Efficacy of a long-acting C-peptide analogue against peripheral neuropathy in streptozotocin-diabetic mice.

AIMS: To investigate the efficacy of a pegylated C-peptide (Peg-C-peptide) against indices of peripheral neuropathy in a mouse model of type 1 diabetes and to compare efficacy of this C-peptide analogue against that of the native molecule. METHODS: C57Bl/6 mice were injected with two consecutive doses of streptozotocin (STZ) to induce type 1 diabetes. Mice were treated twice daily with native C-peptide [0.4-1.3 mg/kg subcutaneously (s.c.)] or twice weekly with Peg-C-peptide (0.1-1.3 mg/kg s.c.) for 20 weeks. Motor and sensory nerve conduction velocities, thermal and tactile responses and rate dependent H-wave depression were assessed after 20 weeks of diabetes. Foot skin intraepidermal fibres and corneal nerves were counted, and sciatic nerve substance P and plasma C-peptide levels were also determined. RESULTS: After 5 months of STZ-induced diabetes, mice exhibited significant motor and sensory nerve conduction slowing, thermal hypoalgesia, tactile allodynia and attenuation of rate-dependent depression of the H reflex. These functional disorders were accompanied by nerve substance P depletion but not loss of small sensory fibres in the hind paw epidermis or the cornea. The efficacy of twice-daily treatment with native C-peptide in preventing these disorders was matched or exceeded by twice-weekly treatment with Peg-C-peptide. Both native and Peg-C-peptide also increased corneal nerve occupancy in the sub-basal nerve plexus of control rats. CONCLUSIONS: These data identify actions of C-peptide against novel and clinically pertinent aspects of diabetic neuropathy in mice and also establish Peg-C-peptide as a long-acting therapeutic method of potential clinical value.

Enhanced insulin action following subcutaneous co-administration of insulin and C-peptide in rats.

BACKGROUND: This study was undertaken to examine if C-peptide (C) may interact with hexameric insulin and facilitate its disaggregation into the physiologically active monomeric form. METHODS: Regular insulin (I) or an insulin analogue (IA) were injected s.c. in rats together with C or its C-terminal pentapeptide (PP). I or IA and C or PP were administered either as a physical mixture or into two separate s.c. depots. Whole body glucose utilization was evaluated using the euglycemic clamp technique. Phosphorylation of Akt/PKB and GSK in liver and skeletal muscles and 86Rb⁺ uptake by L6 cells were measured. RESULTS: S.c. injection of a mixture of I and C or I and PP resulted in a 30-55% greater (P < 0.01-0.001) and 15-27% (P < 0.05-0.001) longer stimulation of whole body glucose utilization than after separate injections. Insulin-stimulated phosphorylation of Akt/PKB in liver increased 35% more after injection of I and C in mixture compared with after separate injections. Phosphorylation of GSK3 was augmented by 50% (P < 0.05) following the injection of I and C in mixture compared with separate injections. Stimulation of myotubes with premixed I and C (1 nM) elicited 20% additional increase in ouabain-sensitive 86Rb⁺ uptake (P < 0.05) in comparison with the effect when I and C were added separately. CONCLUSIONS: Subcutaneous co-administration of insulin and C results in augmented insulin bioactivity at the level of tissue glucose uptake, intracellular signalling, and enzyme activation. These effects may be attributed to augmented C mediated disaggregation of hexameric insulin into its physiologically active monomeric form.

Influence of endogenous insulin secretion on splanchnic glucose and amino acid metabolism in man.

Splanchnic exchange of glucose, 20 individual amino acids, lactate, and pyruvate was studied in normal subjects in the postabsorptive state and after stimulation of endogenous insulin secretion by infusion of glucose at two dose levels. In the basal state, mean splanchnic glucose production was 3.4 mg/kg per min. A net uptake of lactate, pyruvate, and nine amino acids was observed, with alanine accounting for half of the total splanchnic-amino acid extraction. Infusion of glucose at 25 mg/kg per min for 20 min resulted in a fivefold increase in arterial insulin levels and in reversal of splanchnic glucose balance to a net uptake. Splanchnic uptake of alanine, glycine, phenylalanine, lactate, and pyruvate fell by 30-60% due to a reduction in fractional extraction of these substrates, inasmuch as their arterial concentrations did not decline.Administration of glucose at 2 mg/kg per min for 45 min resulted in a 19 mg/100 ml increase in arterial glucose concentration and a doubling of arterial insulin levels. Despite the small increment in insulin, hepatic glucose production fell by 85%. Splanchnic exchange of amino acids, lactate, and pyruvate was unaltered. Estimated total glucose utilization during the infusion was no greater than in the basal state, indicating lack of stimulation of peripheral glucose uptake. IT IS CONCLUDED THAT: (a) inhibition of hepatic glucose production associated with glucose infusion and large increments in insulin levels occurs in the absence of a decrease in the concentration of circulating gluconeogenic substrate, suggesting an hepatic rather than peripheral effect; (b) the liver is the primary target organ whereby glucose homeostasis is achieved with small increments in insulin; (c) the relatively greater sensitivity of the liver's response to insulin as compared with an effect of insulin on the peripheral tissues, may be a consequence of the higher levels of endogenous insulin in portal as compared with peripheral blood.

Amino acid metabolism in exercising man.

Arterial concentration and net exchange across the leg and splanchnic bed of 19 amino acids were determined in healthy, postabsorptive subjects in the resting state and after 10 and 40 min of exercise on a bicycle ergometer at work intensities of 400, 800, and 1200 kg-m/min. Arterio-portal venous differences were measured in five subjects undergoing elective cholecystectomy. In the resting state significant net release from the leg was noted for 13 amino acids, and significant splanchnic uptake was observed for 10 amino acids. Peripheral release and splanchnic uptake of alanine exceeded that of all other amino acids, accounting for 35-40% of total net amino acid exchange. Alanine and other amino acids were released in small amounts (relative to net splanchnic uptake) by the extrahepatic splanchnic tissues drained by the portal vein. During exercise arterial ananine rose 20-25% with mild exertion and 60-96% at the heavier work loads. Both at rest and during exercise a direct correlation was observed between arterial alanine and arterial pyruvate levels. Net amino acid release across the exercising leg was consistently observed at all levels of work intensity only for alanine. Estimated leg alanine output increased above resting levels in proportion to the work load. Splanchnic alanine uptake during exercise exceeded that of all other amino acids and increased by 15-20% during mild and moderate exercise, primarily as a consequence of augmented fractional extraction of alanine. For all other amino acids, there was no change in arterial concentration during mild exercise. At heavier work loads, increases of 8-35% were noted for isoleucine, leucine, methionine, tyrosine, and phenylalanine, which were attributable to altered splanchnic exchange rather than augmented peripheral release. The data suggest that (a) synthesis of alanine in muscle, presumably by transamination of glucose-derived pyruvate, is increased in exercise probably as a consequence of increased availability of pyruvate and amino groups; (b) circulating alanine serves an important carrier function in the transport of amino groups from peripheral muscle to the liver, particularly during exercise; (c) a glucose-alanine cycle exists whereby alanine, synthesized in muscle, is taken up by the liver and its glucose-derived carbon skeleton is reconverted to glucose.

Influence of physiologic hyperglucagonemia on basal and insulin-inhibited splanchnic glucose output in normal man.

To evaluate the effects of physiologic hyperglucagonemia on splanchnic glucose output, glucagon was infused in a dose of 3 ng/kg per min to healthy subjects in the basal state and after splanchnic glucose output had been inhibited by an infusion of glucose (2 mg/kg per min). In the basal state, infusion of glucagon causing a 309 +/- 25 pg/ml rise in plasma concentration was accompanied by a rapid increase in splanchnic glucose output to values two to three times basal by 7-15 min. The rise in arterial blood glucose (0.5-1.5 mM) correlated directly with the increment in splanchnic glucose output. Despite continued glucagon infusion, and in the face of stable insulin levels, splanchnic glucose output declined after 22 min, returning to basal levels by 30-45 min. In the subjects initially receiving the glucose infusion, arterial insulin concentration rose by 5-12 muU/ml, while splanchnic glucose output fell by 85-100%. Infusion of glucagon causing an increment in plasma glucagon concentration of 272 +/- 30 pg/ml reversed the inhibition in splanchnic glucose production within 5 min. Splanchnic glucose output reached a peak increment 60% above basal levels at 10 min, and subsequently declined to levels 20-25% below basal at 30-45 min. These findings provide direct evidence that physiologic increments in plasma glucagon stimulate splanchnic glucose output in the basal state and reverse insulin-mediated inhibition of splanchnic glucose production in normal man. The transient nature of the stimulatory effect of glucagon on splanchnic glucose output suggests the rapid development of inhibition or reversal of glucagon action. This inhibition does not appear to depend on increased insulin secretio.

Splanchnic and peripheral glucose and lactate metabolism during and after prolonged arm exercise.

Splanchnic and peripheral exchange of glucose and gluconeogenic substrates was examined in 12 healthy subjects during 2 h of arm or leg exercise on a bicycle ergometer and during a 40-min postexercise recovery period. The work intensity corresponded to 30% of the maximal pulmonary oxygen uptake. The regional exchange of substrates was evaluated using catheter technique and indicator dilution methods for blood flow measurements. Our findings indicate that prolonged arm exercise as compared with exercise with the legs results in a greater increase in heart rate (25-40%) and a more marked reduction in splanchnic blood flow (10-30%) as well as higher arterial concentrations of lactate, free fatty acids, and catecholamines. The respiratory exchange ratio was consistently higher with arm exercise. In addition, arm exercise results in a greater fractional extraction and utilization of glucose by exercising muscle as well as a greater hepatic gluconeogenesis from lactate and glycerol. During recovery from prolonged arm exercise, leg muscle becomes an important site of lactate release to the splanchnic bed, despite a lack of net glucose uptake by the leg. Simultaneously, arm muscle shows an increase in glucose uptake in the absence of a net release of lactate. These coincident but discordant processes in the leg and arm during recovery suggest the occurrence of a redistribution of muscle glycogen from previously resting (leg) muscle to previously exercising (arm) muscle.

Effect of protein ingestion on splanchnic and leg metabolism in normal man and in patients with diabetes mellitus.

The inter-organ flux of substrates after a protein-rich meal was studied in seven healthy subjects and in eight patients, with diabetes mellitus. Arterial concentrations as well as leg and splanchnic exchange of amino acids, carbohydrate substrates, free fatty acids (FFA), and ketone bodies were examined in the basal state and for 3 h after the ingestion of lean beef (3 g/kg body wt). Insulin was withheld for 24 h before the study in the diabetic patients. In the normal subjects, after protein ingestion, there was a large amino acid release from the splanchnic bed predominantly involving the branched chain amino acids. Valine, isoleucine, and leucine accounted together for more than half of total splanchnic amino acid output. Large increments were seen in the arterial concentrations of the branched chain amino acids (100-200%) and to a smaller extent for other amino acids. Leg exchange of most amino acids reverted from a basal net outut to a net uptake after protein feeding which was most marked for the branched chain amino acids. The latter accounted for more than half of total peripheral amino acid uptake...

Splanchnic and leg exchange of glucose, amino acids, and free fatty acids during exercise in diabetes mellitus.

The influence of exercise on leg and splanchnic exchange of substrates was examined in eight insulin-dependent diabetics 24 h after withdrawal of insulin and in eight healthy controls studied at rest and after 40 min of bicycle ergometer exercise at 55-60% of maximal capacity. In four of the diabetic subjects, basal arterial ketone acid levels were 3-4 mmol/ liter (ketotic diabetics) and in the remainder, below 1 mmol/liter (nonketotic diabetics). ,ree fatty acid (FFA) turnover and regional exchange were evaluated with 14-C- labeled oleic acid. Leg uptake of blood glucose rose 13-18 fold during exercise in both the diabetics and controls and accounted for a similar proportion of the total oxygen uptake by leg muscles (25-28%) in the two groups. In contrast, leg uptake of FFA corresponded to 39% of leg oxygen consumption in the diabetic group but only 27% in controls. Systemic turnover of oleic acid was similar in the two groups. Splanchnic glucose output increased during exercise 3-4 fold above resting levels in both groups. In the diabetics, splanchnic uptake of lactate, pyruvate, glycerol, and glycogenic amino acids rose more than twofold above resting levels and was fourfold greater than in exercising controls. Total precursor uptake could account for 30% of the splanchnic glucose output in the diabetic group. In contrast, in the controls, total splanchnic uptake of glucose precursors was no greater during exercise than in the resting state and could account for no more than 11% of splanchnic glucose output. The augmented precursor uptake during exercise in the diabetics was a consequence of increased splanchnic fractional extraction as well as increased peripheral production of gluconeogenic substrates. The arterial glucagon concentration was unchanged by exercise in both groups, but was higher in the diabetics. In the diabetic subjects with ketosis in the resting state, exercise elicited a rise in arterial glucose and FFA, an augmented splanchnic uptake of FFA, and a 2-3 fold increase in splanchnic output of 3-hydroxybutyrate. Uptake of 3-hydroxybutyrate by the exercising leg rose more rapidly than splanchnic production, resulting in a fall in arterial levels of 3-hydroxybutyrate. It is concluded that (a) glucose uptake by exercising muscle in hyperglycemic diabetics is no different from that of controls; (b) splanchnic glucose output rises during exercise to a similar extent in diabetics and controls, while uptake of gluconeogenic substrates is markedly higher in diabetics and accounts for a greater proportion of total splanchnic glucose output; (c) exercise in diabetic patients with mild ketosis is associated with a rise in blood glucose and FFA levels as well as augmented splanchnic production and peripheral uptake of ketone bodies.

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.

Uptake of individual free fatty acids by skeletal muscle and liver in man.

Arterial-venous concentration differences for individual free fatty acids (FFA) were measured across the deep tissues of the forearm, the splanchnic vascular bed, and the kidney in healthy, postabsorptive subjects. In addition, arterial-portal venous FFA differences were determined in five patients undergoing elective cholecystectomy. The differences in fractional uptake among the individual FFA across the forearm were small and not statistically significant. Splanchnic fractional uptake was high for FFA with short chain lengths and rose with increasing degree of unsaturation. Small, negative arterial-portal venous differences for individual FFA were observed, indicating that arterial-hepatic venous FFA differences mainly reflect hepatic uptake. When the arterial FFA concentration was reduced to approximately 25% of the control values by the administration of nicotinic acid, net uptake of total FFA ceased but there was release of stearic acid and uptake of lauric, myristic, and palmitoleic acid to the splanchnic region. Muscle and liver uptakes of individual FFA were both dependent on their arterial concentrations with the exception of the splanchnic uptake of stearic acid. There was no uptake of free arachidonic acid by either muscle or liver, nor was there significant uptake of any of the free fatty acids by the kidney. It is concluded (a) that there are important quantitative differences between the net exchanges of individual FFA across the splanchnic vascular bed, (b) that tracer studies of FFA metabolism require the determination of individual FFA specific activities, (c) that palmitic and oleic acid appear to be suitable tracers for the entire FFA fraction in most instances.

Free fatty acid metabolism of the human heart at rest.

Myocardial substrate metabolism was studied in 13 subjects at the time of diagnostic cardiac catheterization by means of palmitic acid-(14)C infusion with arterial and coronary sinus sampling. Two subjects were considered free of cardiac pathology and all, with one exception, demonstrated lactate extraction across the portion of heart under study. Data for this single lactate-producing subject were treated separately.The fractional extraction of (14)C-labeled free fatty acids (FFA) (44.4+/-9.5%) was nearly twice that of unlabeled FFA (23.2+/-7.8%) and raised the possibility of release of FFA into the coronary sinus. FFA uptake, based on either the arterial minus coronary sinus concentration difference or the FFA-(14)C fractional extraction, was directly proportional to the arterial FFA concentration. Gas-liquid chromatography failed to demonstrate selective handling of any individual FFA by the heart. Fractional oxidation of FFA was 53.5+/-12.7%, accounting for 53.2+/-14.4% of the heart's oxygen consumption while nonlipid substrates accounted for an additional 30.0+/-17.3%. Determinations of both labeled and unlabeled triglycerides suggested utilization of this substrate by the fasting human heart. Direct measurement of FFA fractional oxidation as well as FFA uptake, exclusive of possible simultaneous FFA release, would appear necessary in studies concerned with human myocardial FFA metabolism.

Glucose metabolism during leg exercise in man.

Arterial concentrations and net substrate exchange across the leg and splanchnic vascular bed were determined for glucose, lactate, pyruvate, and glycerol in healthy postabsorptive subjects at rest and during 40 min of exercise on a bicycle ergometer at work intensities of 400, 800, and 1200 kg-m/min. Rising arterial glucose levels and small decreases in plasma insulin concentrations were found during heavy exercise. Significant arterial-femoral venous differences for glucose were demonstrated both at rest and during exercise, their magnitude increasing with work intensity as well as duration of the exercise performed. Estimated glucose uptake by the leg increased 7-fold after 40 min of light exercise and 10- to 20-fold at moderate to heavy exercise. Blood glucose uptake could at this time account for 28-37% of total substrate oxidation by leg muscle and 75-89% of the estimated carbohydrate oxidation. Splanchnic glucose production increased progressively during exercise reaching levels 3 to 5-fold above resting values at the heavy work loads. Close agreement was observed between estimates of total glucose turnover during exercise based on leg glucose uptake and splanchnic glucose production. Hepatic gluconeogenesis-estimated from splanchnic removal of lactate, pyruvate, glycerol, and glycogenic amino acids-could supply a maximum of 25% of the resting hepatic glucose production but could account for only 6-11% of splanchnic glucose production after 40 min of moderate to heavy exercise. IT IS CONCLUDED THAT: (a) blood glucose becomes an increasingly important substrate for muscle oxidation during prolonged exercise of this type: (b) peripheral glucose utilization increases in exercise despite a reduction in circulating insulin levels: (c) increased hepatic output of glucose, primarily by means of augmented glycogenolysis, contributes to blood glucose homeostasis in exercise and provides an important source of substrate for exercising muscle.

Splanchnic glucose and amino acid metabolism in obesity.

Arterial concentrations and splanchnic exchange of glucose, lactate, pyruvate, glycerol, free fatty acids, and individual acidic and neutral amino acids were determined in obese and nonobese control subjects in the basal state and during a 45 min infusion of glucose. Glucose was administered to the controls at a rate (2 mg/kg/min; 144 +/- 4 mg/min) known to inhibit splanchnic glucose output without influencing peripheral glucose utilization. The obese subjects received glucose at two dose levels (75 and 150 mg/min) which simulated either the rise in insulin or the inhibition in splanchnic glucose production observed in the controls. In the basal state splanchnic glucose production did not differ significantly between obese and control subjects. However splanchnic uptake of lactate, glycerol, alanine, free fatty acids, and oxygen was 50-160% greater in obese subjects. Splanchnic uptake of glucose precursors could account for 33% of hepatic glucose output in the obese group as compared to 19% in controls. The increase in alanine and lactate uptake was due in part, to a 50% increase in splanchnic fractional extraction. Administration of glucose to the control subjects 144 +/- 4 mg/min) resulted in a 50-60% increment in arterial insulin and a 75% reduction in splanchnic glucose output. In the obese group, infusion of glucose at a rate of 75 mg/min resulted in an equivalent rise in arterial insulin, but was accompanied by a less than 40% inhibition in splanchnic glucose output. Glucose infusion at a rate of 150 mg/min in the obese resulted in a 75% reduction in splanchnic glucose output which was equivalent to that observed in controls, but was accompanied by a significantly greater rise (100-200%) in arterial insulin. It is concluded that in obesity (a) despite basal hyperinsulinemia, splanchnic uptake of glucose precursors is increased, the relative contribution to total glucose release attributable to gluconeogenesis being 70% higher than in controls; (b) infusion of glucose at rates causing equivalent increases in arterial insulin induces a smaller inhibition in splanchnic glucose output than in controls; (c) infusion of glucose at rates causing comparable inhibition in splanchnic glucose output is accompanied by a disproportionately greater increase in endogenous insulin than in controls. These data are compatible with hepatic resistance to insulin in obesity.

Splanchnic and peripheral glucose and amino acid metabolism in diabetes mellitus.

Splanchnic and leg exchange of glucose, lactate, pyruvate, and individual plasma amino acids was studied in diabetics 24 hr after withdrawal of insulin and in healthy controls. Measurements were made in the basal postabsorptive state and during the administration of glucose at a rate of 2 mg/kg per min for 45 min. In the basal state, net splanchnic glucose production did not differ significantly between diabetics and controls. However, splanchnic uptake of alanine and other glycogenic amino acids was 1(1/2)-2 times greater in the diabetics, while lactate and pyruvate uptake was increased by 65-115%. Splanchnic uptake of these glucose precursors could account for 32% of hepatic glucose output in the diabetics, as compared to 20% in the controls. This increase in precursor uptake was a consequence of a two- to threefold increment in fractional extraction of these substrates inasmuch as arterial levels of alanine, glycine, and threonine were reduced in the diabetics, while the levels of the remaining substrates were similar in the two groups. Peripheral output of alanine and other glycogenic amino acids as reflected in arterio-femoral venous differences was similar in both groups. An elevation in arterial valine, leucine, and isoleucine was observed in the diabetics, but could not be accounted for on the basis of alterations in splanchnic or peripheral exchange of these amino acids. Administration of glucose (2 mg/kg per min) for 45 min resulted in an 80% reduction in splanchnic glucose output in controls, but failed to inhibit hepatic glucose release in the diabetics despite a twofold greater increment in arterial glucose levels. In both groups no consistent changes in arterial glucagon were observed during the infusion. It is concluded that in nonketotic diabetics (a) total splanchnic output of glucose is comparable to controls, but the relative contribution of gluconeogenesis may be increased by more than 50%; (b) accelerated splanchnic uptake of glucose precursors is a consequence of increased hepatic extraction of available substrates rather than a result of augmented substrate supply; and (c) the failure of glucose infusion to inhibit hepatic glucose output suggests that the exquisite sensitivity of the liver to the infusion of glucose in normal man is a consequence of glucose-induced insulin secretion.

Influence of hyperthyroidism on splanchnic exchange of glucose and gluconeogenic precursors.

Arterial concentrations and splanchnic exchange of glucose, amino acids, lactate, pyruvate, and glycerol were determined in 14 hyperthyroid patients and 12 healthy controls. Seven of the patients were restudied after 5-12 mo of medical management at which time there was chemical and clinical evidence of a euthyroid state. The arterial level of glucose was slightly higher (+10%) in the patient group and the glycerol concentration was three times greater among the patients. The plasma levels of the glycogenic amino acids, alanine, glycine, and serine were decreased by 20-30%, while the concentrations of leucine, isoleucine, and tyrosine were increased by 20-80%. The levels of lactate and pyruvate were similar in patients and controls as were insulin and glucagon concentrations. Splanchnic glucose output in the patient group was 35% lower than in controls. However, total splanchnic uptake of glucogenic precursors was 100% higher than in controls and showed a direct linear correlation with serum triiodothyronine. Total precursor uptake could account for 75% of splanchnic glucose output in the patients, compared to 26% in controls. The increase in uptake of lactate, alanine, and other amino acids was due to a 35-80% rise in splanchnic fractional extraction plus a 20% rise in estimated hepatic blood flow. When the patients were restudied after medical treatment splanchnic exchange of glucose and glucose precursors had reverted to normal values. The present findings demonstrate that in hyperthyroidism (a) total splanchnic glucose output is reduced in relation to controls, (b) splanchnic uptake of gluconeogenic precursors is accelerated, largely due to a rise in fractional extraction of precursor substrates and to a smaller extent, as a result of an increase in hepatic blood flow, and (c) these changes revert to normal when a euthyroid state has been achieved.

Turnover and splanchnic metabolism of free fatty acids in hyperthyroid patients.

The arterial concentration and turnover rate and the splanchnic exchange of FFA were examined after an overnight fast in a group of 11 female patients with clinical and laboratory evidence of hyperthyroidism. [14C]oleic acid was infused intravenously and the hepatic venous catheter technique was used. As compared with healthy control individuals, the arterial concentrations of FFA and oleic acid were elevated by 30--40% in the hyperthyroid group. Both the turnover rate and the fractional turnover of oleic acid were significantly increased. The turnover rate correlated directly with arterial concentration of oleic acid in both the control and the patient group but the slope was steeper in the patients. The splanchnic uptake of oleic acid was three times higher than in the control group. The augmented uptake was a consequence of elevated arterial concentrations and increased hepatic plasma flow, whereas fractional splanchnic uptake remained unchanged. Ketone body production was four- to fivefold greater in the patients and could be largely accounted for by increased splanchnic FFA uptake. In six patients studied after treatment resulting in a return to normal thyroid function, a significant reduction was observed in arterial FFA, estimated hepatic blood flow, oleic acid turnover, and ketone body production. It is concluded that hyperthyroidism is characterized by increased turnover and splanchnic uptake of FFA and augmented ketogenesis. These findings can be explained on the basis of elevated arterial FFA concentrations and increased blood flow, particularly to the splanchnic bed.

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.

Hepatic extraction and renal production of 3,3'-diiodothyronine and 3',5'-diiodothyronine in man.

The sequential deiodination of thyroxine (T4) gives rise to several iodothyronine analogs including 3,3'-diiodothyronine (3,3'-T2) and 3',5'-diiodothyronine (3',5'-T2). In vitro animal studies suggest that the liver and the kidneys are the main sites of both formation and degradation of 3,3'-T2 and 3',5'-T2. To determine the metabolism of 3,3'-T2 and 3',5'-T2 in human liver and kidneys plasma samples were obtained from (a) a brachial artery and a hepatic vein in 20 normal subjects, and from (b) a femoral artery and a renal vein in 11 normal subjects. Further, the hepatic plasma flow (a) and the renal plasma flow (b) were determined. Both plasma 3,3'-T2 and 3',5'-T2 levels were reduced in the hepatic venous blood as compared to arterial values (1.09 +/- 0.40 vs. 1.75 +/- 0.74 ng/dl (P < 0.02)) (mean +/- 1 SD). This resulted in a hepatic extraction of both, 3,3'-T2 and 3',5'-T2, which averaged 8.2 and 5.2 microgram/d, respectively. Plasma 3,3'-T2 as well as 3'5'-T2 levels were higher in the renal vein as compared to arterial values, 1.49 +/- 0.42 vs. 1.39 +/- 0.45 ng/dl (P < 0.05) and 2.35 +/- 0.83 vs. 2.09 +/- 0.81 ng/dl (P < 0.05), respectively. This positive venoarterial difference implies a net production of 3,3'-T2 and 3',5'-T2 in the kidneys of 1.2 and 3.0 microgram/d, respectively. It is concluded that the liver is an important site of 3,3'-T2 and 3',5'-T2 extraction in normal man. In contrast, the renal production of 3,3'-T2 as well as 3'5'-T2 exceeds the degradation and urinary excretion.

Effects of insulin on peripheral and splanchnic glucose metabolism in noninsulin-dependent (type II) diabetes mellitus.

The mechanism(s) and site(s) of the insulin resistance were examined in nine normal-weight noninsulin-dependent diabetic (NIDD) subjects. The euglycemic insulin clamp technique (insulin concentration approximately 100 microU/ml) was employed in combination with hepatic and femoral venous catheterization and measurement of endogenous glucose production using infusion of tritiated glucose. Total body glucose metabolism in the NIDD subjects (4.37 +/- 0.45 mg/kg per min) was 38% (P less than 0.01) lower than in controls (7.04 +/- 0.63 mg/kg per min). Quantitatively, the most important site of the insulin resistance was found to be in peripheral tissues. Leg glucose uptake in the diabetic group was reduced by 45% as compared with that in controls (6.0 +/- 0.2 vs. 11.0 +/- 0.1 mg/kg leg wt per min; P less than 0.01). A strong positive correlation was observed between leg and total body glucose uptake (r = 0.70, P less than 0.001). Assuming that muscle is the primary leg tissue responsible for glucose uptake, it could be estimated that 90 and 87% of the infused glucose was disposed of by peripheral tissues in the control and NIDD subjects, respectively. Net splanchnic glucose balance during insulin stimulation was slightly more positive in the control than in the diabetic subjects (0.31 +/- 0.10 vs. 0.05 +/- 0.19 mg/kg per min; P less than 0.07). The difference (0.26 mg/kg per min) in net splanchnic glucose balance in NIDD represented only 10% of the reduction (2.67 mg/kg per min) in total body glucose uptake in the NIDD group and thus contributed very little to the insulin resistance. The results emphasize the importance of the peripheral tissues in the disposal of infused glucose and indicate that muscle is the most important site of the insulin resistance in NIDD.