Measurement of the rate of substrate cycling between fructose 6-phosphate and fructose 1,6-bisphosphate in skeletal muscle by using a single-isotope technique

Fructose bisphosphatase 2 overexpression increases glucose uptake in skeletal muscle

Skeletal muscle is a major tissue for glucose metabolism and can store glucose as glycogen, convert glucose to lactate via glycolysis and fully oxidise glucose to CO₂ Muscle has a limited capacity for gluconeogenesis but can convert lactate and alanine to glycogen. Gluconeogenesis requires FBP2, a muscle-specific form of fructose bisphosphatase that converts fructose-1,6-bisphosphate (F-1,6-bisP) to fructose-6-phosphate (F-6-P) opposing the activity of the ATP-consuming enzyme phosphofructokinase (PFK). In mammalian muscle, the activity of PFK is normally 100 times higher than FBP2 and therefore energy wasting cycling between PFK and FBP2 is low. In an attempt to increase substrate cycling between F-6-P and F-1,6-bisP and alter glucose metabolism, we overexpressed FBP2 using a muscle-specific adeno-associated virus (AAV-tMCK-FBP2). AAV was injected into the right tibialis muscle of rats, while the control contralateral left tibialis received a saline injection. Rats were fed a chow or 45% fat diet (HFD) for 5 weeks after which, hyperinsulinaemic-euglycaemic clamps were performed. Infection of the right tibialis with AAV-tMCK-FBP2 increased FBP2 activity 10 fold on average in chow and HFD rats (P < 0.0001). Overexpression of FBP2 significantly increased insulin-stimulated glucose uptake in tibialis of chow animals (control 14.3 ± 1.7; FBP2 17.6 ± 1.6 µmol/min/100 g) and HFD animals (control 9.6 ± 1.1; FBP2 11.2 ± 1.1µmol/min/100 g). The results suggest that increasing the capacity for cycling between F-1,6-bisP and F-6-P can increase the metabolism of glucose by introducing a futile cycle in muscle, but this increase is not sufficient to overcome muscle insulin resistance.

Multiple mass isotopomer tracing of acetyl-CoA metabolism in Langendorff-perfused rat hearts: channeling of acetyl-CoA from pyruvate dehydrogenase to carnitine acetyltransferase

We developed an isotopic technique to assess mitochondrial acetyl-CoA turnover (≈citric acid flux) in perfused rat hearts. Hearts are perfused with buffer containing tracer [(13)C2,(2)H3]acetate, which forms M5 + M4 + M3 acetyl-CoA. The buffer may also contain one or two labeled substrates, which generate M2 acetyl-CoA (e.g. [(13)C6]glucose or [1,2-(13)C2]palmitate) or/and M1 acetyl-CoA (e.g. [1-(13)C]octanoate). The total acetyl-CoA turnover and the contributions of fuels to acetyl-CoA are calculated from the uptake of the acetate tracer and the mass isotopomer distribution of acetyl-CoA. The method was applied to measurements of acetyl-CoA turnover under different conditions (glucose ± palmitate ± insulin ± dichloroacetate). The data revealed (i) substrate cycling between glycogen and glucose-6-P and between glucose-6-P and triose phosphates, (ii) the release of small excess acetyl groups as acetylcarnitine and ketone bodies, and (iii) the channeling of mitochondrial acetyl-CoA from pyruvate dehydrogenase to carnitine acetyltransferase. Because of this channeling, the labeling of acetylcarnitine and ketone bodies released by the heart are not proxies of the labeling of mitochondrial acetyl-CoA.

Acute effect of growth hormone to induce peripheral insulin resistance is independent of FFA and insulin levels in rats

To examine whether growth hormone (GH) induces peripheral insulin resistance by altering plasma free fatty acid (FFA) or insulin levels, the effects of GH infusion on insulin-stimulated glucose fluxes were studied in conscious rats under two protocols. In study 1, either saline (n = 7) or human recombinant GH (21 microg. kg(-1). h(-1); n = 8) was infused for 300 min, and insulin-stimulated glucose fluxes were estimated during the final 150-min period of hyperinsulinemic euglycemic clamps. In study 2, hyperinsulinemic euglycemic clamps were first conducted for 150 min (to raise plasma insulin and suppress FFA levels), and saline or GH (n = 7 for each) was subsequently infused for the following 300-min clamp period. In study 1, GH infusion in the basal state did not significantly alter plasma FFA or insulin levels. In contrast, GH infusion decreased insulin-stimulated glucose uptake, glycolysis, and glycogen synthesis by 32, 27, and 40%, respectively (P < 0.05). In study 2, GH infusion during hyperinsulinemic euglycemic clamps did not alter plasma FFA or insulin levels (P > 0.05). GH infusion had no effect on insulin-stimulated glucose uptake during the initial 150 min but eventually decreased insulin-stimulated glucose uptake by 37% (P < 0. 05), similar to the results in study 1. These data indicate that GH induces peripheral insulin resistance independent of plasma FFA and insulin levels. The induction of insulin resistance was preceded by suppression of glycogen synthesis, consistent with the hypothesis that metabolic impairment precedes and causes development of peripheral insulin resistance.

Cachectin/TNF-mediated lactate production in cultured myocytes is linked to activation of a futile substrate cycle

The cytokine cachectin/TNF induces a rapid increase in lactate production and in glucose metabolism in L6 myocytes in culture; glucose uptake was maximal after 17 h, while elevated glucose utilization and lactate production persisted for up to 32 h. These increases are suggestive of increased glycolytic activity, and were associated with a 10% decrease in cellular oxygen consumption and a comparable decrease in the production of 14C-labelled CO2 from 14C-labelled glucose. This decrease in aerobic metabolism, however, could account for only a small fraction of the energetic requirement for increased glycolytic activity. Furthermore, maximal stimulation of pyruvate dehydrogenase (PDH) by dichloroacetate (DCA) treatment in conjunction with cachectin/TNF abolished lactate production, but increased glucose uptake persisted. Taken together, this suggests that the primary effect of cachectin/TNF on myocyte carbohydrate metabolism is to increase glycolysis. Correspondingly, we postulated that cachectin/TNF must activate one or more ATP-depleting cellular processes to account for the lack of feed-back inhibition on glycolysis by the ATP produced. This led to the identification of a futile substrate cycle between fructose 6-phosphate and fructose 1,6-bisphosphate as a novel energy sink that is activated by cachectin/TNF. Cachectin/TNF treatment led to increased activity of both phosphofructokinase (PFK) and fructose bisphosphate phosphatase (FBP) in myocytes in culture, detectable after 1 h of incubation and persisting for up to 16 h. The possible role of cachectin/TNF-mediated futile substrate cycling in increased glycolytic activity, increased energy expenditure, heat production and tissue wasting during bacterial infections is discussed.

Use of [3-3H]glucose and [6-14C]glucose to measure glucose turnover and glucose metabolism in humans

[3-3H]glucose is frequently used to measure glucose turnover in humans. If fructose 6-phosphate-fructose 1,6-diphosphate cycling (Fpc) is negligible in both liver and muscle, then [3-3H]- and [6-14C]glucose (corrected for Cori cycle activity) should provide equivalent measures of glucose turnover. In addition, if glycogenolysis is fully suppressed, then [14C]lactate specific activity should equal that of [6-14C]glucose from which it was derived, and oxidation of [6-14C]glucose, as measured by rate of generation of 14CO2, should equal total glucose oxidation (i.e., that derived from intra- and extracellular pools) as measured by indirect calorimetry. To address these questions, glucose turnover was measured simultaneously with [3-3H]- and [6-14C]glucose in the basal state and in presence of low (approximately 200 pM) and high (approximately 750 pM) insulin concentrations. Glucose turnover rates measured with [3-3H]- and [6-14C]glucose were equivalent at all insulin concentrations, indicating that Fpc had no detectable effect on measurement of glucose appearance. [14C]lactate specific activity was lower (P less than 0.01) than that of [6-14C]glucose in the basal state but not during either low- or high-dose insulin infusion, implying that all lactate was derived from extracellular glucose. On the other hand, glucose oxidation as measured by rate of generation of 14CO2 was lower (P less than 0.05) than glucose oxidation as measured by indirect calorimetry during both insulin infusions, implying either that suppression of glycogenolysis was not complete in all tissues or that one or both of these techniques do not accurately measure glucose oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

The enigmatic presence of all gluconeogenic enzymes in Schistosoma mansoni adults

The activities of glucose-6-phosphatase (G6Pase), fructose-1,6-bisphosphatase (FBPase), phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC) were determined in homogenates of adult Schistosoma mansoni worms and compared with the activities in homogenates of rat liver and rat skeletal muscle, tissues with a high and a low gluconeogenic capacity, respectively. All four gluconeogenic enzymes were present in S. mansoni. The enzymes were less active than in rat liver, but the activities of G6Pase, PEPCK and PC were at least an order of magnitude higher than in rat skeletal muscle whereas FBPase was approximately equally active in S. mansoni and in rat muscle. Experiments with 14C-labelled substrates or [14C]NaHCO3 failed to demonstrate the actual occurrence of gluconeogenesis in S. mansoni. Some possible other functions of the gluconeogenic enzymes were investigated. Experiments with inhibitors of PEPCK gave no indications that this enzyme was involved in the degradation of glucose. This was confirmed by 13C-NMR experiments which indicated that lactate was formed from phosphoenolpyruvate via the actions of pyruvate kinase and lactate dehydrogenase, and that PEPCK did not participate in the formation of lactate. Substrate cycling between fructose-6-dehydrogenase, and fructose-1,6-bisphosphate was demonstrated to occur in adult S. mansoni. This shows that FBPase participates in the glucose metabolism of this parasite.

Cellular energy metabolism and regulation

Consistent with the increased demand for nutrients imposed by lactation and growth, those tissues directly involved in the digestion, absorption, and processing of the required additional nutrients show response to these states. During lactation, the rumen, upper intestine, and liver increase in size, and more energy is spent on Na+,K+ transport and on protein turnover. The massive endocrine influences during lactation suggest that the metabolism of other tissues besides these and mammary tissue would be influenced, but evidence is rather sparse. Ion transport and protein metabolism in some muscles may indeed be increased. Although substrate cycles characteristically account for a substantially smaller portion of the energy expenditure in the intact animal than do ion transport and protein turnover, stage of lactation influences some of these cycles, particularly the triacylglycerol fatty acid cycle. The needs for additional quantitative in vivo measurements of metabolic conversions and for mechanistic model description of metabolic events in nonmammary tissues are discussed.

Hormonal control of substrate cycling in humans

Recent studies have established the existence of substrate cycles in humans, but factors regulating the rate of cycling have not been identified. We have therefore investigated the acute response of glucose/glucose-6P-glucose (glucose) and triglyceride/fatty acid (TG/FA) substrate cycling to the infusion of epinephrine (0.03 microgram/kg.min) and glucagon. The response to a high dose glucagon infusion (2 micrograms/kg.min) was tested, as well as the response to a low dose infusion (5 ng/kg.min), with and without the simultaneous infusion of somatostatin (0.1 microgram/kg.min) and insulin (0.1 mU/kg.min). Additionally, the response to chronic prednisone (50 mg/d) was evaluated, both alone and during glucagon (low dose) and epinephrine infusion. Finally, the response to hyperglycemia, with insulin and glucagon held constant by somatostatin infusion and constant replacement of glucagon and insulin at basal rates, was investigated. Glucose cycling was calculated as the difference between the rate of appearance (Ra) of glucose as determined using 2-d1- and 6,6-d2-glucose as tracers. TG/FA cycling was calculated by first determining the Ra glycerol with d5-glycerol and the Ra FFA with [1-13C]palmitate, then subtracting Ra FFA from three times Ra glycerol. The results indicate that glucagon stimulates glucose cycling, and this stimulatory effect is augmented when the insulin response to glucagon infusion is blocked. Glucagon had minimal effect on TG/FA cycling. In contrast, epinephrine stimulated TG/FA cycling, but affected glucose cycling minimally. Prednisone had no direct effect on either glucose or TG/FA cycling, but blunted the stimulatory effect of glucagon on glucose cycling. Hyperglycemia, per se, had no direct effect on glucose or TG/FA cycling. Calculations revealed that stimulation of TG/FA cycling theoretically amplified the sensitivity of control of fatty acid flux, but no such amplification was evident as a result of the stimulation of glucose cycling by glucagon.

Hormonal regulation of the rate of the glycogen/glucose-1-phosphate cycle in skeletal muscle

A method is presented which allows simultaneous estimation of rates of glycogen synthesis and glycogenolysis in an isolated incubated skeletal muscle, thus allowing measurement of the glycogen/glucose-1-phosphate substrate cycling rate. In the absence of hormonal additions, the measured rates of glycogen synthesis and breakdown were similar [respectively, 0.54 +/- 0.12 (8) and 0.74 +/- 0.10 (8) mumol glucosyl equiv. h-1 (g tissue)-1]. Incremental addition of insulin stimulated glycogen synthesis up to threefold and reduced glycogenolysis by about sevenfold; the half-maximally effective concentration of insulin on both processes was about 100 microU/ml (0.7 nM). Incremental addition of adrenaline (in the presence of 1 mU insulin/ml) caused a dramatic increase in the glycogenolytic rate (about 15-fold), but a much less marked inhibition of glycogen synthetic rate. In addition to hormonal manipulation of the muscle preparation in vitro, the effects of cold exposure, the hyperthyroid state, a single exercise bout and exercise-training of animals in vivo on the rates of glycogen synthesis and breakdown in the isolated incubated muscle preparation have been investigated. Significant changes in measured glycogen synthesis, breakdown and glycogen/glucose-1-phosphate cycling have been observed, both under basal conditions and in response to hormonal additions in vitro. The results are discussed with respect to the possible physiological importance of this substrate cycle.

The rate of substrate cycling between fructose 6-phosphate and fructose 1,6-bisphosphate in skeletal muscle from cold-exposed, hyperthyroid or acutely exercised rats

The effects of cold-exposure, the hyperthyroid state and a single exercise bout in vivo on the maximal enzyme activities of 6-phosphofructokinase and fructose-1,6-bisphosphatase in vastus lateralis muscle and the rates of fructose 6-phosphate/fructose 1,6-bisphosphate cycling measured in epitrochlearis muscle in vitro were investigated. In all cases significant changes in substrate cycling rates were observed, whether in the absence of added hormones in vitro (acute exercise), or when stimulated by insulin plus adrenaline (cold-exposure), or with respect to the catecholamine-sensitivity of the cycling rate (the hyperthyroid state).

The effect of fructose 2,6-bisphosphate on muscle fructose-1,6-bisphosphatase activity

Rat and rabbit muscle fructose 1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) are inhibited by fructose 2,6-bisphosphate. In contrast with the liver isozyme, the inhibition of muscle fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate is not synergistic with that of AMP. Activation of fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate has been observed at high concentrations of substrate. An attempt is made to correlate changes in concentrations of hexose monophosphate, fructose 1,6-bisphosphate and fructose 2,6-bisphosphate with changes in fluxes through 6-phosphofructokinase and fructose-1,6-bisphosphatase in isolated epitrochlearis muscle challenged with insulin and adrenaline.