Effect of lactate and palmitate on substrate utilization of isolated rat soleus

Chronic lactate exposure promotes cardiomyocyte cytoskeleton remodelling

We investigated the effect of growing on lactate instead of glucose in human cardiomyocyte assessing their viability, cell cycle activity, oxidative stress and metabolism by a proteomic and metabolomic approach. In previous studies performed on elite players, we found that adaptation to exercise is characterized by a chronic high plasma level of lactate. Lactate is considered not only an energy source but also a signalling molecule and is referred as "lactormone"; heart is one of the major recipients of exogenous lactate. With this in mind, we used a cardiac cell line AC16 to characterize the lactate metabolic profile and investigate the metabolic flexibility of the heart. Interestingly, our data indicated that cardiomyocytes grown on lactate (72 h) show change in several proteins and metabolites linked to cell hypertrophy and cytoskeleton remodelling. The obtained results could help to understand the effect of this metabolite on heart of high-performance athletes.

Sodium oxamate reduces lactate production to improve the glucose homeostasis of Micropterus salmoides fed high-carbohydrate diets

The study was performed to evaluate the effects of the reduced lactate production by sodium oxamate (SO) on growth performance, lactate and glucose and lipid metabolism, and glucose tolerance of Micropterus salmoides fed high-carbohydrate (CHO) diets. In in vitro study, primary hepatocytes were incubated for 48 h in a control medium (5.5 mM glucose), a high-glucose medium (25 mM glucose, HG), or a SO-containing high-glucose medium (25 mM glucose + 50 mM SO, HG-SO). Results indicated lactate and triglyceride (TG) levels, and lactate dehydrogenase a (LDH-a) expression in the HG-SO group were remarkably lower than those of the HG group. In in vivo study, M. salmoides (5.23 ± 0.03 g) were fed four diets containing a control diet (10% CHO, C) and three SO contents [0 (HC), 100 (HC-SO1), and 200 (HC-SO2) mg·kg^-1, respectively] of high-CHO diets (20% CHO) for 11 wk. High-CHO diets significantly reduced weight gain rate (WGR), specific growth rate (SGR), p-AMPK-to-t-AMPK ratio, and expression of insulin receptor substrate 1 (IRS1), insulin-like growth factor I (IGF-I), insulin-like growth factor I receptor (IGF-IR), fructose-1,6-biphosphatase (FBPase), peroxisome proliferator-activated receptor α (PPARα), and carnitine palmitoyl transferase 1α (CPT1α) compared with the C group, whereas the opposite was true for plasma levels of glucose, TG, lactate, tissue glycogen, and lipid contents, and expression of LDH-a, monocarboxylate transporter 1 and 4 (MCT1 and MCT4), insulin, glucokinase (GK), pyruvate dehydrogenase E1 subunit (PDH), sterol-regulatory element-binding protein 1 (SREBP1), fatty acid synthase (FAS). The HC-SO2 diets remarkably increased WGR, SGR, p-AMPK-to-t-AMPK ratio, and expression of IRS1, IGF-I, IGF-IR, GK, PDHα, PDHβ, FAS, acetyl-CoA carboxylase 1 (ACC1), PPARα, and CPT1α compared with the HC group. Besides, HC-SO2 diets also enhanced glucose tolerance of fish after a glucose loading. Overall, the reduced lactate production by SO benefits growth performance and glucose homeostasis of high-CHO-fed M. salmoides through the enhancement of glycolysis, lipogenesis, and fatty acid β-oxidation coupled with the suppression of glycogenesis and gluconeogenesis.

Determining insulin sensitivity from glucose tolerance tests in Iberian and landrace pigs

As insulin sensitivity may help to explain divergences in growth and body composition between native and modern breeds, metabolic responses to glucose infusion were measured using an intra-arterial glucose tolerance test (IAGTT). Iberian (n = 4) and Landrace (n = 5) barrows (47.0 ± 1.2 kg body weight (BW)), fitted with a permanent carotid artery catheter were injected with glucose (500 mg/kg BW) and blood samples collected at -10, 0, 5, 10, 15, 20, 25, 30, 45, 60, 90, 120 and 180 min following glucose infusion. Plasma samples were analysed for insulin, glucose, lactate, triglycerides, cholesterol, creatinine, albumin and urea. Insulin sensitivity indices were calculated and analysed. Mean plasma glucose, creatinine and cholesterol concentrations were lower (P < 0.01) in Iberian (14, 68 and 22%, respectively) than in Landrace pigs during the IAGTT. However, mean plasma insulin, lactate, triglycerides and urea concentrations were greater (P < 0.001) in Iberian (50, 35, 18 and 23%, respectively) than in Landrace pigs. Iberian pigs had larger area under the curve (AUC) of insulin (P < 0.05) or tended to a greater AUC of lactate (P < 0.10), and a smaller (P < 0.05) AUC for glucose 0-60 min compared with Landrace pigs. Indices for estimating insulin sensitivity in fasting conditions indicated improved β-cell function in Iberian compared with Landrace pigs, but no difference (P > 0.10) in calculated insulin sensitivity index was found after IAGTT between breeds. A time response (P < 0.05) was obtained for insulin, glucose and lactate so that maximum concentration was achieved at 10 and 15 min post-infusion for insulin (Iberian and Landrace pigs, respectively), immediately post-infusion for glucose, and 20 min post-infusion for lactate, decreasing thereafter until basal levels. There was no time effect for the rest of metabolites evaluated. In conclusion, growing Iberian pigs challenged with an IAGTT showed changes in biochemical parameters and insulin response that may indicate an early stage of insulin resistance.

Lactate production is a prioritized feature of adipocyte metabolism

Adipose tissue is essential for whole-body glucose homeostasis, with a primary role in lipid storage. It has been previously observed that lactate production is also an important metabolic feature of adipocytes, but its relationship to adipose and whole-body glucose disposal remains unclear. Therefore, using a combination of metabolic labeling techniques, here we closely examined lactate production of cultured and primary mammalian adipocytes. Insulin treatment increased glucose uptake and conversion to lactate, with the latter responding more to insulin than did other metabolic fates of glucose. However, lactate production did not just serve as a mechanism to dispose of excess glucose, because we also observed that lactate production in adipocytes did not solely depend on glucose availability and even occurred independently of glucose metabolism. This suggests that lactate production is prioritized in adipocytes. Furthermore, knocking down lactate dehydrogenase specifically in the fat body of Drosophila flies lowered circulating lactate and improved whole-body glucose disposal. These results emphasize that lactate production is an additional metabolic role of adipose tissue beyond lipid storage and release.

Metabolomic analysis of male combat veterans with post traumatic stress disorder

Posttraumatic stress disorder (PTSD) is associated with impaired major domains of psychology and behavior. Individuals with PTSD also have increased co-morbidity with several serious medical conditions, including autoimmune diseases, cardiovascular disease, and diabetes, raising the possibility that systemic pathology associated with PTSD might be identified by metabolomic analysis of blood. We sought to identify metabolites that are altered in male combat veterans with PTSD. In this case-control study, we compared metabolomic profiles from age-matched male combat trauma-exposed veterans from the Iraq and Afghanistan conflicts with PTSD (n = 52) and without PTSD (n = 51) (‘Discovery group’). An additional group of 31 PTSD-positive and 31 PTSD-negative male combat-exposed veterans was used for validation of these findings (‘Test group’). Plasma metabolite profiles were measured in all subjects using ultrahigh performance liquid chromatography/tandem mass spectrometry and gas chromatography/mass spectrometry. We identified key differences between PTSD subjects and controls in pathways related to glycolysis and fatty acid uptake and metabolism in the initial ‘Discovery group’, consistent with mitochondrial alterations or dysfunction, which were also confirmed in the ‘Test group’. Other pathways related to urea cycle and amino acid metabolism were different between PTSD subjects and controls in the ‘Discovery’ but not in the smaller ‘Test’ group. These metabolic differences were not explained by comorbid major depression, body mass index, blood glucose, hemoglobin A1c, smoking, or use of analgesics, antidepressants, statins, or anti-inflammatories. These data show replicable, wide-ranging changes in the metabolic profile of combat-exposed males with PTSD, with a suggestion of mitochondrial alterations or dysfunction, that may contribute to the behavioral and somatic phenotypes associated with this disease.

The effect of pre-test carbohydrate ingestion on the anaerobic threshold, as determined by the lactate-minimum test

The purpose of this study was to investigate the effect of pre-test carbohydrate (CHO) ingestion on anaerobic-threshold assessment using the lactate-minimum test (LMT). Fifteen competitive male distance runners capable of running 10 km in 33.5-43 min were used as subjects. LMT was performed following CHO (2x300 mL, 7% solution) or comparable placebo (Pl) ingestion, in a double-blind, randomized order. The LMT consisted of two high-intensity 1 min treadmill runs (17-21 km.h(-1)), followed by an 8 min recovery period. Subsequently, subjects performed 5 min running stages, incremented by 0.6 km.h(-1) and separated by 1 min blood-sampling intervals. Tests were terminated after 3 consecutive increases in blood-lactate concentration ([La]) had been observed. Finger-tip capillary blood was sampled for [La] and blood-glucose determination 30 min before the test's onset, during the recovery phase following the 2 high-intensity runs, and following each of the subsequent 5 min stages. Heart rate (HR) and rating of perceived exertion (RPE) were recorded after each stage. The lactate-minimum speed (LMS) was determined from the individual [La]-velocity plots and was considered reflective of the anaerobic threshold. Pre-test CHO ingestion had no effect on LMS (13.19+/-1.12 km.h(-1) vs. 13.17+/-1.08 km.h(-1) in CHO and Pl, respectively), nor on [La] and glucose concentration at that speed, or on HR and RPE responses. Pre-test CHO ingestion therefore does not affect LMS or the LMT-estimated anaerobic threshold.

Catecholamine response is attenuated during moderate-intensity exercise in response to the "lactate clamp"

Catecholamine release is known to be regulated by feedforward and feedback mechanisms. Norepinephrine (NE) and epinephrine (Epi) concentrations rise in response to stresses, such as exercise, that challenge blood glucose homeostasis. The purpose of this study was to assess the hypothesis that the lactate anion is involved in feedback control of catecholamine concentration. Six healthy active men (26 +/- 2 yr, 82 +/- 2 kg, 50.7 +/- 2.1 ml.kg(-1).min(-1)) were studied on five occasions after an overnight fast. Plasma concentrations of NE and Epi were determined during 90 min of rest and 90 min of exercise at 55% of peak O2 consumption (VO2 peak) two times with exogenous lactate infusion (lactate clamp, LC) and two times without LC (CON). The blood lactate profile ( approximately 4 mM) of a preliminary trial at 65% VO2 peak (65%) was matched during the subsequent LC trials. In resting men, plasma NE concentration was not different between trials, but during exercise all conditions were different with 65% > CON > LC (65%: 2,115 +/- 166 pg/ml, CON: 1,573 +/- 153 pg/ml, LC: 930 +/- 174 pg/ml, P < 0.05). Plasma Epi concentrations at rest were different between conditions, with LC less than 65% and CON (65%: 68 +/- 9 pg/ml, CON: 59 +/- 7 pg/ml, LC: 38 +/- 10 pg/ml, P < 0.05). During exercise, Epi concentration showed the same trend (65%: 262 +/- 37 pg/ml, CON: 190 +/- 34 pg/ml, LC: 113.2 +/- 23 pg/ml, P < 0.05). In conclusion, lactate attenuates the catecholamine response during moderate-intensity exercise, likely by feedback inhibition.

Energy substrates for neurons during neural activity: a critical review of the astrocyte-neuron lactate shuttle hypothesis

Glucose had long been thought to fuel oxidative metabolism in active neurons until the recently proposed astrocyte-neuron lactate shuttle hypothesis (ANLSH) challenged this view. According to the ANLSH, activity-induced uptake of glucose takes place predominantly in astrocytes, which metabolize glucose anaerobically. Lactate produced from anaerobic glycolysis in astrocytes is then released from astrocytes and provides the primary metabolic fuel for neurons. The conventional hypothesis asserts that glucose is the primary substrate for both neurons and astrocytes during neural activity and that lactate produced during activity is removed mainly after neural activity. The conventional hypothesis does not assign any particular fraction of glucose metabolism to the aerobic or anaerobic pathways. In this review, the authors discuss the theoretical background and critically review the experimental evidence regarding these two hypotheses. The authors conclude that the experimental evidence for the ANLSH is weak, and that existing evidence and theoretical considerations support the conventional hypothesis.

Glucose autoregulates its uptake in skeletal muscle: involvement of AMP-activated protein kinase

Preexposure to a low concentration of glucose upregulates glucose transport into skeletal muscle, whereas exposure to a high concentration of glucose has the opposite effect. This autoregulatory process occurs independently of insulin, and the mechanism by which it operates is incompletely understood. Activation of the energy-sensing enzyme AMP-activated protein kinase (AMPK) has been shown to increase insulin-independent glucose transport into skeletal muscle in response to such stimuli as exercise and hypoxia. In the present study, we examined whether AMPK could also mediate glucose autoregulation. The activity of the alpha2 isoform of AMPK and 2-deoxyglucose uptake were assessed in incubated rat extensor digitorum longus muscle after preincubation for 4 h in media containing 0, 3, 6, or 25 mmol/l glucose. The principal findings were as follows. First, AMPK activity was highest in muscles incubated with no added glucose, and it decreased as the concentration of glucose was increased. In keeping with these findings, the concentration of malonyl CoA was increased, and acetyl CoA carboxylase phosphorylation at serine 79 was decreased as the medium glucose concentration was raised. Second, decreases in AMPK activity at the higher glucose concentrations correlated closely with decreases in glucose transport (2-deoxyglucose uptake), measured during a subsequent 20-min incubation at 6 mmol/l glucose (r(2) = 0.93, P < 0.001). Third, the decrease in AMPK activity at the higher glucose concentrations was not associated with changes in whole-tissue concentrations of creatine phosphate or adenine nucleotides; however, it did correlate with increases in the rate of glycolysis, as estimated by lactate release. The results suggest that glucose autoregulates its own transport into skeletal muscle by a mechanism involving AMPK. They also suggest that this autoregulatory mechanism is not paralleled by changes in whole-tissue concentrations of creatine phosphate ATP, or AMP, but they leave open the possibility that alterations in a cytosolic pool of these compounds play a regulatory role.

Lactate and glucose interactions during rest and exercise in men: effect of exogenous lactate infusion

To test the hypothesis that lactate plays a central role in the distribution of carbohydrate (CHO) potential energy for oxidation and glucose production (GP), we performed a lactate clamp (LC) procedure during rest and moderate intensity exercise. Blood [lactate] was clamped at approximately 4 mM by exogenous lactate infusion. Subjects performed 90 min exercise trials at 65 % of the peak rate of oxygen consumption (V(O(2))(,peak); 65 %), 55 % V(O(2))(,peak) (55 %) and 55 % V(O(2))(,peak) with lactate clamped to the blood [lactate] that was measured at 65 % V(O(2))(,peak) (55 %-LC). Lactate and glucose rates of appearance (R(a)), disappearance (R(d)) and oxidation (R(ox)) were measured with a combination of [3-(13)C]lactate, H(13)CO(3)(-), and [6,6-(2)H(2)]glucose tracers. During rest and exercise, lactate R(a) and R(d) were increased at 55 %-LC compared to 55 %. Glucose R(a) and R(d) were decreased during 55 %-LC compared to 55 %. Lactate R(ox) was increased by LC during exercise (55 %: 6.52 +/- 0.65 and 55 %-LC: 10.01 +/- 0.68 mg kg(-1) min(-1)) which was concurrent with a decrease in glucose oxidation (55 %: 7.64 +/- 0.4 and 55 %-LC: 4.35 +/- 0.31 mg kg(-1) min(-1)). With LC, incorporation of (13)C from tracer lactate into blood glucose (L GNG) increased while both GP and calculated hepatic glycogenolysis (GLY) decreased. Therefore, increased blood [lactate] during moderate intensity exercise increased lactate oxidation, spared blood glucose and decreased glucose production. Further, exogenous lactate infusion did not affect rating of perceived exertion (RPE) during exercise. These results demonstrate that lactate is a useful carbohydrate in times of increased energy demand.

Metabolic and cardiorespiratory responses to "the lactate clamp"

To evaluate the hypothesis that precursor supply limits gluconeogenesis (GNG) during exercise, we examined training-induced changes in glucose kinetics [rates of appearance (R(a)) and disappearance (R(d))], oxidation (R(ox)), and recycling (R(r)) with an exogenous lactate infusion to 3.5-4.0 mM during rest and to pretraining 65% peak O(2) consumption (VO(2 peak)) levels during exercise. Control and clamped trials (LC) were performed at rest pre- (P(R)R, P(R)R-LC) and posttraining (P(O)R, P(O)R-LC) and during exercise pre- (P(R)E(X)) and posttraining at absolute (P(O)A(B), P(O)A(B)-LC) and relative (P(O)R(L), P(O)R(L)-LC) intensities. Glucose R(r) was not different in any rest or exercise condition. Glucose R(a) did not differ as a result of LC. Glucose R(ox) was significantly decreased with LC at P(O)R (0.38 +/- 0.03 vs. 0.56 +/- 0.04 mg. kg(-1). min(-1)) and P(O)A(B) (3.82 +/- 0.51 vs. 5.0 +/- 0.62 mg. kg(-1). min(-1)). Percent glucose R(d) oxidized decreased with all LC except P(O)R(L)-LC (P(R)R, 32%; P(R)R-LC, 22%; P(O)R, 27%; P(O)R-LC, 20%; P(O)A(B), 95%; P(O)A(B)-LC, 77%), which resulted in a significant increase in oxidation from alternative carbohydrate (CHO) sources at rest and P(O)A(B). We conclude that 1) increased arterial [lactate] did not increase glucose R(r) measured during rest or exercise after training, 2) glucose disposal or production did not change with increased precursor supply, and 3) infusion of exogenous CHO in the form of lactate resulted in the decrease of glucose R(ox).

Human skeletal muscle fatty acid and glycerol metabolism during rest, exercise and recovery

This study was conducted to investigate skeletal muscle fatty acid (FA) and glycerol kinetics and to determine the contribution of skeletal muscle to whole body FA and glycerol turnover during rest, 2 h of one-leg knee-extensor exercise at 65 % of maximal leg power output, and 3 h of recovery. To this aim, the leg femoral arterial-venous difference technique was used in combination with a continuous infusion of [U-(13)C]palmitate and [(2)H(5)]glycerol in five post-absorptive healthy volunteers (22 +/- 3 years). The influence of contamination from non-skeletal muscle tissues, skin and subcutaneous adipose tissue, on FA and glycerol kinetics was studied by catheterization of the femoral vein in antegrade and retrograde directions. Substantially higher net leg FA and glycerol uptakes were observed with a retrograde compared to an antegrade catheter position, as a result of a much lower tracer-calculated leg FA and glycerol release. The whole body FA rate of appearance (R(a)) increased with exercise and decreased rapidly in recovery but stayed higher compared to pre-exercise. The leg net FA uptake decreased immediately on cessation of exercise to near pre-exercise level, but the tracer FA uptake and release decreased slowly and reached constant values after approximately 1.5 h of recovery similar to pre-exercise. Whole body FA reesterification (FA R(d) - FA oxidation; R(d), rate of disappearance) was approximately 400 micromol min(-1) at rest and during exercise, and increased during recovery to 495 micromol min(-1). Leg FA reesterification was 17 micromol min(-1) at rest and decreased to 9 micromol min(-1) during recovery, due to a larger fraction of leg FA uptake being directed to oxidation. A net glycerol exchange across the leg could not be detected under all conditions, but a substantial leg glycerol uptake was observed, which was substantially higher during exercise. Total body skeletal muscle FA and glycerol uptake/release was estimated to account for 18-25 % of whole body R(d) or R(a).

IN CONCLUSION

(1) skeletal muscle FA and glycerol metabolism, using the leg arterial-venous difference method, can only be studied if contamination from skin and subcutaneous adipose tissue is prevented; (2) whole body FA reesterification is unchanged when going from rest to exercise, but is increased during recovery; (3) in post-absorptive man total body skeletal muscle contributes 17-24 % to whole body FA and glycerol turnover and FA reesterification at rest; (4) glycerol is taken up by skeletal muscle and the uptake increases many fold during exercise.

Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 micromol x kg(-1) x min(-1)) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle.

Increased lactate release per fat cell in normoglycemic first-degree relatives of individuals with type 2 diabetes

The aim of this study was to examine subcutaneous lactate production in the relatives of individuals with type 2 diabetes. Therefore, we recruited seven healthy first-degree relatives of type 2 diabetic patients and seven pairwise, matched, healthy control subjects without any heredity for diabetes. All subjects were studied with a euglycemic insulin clamp at approximately 600 pmol/l, abdominal subcutaneous microdialysis, and (133)Xe clearance. Furthermore, a subcutaneous needle biopsy was performed to determine fat cell size. In the fasting state, interstitial lactate was 40% higher in relatives than in control subjects (P = 0.043), but net lactate production was similar in both groups. However, during the insulin clamp, interstitial lactate (2.50 +/- 0.29 vs. 1.98 +/- 0.26 mmol/l, P = 0.018), interstitial-arterial lactate concentration difference (1.08 +/- 0.30 vs. 0.53 +/- 0.24 mmol/l, P = 0.028), and net lactate release per fat cell (10.9 +/- 3.7 vs. 2.8 +/- 1.3 fmol. cell(-1). min(-1), P = 0.018) were increased in the relatives. We conclude that first-degree relatives of type 2 diabetic patients may have an enhanced net lactate release per fat cell in abdominal subcutaneous tissue. This could suggest a pathological regulation in adipose tissue that is of importance for the metabolic defects known in type 2 diabetic relatives.

Regulation of fatty acid oxidation and glucose metabolism in rat soleus muscle: effects of AICAR

Previous studies have shown that 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a cell-permeable activator of AMP-activated protein kinase, increases the rate of fatty acid oxidation in skeletal muscle of fed rats. The present study investigated the mechanism by which this occurs and, in particular, whether changes in the activity of malonyl-CoA decarboxylase (MCD) and the beta-isoform of acetyl-CoA carboxylase (ACC beta) are involved. In addition, the relationship between changes in fatty acid oxidation induced by AICAR and its effects on glucose uptake and metabolism was examined. In incubated soleus muscles isolated from fed rats, AICAR (2 mM) increased fatty acid oxidation (90%) and decreased ACC beta activity (40%) and malonyl-CoA concentration (50%); however, MCD activity was not significantly altered. In soleus muscles from overnight-fasted rats, AICAR decreased ACC beta activity (40%), as it did in fed rats; however, it had no effect on the already high rate of fatty acid oxidation or the low malonyl-CoA concentration. In keeping with its effect on fatty acid oxidation, AICAR decreased glucose oxidation by 44% in fed rats but did not decrease glucose oxidation in fasted rats. It had no effect on glucose oxidation when fatty acid oxidation was inhibited by 2-bromopalmitate. Surprisingly, AICAR did not significantly increase glucose uptake or assayable AMP-activated protein kinase activity in incubated soleus muscles from fed or fasted rats. These results indicate that, in incubated rat soleus muscle, 1) AICAR does not activate MCD or stimulate glucose uptake as it does in extensor digitorum longus and epitrochlearis muscles, 2) the ability of AICAR to increase fatty acid oxidation and diminish glucose oxidation and malonyl-CoA concentration is dependent on the nutritional status of the rat, and 3) the ability of AICAR to diminish assayable ACC activity is independent of nutritional state.

Skeletal muscle substrate metabolism during exercise: methodological considerations

The aim of the present article is to evaluate critically the various methods employed in studies designed to quantify precisely skeletal muscle substrate utilization during exercise. In general, the pattern of substrate utilization during exercise can be described well from O2 uptake measurements and the respiratory exchange ratio. However, if the aim is to quantify limb or muscle metabolism, invasive measurements have to be carried out, such as the determination of blood flow, arterio-venous (a-v) difference measurements for O2 and relevant substrates, and biopsies of the active muscle. As many substrates and metabolites may be both taken up and released by muscle at rest and during exercise, isotopes can be used to determine uptake and/or release and also fractional uptake can be accounted for. Furthermore, the use of isotopes opens up further possibilities for the estimation of oxidation rates of various substrates. There are several methodological concerns to be aware of when studying the metabolic response to exercise in human subjects. These concerns include: (1) the muscle mass involved in the exercise is largely unknown (bicycle or treadmill). Moreover, whether the muscle sample obtained from a limb muscle and the substrate and metabolite concentrations are representative can be a problem; (2) the placement of the venous catheter can be critical, and it should be secured so that the blood sample represents blood from the active muscle with a minimum of contamination from other muscles and tissues; (3) the use of net limb glycerol release to estimate lipolysis is probably not valid (triacylglycerol utilization by muscle), since glycerol can be metabolized in skeletal muscle; (4) the precision of blood-borne substrate concentrations during exercise measured by a-v difference is hampered since they become very small due to the high blood flow. Recommendations are given in order to obtain more quantitative and conclusive data in studies investigating the regulatory mechanisms for substrate choice by muscle.

Leptin opposes insulin's effects on fatty acid partitioning in muscles isolated from obese ob/ob mice

Because muscle triacylglycerol (TAG) accumulation might contribute to insulin resistance in leptin-deficient ob/ob mice, we studied the acute (60- to 90-min) effects of leptin and insulin on [14C]glucose and [14C]oleate metabolism in muscles isolated from lean and obese ob/ob mice. In ob/ob soleus, leptin decreased glycogen synthesis 36-46% (P < 0.05), increased oleate oxidation 26% (P < 0.05), decreased oleate incorporation into TAG 32% (P < 0.05), and decreased the oleate partitioning ratio (oleate partitioned into TAG/CO2) 44% (P < 0.05). Insulin decreased oleate oxidation 31% (P < 0.05), increased oleate incorporation into TAG 46% (P < 0.05), and increased the partitioning ratio 125% (P < 0.01). Adding leptin diminished insulin's antioxidative, lipogenic effects. In soleus from lean mice, insulin increased the partitioning ratio 142%, whereas leptin decreased it 51%, as previously reported (Muoio, D. M. , G. L. Dohm, F. T. Fiedorek, E. B. Tapscott, and R. A. Coleman. Diabetes 46: 1360-1363, 1997). The phosphatidylinositol 3-kinase inhibitor wortmannin blocked insulin's effects on lipid metabolism but only attenuated leptin's effects. Increasing glucose concentration from 5 to 10 mM did not affect TAG synthesis, suggesting that insulin-induced lipogenesis is independent of increased glucose uptake. These data indicate that leptin opposes insulin's promotion of TAG accumulation in lean and ob/ob muscles. Because acute leptin exposure does not correct insulin resistance in ob/ob muscles, in vivo improvements in glucose homeostasis appear to require other long-term factors, possibly TAG depletion.

Hyperlactatemia reduces muscle glucose uptake and GLUT-4 mRNA while increasing (E1alpha)PDH gene expression in rat

An increased basal plasma lactate concentration is present in many physiological and pathological conditions, including obesity and diabetes. We previously demonstrated that acute lactate infusion in rats produced a decrease in overall glucose uptake. The present study was carried out to further investigate the effect of lactate on glucose transport and utilization in skeletal muscle. In chronically catheterized rats, a 24-h sodium lactate or bicarbonate infusion was performed. To study glucose uptake in muscle, a bolus of 2-deoxy-[3H]glucose was injected in basal condition and during euglycemic-hyperinsulinemic clamp. Our results show that hyperlactatemia decreased glucose uptake in muscles (i.e., red quadriceps; P < 0.05). Moreover in red muscles, both GLUT-4 mRNA (-30% in red quadriceps and -60% in soleus; P < 0.025) and protein (-40% in red quadriceps; P < 0.05) were decreased, whereas the (E1alpha)pyruvate dehydrogenase (PDH) mRNA was increased (+40% in red quadriceps; P < 0.001) in lactate-infused animals. PDH protein was also increased (4-fold in red gastrocnemius and 2-fold in red quadriceps). These results indicate that chronic hyperlactatemia reduces glucose uptake by affecting the expression of genes involved in glucose metabolism in muscle, suggesting a role for lactate in the development of insulin resistance.

Lactate infusion to normal rats during hyperglycemia enhances in vivo muscle glycogen synthesis

Both hyperglycemia and hyperinsulinemia stimulate whole body and muscle glucose disposal. To define the impact of increased lactate concentration (4-5 mM) on muscle glucose disposal during hyperglycemia, we studied anesthetized normal rats infused with either sodium lactate or sodium bicarbonate as control. Animals were studied under hyperglycemic clamp (13 mM) using [3-3H]glucose (study 1) and 2-deoxy-[1-3H]glucose (study 2) to assess glucose rate of disappearance (Rd), glycolytic flux (GF), glycogen synthesis, and glucose utilization index by different tissues. Moreover, in study 3, the effect of lactate on the pattern of plasma insulin response to hyperglycemia was evaluated. In study 1, lactate infusion resulted in an increased Rd (38.7 +/- 1.7 vs. 32.3 +/- 1.3 mg.min-1.kg-1; P < 0.01), which was explained by an enhanced rate of glycogen synthesis (23.0 +/- 1.7 vs. 14.7 +/- 1.2 mg.min-1.kg-1; P < 0.001), whereas GF was unchanged. In study 2, lactate-infused animals showed an increased 2-deoxy-glucose disposal and a stimulated glycogen synthase activity as well as an increased glycogen accumulation at the end of the study in several skeletal muscles. In study 3, lactate did not induce any change in either early or late insulin response to hyperglycemia. In conclusion, our results show that muscle glycogen deposition may be enhanced by elevated lactate levels under hyperglycemic conditions and support a role for lactate in the regulation of glucose homeostasis.

Lactate infusion in anesthetized rats produces insulin resistance in heart and skeletal muscles

Plasma lactate is elevated in many physiological and pathological conditions, such as physical exercise, obesity, and diabetes, in which a reduction of insulin sensitivity is also present. Furthermore, an increased production of lactate from muscle and adipose tissue together with increased gluconeogenic substrate flux to the liver plays a primary role in enhancing hepatic glucose production (HGP) in diabetes. It has been shown that lactate may interfere with the utilization and oxidation of other substrates such as free fatty acids (FFAs). The aim of this study was to investigate if lactate infusion affects peripheral glucose utilization in rats. Animals were acutely infused with lactate to achieve a final lactate concentration of 4 mmol/L. They were then submitted to a euglycemic-hyperinsulinemic clamp to study HGP and overall glucose metabolism (rate of disappearance [Rd]). At the end of the clamp, a bolus of 2-deoxy-[1-3H]-glucose was injected to study insulin-dependent glucose uptake in different tissues. The results show that lactate infusion did not affect HGP either in the basal state or at the end of clamp, whereas glucose utilization significantly decreased in lactate-infused rats (26.6 +/- 1.1 v 19.5 +/- 1.4 mg.kg-1.min-1, P < .01). A reduction in the tissue glucose utilization index was noted in heart (18.01 +/- 4.44 v 46.21 +/- 6.51 ng.mg-1.min-1, P < .01), diaphragm (5.56 +/- 0.74 v 9.01 +/- 0.93 ng.mg-1.min-1, P < .01), soleus (13.62 +/- 2.29 v 34.05 +/- 6.08 ng.mg-1.min-1, P < .01), and red quadricep (4.43 +/- 0.73 v 5.88 +/- 0.32 ng.mg-1.min-1, P < .05) muscle in lactate-infused animals, whereas no alterations were observed in other muscles or in adipose tissue. Therefore, we suggest that acute lactate infusion induces insulin resistance in the heart and some muscles, thus supporting a role for lactate in the regulation of peripheral glucose metabolism.

Lactate transport in rat sarcolemmal vesicles and intact skeletal muscle, and after muscle contraction

To determine whether it was possible to measure lactate transport rates into intact skeletal muscles, the transport of lactate (zero-trans) was determined in soleus muscle strips incubated in vitro and compared with lactate transport in sarcolemmal vesicles. In addition, the effects of muscle contractility on lactate transport were investigated in electrically-stimulated soleus muscle strips. In both the intact muscle and the sarcolemmal preparations the rates of transport were saturable, stereospecific, and inhibitable by monocarboxylates (pyruvate, alpha-cyano-4-hydroxycinnamate) and a protein modifier (N-ethylmaleimide; P < 0.05). The anion exchange inhibitor SITS had no effect on lactate uptake (P > 0.05). In both preparations lactate transport followed an inwardly directed proton gradient. Relative comparisons (%) between the preparations indicated a similar slope of increasing transport rates with increasing lactate concentrations and similar responses to a changing pH environment. These characterizations of L-lactate transport into isolated sarcolemmal vesicles and muscle strips revealed that both preparations yielded similar conclusions regarding the transmembrane movement of L-lactate. By using this more physiological muscle preparation, contractile activity, induced by electrical stimulation, did not increase lactate uptake in skeletal muscle in the post-exercise period whereas under similar conditions a marked increase in 2-deoxy-D-glucose uptake occurred (+ 47%; P < 0.05). These data suggest that the transport of glucose and lactate in contracting muscle is regulated differently. These studies also show that the incubated muscle strip preparation may be useful for studying lactate transport in an intact cell system during physiological experiments.

Control of muscle pyruvate oxidation during late pregnancy

Despite significant increases in circulating concentrations of lipid fuels (triacylglycerol, non-esterified fatty acids (NEFA) and ketone bodies) in late-pregnant rats sampled in the fed (absorptive) state, cardiac and skeletal muscle active pyruvate dehydrogenase (PDHa) activities remained comparable with those observed in fed, age-matched virgin controls. Cardiac PDHa activity was suppressed in response to acute (6 h) starvation in late-pregnant (as well as virgin) rats: this inactivation was opposed by inhibition of mitochondrial long-chain FA oxidation. Starvation (6 h) also led to PDH inactivation in skeletal muscles of late-pregnant, but not virgin, rats. Starvation for 24 h led to further suppression of cardiac PDHa activity and was associated with significant increases in PDH kinase activities in both virgin and late-pregnant rats. Late pregnancy did not itself influence cardiac PDH kinase activity.

Insulin resistance in obesity is associated with elevated basal lactate levels and diminished lactate appearance following intravenous glucose and insulin

Lactate metabolism is altered in obesity. Increasing obesity is associated with increased blood lactate levels after an overnight fast. In contrast, we have recently shown a marked decrease in the capacity for acute lactate generation in obese subjects following an oral glucose load, which we postulated might be linked to altered insulin sensitivity. In the present study, we systematically analyzed the relationship between insulin sensitivity (the Sensitivity Index [SI] derived using the minimal model), body mass index (BMI), and glucose, insulin, and lactate levels in the basal state and following intravenous (IV) glucose and insulin administration in lean and obese subjects. The results showed that SI and BMI were inversely related, as expected. Insulin sensitivity was more tightly associated with glucose, insulin, and lactate levels (both basal and integrated) than obesity per se. A significant inverse relationship was found between SI and basal lactate levels (r = -.56). Moreover, a significant and positive relationship was found between SI and incremental lactate area under the curve (reflecting acute lactate production) (r = .41). In a multiple regression analysis to separate the independent effects of obesity (BMI) and insulin sensitivity, after adjusting for age, sex, and race, SI accounted for 34% of the variance in basal lactate and 24% of the variance in incremental lactate area. Obesity independently accounted for 10% of the variance in basal lactate and 11% of the variance in incremental lactate area, neither of which were statistically significant. We conclude that elevations in basal lactate are associated with the development of insulin resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycerol and lactate uptake in human forearm

Arteriovenous differences for lactate and glycerol reported across the human forearm are inconsistent in direction and magnitude. Such inconsistency could represent the effects of differing forearm compositions. The hypothesis was tested by examination of 37 studies of forearm arteriovenous differences for lactate, glycerol, glucose, and oxygen (only measured in 25 studies) in 23 normal subjects after overnight fast. In 16 studies, glycerol was taken up rather than released by the forearm, and in 12 of these the arteriovenous difference was greater than could be accounted for by analytical variation. The arteriovenous differences for glycerol and lactate were positively correlated (r = .44, P less than .01). The hypothesis that glycerol and lactate uptake might reflect a more "oxidative" forearm was not borne out, since neither glycerol nor lactate arteriovenous differences correlated with that for oxygen, although oxygen and glucose arteriovenous differences and fluxes were correlated (fluxes: r = .60, P less than .01). The arteriovenous difference for glycerol was positively related to body mass index, arguing against a variable contribution from fat. The hypothesis that the direction of glycerol and lactate exchange would reflect the forearm composition was not borne out by the analysis of repeated studies on the same individual, which showed that the variation within subjects was not significantly less than that between subjects. Therefore, we conclude that in approximately 40% of studies in normal subjects after an overnight fast, the forearm will show glycerol uptake, although we have been unable to identify any physiological reason for this phenomenon. Peripheral glycerol uptake has implications for studies in which glycerol release is taken as a measure of lipolysis.

Effect of the fatty acid oxidation inhibitor 2-tetradecylglycidic acid (TDGA) on glucose and fatty acid oxidation in isolated rat soleus muscle

1. The effect of 2-tetradecylglycidic acid (TDGA), a potent, specific inhibitor of long-chain fatty acid oxidation, on fatty acid and glucose oxidation by isolated rat soleus muscle was studied. 2. TDGA inhibited [1-14C]palmitate oxidation by soleus muscle in a concentration-dependent manner. 3. TDGA inhibited the activity of soleus muscle mitochondrial carnitine palmitoyltransferase A (CPT-A). 4. Added palmitate (0.5 mM) significantly inhibited D-[U-14C]glucose oxidation and, under conditions where TDGA inhibited palmitate oxidation, the oxidation of D-[U-14C]glucose by isolated soleus muscle was significantly stimulated. 5. TDGA stimulation of glucose oxidation was reversed by octanoate, a medium-chain fatty acid whose oxidation is not inhibited by TDGA. 6. When nondiabetic rats were treated with TDGA (10 mg/kg p.o./day x 3 days), fasting plasma glucose was significantly lowered and the ability of isolated contralateral soleus muscles to oxidize palmitate was inhibited while glucose oxidation was significantly stimulated.

Effects of starvation and exercise on concentrations of citrate, hexose phosphates and glycogen in skeletal muscle and heart. Evidence for selective operation of the glucose-fatty acid cycle

Concentrations of citrate, hexose phosphates and glycogen were measured in skeletal muscle and heart under conditions in which plasma non-esterified fatty acids and ketone bodies were physiologically increased. The aim was to determine under what conditions the glucose-fatty acid cycle might operative in skeletal muscle in vivo. In keeping with the findings of others, starvation increased the concentrations of glycogen, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in heart, indicating that the cycle was operative. In contrast, it decreased glycogen and had no effect on the concentration of citrate or the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in the soleus, a slow-twitch red muscle in which the glucose-fatty acid cycle has been demonstrated in vitro. In fed rats, exercise of moderate intensity caused glycogen depletion in the soleus and red portion of gastrocnemius muscle, but not in heart. In starved rats the same exercise had no effect on the already diminished glycogen contents in skeletal muscle, but it decreased cardiac glycogen by 25-30%. After exercise, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio were increased in the soleus of the starved rat. Significant changes were not observed in fed rats. The data suggest that in the resting state the glucose-fatty acid cycle operates in the heart, but not in the soleus muscle, of a starved rat. In contrast, the metabolite profile in the soleus was consistent with activation of the glucose-fatty acid cycle in the starved rat during the recovery period after exercise. Whether the cycle operates during exercise itself is unclear.

Phase-related changes in tissue energy reserves during hemorrhagic shock

In view of the well-known fact that the liver is more sensitive to ischemia than skeletal muscle, it was the purpose of the present study to determine the relationship between the hemorrhage-induced changes in plasma glucose and lactate concentrations and the status of the energy reserves of these two tissues. Sprague-Dawley rats were bled to a constant mean arterial blood pressure of 40 mm Hg and held there by removal or reinfusion of blood. The stages of shock defined on the basis of the net blood loss were early compensatory, maximal compensatory, early decompensatory, and late decompensatory phases. The results showed a depletion of hepatic ATP levels which occurred between the early compensatory and maximal compensatory phases of shock, coincident with the most dramatic increases in plasma glucose and lactate seen during the shock protocol. Hepatic ATP levels fell no further through the decompensatory phases of shock while plasma glucose declined to hypoglycemic levels and plasma lactate was maintained at the same high level attained at the maximal compensatory phase. Since hepatic sources of glucose were exhausted by the maximal compensatory phase and hepatic energy stores were depleted to a point which precludes significant gluconeogenesis, the large increase in plasma lactate was probably largely due to loss of the hepatic "sink" for lactate during this phase of shock. In contrast to the liver, soleus muscle showed no change in the levels of glycogen, ATP, CrP, free creatine, or total creatine compared to time-matched controls in any phase of hemorrhagic shock suggesting the absence of significant muscle ischemia. The possibility that red skeletal muscle may act as a "sink" for lactate is considered.

Denervated frog skeletal muscle. Some electrical and mechanical properties

The effects of denervation on several mechanical and electrical parameters of frog sartorius muscle have been investigated. In denervated muscles, there is no change in the resting potential and a relatively small change in the action potential. The first alteration in the action potential is a reduction of about 30% in the maximum rate of repolarization in muscles that have been denervated for 40 days or longer. Later, the overshoot and maximum rate of depolarization also decline. No tetrodotoxin resistant action potentials could be detected. Fibrillatory potentials were observed infrequently and in most cases in depolarized fibers. Twitch tension is significantly reduced by denervation while the tetanus tension is practically unaffected by denervation. The experiments suggest that the decline in twitch tension produced by denervation reflect a defect in some step of the excitation contraction coupling sequence. On the other hand, post-tetanic potentiation of the twitch is much larger in denervated than in control muscles. This potentiation in denervated muscles is paralleled by an increased action potential duration which returns to its pretetanic duration with a time course indistinguishable from that of the twitch potentiation.

Skeletal muscle fibers in suspension: a new approach to the study of oxidative and glycolytic metabolism in differentiated muscle

A preparation of suspended fibers from m. flexor digitorum brevis of the rat was characterized with respect to morphological features, and its relevance for the study of muscular metabolism investigated. The activities of oxidative (palmitate and pyruvate oxidation) and glycolytic (lactate formation) pathways were enhanced in myofiber suspensions when compared to intact whole muscle. The rate of glycolysis was stimulated about two-fold by insulin in both the myofiber suspensions and intact muscle. Parameters of oxidative metabolism responded similarly to metabolic effectors in the myofiber suspensions and in intact muscle. It is concluded that the myofiber suspensions have distinct advantages over intact muscle for biochemical and pharmacological studies.

Twitch potentiation and caffeine contractures in isolated rat soleus muscle

1. Electrically-evoked twitch and tetanic tension were measured in isolated rat soleus muscle after exposure to caffeine. 2. Between 0.01 and 2.5 mM caffeine twitch tension was potentiated, reaching a peak of 150% of Resting Tension at 0.5 mM. 3. Biphasic Tension development with relaxation was observed at 2.5 mM caffeine with maximal contractures (110% tetanic tension) occurring at 20 mM. 4. Creatine phosphate and ATP stores were maintained throughout the period of tension development and relaxation. 5. In contrast with amphibian muscle, the isolated soleus is very sensitive to low doses of caffeine and produces biphasic caffeine contractures which relax in the presence of caffeine.

Palmitate oxidation by intact preparations of skeletal muscle

1. The palmitate oxidation by intact preparations of rat hemidaphragm, m.soleus and m.flexor digitorum brevis and by teased fibers of human m.pectoralis was studied. 2. The structural and metabolic viability of the in vitro preparations was shown by a low leakage of soluble creatine kinase, a constant rate of palmitate oxidation and only a small stimulatory effect of L-carnitine. 3. With hemidaphragm the palmitate oxidation rate increases with both the palmitate concentration (0-3 mM) and the palmitate/albumin molar ratio (0.5-5.0). 4. The apparent Km for palmitate oxidation was about 1.5 mM at 0.1 and 0.2 mM albumin and about 2.7 mM at 0.4 and 0.6 mM albumin, which correlates with the higher affinity of albumin for palmitate at lower palmitate/albumin molar ratios. 5. After prolonged starvation the apparent Km at 0.4 mM albumin is markedly lower. In whole homogenates of diaphragm the apparent Km at 0.4 mM albumin is only about 370 microM. 6. The calculated maximal oxidation rate was not significantly different for all albumin concentrations examined (23-32 nmol/min per g), did not change after starvation and appears to be of the same order of magnitude as the rate of endogenous fatty acid consumption (30-40 nmol/min per g). 7. Results suggest that substrate availability is a main factor for the oxidation rate of exogenous palmitate by hemidiaphragm in vitro and that the magnitude of the apparent Km is largely dependent upon the degree of label dilution with fatty acids of endogenous origin.