essentially constant in the present studies, interpretation of clamps that necessitate a rapid fall in glucose concentration also potentially can be confounded by an increase in counterregulatory hormone secretion. An alternative tactic is to normalize glucose concentrations in diabetic subjects before initiation of the euglycemic clamp. Although this approach avoids the uncertainty introduced by differences in baseline glucose concentrations, at least in humans, it requires administration of insulin, which in itself may alter insulin action (34, 35, 36, 37). Until now it has not been clear whether the two approaches provide concordant measures of insulin action. The present experiments indicate that they provide concordant of the effects of insulin on glucose oxidation and disappearance but not for the effects of insulin on endogenous glucose production. Although nondiabetic controls were not studied, we and others have shown that hepatic and extrahepatic insulin resistance persists in diabetic subjects after overnight insulin infusion (24, 25, 27, 28, 29, 30). Thus, overnight insulin infusion and its resultant euglycemia improve, but do not totally correct, the hepatic response to insulin. We have not performed time- and dose-response curves and therefore do not know how marked a change in nocturnal glucose concentration is required to alter the hepatic response to insulin. Nevertheless, it probably would be prudent to be sure that fasting glucose concentrations are as consistent as possible from study to study if insulin action (whatever the method used to measure it) is to be compared between groups of diabetic subjects or over time in one group of diabetic subjects.

The present study has several limitations. We only measured plasma glucose concentrations at 2-h intervals during the night. Although it is possible that the diabetic subjects experienced asymptomatic hypoglycemia, we doubt that this occurred because blood glucose concentrations also were measured with a reflectance meter every 30–60 min to adjust the insulin infusion rate, and no episodes of hypoglycemia were detected. Furthermore, we would anticipate that hypoglycemia would impair, rather than improve, the subsequent hepatic response to insulin. We only measured insulin action at insulin concentrations of about 300 pmol/L. We chose this concentration because we have previously shown that it causes submaximal stimulation of glucose uptake and submaximal suppression of glucose production (39, 62). It also approximates insulin concentrations commonly observed in patients with type 2 diabetes after food ingestion (63, 64, 65, 66, 67). Nevertheless, it remains possible that different results would have been observed if we had used a different insulin infusion rate. Of note, the improvement in insulin action that has been demonstrated after chronic improvement in glycemic control has been evident primarily at high insulin concentrations (7, 8, 51, 68). The physiological significance of an increase in glucose uptake at high insulin concentrations is open to question. We infused insulin into the peripheral, rather than the portal, venous circulation, which perhaps accounts for the lower free fatty acid concentrations after overnight insulin than after overnight saline infusion. Although this should not alter our ability to measure insulin action in peripheral tissues, it may have obscured a difference in hepatic response. Thus, we may have underestimated the degree of improvement in hepatic insulin action that occurred after overnight insulin infusion. We only measured the systemic rate of appearance of glucose. Although we refer to the improvement as hepatic, enhanced insulin-induced suppression of renal glucose production also may have occurred (69, 70).

In summary, acute restoration of euglycemia by means of an overnight insulin infusion improves hepatic (and perhaps renal) but not extrahepatic insulin action. This observation not only has implications for the design of studies examining insulin action but also may explain the remarkable success of therapeutic programs that use insulin to achieve and maintain nocturnal euglycemia. It also may account for the clinical observation that it is difficult to achieve adequate glycemic control during the day if the patient begins the morning hyperglycemic.


   Acknowledgments
 
We thank P. Berg, T. Madson, D. Nash, and C. Nordyke for technical assistance; A. Wagner for assistance with the preparation of the manuscript; and the staff of the Mayo General Clinical Research Center for their assistance with performing the studies.


   Footnotes
 
1 This work was supported by the USPHS (Grants DK-29953 and RR-00585), the Mayo Foundation, and research grants from the Danish Medical Research Council, the University of Aarhus, and the Institute for Experimental Clinical Research, Aarhus University Hospital (Aarhus, Denmark; to M.F.N.). Back

Received January 12, 1998.

Revised March 27, 1998.

Accepted April 6, 1998.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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