Tryptophan hydroxylation: measurement in pineal gland, brainstem, and carcinoid tumor.

Development of a rapid and sensitive radioassay has permitted study of the conversion of tryptophan to 5-hydroxytryptophan in mammalian tissues. Of normal tissues examined, beef and rat pineal gland contained the highest activity. This is the first direct demonstration of tryptophan hydroxylase in this hydroxyindole-rich tissue. Rat and rabbit brainstem and human carcinoid tumor also had quantities of enzyme that could be measured easily. The reaction requires a reduced pteridine and oxygen and is inhibired by Para-Chorophenylalanine.

Influence of beta-adrenergic blockade on glucose-induced thermogenesis in man.

The role of beta-adrenergically mediated sympathetic nervous activity in the regulation of glucose-induced thermogenesis was examined in healthy male subjects. Respiratory gas exchange was measured continuously, using the ventilated hood technique, under conditions of hyperinsulinemia and hyperglycemia (glucose clamp technique, insulin infusion 1 mU/kg per min, glucose levels 125 mg/dl above basal) before and after beta-adrenergic blockade (i.v. propranolol, 3-mg bolus plus 0.1 mg/min for 2 h). After 2 h of insulin and glucose infusion in series 1, glucose uptake had increased to 23.5 +/- 2.3 mg/kg per min and insulin concentration to 199 +/- 21 microU/ml. Simultaneously, the energy expenditure had risen by 0.39 +/- 0.05 kcal/min above basal. After propranolol administration, glucose uptake did not change, while energy expenditure fell significantly, to a level 0.28 +/- 0.04 kcal/min above basal. The glucose-induced thermogenesis (GIT) was 6.5 +/- 0.3% before and 4.6 +/- 0.5% (P less than 0.02) after propranolol. In series 2, insulin and glucose infusion was continued for 4 h without propranolol administration. Glucose uptake rose (+12%) and insulin levels increased (+40%) between the 2nd and 4th h but energy expenditure and GIT remained unchanged. Subjects in series 3 received saline infusion alone for 3 h, at which time propranolol administration as in series 1 was added during a further 2-h period. No changes in energy expenditure were seen during saline or propranolol infusion. These data demonstrate the presence of a beta-adrenergically mediated sympathetic nervous component in glucose-induced thermogenesis in healthy human subjects. This factor may be of importance in the regulation of normal body weight in man.

Metabolism of alpha-methyltyrosine in man: relationship to its potency as an inhibitor of catecholamine biosynthesis.

The metabolic fate of the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (alpha-MPT), was studied after oral administration of single and multiple doses to patients with pheochromocytoma and essential hypertension. No major urinary excretion product was found other than the drug itself, which accounted for 44-88% of the fate of single or repeated oral doses. Though less than 1% of the administered drug could be recovered in the urine as catechol metabolites, it was possible to identify alpha-methyldopa, alpha-methyldopamine, and alpha-methylnorepinephrine and to quantify the excretion of the first two of these compounds. This minor route of metabolism required revision of methodology (presented herein) for measuring urinary catecholamines during alpha-MPT treatment since these compounds produce spurious fluorescence in routine methods of assay for catecholamines. The catechol metabolites probably are not present in sufficient amounts to contribute to the biochemical effects of the drug. Determination of plasma concentrations of alpha-MPT during maintenance therapy and considerations of the kinetics of enzyme inhibition enabled a calculation to be made of the degree of inhibition of catecholamine synthesis to be expected in the patient. This was calculated to be about 75% for the highest doses employed and is similar in magnitude to experimentally determined values.

Biochemical and pharmacologic effects of alpha-methyltyrosine in man.

Alpha methyltyrosine (alpha-MPT) was administered to 52 patients from 4 days to 10 months; 22 patients were cases of pheochromocytoma and 20 had essential hypertension. Inhibition of catecholamine synthesis in the range of 50-80% was achieved with divided daily drug dosage of from 1.0 to 4.0 g. Striking clinical benefit was noted in patients with pheochromocytoma in whom the drug was used in preparation for surgery and during chronic medical management. The drug appeared to have limited usefulness when used in essential hypertension, unless added to existing therapy with conventional agents. No beneficial effects were noted in thyrotoxicosis, glaucoma, and Raynaud's phenomenon. Untoward effects in order of decreasing incidence were: sedation (with insomnia on withdrawal), anxiety, tremor, diarrhea, and galactorrhea. Drug crystalluria, which has been observed in animals and is currently restrictive of clinical trials, was not observed in these studies. Evidence is presented that the minor conversion of alpha-MPT to methyldopa probably does not contribute significantly to the central and peripheral effects of the drug.

Thermic effect of glucose in man. Obligatory and facultative thermogenesis.

The contribution of the sympathetic nervous system to the thermic effect of intravenously infused glucose and insulin was studied in 10 healthy young men before and after beta-adrenergic receptor blockade with propranolol during conditions of normoglycemia (90 mg/dl) at two levels of hyperinsulinemia (approximately 90 microU/ml and approximately 620 microU/ml). During steady state conditions of glucose uptake (0.515 +/- 0.046 and 0.754 +/- 0.056 g/min), significant increases were observed in energy expenditure (0.10 +/- 0.02 kcal/min, P less than 0.001, and 0.21 +/- 0.02 kcal/min, P less than 0.01, respectively). Similarly, glucose oxidation increased from 0.100 +/- 0.015 to 0.266 +/- 0.022 g/min (P less than 0.001) at approximately microU/ml insulin and from 0.082 +/- 0.013 to 0.295 +/- 0.018 g/min (P less than 0.001) at approximately 620 microU/ml insulin. Concomitantly, the rate of nonoxidative glucose disposal or "glucose storage" was 0.249 +/- 0.033 and 0.459 +/- 0.048 g/min, respectively. At this time the thermic effect of infused glucose/insulin was 5.3 +/- 0.9 and 7.5 +/- 0.7%, and the energy cost of "glucose storage" was 0.50 +/- 0.16 kcal/g and 0.47 +/- 0.04 kcal/g at the two different levels of glucose uptake. After beta-adrenergic receptor blockade with propranolol, glucose uptake, oxidation, and "storage" were unchanged in both studies, but significant decreases in energy expenditure were observed (1.41 +/- 0.06-1.36 +/- 0.05 kcal/min, P less than 0.01 at approximately 90 microU/ml insulin, and 1.52 +/- 0.07-1.43 +/- 0.05 kcal/min, P less than 0.005 at approximately 620 microU/ml insulin) causing significant falls in both the estimated thermic effect of infused glucose/insulin and the energy cost of "glucose storage". Regression analysis of the results from both studies indicated a mean energy cost for "glucose storage" of 0.36 kcal/g (r = 0.74, P less than 0.001), which fell significantly (P less than 0.005) to 0.21 kcal/g (r = 0.49, P less than 0.05) during beta-adrenergic receptor blockade with propranolol. The latter is in close agreement with that calculated on theoretical grounds for the metabolic cost of glucose storage as glycogen, i.e., obligatory thermogenesis. It is concluded that beta-adrenergically mediated sympathetic nervous activity is responsible for almost the entire rise in energy expenditure in excess of the obligatory requirements for processing and storing glucose during conditions of normoglycemia and hyperinsulinemia in healthy man, and that the energy cost of "glucose storage" is not different at normal (approximately 90 microU/ml) and supraphysiological (approximately 620 microU/ml) plasma insulin concentrations.

Effects of dietary fat on postprandial substrate oxidation and on carbohydrate and fat balances.

To study the effect of dietary fat on postprandial substrate utilization and nutrient balance, respiratory exchange was determined in seven young men for 1 h before and 9 h after the ingestion of one of three different breakfasts: i.e., bread, jam, and dried meat (482 kcal: 27% protein, 62% carbohydrate, and 11% fat); bread, jam, and dried meat plus 50 g of margarine containing long-chain triglycerides (LCT); or bread, jam, and dried meat plus 40 g medium-chain triglycerides (MCT) and 10 g LCT margarine (858 kcal: 15% protein, 35% carbohydrate, and 50% fat). Plasma glucose concentrations peaked 45 min after the start of the meals. When compared with the low fat meal, the LCT margarine supplement had no effect at any time on circulating glucose and insulin concentrations, nor on the respiratory quotient. When MCTs were consumed, plasma glucose and insulin concentrations remained lower and plasma FFA concentrations higher during the first 2 h. 9 h after the breakfasts, the amounts of substrates oxidized were similar in each case, i.e., approximately 320, 355, and 125 kcal for carbohydrate, fat, and protein, respectively. This resulted in comparable carbohydrate (mean +/- SD = -22 +/- 32, -22 +/- 37, and -24 +/- 22 kcal) and protein balances (-7 +/- 9, +7 +/- 7, and -8 +/- 11 kcal) after the low fat, LCT- and MCT-supplemented test meals, respectively. However, after the low fat meal, the lipid balance was negative (-287 +/- 60 kcal), which differed significantly (P less than 0.001) from the fat balances after the LCT- and MCT-supplemented meals, i.e., +60 +/- 33 and +57 +/- 25 kcal, respectively. The results demonstrate that the rates of fat and of carbohydrate oxidation are not influenced by the fat content of a meal.

Evidence that insulin resistance is responsible for the decreased thermic effect of glucose in human obesity.

The thermic effect of glucose was investigated in nine obese and six lean subjects in whom the same rate of glucose uptake was imposed. Continuous indirect calorimetry was performed for 240 min on the supine subject. After 45 min, 20% glucose was infused (609 mg/min) for 195 min and normoglycemia was maintained by adjusting the insulin infusion rate. At 2 h, propranolol was infused (bolus 100 micrograms/kg; 1 microgram/kg X min) for the remaining 75 min. To maintain the same glucose uptake (0.624 g/min), it was necessary to infuse insulin at 3.0 +/- 0.6 (leans) and 6.6 +/- 1.2 mU/kg X min (obese) (P less than 0.02). At this time, glucose oxidation was 0.248 +/- 0.019 (leans) and 0.253 +/- 0.022 g/min (obese) (NS), and nonoxidative glucose disposal was 0.375 +/- 0.011 and 0.372 +/- 0.029 g/min, respectively. Resting metabolic rate (RMR) rose significantly by 0.13 +/- 0.02 kcal/min in both groups, resulting in similar thermic effects, i.e., 5.5 +/- 0.7% (leans) 5.4 +/- 0.9% (obese) (NS) and energy costs of glucose storage 0.35 +/- 0.06 and 0.39 +/- 0.09 kcal/g (NS), respectively. With propranolol, glucose uptake and storage remained the same, while RMR fell significantly in both groups, with corresponding decreases (P less than 0.05) in the thermic effects of glucose to 3.7 +/- 0.6% and 2.9 +/- 0.8% (NS) and the energy costs of glucose storage 0.23 +/- 0.04 and 0.17 +/- 0.05 kcal/g (NS) in the lean and obese subjects, respectively. These results suggest that the defect in the thermic effect of glucose observed in obese subjects is due to their insulin resistance, which is responsible for a lower rate of glucose uptake and hence decreased rate of glucose storage, which is an energy-requiring process.

Impaired insulin-induced sympathetic neural activation and vasodilation in skeletal muscle in obese humans.

The sympathetic nervous system is an important regulatory mechanism of both metabolic and cardiovascular function, and altered sympathetic activity may play a role in the etiology and/or complications of obesity. In lean subjects, insulin evokes sympathetic activation and vasodilation in skeletal muscle. In obese subjects such vasodilation is impaired and, in turn, may contribute to insulin resistance. To examine the relationship between sympathetic and vasodilatory responses in skeletal muscle to hyperinsulinemia, we simultaneously measured muscle sympathetic nerve activity (MSNA) and calf blood flow at basal and during a 2-h hyperinsulinemic (6 pmol/kg per min) euglycemic clamp in eight lean and eight obese subjects. The major findings of this study are twofold: obese subjects had a 2.2 times higher fasting rate of MSNA, and euglycemic hyperinsulinemia, which more than doubled MSNA and increased calf blood flow by roughly 30% in lean subjects, had only a minor vasodilatory and sympathoexcitatory effect in obese subjects. In contrast, two non-insulin-sympathetic stimuli evoked comparably large increases in MSNA in lean and obese subjects. We conclude that insulin resistance in obese subjects is associated with increased fasting MSNA and a specific impairment of sympathetic neural responsiveness to physiological hyperinsulinemia in skeletal muscle tissue.

Insulin. Its role in the thermic effect of glucose.

To investigate the possible role of insulin per se in the thermic response to glucose/insulin infusions, respiratory exchange measurements were performed on eight healthy young men for 45 min before and 210 min after somatostatin infusion. Two tests were performed on separate days and each had two consecutive phases of 90 min each. Test 1. Two different rates of glucose uptake were imposed, one at euglycemia (phase 1) and the other at hyperglycemia (phase 2) while insulinemia was maintained constant throughout. Test 2. Glucose uptake was maintained constant throughout while insulin was infused at two different rates: 1 mU/kg per min with hyperglycemia (phase 1) and 6.45 mU/kg per min with "euglycemia" (phase 2). The thermic effect of glucose and insulin, obtained from phase 1 in both tests, was 5.9 +/- 1.2 and 5.8 +/- 0.5% (NS) of the energy infused, respectively. A step increase in glucose uptake alone, test 1, phase 2, (0.469 +/- 0.039 to 1.069 +/- 0.094 g/min) caused an increase in energy expenditure of 0.14 +/- 0.03 kcal/min (thermic effect 5.9 +/- 1.1%). When insulin was increased by 752 +/- 115 microU/ml, with no change in glucose uptake, energy expenditure rose by 0.05 +/- 0.02 kcal/min, which correlated with the increase in plasma catecholamines. It is concluded that a large proportion of the thermic response to glucose/insulin infusions is due to glucose metabolism alone. The thermic effect of insulin is small and appears to be mediated by the sympathetic nervous system; thus at physiological insulin concentrations, the thermic effect of insulin per se is negligible.

Differential effects of hyperinsulinemia and carbohydrate metabolism on sympathetic nerve activity and muscle blood flow in humans.

Euglycemic hyperinsulinemia evokes both sympathetic activation and vasodilation in skeletal muscle, but the mechanism remains unknown. To determine whether insulin per se or insulin-induced stimulation of carbohydrate metabolism is the main excitatory stimulus, we performed, in six healthy lean subjects, simultaneous microneurographic recordings of muscle sympathetic nerve activity, plethysmographic measurements of calf blood flow, and calorimetric determinations of carbohydrate oxidation rate. Measurements were made during 2 h of: (a) insulin/glucose infusion (hyperinsulinemic [6 pmol/kg per min] euglycemic clamp), (b) exogenous glucose infusion at a rate matched to that attained during protocol a, and (c) exogenous fructose infusion at the same rate as for glucose infusion in protocol b. For a comparable rise in carbohydrate oxidation, insulin/glucose infusion that resulted in twofold greater increases in plasma insulin concentrations than did glucose infusion alone, evoked twofold greater increases in both muscle sympathetic nerve activity and calf blood flow. Fructose infusion, which increased carbohydrate oxidation comparably, but had only a minor effect on insulinemia, did not stimulate either muscle sympathetic nerve activity or calf blood flow. These observations suggest that in humans hyperinsulinemia per se, rather than insulin-induced stimulation of carbohydrate metabolism, is the main mechanism that triggers both sympathetic activation and vasodilation in skeletal muscle.

Effect of beta and alpha adrenergic blockade on glucose-induced thermogenesis in man.

After intravenous glucose/insulin infusion there is an increase in oxygen consumption and energy expenditure that has been referred to as thermogenesis. To examine the contribution of the beta and alpha adrenergic nervous system to this thermogenic response, 12 healthy volunteers participated in three studies: (a) euglycemic insulin (plasma insulin approximately 100 microunits/ml) clamp study (n = 12); (b) insulin clamp study after beta adrenergic blockade with intravenous propranolol for 1 h (n = 12); (c) insulin clamp study after alpha adrenergic blockade with phentolamine for 1 h (n = 5). During the control insulin clamp study total glucose uptake, glucose oxidation and nonoxidative glucose uptake averaged 7.85 +/- 0.47, 2.62 +/- 0.22, and 5.23 +/- 0.51 mg/kg X min. After propranolol infusion, insulin-mediated glucose uptake was significantly reduced, 6.89 +/- 0.41 (P less than 0.02). This decrease was primarily the result of a decrease in glucose oxidation (1.97 +/- 0.19 mg/kg X min, P less than 0.01) without any change in nonoxidative glucose metabolism. Phentolamine administration had no effect on total glucose uptake, glucose oxidation, or nonoxidative glucose disposal. The increments in energy expenditure (0.10 +/- 0.01 vs. 0.03 +/- 0.01 kcal/min) and glucose/insulin-induced thermogenesis (4.9 +/- 0.5 vs. 1.5 +/- 0.5%) were reduced by 70% during the propranolol/insulin clamp study. The increments in energy expenditure (0.12 +/- 0.03 kcal/min) and thermogenesis (5.0 +/- 1.5%) were not affected by phentolamine. These results indicate that activation of the beta adrenergic receptor plays an important role in the insulin/glucose-mediated increase in energy expenditure and thermogenesis. In contrast, the alpha adrenergic receptor does not appear to participate in this response.

Insulin sensitivity and exogenous insulin clearance in Graves' disease. Measurement by the glucose clamp technique and continuous indirect calorimetry.

Insulin sensitivity was measured in a group of seven thyrotoxic patients and in a group of seven normal subjects by means of the glucose clamp technique. Infusion of insulin at a rate of 0.80 +/- 0.05 mU/kg X min in the hyperthyroid patients and of 0.55 +/- 0.04 mU/kg X min in the control group was performed to obtain a steady-state plasma insulin concentration of approximately 50 microU/ml. Substrate oxidation rates were measured in the postabsorptive state and during the 2 h of the clamp by means of continuous indirect calorimetry. In the postabsorptive state, hyperthyroid patients presented a preferential oxidation of lipids. During the period 60-120 min of the clamp, mean plasma glucose (92 +/- 2 versus 93 +/- 2 mg/dl), insulin (50 +/- 5 versus 58 +/- 3 microU/ml), and total glucose metabolism (5.8 +/- 0.7 versus 6.1 +/- 0.3 mg/kg X min) were similar in the hyperthyroid patients and the control subjects. The rate of glucose oxidation was higher in hyperthyroid patients than in control subjects (4.3 +/- 0.5 versus 2.2 +/- 0.2 mg/kg X min, P less than 0.001), while that of lipid oxidation was similar in both groups (0.6 +/- 0.2 versus control 0.7 +/- 0.1 mg/kg X min). The calculated metabolic clearance rate of insulin was markedly higher in the hyperthyroid patients (1144 +/- 132 ml/min) than in the normal subjects (812 +/- 56 ml/min, P less than 0.025). It is concluded that insulin sensitivity is not altered in the thyrotoxic state. The major route of insulin-stimulated glucose disposal in the hyperthyroid patients appears to be glucose oxidation.

Insulin-induced sympathetic activation and vasodilation in skeletal muscle. Effects of insulin resistance in lean subjects.

Insulin-induced stimulation of blood flow and sympathetic nerve activity in skeletal muscle tissue is impaired in obesity, but the underlying mechanism is unknown. To determine whether insulin resistance alters sympathetic and vasodilatory responses to euglycemic hyperinsulinemia, in eight healthy subjects we measured calf blood flow and muscle sympathetic nerve activity (MSNA) (n = 5) during insulin/glucose infusion (euglycemic hyperinsulinemic [6 pmol.kg-1.min-1] clamp) performed alone and performed during concomitant fat emulsion infusion, a maneuver designed to induce insulin resistance. The major new finding is that fat emulsion infusion, which attenuated insulin-induced stimulation of carbohydrate oxidation by 39 +/- 7% (P < 0.01), did not have any detectable effect on insulin-induced vasodilatory and sympathetic responses: at the end of the 2-h clamp, blood flow and MSNA had increased by 35 +/- 6% (P < 0.01) and 152 +/- 58% (P < 0.01), respectively, during insulin infusion alone and by 35 +/- 7% (P < 0.01) and 244 +/- 90% (P < 0.01), respectively, during insulin infusion superimposed on free fatty acid infusion. These observations in lean healthy subjects indicate that induction of resistance to the stimulatory effects of insulin on carbohydrate metabolism does not attenuate muscle blood flow and MSNA responses evoked by acute euglycemic hyperinsulinemia. These findings provide further evidence that hyperinsulinemia per se is the primary stimulus that triggers stimulation of muscle blood flow and MSNA during insulin/glucose infusion in humans and suggest that the impaired insulin-induced vasodilation in obese subjects is not related primarily to impaired stimulation of muscle carbohydrate metabolism.

Modifications of glucose storage and oxidation in nonobese diabetics, measured by continuous indirect calorimetry.

A new application of continuous indirect calorimetry is described for measuring the disposal of a glucose load. In a group of 10 normal subjects, 3 h after a 100 g oral glucose load, 20 g glucose was oxidized at basal rate, 19 g in response to the load and 63 g stored, while a decrease of 2 g was observed in the glucose space (GS). In a group of four type I, insulin-dependent diabetics, both glucose oxidation (9 g at the basal rate and 4 g in response to the load) and glucose storage (9 g) were markedly decreased, with the remainder either being lost in the urine (36 g) or remaining in the glucose space (42 g). In a group of eight nonobese type II, non-insulin-dependent diabetics, glucose oxidation both in the basal rate and in response to the load was slightly decreased (13 and 14 g, respectively) and glucose storage decreased to 40 g. These results suggest that, in type I diabetics, complete insulin deficiency seriously impairs two major mechanisms regulating glucose homeostasis, i.e., glucose storage and oxidation, while, in type II diabetics, the remaining insulin secretion attentuates these disturbances.

Abnormal regulation of hepatic glucose output in maturity-onset diabetes of the young caused by a specific mutation of the glucokinase gene.

A subtype of maturity-onset diabetes of the young (MODY) is caused by mutations of the glucokinase gene, an enzyme expressed in pancreatic beta-cells and the liver. To assess the consequences of a functional alteration of glucokinase at the level of the liver, endogenous (hepatic) glucose production and glucose cycling (an indirect assessment of hepatic glucokinase activity) were measured with 2-2H glucose and 6,6-2H glucose in patients who developed MODY because of the V203A mutation of glucokinase, and in control subjects at similar levels of glycemia. Measurements were performed in the postabsorptive state and after ingestion of 13C-labeled glucose. In the postabsorptive state, MODY patients had normal glucose production (10.9 +/- 1.3 vs. 11.3 +/- 0.6 micromol x kg(-1) x min(-1)) but decreased glucose cycling (0.6 +/- 0.3 vs. 1.5 +/- 0.3 micromol x kg(-1) x min(-1); P < 0.05) when compared with control subjects. However, at plasma glucose and insulin levels similar to those observed in MODY patients, control subjects' glucose production was markedly lower (3.2 +/- 1.5 micromol x kg(-1) x min(-1). After glucose ingestion, endogenous glucose production was reduced by only 29% in MODY patients compared with 80% in control subjects at a similar level of hyperglycemia (P < 0.05). This suggests that the V203A mutation of glucokinase results in decreased activity of glucokinase in liver cells. Thus endogenous glucose production is inadequately inhibited by hyperglycemia in MODY patients, possibly as a result of impaired hepatic glucokinase activity. These alterations contribute to the pathogenesis of hyperglycemia.

In normal men, free fatty acids reduce peripheral but not splanchnic glucose uptake.

Raising plasma free fatty acid (FFA) levels reduces muscle glucose uptake, but the effect of FFAs on splanchnic glucose uptake, total glucose output, and glucose cycling may also be critical to producing lipid-induced glucose intolerance. In eight normal volunteers, we measured glucose turnover and cycling rates ([2H7]glucose infusion) during a moderately hyperglycemic (7.7 mmol/l) hyperinsulinemic clamp, before and after ingestion of a labeled (dideuterated) oral glucose load (700 mg/kg). Each test was performed twice, with either a lipid or a saline infusion; four subjects also had a third test with a glycerol infusion. As shown by similar rates of exogenous glucose appearance, the lipid infusion did not reduce first-pass splanchnic glucose uptake (saline 1.48+/-0.18, lipid 1.69+/-0.17, and glycerol 1.88+/-0.17 mmol/kg per 180 min; NS), but it reduced peripheral glucose uptake by 40% (P < 0.01 vs. both saline and glycerol infusions). Before oral ingestion of glucose, total glucose output was similarly increased by the lipid and glycerol infusions. Total glucose output was significantly increased by FFAs after oral ingestion of glucose (saline 3.68+/-1.15, glycerol 3.68+/-1.70, and lipid 7.92+/-0.88 micromol x kg(-1) x min(-1); P < 0.01 vs. saline and P < 0.05 vs. glycerol). The glucose cycling rate was approximately 2.7 micromol x kg(-1) x min(-1) with the three infusions and tended to decrease all along the lipid infusion, which argues against a stimulation of glucose-6-phosphatase by FFAs. It is concluded that in situations of moderate hyperinsulinemia-hyperglycemia, FFAs reduce peripheral but not splanchnic glucose uptake. Total glucose output is increased by FFAs, by a mechanism that does not seem to involve stimulation of glucose-6-phosphatase.

Role of lipid oxidation in pathogenesis of insulin resistance of obesity and type II diabetes.

Increased lipid oxidation is generally observed in subjects with obesity and diabetes and has been suggested to be responsible for the insulin resistance associated with these conditions. We measured, by continuous indirect calorimetry, lipid and glucose oxidation and nonoxidative glucose disposal in 82 obese subjects during a 100-g oral glucose tolerance test (OGTT) and in 26 during a euglycemic insulin (40 mU.min-1.m-2) clamp. The obese subjects were subdivided into those with normal glucose tolerance (group A), those with impaired glucose tolerance (group B), and those with overt diabetes (group C). Forty-five healthy nonobese subjects were subdivided into a young and an older control group, which were age-matched to the nondiabetic obese (groups A and B) and diabetic obese (group C) subjects, respectively. In the postabsorptive state, as well as in response to insulin stimulation (both OGTT and insulin clamp), lipid oxidation was significantly increased in all three obese groups in comparison with either young or older controls. Basal glucose oxidation was significantly decreased in obese nondiabetic and obese glucose--intolerant subjects (groups A and B) compared with age-matched controls. During the OGTT and during the insulin clamp, insulin-stimulated glucose oxidation was decreased in all three obese groups. In contrast, nonoxidative glucose disposal was markedly inhibited in nondiabetic and diabetic obese patients during the euglycemic insulin clamp but not during the OGTT. After glucose ingestion, nonoxidative glucose uptake was normal in nondiabetic obese and glucose-intolerant obese subjects and decreased in diabetic obese subjects. Statistical analysis revealed that lipid and glucose oxidation were strongly and inversely related in the basal state, during euglycemic insulin clamp, and during OGTT. The negative correlation between lipid oxidation and nonoxidative glucose uptake, although significant, was much weaker. Fasting and post-OGTT hyperglycemia were the strongest (negative) correlates of nonoxidative glucose disposal in both single and multiple regression models. We conclude that 1) reduced glucose oxidation and reduced nonoxidative glucose disposal partake of the insulin resistance of nondiabetic obese and diabetic obese individuals; 2) hyperglycemia provides a compensatory mechanism for the defect in nonoxidative glucose disposal in nondiabetic obese subjects; however, this compensation is characteristically lost when overt diabetes ensues; and 3) increased lipid oxidation may contribute, in part, to the defects in glucose oxidation and nonoxidative glucose uptake in obesity.

Glucose utilization and production in patients with maturity-onset diabetes of the young caused by a mutation of the hepatocyte nuclear factor-1alpha gene.

Mutations of the hepatocyte nuclear factor (HNF)-1alpha gene cause impaired insulin secretion and hyperglycemia in patients with maturity-onset diabetes of the young (MODY)3. Whether these mutations also affect glucose metabolism in tissues other than the beta-cell has not yet been documented. We therefore assessed, in five MODY3 patients and a dozen healthy control subjects, insulin secretion, oxidative and nonoxidative glucose disposal, and glucose production during a two-step hyperglycemic clamp and a euglycemic hyperinsulinemic (0.4 mU x kg(-1) x min(-1)) clamp. Compared with healthy control subjects, MODY3 patients had higher fasting plasma glucose (+100%) but similar rates of fasting glucose production and oxidation. Both the early and late phases of insulin secretion were virtually abolished during the hyperglycemic clamp, and glucose production was suppressed by only 43% in MODY3 patients vs. 100% in healthy control subjects. The rate of glucose infusion required to produce a 5 mmol/l increase above basal glycemia was reduced by 30%, net nonoxidative glucose disposal (which is equal to net glycogen deposition) was inhibited by 39%, and net carbohydrate oxidation during hyperglycemia was 25% lower in MODY3 patients compared with control subjects. Insulin-stimulated glucose utilization and oxidation measured during the hyperinsulinemic clamp (at approximately 200 pmol/l insulin) were identical in MODY3 patients and in healthy control subjects, indicating that peripheral insulin sensitivity was not altered. Suppression of endogenous glucose production was, however, mildly impaired. It is concluded that MODY3 patients have severely depressed glucose-induced insulin secretion. The development of hyperglycemia in these patients appears to be caused by a decreased stimulation of glucose utilization, oxidation, and nonoxidative glucose disposal as well as by a blunted suppression of endogenous glucose output. These phenomena are essentially secondary to insulinopenia, whereas insulin sensitivity remains intact.

Energy and substrate metabolism in obesity and postobese state.

The total energy expenditure (EE) of human beings is represented by basal metabolic rate (which corresponds to 60-70% of EE), dietary-induced thermogenesis (10% of EE), and the energy expended in physical activity (20-30% of EE). Obese individuals have an increased total EE compared with lean subjects; this increase is essentially due to an increased lean body mass concomitant with obesity, and is completely reverted after weight loss. Glucose-induced thermogenesis (GIT), measured during an oral glucose tolerance test (OGTT) or hyperinsulinemic-euglycemic clamps, has been found to be decreased in obese individuals, although some discrepancy exists between studies. The observed decreases in GIT show a gradation, increasing progressively from obese patients with normal glucose tolerance to obese patients with impaired glucose tolerance (IGT) to obese patients with non-insulin-dependent diabetes mellitus (NIDDM) and an increased insulin response after OGTT to obese patients with NIDDM and a hypoinsulinemic response after OGTT. The defect in GIT appears to be related to impairment in nonoxidative glucose storage and with the degree of insulin resistance. Obese patients after weight loss show a further decrease in GIT after OGTT or during a euglycemic clamp, which remains unclear. Obese patients have an increased basal lipid oxidation and a decreased suppression of lipid oxidation after OGTT or during a euglycemic clamp. Glucose oxidation and storage are both markedly decreased during a euglycemic clamp. In contrast, the defect in glucose storage is less apparent after OGTT, due to the compensatory effect of hyperglycemia and hyperinsulinemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hyperinsulinemia and hyperglycemia on lactate release and local blood flow in subcutaneous adipose tissue of healthy humans.

To determine the effects of hyperglycemia and hyperinsulinemia on lactate production by adipocytes, healthy volunteers were studied during three experimental protocols. In protocol 1, the changes in interstitial lactate concentrations were measured by microdialysis (sc tissue) after oral glucose administration. The plasma lactate concentration increased by 39.4 +/- 6.0%, and the dialysate lactate concentration increased by 117.9 +/- 16.3%. In protocol 2, a 2.5-h hyperinsulinemic euglycemic clamp and somatostatin infusion were performed. The plasma and dialysate lactate concentrations increased by 27.1 +/- 5.5% and 146.8 +/- 44.5%, respectively. In addition, [U-13C]glucose was infused through the probe, and dialysate lactate was enriched in 13C at 2.5 +/- 0.3 molar percent excess basally and at 3.4 +/- 0.3 molar percent excess during the clamp (P < 0.05 vs. basal). [13C]Urea was also infused through the probe, and the outflow to inflow ratio of [13C]urea was used as an index of local blood flow. It decreased by 10.2 +/- 3.6% (P < 0.001) at the end of the hyperinsulinemic euglycemic clamp, indicating an increase in blood flow. In protocol 3, a hyperglycemic clamp (10.0 mmol/L) at the basal insulin concentration was performed. It increased the dialysate lactate concentration by 43.5 +/- 15.9% and did not alter the plasma lactate concentration or local blood flow. It is concluded that hyperinsulinemia and, to a lesser extent, hyperglycemia stimulate glucose conversion into lactate in adipocytes. Hyperinsulinemia, but not hyperglycemia, also increases adipose tissue blood flow.