Stefano Bevilacqua

Insulin Resistance in Essential Hypertension

High blood pressure is prevalent in obesity and in diabetes, both conditions with insulin resistance. To test whether hypertension is associated with insulin resistance independently of obesity and glucose intolerance, we measured insulin sensitivity (using the euglycemic insulin-clamp technique), glucose turnover (using [3H]glucose isotope dilution), and whole-body glucose oxidation (using indirect calorimetry) in 13 young subjects (38 +/- 2 years [+/- SEM]) with untreated essential hypertension (165 +/- 6/112 +/- 3 mm Hg), normal body weight, and normal glucose tolerance. In the postabsorptive state, all measures of glucose metabolism were normal. During steady-state euglycemic hyperinsulinemia (about 60 microU per milliliter), hepatic glucose production and lipolysis were effectively suppressed, and glucose oxidation and potassium disposal were normally stimulated. However, total insulin-induced glucose uptake was markedly impaired (3.80 +/- 0.32 vs. 6.31 +/- 0.42 mg per minute per kilogram of body weight in 11 age- and weight-matched controls, P less than 0.001). Thus, reduced nonoxidative glucose disposal (glycogen synthesis and glycolysis) accounted for virtually all the defect in overall glucose uptake (1.19 +/- 0.24 vs. 3.34 +/- 0.44 mg per minute per kilogram, P less than 0.001). Total glucose uptake was inversely related to systolic or mean blood pressure (r = 0.76 for both, P less than 0.001). These results provide preliminary evidence that essential hypertension is an insulin-resistant state. We conclude that this insulin resistance involves glucose but not lipid or potassium metabolism, is located in peripheral tissues but not the liver, is limited to nonoxidative pathways of intracellular glucose disposal, and is directly correlated with the severity of hypertension.

Effect of fatty acids on glucose production and utilization in man.

Since the initial proposal of the glucose fatty acid cycle, considerable controversy has arisen concerning its physiologic significance in vivo. In the present study, we examined the effect of acute, physiologic elevations of FFA concentrations on glucose production and uptake in normal subjects under three controlled experimental conditions. In group A, plasma insulin levels were raised and maintained at approximately 100 microU/ml above base line by an insulin infusion, while holding plasma glucose at the fasting level by a variable glucose infusion. In group B, plasma glucose concentration was raised by 125 mg/100 ml and plasma insulin was clamped at approximately 50 microU/ml by a combined infusion of somatostatin and insulin. In group C, plasma glucose was raised by 200 mg/100 ml above the fasting level, while insulin secretion was inhibited with somatostatin and peripheral glucagon levels were replaced with a glucagon infusion (1 ng/min X kg). Each protocol was repeated in the same subject in combination with a lipid-heparin infusion designed to raise plasma FFA levels by 1.5-2.0 mumol/ml. With euglycemic hyperinsulinemia (study A), lipid infusion caused a significant inhibition of total glucose uptake (6.3 +/- 1.3 vs. 7.4 +/- 0.6 mg/min X kg, P less than 0.02). Endogenous glucose production (estimated by the [3-3H]glucose technique) was completely suppressed both with and without lipid infusion. With hyperglycemic hyperinsulinemia (study B), lipid infusion also induced a marked impairment in glucose utilization (6.2 +/- 1.1 vs. 9.8 +/- 1.9 mg/min X kg, P less than 0.05); endogenous glucose production was again completely inhibited despite the increase in FFA concentrations. Under both conditions (A and B), the percentage inhibition of glucose uptake by FFA was positively correlated with the total rate of glucose uptake (r = 0.69, P less than 0.01). In contrast, when hyperglycemia was associated with relative insulinopenia and hyperglucagonemia (study C), thus simulating a diabetic state, lipid infusion had no effect on glucose uptake (2.9 +/- 0.2 vs. 2.6 +/- 0.2 mg/min X kg) but markedly stimulated endogenous glucose production (1.4 +/- 0.5 vs. 0.5 +/- 0.4 mg/min X kg, P less than 0.005). Under the same conditions as study C, a glycerol infusion producing plasma glycerol levels similar to those achieved with lipid-heparin, enhanced endogenous glucose production (1.5 +/- 0.5 vs. 0.7 +/- 0.6 mg/min X kg, P less than 0.05). We conclude that, in the well-insulinized state raised FFA levels effectively compete with glucose for uptake by peripheral tissues, regardless of the presence of hyperglycemia. When insulin is deficient, on the other hand, elevated rates of lipolysis may contribute to hyperglycemia not by competition for fuel utilization, but through an enhancement of endogenous glucose output.

Acute elevation of free fatty acid levels leads to hepatic insulin resistance in obese subjects

Raised levels of free fatty acids (FFA) compete with glucose for utilization by insulin-sensitive tissues, and, therefore, they may induce insulin resistance in the normal subject. The influence of experimental elevations in FFA levels on glucose metabolism in native insulin-resistant states is not known. We studied seven women with moderate obesity (63% above their ideal body weight) but normal glucose tolerance with the use of the insulin clamp technique with or without an infusion of Intralipid + heparin. Upon raising plasma insulin levels to approximately 60 microU/mL while maintaining euglycemia, whole body glucose utilization (3H-3-glucose) rose similarly without (from 66 +/- 7 to 113 +/- 11 mg/min m2, P less than .02) or with (from 70 +/- 7 to 137 +/- 19 mg/min m2, P less than .02) concomitant lipid infusion. In contrast, endogenous glucose production was considerably (73%) suppressed (from 66 +/- 7 to 15 +/- 8 mg/min m2, P less than .001) during the clamp without lipid, but declined only marginally (from 70 +/- 7 to 48 +/- 7 mg/min m2, NS) with lipid administration. The difference between the control and the lipid study was highly significant (P less than .02), and amounted to an average of 3.8 g of relative glucose overproduction during the second hour of the clamp. Blood levels of lactate rose by 34 +/- 15% (.1 greater than P greater than .05) in the control study but only by 17 +/- 10% (NS) during lipid infusion. Blood pyruvate concentrations fell in both sets of experiments (by approximately 45% at the end of the study) with similar time courses.(ABSTRACT TRUNCATED AT 250 WORDS)

The disposal of an oral glucose load in patients with non-insulin-dependent diabetes.

Following glucose ingestion, tissue glucose uptake is enhanced and endogenous glucose production is inhibited, thus contributing to the maintenance of normal glucose tolerance. To examine whether these responses are disturbed in diabetes, glucose kinetics after oral glucose administration were studied in 12 non-insulin-dependent diabetic and 10 age- and weight-matched control subjects. A double tracer approach was used, whereby the endogenous glucose pool was labeled with 3-3H-glucose and the oral load with 1-14C-glucose. The two glucose tracers were separated in plasma by a two-step chromatographic procedure, and the two sets of isotopic data were analyzed according to a two-compartment model for the glucose system. Basally, glucose production was slightly higher in diabetics than in controls (2.51 +/- 0.24 v 2.28 +/- 0.11 mg/kg.min, NS) even though the former had higher plasma glucose (189 +/- 19 v 93 +/- 2 mg/dL, P less than .001) and insulin (23 +/- 4 v 12 +/- 1 microU/mL, P less than .05) concentrations. Following the ingestion of 1 g/kg of glucose, oral glucose appeared in the peripheral circulation in similar time-course and amount in the two groups (75 +/- 2% of the load over 3.5 hours in the diabetics v 76 +/- 3% in controls). Endogenous glucose production was promptly inhibited in diabetic and normal subjects alike, but the mean residual hepatic glucose production after glucose ingestion was significantly greater in the diabetic group (17 +/- 2 v 10 +/- 3 g/3.5 h, P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of Improvement in Glucose Metabolism After Chronic Glyburide Therapy

The effect of glyburide on glucose metabolism was examined in 10 non-insulin-dependent diabetic subjects (NIDDM) and 7 young, control subjects. After 3 mo of glyburide treatment in NIDDM, fasting plasma glucose declined from 198 to 141 mg/dl (P less than 0.01) without change in fasting insulin levels. Basal hepatic glucose production (HGP) was slightly elevated in NIDDM versus controls (2.35 versus 2.18 mg/kg X min, P = NS) and was positively correlated with the fasting glucose concentration (r = 0.93, P less than 0.001). With chronic glyburide therapy, HGP declined to 1.72 mg/kg X min (P less than 0.01 versus preglyburide) and remained highly correlated with the fasting glucose concentration (r = 0.85, P less than 0.005). Basal glucose clearance in NIDDM was reduced by 48% compared with age-matched controls (1.22 versus 2.32 ml/kg X min, P less than 0.001) and was unchanged after 3 mo of glyburide. Thus, the most important factor responsible for the decline in fasting plasma glucose concentration was an inhibition of hepatic glucose output. The decrease in basal hepatic glucose production and fasting plasma glucose concentration occurred without any change in fasting plasma insulin or C-peptide concentration. Insulin-mediated glucose metabolism (insulin clamp technique) was reduced by 55% in NIDDM (2.91 versus 6.39 mg/kg X min, P less than 0.001). After glyburide, insulin-mediated glucose metabolism increased by 26% to 3.67 mg/kg X min (P less than 0.01). This increase in tissue sensitivity to insulin was unassociated with any change in insulin binding to monocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Operation of Randle's Cycle in Patients With NIDDM

It has been suggested that the insulin resistance of non-insulin-dependent diabetes mellitus (NIDDM) may be caused by substrate competition between glucose and free fatty acids (FFAs) (Randles cycle). We measured substrate oxidation and energy metabolism in 10 nonobese untreated NIDDM patients with fasting glucose levels of 7–8 mM with indirect calorimetry in the basal state and during an isoglycemic-hyperinsulinemic (∼100 mU/L) clamp without (control) and with a concomitant infusion (∼0.35 mmol/min) of Intralipid, a triglyceride emulsion. In the control study, fasting rates of total glucose turnover ([3−3H]glucose) and glucose and lipid oxidation (9.4 ± 1.4, 7.3 ± 1.3, and 3.0 ± 0.4 μmol · kg−1 · min−1, respectively) were comparable with those of nondiabetic individuals. After insulin administration, lipid oxidation was normally suppressed (to 1.3 ± 0.3 μ · kg−1 · min−1 P < 0.01), as were the circulating levels of FFA, glycerol, and β-hydroxybutyrate, whereas glucose oxidation doubled (14.1 ± 1.8 μmol; · kg−1 · min−1 P <0.01). Because glycemia was clamped at 7.5 mM, endogenous glucose production (EGP) was completely suppressed, and total glucose disposal was stimulated (to 25.7 ± 5.2 μmol · kg−1 · min−1 P < 0.01 vs. baseline), but glucose clearance (3.6 ± 0.8 ml · kg−1 · min−1) was 30% reduced compared with normal. With concomitant lipid infusion, FFA, glycerol, and β-hydroxybutyrate all rose during the clamp; correspondingly, lipid oxidation was maintained at fasting rates (3.6 ± 0.2 μmol · kg−1 · min−1 P < 0.01 vs. control). As a consequence, the insulin-induced increase in glucose oxidation was abolished (7.9 ±1.3 μmol · kg−1 · min−1 P < 0.01 vs. control), and total glucose disposal was inhibited (21.8 ± 4.6 μmol · kg−1 · min−1 P < 0.05 vs. control) by an amount almost equal to the decrement in glucose oxidation. Lipid infusion did not detectably interfere with insulin-induced suppression of EGP. Energy expenditure failed to increase during the control insulin clamp but was significantly stimulated (∼10%, P < 0.01) by concomitant lipid administration (diet-induced thermogenesis). We conclude that in mildly hyperglycemic, nonobese NIDDM patients, excessive fatty substrate oxidation is unlikely to be responsible for the insulin resistance; increased lipid provision, however, enhances lipid oxidation and energy expenditure and inhibits glucose oxidation and total disposal. Thus, in this type of diabetes, Randles cycle does not appear to be spontaneously overactive but can be induced acutely, with metabolic and energetic consequences similar to those observed in nondiabetic subjects.

Insulin Resistance in Diabetic Ketoacidosis

The effect of “low-dose” (6–10 U/h) insulin treatment on the rate of decline of plasma glucose concentration was determined in 15 diabetic subjects admitted in ketoacidosis (plasma glucose = 948 ± 79 mg/dl) and in six normal volunteers rendered hyperglycemic by a combined infusion of somatostatin and glucose (plasma glucose = 653 ± 28 mg/dl). The fractional glucose turnover and the half-time of the fall in plasma glucose during insulin treatment were both 10-fold reduced (P < 0.001) in the diabetics as compared with the controls. In the ketoacidotic subjects, the mean glucose clearance during insulin treatment was only 8% of that in the controls (P < 0.001). In the normal subjects, tissue glucose clearance during insulin treatment of the hyperglycemia (5.8 ± 0.7 ml/min · kg) was similar to that measured in the same subjects using a standard technique to quantitate insulin sensitivity (euglycemic insulin clamp). In the ketoacidotic patients, a history of prior insulin therapy, but not the degree of hyperglycemia at the time of admission, was associated with a more rapid rate of decline of plasma glucose in response to insulin treatment. We conclude that marked insulin resistance is present in virtually all diabetics in ketoacidosis.

Hepatic and extrahepatic splanchnic glucose metabolism in the postabsorptive and glucose fed dog

In awake dogs we measured the glucose balance across the liver and extrahepatic splanchnic tissues in the postabsorptive state and during two hours of IV infusion of glucose or for three hours following ingestion of oral glucose and during four hours of sequential intraportal followed by oral glucose. The IV glucose infusion rate was adjusted to maintain a steady state glucose concentration of either euglycemic levels (insulin clamp, group 1, N = 4), 125 mg/100 mL above the postabsorptive glucose concentration (+125 mg glucose clamp, group 2, N = 3) or 200 mg/100 mL above basal glucose levels (+200 mg glucose clamp, group 3, N = 7). Oral glucose was given at a dose of either 1.5 g/kg (group 4, N = 7) or 2.5 g/kg (group 5, N = 12). In dogs that received IV glucose, basal gut glucose uptake (0.5 +/- 0.1 mg/min X kg) was stimulated by hyperglycemia (1.5 +/- 0.5 and 1.4 +/- 0.1 mg/min X kg for group 2 and 3, respectively, P less than 0.05). In these same animals basal hepatic glucose output (-2.7 +/- 0.3 mg/min X kg) was promptly suppressed and net hepatic glucose uptake occurred (2.8 +/- 0.2 and 2.4 +/- 0.5 mg/min X kg in group 2 and 3 respectively). Euglycemic hyperinsulinemia (group 1) suppressed postabsorptive hepatic glucose release but did not enhance glucose removal by either the liver or gut tissues. After oral glucose gut tissues released absorbed glucose into portal blood. Over three hours following the glucose meal 74% and 59% of the ingested glucose was absorbed in group 4 and 5, respectively. As with IV glucose, postabsorptive hepatic glucose production was suppressed and over the first two hours after feeding the liver took up glucose (3.4 +/- 1.0 and 3.1 +/- 0.7 mg/min X kg groups 4 and 5, respectively) at a rate similar to that seen with IV glucose. To further examine the effect of the route of glucose administration on liver glucose handling, hepatic glucose balance was measured serially over four hours in three dogs that received IV glucose into a mesenteric vein to produce portal hyperglycemia (+125 mg/dL portal glucose clamp N = 3). Oral glucose (2.5 mg/kg) was given at two hours, and the rate of the mesenteric glucose infusion adjusted to maintain portal glycemia constant. The hepatic glucose balance averaged 5.5 mg/min X kg over the 0 to 2 hour period and 4.2 +/- 1.0 mg/min X kg over the 2 to 4 hour time.(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatic and peripheral insulin resistance following streptozotocin-induced insulin deficiency in the dog

Insulin resistance and insulin deficiency are both present in many patients with diabetes mellitus. We tested the hypothesis that insulin resistance can evolve from a primary lesion of the beta-cell secretory function. Insulin-mediated glucose uptake (insulin clamp), endogenous glucose production, and glucose-stimulated insulin secretion (hyperglycemic clamp) were measured in awake dogs before and four to six weeks after streptozotocin-induced diabetes mellitus. Streptozotocin (30 mg/kg) resulted in a significant rise in the mean fasting plasma glucose concentration from 104 +/- 2 mg/100 mL to 200 +/- 34 mg/100 mL, (P less than 0.05), and a slight decrease in the mean fasting plasma insulin concentration (from 21 +/- 2 microU/mL to 15 +/- 2 microU/mL). Under conditions of steady-state hyperglycemia (+75 mg/100 mL hyperglycemic clamp, insulin secretion was reduced by 75% in the streptozotocin-treated dogs (P less than 0.025), and the total amount of glucose metabolized decreased from 13.56 +/- 1.04 to 4.74 +/- 0.70 mg/min X kg (P less than 0.001). In the postabsorptive state, endogenous glucose production was slightly, although not significantly, higher in the diabetic dogs (3.05 +/- 0.46 v 2.51 +/- 0.22 mg/min . kg), while the glucose clearance rate was 35% lower (P less than 0.001). When the plasma insulin concentration was increased to approximately 45 microU/mL (insulin clamp) while holding plasma glucose constant at the respective fasting levels (99 +/- 1 and 186 +/- 30 mg/100 mL), endogenous glucose production was completely suppressed in control dogs but suppressed by only 51% (1.46 +/- 0.37 mg/min . kg, P less than 0.025) in diabetic animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Energy metabolism of surgical patients in the early postoperative period: a reappraisal.

Energy metabolism was measured at the bedside in 22 uncomplicated surgical patients in the early (24 to 48 h) postoperative period with the use of continuous computerized indirect calorimetry with a canopy system. Energy production rates were higher than those predicted by the Harris-Benedict formula both in absolute value (1516 +/- 61 vs. 1387 +/- 49 kcal/day, p less than .05) and when normalized by body weight (BW; 23.5 +/- 0.5 vs. 21.7 +/- 0.5 kcal/day.kg BW, p less than .01) or by lean body mass (LBM; 32.8 +/- 0.8 vs. 30.2 +/- 0.9 kcal/day.kg LBM, p less than .01). Furthermore, surgical patients had higher energy production rates than those measured in 22 overnight fasted, resting healthy subjects matched for age, sex, and body size (23.5 +/- 0.5 vs. 21.8 +/- 0.6 kcal/day.kg BW, p less than .05). In both the patients and the control group, measured energy production bore a direct relation to LBM. We conclude that the early postoperative period of uncomplicated surgery is associated with a small (about 7%) but consistent increase in energy metabolism above the level observed in the overnight fasted, resting healthy individual. This increase appears to be an effect of surgery itself, and is not predicted by Harris-Benedict equations.