Effect of increased free fatty acid supply on glucose metabolism and skeletal muscle glycogen synthase activity in normal man.

1. Experimental elevation of plasma non-esterified fatty acid concentrations has been postulated to decrease insulin-stimulated glucose oxidation and storage rates. Possible mechanisms were examined by measuring skeletal muscle glycogen synthase activity and muscle glycogen content before and during hyperinsulinaemia while fasting plasma non-esterified fatty acid levels were maintained. 2. Fasting plasma non-esterified fatty acid levels were maintained in seven healthy male subjects by infusion of 20% (w/v) Intralipid (1 ml/min) for 120 min before and during a 240 min hyperinsulinaemic euglycaemic clamp (100 m-units h-1 kg-1) combined with indirect calorimetry. On the control day, 0.154 mol/l NaCl was infused. Vastus lateralis muscle biopsy was performed before and at the end of the insulin infusion. 3. On the Intralipid study day serum triacylglycerol (2.24 +/- 0.20 versus 0.67 +/- 0.10 mmol/l), plasma nonesterified fatty acid (395 +/- 13 versus 51 +/- 1 mumol/l), blood glycerol (152 +/- 2 versus 11 +/- 1 mumol/l) and blood 3-hydroxybutyrate clamp levels [mean (95% confidence interval)] [81 (64-104) versus 4 (3-5) mumol/l] were all significantly higher (all P less than 0.001) than on the control study day. Lipid oxidation rates were also elevated (1.07 +/- 0.07 versus 0.27 +/- 0.08 mg min-1 kg-1, P less than 0.001). During the clamp with Intralipid infusion, insulin-stimulated whole-body glucose disposal decreased by 28% (from 8.53 +/- 0.77 to 6.17 +/- 0.71 mg min-1 kg-1, P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)     

Insulin sensitivity of protein and glucose metabolism in human forearm skeletal muscle.

Physiologic increases of insulin promote net amino acid uptake and protein anabolism in forearm skeletal muscle by restraining protein degradation. The sensitivity of this process to insulin is not known. Using the forearm perfusion method, we infused insulin locally in the brachial artery at rates of 0.00 (saline control), 0.01, 0.02, 0.035, or 0.05 mU/min per kg for 150 min to increase local forearm plasma insulin concentration by 0, approximately 20, approximately 35, approximately 60, and approximately 120 microU/ml (n = 35). L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine were infused systemically, and the net forearm balance, rate of appearance (Ra) and rate of disposal (R(d)) of phenylalanine and leucine, and forearm glucose balance were measured basally and in response to insulin infusion. Compared to saline, increasing rates of insulin infusion progressively increased net forearm glucose uptake from 0.9 mumol/min per 100 ml (saline) to 1.0, 1.8, 2.4, and 4.7 mumol/min per 100 ml forearm, respectively. Net forearm balance for phenylalanine and leucine was significantly less negative than basal (P < 0.01 for each) in response to the lowest dose insulin infusion, 0.01 mU/min per kg, and all higher rates of insulin infusion. Phenylalanine and leucine R(a) declined by approximately 38 and 40% with the lowest dose insulin infusion. Higher doses of insulin produced no greater effect (decline in R(a) varied between 26 and 42% for phenylalanine and 30-50% for leucine). In contrast, R(d) for phenylalanine and leucine did not change with insulin. We conclude that even modest increases of plasma insulin can markedly suppress proteolysis, measured by phenylalanine R(a), in human forearm skeletal muscle. Further increments of insulin within the physiologic range augment glucose uptake but have little additional effect on phenylalanine R(a) or balance. These results suggest that proteolysis in human skeletal muscle is more sensitive than glucose uptake to physiologic increments in insulin.     

Impaired free fatty acid utilization by skeletal muscle in non-insulin-dependent diabetes mellitus.

This study was undertaken to assess utilization of FFA by skeletal muscle in patients with non-insulin-dependent diabetes mellitus (NIDDM). 11 NIDDM and 9 nondiabetic subjects were studied using leg balance methods to measure the fractional extraction of [3H]oleate. Limb indirect calorimetry was used to estimate RQ. Percutaneous muscle biopsy samples of vastus lateralis were analyzed for muscle fiber type distribution, capillary density, and metabolic potential as reflected by measurements of the activity of seven muscle enzyme markers of glycolytic and aerobic-oxidative pathways. During postabsorptive conditions, fractional extraction of oleate across the leg was lower in NIDDM subjects (0.31 +/- 0.08 vs. 0.43 +/- 0.10, P < 0.01), and there was reduced oleate uptake across the leg (66 +/- 8 vs. 82 +/- 13 nmol/min, P < 0.01). Postabsorptive leg RQ was increased in NIDDM (0.85 +/- 0.03 vs. 0.77 +/- 0.02, P < 0.01), and rates of lipid oxidation by skeletal muscle were lower while glucose oxidation was increased (P < 0.05). In subjects with NIDDM, proportions of type I, IIa, and IIb fibers were 37 +/- 2, 37 +/- 6, and 26 +/- 5%, respectively, which did not differ from nondiabetics; and capillary density, glycolytic, and aerobic-oxidative potentials were similar. During 6 h after ingestion of a mixed meal, arterial FFA remained greater in NIDDM subjects. Therefore, despite persistent reduced fractional extraction of oleate across the leg in NIDDM (0.34 +/- 0.04 vs. 0.38 +/- 0.03, P < 0.05), rates of oleate uptake across the leg were greater in NIDDM (54 +/- 7 vs. 45 +/- 8 nmol/min, P < 0.01). In summary, during postabsorptive conditions there is reduced utilization of FFA by muscle, while during postprandial conditions there is impaired suppression of FFA uptake across the leg in NIDDM. During both fasting and postprandial conditions, NIDDM subjects have reduced rates of lipid oxidation by muscle.     

Glucose-free fatty acid cycle operates in human heart and skeletal muscle in vivo.

Positron emission tomography permits noninvasive measurement of regional glucose uptake in vivo in humans. We employed this technique to determine the effect of FFA on glucose uptake in leg, arm, and heart muscles. Six normal men were studied twice under euglycemic hyperinsulinemic (serum insulin approximately 500 pmol/liter) conditions, once during elevation of serum FFA by infusions of heparin and Intralipid (serum FFA 2.0 +/- 0.4 mmol/liter), and once during infusion of saline (serum FFA 0.1 +/- 0.01 mmol/liter). Regional glucose uptake rates were measured using positron emission tomography-derived 18F-fluoro-2-deoxy-D-glucose kinetics and the three-compartment model described by Sokoloff (Sokoloff, L., M. Reivich, C. Kennedy, M. C. Des Rosiers, C. S. Patlak, K. D. Pettigrew, O. Sakurada, and M. Shinohara. 1977. J. Neurochem. 28: 897-916). Elevation of plasma FFA decreased whole body glucose uptake by 31 +/- 2% (1,960 +/- 130 vs. 2,860 +/- 250 mumol/min, P less than 0.01, FFA vs. saline study). This decrease was due to inhibition of glucose uptake in the heart by 30 +/- 8% (150 +/- 33 vs. 200 +/- 28 mumol/min, P less than 0.02), and in skeletal muscles; both when measured in femoral (1,594 +/- 261 vs. 2,272 +/- 328 mumol/min, 25 +/- 13%) and arm muscles (1,617 +/- 411 to 2,305 +/- 517 mumol/min, P less than 0.02, 31 +/- 6%). Whole body glucose uptake correlated with glucose uptake in femoral (r = 0.75, P less than 0.005), and arm muscles (r = 0.69, P less than 0.05) but not with glucose uptake in the heart (r = 0.04, NS). These data demonstrate that the glucose-FFA cycle operates in vivo in both heart and skeletal muscles in humans.     

Regulation of glucose transport in human skeletal muscle

Glucose transport, the rate limiting step in glucose metabolism in skeletal muscle, is mediated by insulin-sensitive glucose transporter 4 (GLUT4) and can be activated in skeletal muscle by two separate and distinct signalling pathways: one stimulated by insulin and the second by muscle contractions. Skeletal muscle is the principal tissue responsible for insulin-stimulated glucose disposal and thus the major site of peripheral insulin resistance. Impaired glucose transport in skeletal muscle leads to impaired whole body glucose uptake, and contributes to the pathogenesis of Type 2 diabetes mellitus. A combination of genetic and environmental factors is likely to contribute to the pathogenesis of Type 2 diabetes mellitus; however, the primary defect is still unknown. Intense efforts are underway to define the molecular mechanisms that regulate glucose metabolism in insulin sensitive tissues. This review will present our current understanding of mechanisms regulating glucose transport in skeletal muscle in humans. Elucidation of the pathways involved in the regulation of glucose homeostasis will offer insight into the pathogenesis of insulin resistance and Type 2 diabetes mellitus and may lead to the identification of biochemical entry points for drug intervention to improve glucose homeostasis.     

Regulation of glucose transport in human skeletal muscle

Glucose transport, the rate limiting step in glucose metabolism in skeletal muscle, is mediated by insulin-sensitive glucose transporter 4 (GLUT4) and can be activated in skeletal muscle by two separate and distinct signalling pathways: one stimulated by insulin and the second by muscle contractions. Skeletal muscle is the principal tissue responsible for insulin-stimulated glucose disposal and thus the major site of peripheral insulin resistance. Impaired glucose transport in skeletal muscle leads to impaired whole body glucose uptake, and contributes to the pathogenesis of Type 2 diabetes mellitus. A combination of genetic and environmental factors is likely to contribute to the pathogenesis of Type 2 diabetes mellitus; however, the primary defect is still unknown. Intense efforts are underway to define the molecular mechanisms that regulate glucose metabolism in insulin sensitive tissues. This review will present our current understanding of mechanisms regulating glucose transport in skeletal muscle in humans. Elucidation of the pathways involved in the regulation of glucose homeostasis will offer insight into the pathogenesis of insulin resistance and Type 2 diabetes mellitus and may lead to the identification of biochemical entry points for drug intervention to improve glucose homeostasis.     

Lipoprotein metabolism influenced by training-induced changes in human skeletal muscle.

The influence of training-induced adaptations in skeletal muscle tissue on lipoprotein metabolism was investigated in six healthy men. The knee extensors were studied at rest and during exercise after 8 wk of dynamic exercise training of the knee extensors of one leg, while the other leg served as a control. The trained and nontrained thighs were investigated on different occasions. In the trained knee extensors, muscle (m) lipoprotein lipase activity (LPLA) was 70 +/- 29% higher compared with the nontrained (P less than 0.05), and correlated positively with the capillary density (r = 0.84). At rest there was a markedly higher arteriovenous (A-V) VLDL triacylglycerol (TG) difference over the trained thigh, averaging 55 mumol/liter (range 30-123), than over the nontrained, averaging 30 mumol/liter (4-72). In addition to the higher LPLA and VLDL-TG uptake in the trained thigh, a higher production of HDL cholesterol (C) and HDL2-C was also observed (P less than 0.05). Positive correlations between m-LPLA and A-V differences of VLDL-TG (r = 0.90; P less than 0.05) were observed only in the trained thigh. During exercise with the trained thigh the venous concentration of HDL2-C was invariably higher than the arterial, and after 110 min of exercise a production of 88 mumol/min (54-199) of HDL2-C was revealed. Even though a consistent degradation of VLDL-TG was not found during exercise, the total production of HDL-C across the trained and nontrained thigh, estimated from A-V differences times venous blood flow for the whole exercise period, correlated closely with the total estimated degradation of VLDL-TG (r = 0.91). At the end of 2 h of exercise m-LPLA did not differ from the preexercise value in either the nontrained or the trained muscle. We conclude that changes in the lipoprotein profile associated with endurance training to a large extent are explainable by training-induced adaptations in skeletal muscle tissue.     

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.     

Influence of body fat distribution on free fatty acid metabolism in obesity.

In order to determine whether differences in body fat distribution result in specific abnormalities of free fatty acid (FFA) metabolism, palmitate turnover, a measure of systemic adipose tissue lipolysis, was measured in 10 women with upper body obesity, 9 women with lower body obesity, and 8 nonobese women under overnight postabsorptive (basal), epinephrine stimulated and insulin suppressed conditions. Results: Upper body obese women had greater (P less than 0.005) basal palmitate turnover than lower body obese or nonobese women (2.8 +/- 0.2 vs. 2.1 +/- 0.2 vs. 1.8 +/- 0.2 mumol.kg lean body mass (LBM)-1.min-1, respectively), but a reduced (P less than 0.05) net lipolytic response to epinephrine (59 +/- 7 vs. 79 +/- 5 vs. 81 +/- 7 mumol palmitate/kg LBM, respectively). Both types of obesity were associated with impaired suppression of FFA turnover in response to euglycemic hyperinsulinemia compared to nonobese women (P less than 0.005). These specific differences in FFA metabolism may reflect adipocyte heterogeneity, which may in turn affect the metabolic aberrations associated with different types of obesity. These findings emphasize the need to characterize obese subjects before studies.     

Stimulation of glucose metabolism in human blood cells by inhibitors of carnitine-dependent fatty acid transport

According to a well accepted hypothesis, increased fatty acid oxidation can lead to hyperglycaemia by stimulating gluconeogenesis and reducing glycolysis. Therefore, inhibitors of fatty acid metabolism should cause hypoglycaemia by inhibiting gluconeogenesis and activating glycolysis. Various substances were tested to validate this hypothesis with regard to glucose oxidation in human mononuclear leukocytes and thrombocytes. 2-(3-Methyl-cinnamyl-hydrazono)-propionate, an inhibitor of the carnitine acyltransfer system was found to cause hypoglycaemia in whole animals and to inhibit gluconeogensis in the perfused guinea pig liver, while the acetyl-CoA/CoASH ratio was decreased. This substance stimulated the metabolism of glucose to CO2 in human mononuclear leukocytes and especially in platelets. This effect could be potentiated if concanavalin A and 2-(3-methyl-cinnamyl-hydrazono)-propionate were applied simultaneously. Under these conditions, however, fatty acid oxidation was no longer inhibited. From these results, it can be concluded that the activation of glucose oxidation by 2-(3-methyl-cinnamyl-hydrazono)-propionate is independent of its effect on fatty acid metabolism. Other inhibitors of fatty acid metabolism which were also investigated behaved similarly.     

Insulin resistance in liver cirrhosis. Positron-emission tomography scan analysis of skeletal muscle glucose metabolism.

BACKGROUND. Insulin resistance and glucose intolerance are a major feature of patients with liver cirrhosis. However, site and mechanism of insulin resistance in cirrhosis are unknown. We investigated insulin-induced glucose metabolism of skeletal muscle by positron-emission tomography to identify possible defects of muscle glucose metabolism in these patients. METHODS. Whole body glucose disposal and oxidation were determined by the combined use of the euglycemic-hyperinsulinemic clamp technique (insulin infusion rate: 1 mU/kg body wt per min) and indirect calorimetry in seven patients with biopsy-proven liver cirrhosis (Child: 1A, 5B, and 1C) and five healthy volunteers. Muscle glucose uptake of the thighs was measured simultaneously by dynamic [18F]fluorodeoxyglucose positron-emission tomography scan. RESULTS. Both whole body and nonoxidative glucose disposal were significantly reduced in patients with liver cirrhosis (by 48%, P < 0.001, and 79%, P < 0.0001, respectively), whereas glucose oxidation and the increase in plasma lactate were normal. Concomitantly, skeletal muscle glucose uptake was reduced by 69% in liver cirrhosis (P < 0.003) and explained 55 or 92% of whole body glucose disposal in cirrhotics and controls, respectively. Analysis of kinetic constants using a three-compartment model further indicated reduced glucose transport (P < 0.05) but unchanged phosphorylation of glucose in patients with liver cirrhosis. CONCLUSIONS. Patients with liver cirrhosis show significant insulin resistance that is characterized by both decreased glucose transport and decreased nonoxidative glucose metabolism in skeletal muscle.     

Pathways of fatty acid metabolism in human platelets

The metabolic fate of ¹⁴C-labeled fatty acids which have been incubated with human platelets, has been traced. The following has been shown. (a) Intact platelets have a considerable capacity to oxidize fatty acids. (b) When tracer amounts of four of the most common fatty acids in normal plasma were incubated with platelets, each showed a distinctive pattern of uptake among neutral lipids and phospholipids. With regard to the latter, it was shown that these distribution patterns were, in most cases, similar to those of the fatty acids found in natural platelet phospholipids. (c) By increasing the time of incubation or the amount of added oleic acid, the distribution of oleic acid uptake between lecithin and other phosphoglycerides was altered so that a larger share was incorporated into the latter. (d) The effects of added lysolecithin or lysoethanolamine phosphoglycerides on oleic acid incorporation into platelet phosphoglycerides are quite variable. At low concentrations, added lysolecithin functions chiefly as a reaction partner for oleic acid. Added adenosine triphosphate and CoASH augment the incorporation of oleic acid into lecithin over a wide range of added lysolecithin (12.5-500 μmoles/liter). At higher concentrations of added lysolecithin, in the absence of ATP and CoASH, oleic acid incorporation into lecithin is considerably reduced. Also, added lysolecithin and lysoethanolamine phosphoglycerides, in the absence of ATP and CoASH, are able, at certain concentrations, to stimulate oleic acid incorporation into all except the serine phosphoglycerides. (e) Platelets appear to have a de novo pathway for renewal of lecithin.     

Effect of the antilipolytic nicotinic acid analogue acipimox on whole-body and skeletal muscle glucose metabolism in patients with non-insulin-dependent diabetes mellitus.

Increased nonesterified fatty acid (NEFA) levels may be important in causing insulin resistance in skeletal muscles in patients with non-insulin-dependent diabetes mellitus (NIDDM). The acute effect of the antilipolytic nicotinic acid analogue Acipimox (2 X 250 mg) on basal and insulin-stimulated (3 h, 40 mU/m2 per min) glucose metabolism was therefore studied in 12 patients with NIDDM. Whole-body glucose metabolism was assessed using [3-3H]glucose and indirect calorimetry. Biopsies were taken from the vastus lateralis muscle during basal and insulin-stimulated steady-state periods. Acipimox reduced NEFA in the basal state and during insulin stimulation. Lipid oxidation was inhibited by Acipimox in all patients in the basal state (20 +/- 2 vs. 33 +/- 3 mg/m2 per min, P less than 0.01) and during insulin infusion (8 +/- 2 vs. 17 +/- 2 mg/m2 per min, P less than 0.01). Acipimox increased the insulin-stimulated glucose disposal rate (369 +/- 49 vs. 262 +/- 31 mg/m2 per min, P less than 0.01), whereas the glucose disposal rate was unaffected by Acipimox in the basal state. Acipimox increased glucose oxidation in the basal state (76 +/- 4 vs. 50 +/- 4 mg/m2 per min, P less than 0.01). During insulin infusion Acipimox increased both glucose oxidation (121 +/- 7 vs. 95 +/- 4 mg/m2 per min, P less than 0.01) and nonoxidative glucose disposal (248 +/- 47 vs. 167 +/- 29 mg/m2 per min, P less than 0.01). Acipimox enhanced basal and insulin-stimulated muscle fractional glycogen synthase activities (32 +/- 2 vs. 25 +/- 3%, P less than 0.05, and 50 +/- 5 vs. 41 +/- 4%, P less than 0.05). Activities of muscle pyruvate dehydrogenase and phosphofructokinase were unaffected by Acipimox. In conclusion, Acipimox acutely improved insulin action in patients with NIDDM by increasing both glucose oxidation and nonoxidative glucose disposal. This supports the hypothesis that elevated NEFA concentrations may be important for the insulin resistance in NIDDM. The mechanism responsible for the increased insulin-stimulated nonoxidative glucose disposal may be a stimulatory effect of Acipimox on glycogen synthase activity in skeletal muscles.     

Regulation by insulin of myocardial glucose and fatty acid metabolism in the conscious dog.

In vivo small doses of insulin inhibit lipolysis, lower plasma FFA, and stimulate glucose disposal. Lowering of plasma FFA, either in the absence of a change in insulin or during combined hyperglycemia and hyperinsulinemia, promotes glucose uptake by heart muscle in vivo. In the isolated perfused heart, large doses of insulin directly stimulate heart glucose uptake. To assess the effect of physiological elevations of plasma insulin upon myocardial glucose and FFA uptake in vivo independent of changes in plasma substrate concentration, we measured arterial and coronary sinus concentrations of glucose, lactate, and FFA, and coronary blood flow in conscious dogs during a 30 min basal and a 2 h experimental period employing three protocols: (a) euglycemic hyperinsulinemia (insulin clamp, n = 5), (b) euglycemic hyperinsulinemia with FFA replacement (n = 5), (c) hyperglycemic euinsulinemia (hyperglycemic clamp with somatostatin, n = 5). In group 1, hyperinsulinemia (insulin = 73 +/- 13 microU/ml) stimulated heart glucose uptake (7.3 +/- 4.4 vs. 28.2 +/- 2.8 mumol/min, P less than 0.002), lowered plasma FFA levels by 80% (P less than 0.05), and decreased heart FFA uptake (28.4 +/- 4 vs. 1.5 +/- 0.9, P less than 0.01). When the fall in plasma FFA was prevented by FFA infusion (group 2), hyperinsulinemia (86 +/- 10 microU/ml) provoked a lesser (P less than 0.05) stimulation of glucose uptake (delta = 8.2 +/- 4.2 mumol/min) than in group 1, and there was no significant change in FFA uptake (25.3 +/- 16 vs. 16.5 +/- 4). Hyperglycemia (plasma glucose = 186 +/- 8 mg/100 ml) during somatostatin infusion resulted in only a small rise in plasma insulin (delta = 12 +/- 7 microU/ml), and although plasma FFA tended to decline, heart glucose uptake did not rise significantly (delta = 5.5 +/- 3.2 mumol/min, P = NS). There was no significant change in coronary blood flow during any of the three study protocols. We conclude that, in the dog, insulin at physiologic concentrations: (a) stimulates heart glucose uptake, both directly and by suppressing the plasma FFA concentration, and (b) does not alter coronary blood flow. Hyperglycemia per se has little effect on heart glucose uptake.     

Kinetic studies of plasma free fatty acid and triglyceride metabolism in man

Plasma transport of free fatty acids (FFA) and triglyceride fatty acids (TGFA) was studied in seven subjects with normal lipid metabolism, one case of total lipodystrophy, and one case of familial hyperlipemia (Type V). Studies were carried out after intravenous injection of radioactive FFA, of lipoproteins previously labeled in vitro in the triglyceride moiety, or both.Computer techniques were used to evaluate a series of multicompartmental models, and a general model is proposed that yields optimum fitting of experimental data for both FFA and TGFA. The results show that as much as 20-30% of FFA leaving the plasma compartment in normal subjects is transported to an exchanging extravascular pool and quickly reenters the plasma pool as FFA. The rate of irreversible delivery of FFA from plasma to tissues averaged 358 muEq/min in normals. The lipodystrophy patient, despite the virtual absence of adipose tissue (confirmed at autopsy), had a plasma FFA concentration and a total FFA transport, both more than twice normal. Total TGFA transport ranged from 25 to 81 muEq/min in four normal controls. The rate constant for TGFA turnover in the patient with Type V hyperlipemia was so small that total transport could not be quantified from the data available; the TGFA half-life was over 500 min.In two normal subjects given injections of autologous lipoproteins labeled in vitro with triolein-(14)C and simultaneously given oleic acid-(3)H, it was shown that the time course for the disappearance of the TGFA in the in vitro labeled samples conformed almost exactly to that of the physiologically labeled lipoprotein TGFA synthesized from injected FFA (as evidenced by the simultaneous fitting of both sets of data using the same multicompartmental model and the same rate constants). Radioactivity appeared in the plasma FFA fraction at a significant rate after injection of plasma labeled in vitro with TGFA. It was estimated that as much as 50% of the total TGFA transported underwent rapid and rather direct conversion to FFA in the two normal subjects studied this way. The kinetic data suggest that such conversion of TGFA to FFA was not preceded by any extensive dilution, such as would result from complete mixing with tissue triglyceride stores.     

Effects of uridine and inosine on glucose metabolism in skeletal muscle and activated lipolysis in adipose tissue.

In a first series of experiments, the effects of uridine and inosine on glucose metabolism in rat diaphragm muscle incubated in Krebs-bicarbonate buffer were studied. Uridine in concentrations of 10(-4) to 10(-6) M stimulated the uptake of glucose and increased the content of glycogen, but had no effect on the production of lactate. When diaphragm muscles were incubated in the buffer without glucose, uridine (10(-4)-10(-6) M) had no effects on the content of glycogen and on the production of lactate. On the other hand, inosine in concentrations of 10(-4) to 10(-6) M stimulated the uptake of glucose and the production of lactate, but had no effect on the content of glycogen in the muscle. In a second series of experiments, uridine (10(-4)-10(-5) M) and inosine (10(-4)-10(-7) M) inhibited the relase of glycerol from isolated rat epididymal adipose tissue in Krebs-bicarbonate buffer. Uridine and inosine in concentrations of 10(-4) M inhibited the epinephrine (10(-5) M)-, the norepinephrine (10(-5) M)- and the theophylline (10(-3) M)-stimulated lipolysis. Dibutyryl 3',5'-adenosine monophosphate-stimulated lipolysis was further activated in the presence of 10(-4) M uridine or inosine. Dose-response curves studies suggested that inosine, but not uridine, has a common receptor site with epinephrine in adipose tissue. These results demonstrated that both nucleosides stimulated the glucose uptake, but only uridine increased the synthesis of glycogen in the muscle. Both nucleosides also inhibited lipolysis in adipose tissue. The mechanism of antilipolytic action of these nucleosides is unknown, but one of the receptor sites for inosine might be adenylate cyclase.     

Effects of insulin on glucose metabolism in skeletal muscle from septic and endotoxaemic rats

1. The effects of non-lethal bacteraemia or endotoxaemia on insulin-stimulated glucose metabolism were studied in isolated, incubated soleus muscle of rats after 24 and 48 h. 2. The insulin-stimulated rates of lactate formation and glycogen synthesis were similar in muscles isolated from control and bacteraemic rats. 3. Endotoxaemia increased the rates of lactate formation, at all levels of insulin, both at 24 h (approximately 32%) and 48 h (approximately 26%). Endotoxaemia did not alter the sensitivity of glycolysis to insulin. 4. Endotoxaemia decreased the rates of glycogen synthesis at all concentrations of insulin both at 24 h (approximately 39%) and 48 h (approximately 23%). 5. The increase in the rate of glycolysis was related in a dose-dependent manner to the amount of endotoxin given to the animals. 6. Endotoxaemia decreased plasma tri-iodothyronine levels (41%). However, the effects of endotoxaemia (48 h) on glucose metabolism in muscle are similar to those caused by hyperthyroidism. In hypothyroid rats, endotoxin administration increased the rates of glycolysis in muscle in vitro. 7. It is concluded that there are enhanced basal and insulin-stimulated rates of glycolysis in soleus muscle from endotoxaemic rats. This may be due to both increased glucose transport and decreased glycogen synthesis.