Enzymatic activities in slow and fast denervated old rat muscles

The activities of five enzymes have been studied quantitatively in denervated extensor digitorum longus, gastrocnemius and soleus muscles of 24-month-old rats. The results have been compared with those obtained from normal muscles of a similar age group of rats. Three weeks after denervation, the activity of hexokinase was increased in gastrocnemius and extensor digitorum longus. Phosphofructokinase, lactate dehydrogenase, malate dehydrogenase and 3-hydroxyacyl-CoA-dehydrogenase showed decreased activities. These results suggest that enzyme which represents glucose uptake increased its activity in fast muscles and that enzymes for anaerobic glycolysis, lactate fermentation, citric acid cycle and beta-oxidation had a decreased activity in slow and fast muscles.     

Metabolic interactions of glucose, acetoacetate and adrenaline in rat submaxillary gland in vitro.

1. The metabolic interactions between glucose, acetoacetate and adrenaline were studied in submaxillary-gland slices. 2. Acetoacetate (2.5 mM) inhibited glucose removal by 22% and entry of glucose carbon into the tricarboxylic acid cycle by 54%. 3. Acetoacetate caused an increase in (glucose 6-phosphate) together with an increase in (citrate), a finding that suggests that the phosphofructokinase step might be inhibited by the elevated (citrate). Support for this suggestion was obtained in experiments in which fluoracetate was used to elevate (citrate). 4. A further site of action of acetoacetate at the pyruvate dehydrogenase step was suggested by an increase in the lactate+pyruvate pool, and the finding that pyruvate removal and (3-14C)pyruvate oxidation were inhibited by acetoacetate. 5. Adrenaline, a stimulator of secretion by this tissue, increased glucose removal by 25%. Adrenaline increased glucose removal to the same extent when acetoacetate was also present in the incubation medium. In both cases the increase was accompanied by a fall in (glucose 6-phosphate). 6. Adrenaline also overcame the inhibition of pyruvate removal caused by acetoacetate. 7. The tissue (ATP) decreased by about 50% on addition of adrenaline, and a similar fall was observed in vivo after adrenergic stimulation by isoproterenol. 8. Omission of Ca-2+ from the medium prevented the fall in (glucose 6-phosphate) and (ATP) caused by adrenaline, although adrenaline was still able to stimulate glucose removal. The inhibitory effect of acetoacetate on gluocse removal was reversed by adrenaline, but there was no stimulation above the control rates. Inhibition of pyruvate removal by acetoacetate was not overcome by adrenaline in the absence of Ca-2+. 9. Dibutyryl cyclic AMP had no effect on glucose removal or on (ATP). 10. Possible mechanisms by which adrenaline can bring about its metabolic effects are discussed.     

Effects of dexamethasone treatment on insulin-stimulated rates of glycolysis and glycogen synthesis in isolated incubated skeletal muscles of the rat.

1. Rats were treated with dexamethasone for 4 days before measurement of the rates of lactate formation [which is an index of hexose transport; see Challiss, Lozeman, Leighton & Newsholme (1986) Biochem. J. 233, 377-381] and glycogen synthesis in response to various concentrations of insulin in isolated incubated soleus and extensor digitorum longus muscle preparations. 2. The concentration of insulin required to stimulate these processes half-maximally in soleus and extensor digitorum longus muscles isolated from control rats was about 100 muunits/ml. 3. Dexamethasone increases the concentration of insulin required to stimulate glycolysis half-maximally in soleus and extensor digitorum longus preparations to 250 and 300 muunits/ml respectively. The respective insulin concentrations necessary to stimulate glycogen synthesis half-maximally were about 430 and 370 muunits/ml for soleus and extensor digitorum longus muscle preparations isolated from steroid-treated rats. 5. Dexamethasone treatment did not change the amount of insulin bound to soleus muscle.     

Renal tissue metabolism in the rat during chronic metabolic alkalosis: importance of glycolysis

Chronic metabolic alkalosis was induced in rats drinking 0.3 M NaHCO3 and receiving 1 mg furosemide/100 g body weight per day intraperitoneally. Another group of animals received a potassium supplement in the form of 0.3 M KHCO3. In this group, hypokalemia did not develop and muscle potassium fell by only 18% versus 50% in those not receiving potassium. In vitro renal production of ammonia and uptake of glutamine fell by 40% with a decrease in the activity of glutaminase I and glutamate dehydrogenase. Activity of phosphofructokinase, a major enzyme of glycolysis, rose only in the kidney of animals receiving a potassium supplement. Fructose-1,6-diphosphatase fell as well as phosphoenolpyruvate carboxykinase. Malate dehydrogenase also fell. The activity of phosphofructokinase also rose in the liver, heart, and leg muscle. The major biochemical changes in the renal cortex were the following: glutamate, alpha-ketoglutarate, malate, lactate, pyruvate, alanine, aspartate, and citrate rose as well as calculated oxaloacetate. The concentration of intermediates like 2-phosphoglycerate, 3-phosphoglycerate, and glucose-6-phosphate fell. The cytosolic redox potential (NAD+/NADH) decreased. In addition to the fall in ammoniagenesis, it could be demonstrated in vitro that the renal tubules incubated with glutamine showed decreased glucose production and increased production of lactate and pyruvate. The concentration of lactate was elevated in all tissues examined including liver, heart, and leg muscle. This study confirms in the rat that decreased renal ammoniagenesis takes place following decreased uptake of glutamine in metabolic alkalosis. All other changes are accounted for by the process of increased glycolysis, which appears to take place in all tissues in metabolic alkalosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

The in vitro effect of corticosterone on insulin binding and glucose metabolism in mouse skeletal muscles

We studied the in vitro effect of corticosterone on insulin binding, uptake of 2-deoxy-D-glucose, glycolysis, and glycogenesis in the soleus and extensor digitorum longus (EDL) of Swiss-Webster mice. In each experiment, one muscle (soleus/EDL) was incubated with corticosterone (0.1, 1, 50, and 100 micrograms/mL) and the respective contralateral muscle was incubated without corticosterone, but at the same insulin and pH levels. Corticosterone did not affect insulin binding in both muscles. However, corticosterone decreased the uptake of 2-deoxy-D-glucose and the rate of glycolysis and glycogenesis in both muscles when the dose was pharmacologic (50 and 100 micrograms/mL), but not when it was physiologic (0.1 and 1 microgram/mL). For glycolysis and glycogenesis, the suppression was greater in the EDL when compared with the soleus. This suppression was seen in both basal and insulin-stimulated conditions. In this in vitro system, where the experimental muscle is not exposed to prior hyperinsulinemia as in the in vivo model, corticosterone, at pharmacologic doses, affects postreceptor events without altering the insulin binding in the skeletal muscle.     

Mechanism for the oleate stimulation of gluconeogenesis from dihydroxyacetone by hepatocytes from fasted rats

Oleate stimulates glucose production and concomitantly decreases lactate and pyruvate production by rat hepatocyte suspensions incubated with dihydroxyacetone as substrate. The actions of oleate could be blocked by D-(+)dodecanoylcarnitine, which inhibits transport of the fatty acid into the mitochondria and the subsequent oxidation. beta-Hydroxybutyrate, but not acetoacetate, also stimulated glucose synthesis and inhibited lactate and pyruvate production. Furthermore, both beta-hydroxybutyrate and oleate stimulated oxygen consumption to the same extent. This suggests that oleate stimulates glucose production by the provision of energy subsequent to mitochondrial beta-oxidation of the fatty acids. The content of ATP itself did not appear to be responsible for the effects of oleate. Crossover analysis of the gluconeogenic intermediates implicated a site of oleate action between fructose 1,6-bisphosphate and fructose 6-phosphate, suggesting phosphofructokinase and/or fructose-bisphosphatase as possible regulatory sites. Coupled with the finding that intracellular citrate accumulates upon addition of oleate or beta-hydroxybutyrate, but not acetoacetate, the results suggest that citrate inhibition of phosphofructokinase accounts for the redirection of carbon flow from lactate and pyruvate formation and towards that of glucose.     

Fibre-type specificity and effect of thyroid hormone on glucose 6-phosphate dehydrogenase activity in normal and denervated skeletal muscles of the rat

Summary Glucose-6-phosphate dehydrogenase activity increases following denervation of rat skeletal muscle. The specificity of this effect to muscle fibre type was studied. Basal activity of the dehydrogenase was higher in soleus, a muscle composed predominantly of type I fibres, than in extensor digitorum longus, a muscle composed predominantly of type IIa and b fibres. The enzymatic activity of the soleus was also greater than that of the red (RQ) and white (WQ) portions of quadriceps muscle (predominantly type IIa and type IIb fibres, respectively). Following denervation, glucose-6-phosphate dehydrogenase increased in extensor digitorum longus and RQ, but not in WQ or the soleus. Following chronic treatment of rats with 3,3,5-triiodothyronine, which converts type I muscle fibres to type II, the dehydrogenase activity increased in both denervated soleus and extensor digitorum longus. It is concluded that the effect of denervation on glucose-6-phosphate dehydrogenase activity is selective for type IIa (fast oxidative-glycolytic) muscle fibres.     

Epinephrine-mediated stimulation of glucose uptake and lactate release by the perfused rat heart. Evidence for alpha- and beta-adrenergic mechanisms

Epinephrine treatment of the perfused rat heart led to an increase in the rate of glucose uptake and lactate release as well as increases in the rate of beating and the activity ratio of phosphofructokinase. The dose of epinephrine required for half maximal increases in the rate of beating, and glucose uptake and the activity ratio of phosphofructokinase was approx.10^?7M. Glucose uptake, lactate release and the activity ratio of phosphofructokinase were increased by the α-agonists methoxamine and phenylephrine, and the β agonist, isoproterenol. Propranolol and phenoxybenzamine each partially blocked the stimulatory effects of epinephrine on glucose uptake and lactate production. Phenoxybenzamine blocked the stimulatory effects of methoxamine but had no effect on those produced by isoproterenol which were blocked by propranolol. It is concluded that dual α and β adrenergic control of glycolysis occurs in cardiac muscle. It is proposed that the previously reported α-adrenergic control of phosphofructokinase plays a key role in the control of heart muscle glycolysis.     

Oxygen consumption in perfused skeletal muscle. Effect of perfusion with aged, fresh and aged-rejuvenated erythrocytes on oxygen consumption, tissue metabolites and inhibition of glucose utilization by acetoacetate.

1. O2 consumption, glucose metabolism and the energy status of skeletal muscle were compared in isolated rat hindquarters perfused with aged (21--35 days), fresh and aged-rejuvenated human erythrocytes. 2. The age of the erythrocytes did not affect O2 consumption, glucose utilization or lactate release either at rest or during exercise. The concentrations of ATP, phosphocreatine and lactate within the muscle were also unaffected by the use of aged erythrocytes. 3. Perfusion with acetoacetate did not inhibit glucose utilization; but, it caused a marked increase in the tissue concentration of citrate in the soleus, a slow-twitch red muscle, and a smaller increase in the gastrocnemius, which contains fast-twitch red and white fibres. Results were similar in hindquarters perfused with aged and aged-rejuvenated erythrocytes. 4. These findings suggest that perfusion with aged human erythrocytes does not cause major alterations in the metabolic performance of the isolated rat hindquarter.     

Evaluation of enzyme histochemical observations for metabolic studies. A combined histochemical and biochemical investigation of experimentally induced skeletal muscle-changes

Summary The reliability of enzyme histochemical observations for metabolic studies on skeletal muscle tissue was investigated with a combined histochemical and biochemical study. Specimens of musculus soleus with a predominantly aerobic metabolism and of musculus flexor digitorum longus with a predominantly anaerobic metabolism of rabbits in which both muscles were surgically cross-reinnervated or auto-reinnervated were used. For the histochemical investigation activities and localisations of succinate dehydrogenase, l-glycerol-3-phosphate: acceptor oxidoreductase, nicotinamide adenine dinucleotide: tetrazolium oxidoreductase and of -glucan phosphorylase were examined. For the biochemical investigation maximal activity of phosphofructokinase, the rate limiting enzyme for the regulation of the glycolysis was measured. In addition the activities of succinate dehydrogenase and l-glycerol-3-phosphate: acceptor oxidoreductase to characterize the aerobic metabolism and the key role in gearing energy requirements to glycolysis respectively were biochemically determined. For further information about metabolic aspects the isoenzyme ratio of lactate dehydrogenase was established. In the present paper the histochemical findings are reported and discussed.Part of this study was taken from the Ph. D. thesis of A. C. Jöbsis (1971).     

Increased response to insulin of glucose metabolism in the 6-day unloaded rat soleus muscle

Hind leg muscles of female rats (85-99 g) were unloaded by tail cast suspension for 6 days. In the fresh-frozen unloaded soleus, the significantly greater concentration of glycogen correlated with a lower activity ratio of glycogen phosphorylase (p less than 0.02). The activity ratio of glycogen synthase also was lower (p less than 0.001), possibly due to the higher concentration of glycogen. In isolated unloaded soleus, insulin (0.1 milliunit/ml) increased the oxidation of D-[U-14C]glucose, release of lactate and pyruvate, incorporation of D-[U-14C]glucose into glycogen, and the concentration of glucose 6-phosphate more (p less than 0.05) than in the weight-bearing soleus. At physiological doses of insulin, the percent of maximal uptake of 2-deoxy-D-[1,2-3H]glucose/muscle also was greater in the unloaded soleus. Unloading of the soleus increased by 50% the concentration of insulin receptors, due to no decrease in total receptor number during muscle atrophy. This increase may account for the greater response of glucose metabolism to insulin in this muscle. The extensor digitorum longus, which generally shows little response to unloading, displayed no differential response of glucose metabolism to insulin.     

Endurance training increases lactate transport in male Zucker fa/fa rats

The purpose of this study was to investigate the effect of endurance training (10 weeks) on previously reported alterations of lactate exchange in obese Zucker fa/fa rats. We used sarcolemmal vesicles to measure lactate transport capacity in control sedentary rats, Zucker (fa/fa), and endurance trained Zucker (fa/fa) rats. Monocarboxylate transporter (MCT) 1 and 4 content was measured in sarcolemmal vesicles and skeletal muscle. Training increased citrate synthase activity in soleus and in red tibialis anterior, and improved insulin sensitivity measured by intraperitoneal glucose tolerance test. Endurance training increased lactate influx in sarcolemmal vesicles at 1 mM of external lactate concentration and increased MCT1 expression on sarcolemmal vesicles. Furthermore, muscular lactate level was significantly decreased after training in red tibialis anterior and extensor digitorum longus. This study shows that endurance training improves impairment of lactate transport capacity that is found in insulin resistance state like obesity and type 2 diabetes.     

Islet-cell metabolism during insulin release. Effects of glucose, citrate, octanoate, tolbutamide, glucagon and theophylline

1. Concentrations of glucose 6-phosphate and 6-phosphogluconate were studied in islets of Langerhans isolated from rat pancreas and incubated in the presence of various agents that induce insulin release. 2. In response to rising concentrations of extracellular glucose (2-10mm) there is a linear increase in the intracellular concentration of glucose 6-phosphate, though this is not the case for 6-phosphogluconate, the intracellular concentration of which only increases when the external glucose concentration exceeds 5mm. 3. Tolbutamide, octanoate and citrate, all of which promote insulin secretion from isolated islets, increase the intracellular concentrations of glucose 6-phosphate and 6-phosphogluconate. The results obtained in the presence of octanoate and citrate are compatible with an inhibitory effect of citrate on islet-cell phosphofructokinase. 4. Theophylline and glucagon when incubated with islets in vitro promote insulin release and cause a rise in 6-phosphogluconate concentration and not in that of glucose 6-phosphate. 5. It is suggested that the further metabolism of glucose 6-phosphate through a pathway other than glycolysis is essential for insulin release. One such pathway involves its oxidation to 6-phosphogluconate, which seems to be a necessary accompaniment of insulin secretion due to glucose. The possibility that agents other than glucose promote insulin release by enhancing the oxidation of glucose 6-phosphate through this pathway is discussed.     

The effect of hyperosmolarity on muscle glycogen accumulation.

Soleus (red) and extensor digitorum longus (white) muscles from Sprague Dawley rats were incubated with 6-14C-labelled glucose in normal and in hyperosmotic media. Hyperosmolarity decreased 6-14C-glucose incorporation into muscle glycogen in a dose dependent manner and increased glycolysis and glucose oxidation. Increased glycogenolysis rather than decreased glycogenesis was responsible for the reduction in labelled glycogen accumulation.     

The regulation of triglyceride synthesis and fatty acid synthesis in rat epididymal adipose tissue. Effects of insulin, adrenaline and some metabolites in vitro

1. Adipose tissues from rats fed a balanced diet were incubated in the presence of glucose (20mm) with the following additions: insulin, anti-insulin serum, insulin+acetate, insulin+pyruvate, insulin+lactate, insulin+phenazine methosulphate, insulin+oleate+albumin, insulin+adrenaline+albumin, insulin+6-N-2'-O-dibutyryl 3':5'-cyclic AMP+albumin. 2. Measurements were made of the whole tissue concentrations of adenine nucleotides, hexose phosphates, triose phosphates, glycerol 1-phosphate, 3 phosphoglycerate, 6-phosphogluconate, long-chain fatty acyl-CoA, acid-soluble CoA, citrate, isocitrate, malate and 2-oxoglutarate, and of the release into the incubation medium of lactate, pyruvate and glycerol after 1h of incubation. 3. Fluxes of [(14)C]glucose carbon through the major pathways of glucose metabolism were calculated from the yields of (14)C in various products after 2h of incubation. Fluxes of [(14)C]acetate, [(14)C]pyruvate or [(14)C]lactate carbon in the presence of glucose were also determined. 4. Measurements were also made of the whole-tissue concentrations of metabolites in tissues taken directly from Nembutal-anaesthetized rats. 5. Whole tissue mass-action ratios for phosphofructokinase, phosphoglucose isomerase and the combined (aldolasextriose phosphate isomerase) reaction were similar in vivo and in vitro. The reactants of phosphofructokinase appeared to be far from mass-action equilibrium. In vitro, the reactants of hexokinase also appeared to be far from mass-action equilibrium. 6. Correlation of observed changes in glycolytic flux with changes in fructose 6-phosphate concentration suggested that phosphofructokinase may show regulatory behaviour. The enzyme appeared to be activated in the presence of oleate or adrenaline and to be inhibited in the presence of lactate or pyruvate. 7. Evidence is presented that the reactants of lactate dehydrogenase and glycerol 1-phosphate dehydrogenase may be near to mass-action equilibrium in the cytoplasm. 8. No satisfactory correlations could be drawn between the whole-tissue concentrations of long-chain fatty acyl-CoA, citrate and glycerol 1-phosphate and the observed rates of triglyceride and fatty acid synthesis. Under the conditions employed, the concentration of glycerol 1-phosphate appeared to depend mainly on the cytoplasmic [NAD(+)]/[NADH] ratios. 9. Calculated hexose monophosphate pathway flux rates roughly correlated with fatty acid synthesis rates and with whole tissue [6-phosphogluconate]/[glucose 6-phosphate] ratios. The relative rates of production of NADPH for fatty acid synthesis by the hexose monophosphate pathway and by the ;malic enzyme' are discussed. It is suggested that all NADH produced in the cytoplasm may be used in that compartment for reductive synthesis of fatty acids, lactate or glycerol 1-phosphate.     

Glucose metabolism in perfused skeletal muscle. Effects of starvation, diabetes, fatty acids, acetoacetate, insulin and exercise on glucose uptake and disposition.

1. The regulation of glucose uptake and disposition in skeletal muscle was studied in the isolated perfused rat hindquarter. 2. Insulin and exercise, induced by sciatic-nerve stimulation, enhanced glucose uptake about tenfold in fed and starved rats, but were without effect in rats with diabetic ketoacidosis. 3. At rest, the oxidation of lactate (0.44 mumol/min per 30 g muscle in fed rats) was decreased by 75% in both starved and diabetic rats, whereas the release of alanine and lactate (0.41 and 1.35 mumol/min per 30 g respectively in the fed state) was increased. Glycolysis, defined as the sum of lactate+alanine release and lactate oxidation, was not decreased in either starvation or diabetes. 4. In all groups, exercise tripled O2 consumption (from approximately 8 to approximately 25 mumol/min per 30 g of muscle) and increased the release and oxidation of lactate five- to ten-fold. The differences in lactate release between fed, starved and diabetic rats observed at rest were no longer apparent; however, lactate oxidation was still several times greater in the fed group. 5. Perfusion of the hindquarter of a fed rat with palmitate, octanoate or acetoacetate did not alter glucose uptake or lactate release in either resting or exercising muslce; however, lactate oxidation was significantly inhibited by acetoacetate, which also increased the intracellular concentration of acetyl-CoA. 6. The data suggest that neither that neither glycolysis nor the capacity for glucose transport are inhbitied in the perfused hindquarter during starvation or perfusion with fatty acids or ketone bodies. On the other hand, lactate oxidation is inhibited, suggesting diminished activity of pyruvate dehydrogenase. 7. Differences in the regulation of glucose metabolism in heart and skeletal muscle and the role of the glucose/fatty acid cycle in each tissue are discussed.     

Effects of clofibrate on basal and insulin-activated glucose uptake in fast and slow skeletal muscles of rats.

1. The basal uptake of glucose was increased significantly in the extensor digitorum longus muscle (EDL) of rats by clofibrate administration. 2. The insulin-activated uptake of glucose was increased in the soleus muscle (Sol) by clofibrate. 3. The insulin-induced increment of glucose uptake was increased significantly in Sol and decreased significantly in EDL by clofibrate.     

Evaluation of the isolated perfused rat hindquarter for the study of muscle metabolism

1. The metabolic integrity of a new isolated rat hindquarter preparation was studied. The hindquarter was perfused with a semi-synthetic medium containing aged human erythrocytes. More than 95% of the oxidative metabolism of the preparation was due to muscle, the remainder being due to bone, adipose tissue and, where present, skin. 2. Consumption of O(2), glucose utilization, glycerol release and lactate production were similar in the presence and in the absence of the skin, indicating that the latter contributed little to the overall metabolism of the preparation. 3. After 40min of perfusion, tissue concentrations of creatine phosphate, ATP and ADP were similar to those found in muscle taken directly from intact animals. The muscle also appeared normal under the electron microscope. 4. The hindquarter did not lose K(+) to the medium during a 30min perfusion. In the presence of insulin it had a net K(+) uptake. 5. Insulin caused a sixfold increase in glucose uptake, stimulated O(2) consumption by nearly 40% and depressed glycerol release to less than half the control value. 6. Bilateral sciatic-nerve stimulation caused severalfold increases in O(2) consumption and lactate production. In the absence of insulin nerve stimulation also enhanced glucose uptake; in the presence of insulin it did not further increase the already high rate of glucose uptake. 7. Rates of lactate production and O(2) consumption of the rat hindquarter in vivo and the isolated perfused hindquarter were very similar. 8. Ketone bodies were a major oxidative fuel in vivo of the hindquarter of a rat starved for 2 days. If the acetoacetate and 3-hydroxybutyrate removed by the tissue were completely oxidized, they would have accounted for 77% of the O(2) consumption. 9. Acetoacetate accounted for 84% of the ketone bodies removed by the hindquarter in vivo even though its arterial concentration was half that of 3-hydroxybutyrate. 10. Similar rates of acetoacetate and 3-hydroxybutyrate utilization were observed in the perfused hindquarter. 11. Acetoacetate utilization by the perfused hindquarter was not diminished by the addition of either oleate or insulin to the perfusate. 12. Oxidation of glucose to CO(2) accounted for less than 4% of the O(2) consumed by the perfused hindquarter in both the presence and the absence of insulin. 13. The results indicate that the isolated perfused hindquarter is a useful tool for studying muscle metabolism. They also suggest that ketone bodies, if present in sufficient concentration, are the preferred oxidative fuel of resting muscle.     

The metabolic effects of sodium dichloroacetate in the starved rat

1. Sodium dichloroacetate (300mg/kg body wt. per h) was infused in 24h-starved rats for 4h. 2. Blood glucose decreased significantly, an effect that had previously only been noted in diabetic animals 3. Plasma insulin concentration decreased by 63%; blood lactate and pyruvate concentrations decreased by 50 and 33%, whereas concentrations of 3-hydroxybutyrate and acetoacetate increased by 81 and 73% respectively. 4. Livers were freeze-clamped at the end of the 4h infusion. There were significant decreases in hepatic [glucose], [glucose 6-phosphate], [2-phosphoglycerate], the [lactate]/[pyruvate] ratio, [citrate] and [malate], and also [alanine], [glutamate] and [glutamine], suggesting a diminished supply of gluconeogenic substrates. 5. Animals subjected to a functional hepatectomy at the end of 2h infusions showed no difference in blood-glucose disappearance but a highly significant decrease in the rate of accumulation of lactate, pyruvate, glycerol and alanine, compared with control animals. Dichloroacetate decreased ketone-body clearance. 6. After functional hepatectomy an increase in glutamine accumulation appeared to compensate for the decrease in alanine accumulation. 7. It is concluded that dichloroacetate causes hypoglycaemia by decreasing the net release of gluconeogenic precursors from extrahepatic tissues while inhibiting peripheral ketone-body uptake. 8. These findings are consistent with the activation of pyruvate dehydrogenase (EC 1.2.4.1) in rat muscle by dichloroacetate previously described by Whitehouse & Randle (1973).     

Effect of noradrenaline on triacylglycerol synthesis in rat brown adipocytes.

1. The effects of hypothyroidism on the sensitivity of glycolysis and glycogen synthesis to insulin were investigated in the isolated, incubated soleus muscle of the rat. 2. Hypothyroidism, which was induced by administration of propylthiouracil to the rats, decreased fasting plasma levels of free fatty acids and increased plasma levels of glucose but did not significantly change plasma levels of insulin. 3. The sensitivity of the rates of glycogen synthesis to insulin was increased at physiological, but decreased at supraphysiological, concentrations of insulin. 4. The rates of glycolysis in the hypothyroid muscles were decreased at all insulin concentrations studied and the EC50 for insulin was increased more than 8-fold; the latter indicates decreased sensitivity of this process to insulin. However, at physiological concentrations of insulin, the rates of glucose phosphorylation in the soleus muscles of hypothyroid rats were not different from controls. This suggests that hypothyroidism affects glucose metabolism in muscle not by affecting glucose transport but by decreasing the rate of glucose 6-phosphate conversion to lactate and increasing the rate of conversion of glucose 6-phosphate to glycogen. 5. The rates of glucose oxidation were decreased in the hypothyroid muscles at all insulin concentrations.