Differential modulation of glucose,lactate, and pyruvate oxidation by insulin and dichloroacetate in the rat heart

Despite the fact that lactate and pyruvate are potential substrates for energy production in vivo, our understanding of the control and regulation of carbohydrate metabolism is based principally on studies where glucose is the only available carbohydrate. Therefore, the purpose of this study was to determine the contributions of lactate, pyruvate, and glucose to energy production in the isolated, perfused rat heart over a range of insulin concentrations and after activation of pyruvate dehydrogenase with dichloroacetate (DCA). Hearts were perfused with physiological concentrations of [1-13C]glucose, [U-13C]lactate, [2-13C]pyruvate, and unlabeled palmitate for 45 min. Hearts were freeze clamped, and 13C NMR glutamate isotopomer analysis was performed on tissue extracts. Glucose, lactate, and pyruvate all contributed significantly to myocardial energy production; however, in the absence of insulin, glucose contributed only 25-30% of total pyruvate oxidation. Even under conditions where carbohydrates represented >95% of substrate entering the tricarboxylic acid (TCA) cycle, we found that glucose contributed at most 50-60% of total carbohydrate oxidation. Despite being present at only 0.1 mM, pyruvate contributed between approximately 10% and 30% of total acetyl-CoA entry into the TCA cycle. We also found that insulin and DCA not only increased glucose oxidation but also exogenous pyruvate oxidation; however, lactate oxidation was not increased. The differential effects of insulin and DCA on pyruvate and lactate oxidation provide further evidence for compartmentation of cardiac carbohydrate metabolism. These results may have important implications for understanding the mechanisms underlying the beneficial effects of increasing cardiac carbohydrate metabolism.     

Role of fructose 2,6-bisphosphate in the regulation of glycolysis and gluconeogenesis in chicken liver

Glucagon and dibutyryl cyclic AMP inhibited glucose utilization and lowered fructose 2,6-bisphosphate levels of hepatocytes prepared from fed chickens. Partially purified preparations of chicken liver 6-phosphofructo-1-kinase and fructose 1,6-bisphosphatase were activated and inhibited by fructose 2,6-bisphosphate, respectively. The sensitivities of these enzymes and the changes observed in fructose 2,6-bisphosphate levels are consistent with an important role for this allosteric effector in hormonal regulation of carbohydrate metabolism in chicken liver. In contrast, oleate inhibition of glucose utilization by chicken hepatocytes occurred without change in fructose, 2,6-bisphosphate levels. Likewise, pyruvate inhibition of lactate gluconeogenesis in chicken hepatocytes cannot be explained by changes in fructose 2,6-bisphosphate levels. Exogenous glucose caused a marked increase in fructose 2,6-bisphosphate content of hepatocytes from fasted but not fed birds. Both glucagon and lactate prevented this glucose effect. Fasted chicken hepatocytes responded to lower glucose concentrations than fasted rat hepatocytes, perhaps reflecting the species difference in hexokinase isozymes.     

COMPARATIVE EFFECTS OF ORGANIC AND INORGANIC MERCURY ON BRAIN SLICE RESPIRATION AND METABOLISM

—Respiration and carbohydrate metabolism were measured in guinea-pig brain slices exposed to organic and inorganic mercury. Organic mercury decreased oxygen uptake and ¹⁴CO₂ production at consistently lower concentrations than inorganic mercury. Organic mercury also caused a striking elevation of pyruvate and lactate at low doses where inorganic mercury had no effect on respiration or metabolism. A unique inhibition of tricarboxylic acid cycle function is suggested and might partially explain the distinctive neurotoxicity of organic mercury.     

Schistosoma mansoni: effects of in vitro serotonin (5-HT) on aerobic and anaerobic carbohydrate metabolism

The effect of Serotonin on carbohydrate metabolism, excreted end products, and adenine nucleotide pools in Schistosoma mansoni was determined following 60 min in vitro incubations under air (= 21% O2) and anaerobic (95% N2:5% CO2) conditions. In the presence of 0.25 mM Serotonin, glucose uptake increased by 82-84% and lactate excretion increased by 77-78%; levels of excreted lactate were significantly higher under aerobic than under anaerobic conditions. The tissue pools of glucose, hexosephosphates, fructose 1,6-bisphosphate, pyruvate, and lactate were significantly increased under anaerobic conditions compared to air incubation; the presence of Serotonin decreased tissue glucose pools and increased the size of the pyruvate and lactate tissue pools. The glycolytic carbon pool was significantly greater under anaerobic than under aerobic conditions, irrespective of Serotonin. Serotonin increased adenosine 5'-diphosphate and adenosine 5'-monophosphate levels under aerobic conditions; neither Serotonin nor gas phase significantly affected total adenine nucleotide levels or the adenylate energy charge. Serotonin increased energy requirements by S. mansoni due to increased muscle contractions; demand was met by enhanced rates of carbohydrate metabolism. Irrespective of gas phase, 74-78% of available carbohydrate was converted to lactate. In the presence of Serotonin, conversion of glucose to lactate was reduced to 63-67%. In view of the requirements by S. mansoni for an abundant supply of glycoprotein and glycolipid precursors for surface membrane renewal, it is suggested that carbohydrate (glucose and glycogen) that was not converted to lactate may have been incorporated into biosynthetic processes leading to membrane synthesis.     

The effect of propionate on the metabolism of pyruvate and lactate in the perfused rat liver

1. Rates of gluconeogenesis in the perfused rat liver from propionate, l-lactate, pyruvate and the combination of propionate with either lactate or pyruvate were measured. Less than additive rates were obtained with either propionate plus lactate or propionate plus pyruvate. 2. The uptake of pyruvate plus lactate from the perfusion medium was decreased more seriously when propionate was present with lactate than with pyruvate. 3. The use of [2-(14)C]pyruvate in the presence of propionate showed that the decreased disappearance of pyruvate plus lactate did not result in their formation from propionate. 4. The addition of sodium butyrate to the perfusion medium caused an inhibition of gluconeogenesis from propionate and stimulated gluconeogenesis and uptake of pyruvate and lactate. 5. The observations are consistent with there being a sparing effect of propionate on lactate and pyruvate metabolism.     

Regulation of mitochondrial pyruvate carboxylation and gluconeogenesis in rat hepatocytes via an alpha-adrenergic, adenosine 3':5'-monophosphate-independent mechanism.

Experiments were performed to determine if catecholamines can regulate control points in the gluconeogenic pathway, such as mitochondrial pyruvate carboxylation and pyruvate kinase activity, via an alpha-adrenergic, adenosine 3':5'-monophosphate-independent mechanism. Of a number of alpha agonists tested, only norepinephrine, epinephrine, and phenylephrine caused an increase in mitochondrial pyruvate metabolism. The effects of catecholamines on pyruvate carboxylation were not attenuated by 1-propranolol which abolishes changes in cyclic nucleotide levels but were blocked by alpha antagonists such as ergotamine, phenoxybenzamine, and phentolamine. Time course experiments demonstrated that the effects of catecholamines on the mitochondria and on carbohydrate metabolism correlated temporally with the concentration of epinephrine in the medium but not with the small changes in adenosine 3':5'-monophosphate. The effects of catecholamines appeared to require extracellular Ca2+ ion. The observation that catecholamines do not increase gluconeogenesis to the same extent as glucagon was not due to a differential effect on mitochondrial CO2 fixation. Rather, catecholamines caused a smaller inhibition of pyruvate kinase activity than did glucagon. The effects of catecholamines on pyruvate kinase also appeared to be mediated by an alpha-adrenergic, adenosine 3':5'-monophosphate-independent mechanism.     

Effect of dilution rate and carbon availability on Bifidobacterium breve fermentation

Bifidobacterium breve NCFB 2257 was grown in glucose-limited and nitrogen (N)-limited chemostats at dilution rates (D) from 0.04 to 0.60 h^–1, to study the effect of nutrient availability on carbohydrate metabolism. The results showed that D had little effect on fermentation product formation, irrespective of the form of nutrient limitation. However, marked differeces were observed in the distribution of fermentation products, that were attributable to glucose availability. In glucose-limited cultures, formate and acetate were the principal end-products of metabolism. Lactate was never detected under these growth conditions. In contrast, lactate and acetate were mainly formed when glucose was in excess, and formate was not produced. These results are explained by the metabolic fate of pyruvate, which can be dissimilated by either phosphoroclastic cleavage to acetyl phosphate and formate, or alternatively, it may be reduced to lactate. Enzymic studies were made to establish the mechanisms that regulated pyruvate metabolism. The data demonstrated that control was not exercised through regulation of the synthesis and activity of lactate dehydrogenase (LDH), phosphofructokinase or alcohol dehydrogenase. It is possible however, that there was competition for pyruvate by LDH and the phosphoroclastic enzyme, which would determine the levels of lactate and formate produced respectively. These results demonstrate the metabolic flexibility of B. breve, which preferentially uses lactate as an electron sink during N-limited growth, whereas under energy-limitation, carbon flow is directed towards acetyl phosphate to maximise ATP synthesis. Correspondence to: B. A. Degnan     

Activity of enzymes related to carbohydrate metabolism in the HT 29 colon adenocarcinoma cell line and tumor

Activity of several enzymes of the glycogen and carbohydrate metabolism is studied in HT 29 colon adenocarcinoma cell line and in HT 29 tumors developed in nude mice, by reference to the normal human colon mucosa. Activity of glycogen synthase, glycogen phosphorylase, pyruvate kinase, fructose-1,6-diphosphatase, glucose-6-phosphate dehydrogenase and lactate dehydrogenase is found to be increased in both the cultured cells and the tumors. It indicates that the biochemical strategy of malignant cells, due to the neoplastic transformation process, involves specific changes in the carbohydrate metabolism of tumor as well as in vitro growing correspondent cell line.     

Functional and biochemical parallels in two-stage devascularization of the liver]

Experiments were performed on mongrel dogs. Biochemical changes of the blood and functional disturbances in the organism were studied after a two-stage devascularization of the liver at different periods after the second stage, i.e. after the ligation of the hepatic artery. Early disturbances in the intermediary metabolism were the consequence of the two-stage devascularization, i.e. of the rise of the pyruvate and the lactate, an increase of ammonia in the blood and serum transaminases. Biochemical changes of the blood precede and accompany the encephalopathia and serious hemodynamic disturbances in the organism. The mentioned method was capable of causing hepatic coma.     

Comparison of glucose metabolism in the lactating mammary gland of the rat in vivo and in vitro. Effects of starvation, prolactin or insulin deficiency

1. Measurements of arteriovenous differences across mammary glands of normal and starved lactating rats, and lactating rats made short-term insulin-deficient with streptozotocin or prolactin-deficient with bromocryptine, showed that only in the starved animals was there a significant decrease in glucose uptake. This decrease was accompanied by release of lactate and pyruvate from the gland, in contrast with the uptake of these metabolites by glands of normal lactating rats. 2. There were no marked differences in metabolite concentrations in freeze-clamped glands in the four conditions studied, apart from a decrease in [lactate] and [pyruvate] and an increase in [glucose] in the glands of the streptozotocin-treated group. 3. Acini isolated from the glands of starved, insulin or prolactin-deficient rats had a higher production of lactate and pyruvate from glucose than did glands from normal rats; this is in agreement with the reported decrease in the proportion of active pyruvate dehydrogenase in these situations [Field & Coore (1976) Biochem. J.156, 333-337; Kankel & Reinauer (1976) Diabetologia12, 149-154]. 4. Addition of insulin did not increase the uptake of glucose by acini from normal glands, but it caused a significant increase in the utilization of glucose by acini from glands of starved rats. Insulin did not decrease the accumulation of lactate and pyruvate in any of the experiments. 5. It is concluded that isolated acini represent a suitable model for the study of mammary-gland carbohydrate metabolism in that they reflect metabolism of the gland in vivo.     

Profiling substrate fluxes in the isolated working mouse heart using 13C-labeled substrates: focusing on the origin and fate of pyruvate and citrate carbons

The availability of genetically modified mice requires the development of methods to assess heart function and metabolism in the intact beating organ. With the use of radioactive substrates and ex vivo perfusion of the mouse heart in the working mode, previous studies have documented glucose and fatty acid oxidation pathways. This study was aimed at characterizing the metabolism of other potentially important exogenous carbohydrate sources, namely, lactate and pyruvate. This was achieved by using (13)C-labeling methods. The mouse heart perfusion setup and buffer composition were optimized to reproduce conditions close to the in vivo milieu in terms of workload, cardiac functions, and substrate-hormone supply to the heart (11 mM glucose, 0.8 nM insulin, 50 microM carnitine, 1.5 mM lactate, 0.2 mM pyruvate, 5 nM epinephrine, 0.7 mM oleate, and 3% albumin). The use of three differentially (13)C-labeled carbohydrates and a (13)C-labeled long-chain fatty acid allowed the quantitative assessment of the metabolic origin and fate of tissue pyruvate as well as the relative contribution of substrates feeding acetyl-CoA (pyruvate and fatty acids) and oxaloacetate (pyruvate) for mitochondrial citrate synthesis. Beyond concurring with the notion that the mouse heart preferentially uses fatty acids for energy production (63.5 +/- 3.9%) and regulates its fuel selection according to the Randle cycle, our study reports for the first time in the mouse heart the following findings. First, exogenous lactate is the major carbohydrate contributing to pyruvate formation (42.0 +/- 2.3%). Second, lactate and pyruvate are constantly being taken up and released by the heart, supporting the concept of compartmentation of lactate and glucose metabolism. Finally, mitochondrial anaplerotic pyruvate carboxylation and citrate efflux represent 4.9 +/- 1.8 and 0.8 +/- 0.1%, respectively, of the citric acid cycle flux and are modulated by substrate supply. The described (13)C-labeling strategy combined with an experimental setup that enables continuous monitoring of physiological parameters offers a unique model to clarify the link between metabolic alterations, cardiac dysfunction, and disease development.     

Stimulation of alanine metabolism by ammonia in the perfused rat liver. Quantitative analysis by means of a mathematical model

The effect of ammonia on the catabolism of alanine was studied in the perfused rat liver. Addition of 0.5 mM NH4Cl to the perfusion medium containing 5 mM alanine plus 0.1 mM octanoate produced drastic changes in the metabolite concentrations in the efflux medium. Not only the rate of ureogenesis was activated, but also the formation of glucose, lactate and pyruvate. Additionally, respiration was stimulated, the output of ketone bodies decreased, and the redox ratios lactate/pyruvate as well as 3-hydroxybutyrate/acetoacetate became more oxidized. To interpret the causes of these metabolic changes, a mathematical model was developed. It contains kinetic equations by which fluxes through essential pathways of alanine catabolism, gluconeogenesis and energy metabolism were related to the intracellular concentrations of pyruvate, oxaloacetate and ammonia, as well as to the redox ratios lactate/pyruvate and 3-hydroxybutyrate/acetoacetate. Using a nonlinear regression procedure, the model was suitable to be fitted to the data found in the experiments. The consistency of the model and experiment allowed the changes caused by ammonia to be explained. Primarily, ammonia stimulated ureogenesis hence accelerating the deamination of alanine which led to the increased formation of pyruvate, lactate and glucose. The enhanced energetic load resulting from ureogenesis and gluconeogenesis shifted the mitochondrial and cytosolic NAD systems towards more oxidized states which additionally modified the flux rates. The results demonstrate that there is a high degree of cooperativity between the metabolic pathways.     

Splanchnic and muscle fructose metabolism during and after exercise

Regional substrate exchange was studied in 12 healthy males during 90 min of bicycle exercise at 30% of maximal O2 consumption with a 20-min recovery. Six subjects received an intravenous fructose infusion (8.5 mmol/min) from 40 min of exercise to the end of recovery. Splanchnic glucose output, muscle glucose uptake, arterial glucose, and insulin were uninfluenced by the infusion. The respiratory exchange ratio rose to 0.93 +/- 0.04, and arterial free fatty acids fell by 50% (P less than 0.05). Fructose was taken up by splanchnic tissues (45% of administered load), leg muscle (28%), and resting muscle (28%). During infusion, arterial lactate and pyruvate rose two- to threefold, and these substrates were released from splanchnic tissues and taken up by exercising and resting muscle. Splanchnic release of lactate, pyruvate, and glucose accounted for 78% of fructose uptake at 90 min of exercise. Uptake of fructose, lactate, and pyruvate accounted for 55% and together with glucose for 103% of the total oxidative metabolism by exercising muscle. The regional fructose uptakes and lactate exchanges persisted throughout recovery. The present results indicate that fructose infusion during leg exercise 1) results in increased carbohydrate oxidation from fructose, lactate, and pyruvate in exercising muscle, 2) exerts a glycogenic effect in resting muscle and liver during exercise and in liver and muscle recovering from exercise, and 3) does not interfere with glucose metabolism, and that fructose transport into muscle differs from that of glucose.     

Impact of aeration on the metabolic end-products formed from glucose and galactose by Streptococcus lactis

Compared with cultures grown aerobically in batch culture with glucose, aerated cultures of lactic streptococci had a less homolactic type of metabolism when galactose was the carbohydrate source in batch cultures, or when glucose was limiting in chemostat cultures. Differences in end-products of sugar metabolism between aerated and unaerated cultures were observed. In addition to lactate, formate, acetate and ethanol were produced in anaerobic cultures, whereas acetate and acetoin were formed in aerated cultures. Acetate production in aerated cultures depended on lipoic acid, an essential cofactor of the pyruvate dehydrogenase complex. In a chemically defined medium with glucose as the energy substrate, lipoic acid (or acetate) was an essential growth factor. Formation of acetoin was inversely related to lipoic acid concentration in the growth medium. Although not observed in unaerated cultures, acetoin (and 2,3-butanediol) was produced in unaerated buffered suspensions metabolizing pyruvate. Aeration caused a modest increase in the activities of aP-acetolactate synthetase and phosphate acetyl trans-ferase, but it is unlikely that the increases were sufficient to account for the changes in end-products of sugar metabolism observed.     

Acetylcholine affects rat liver metabolism via type 3 muscarinic receptors in hepatocytes

Although the role of acetylcholine (Ach) in hepatic glucose metabolism is well elucidated, it is still unclear if it influences gluconeogenesis, glycogenolysis and high-energy phosphate metabolism, and if it does what the mechanisms of this influence are. Therefore, using isolated perfused rat liver as a model, we have studied the effect of Ach on oxygen consumption, synthesis of glucose from lactate and pyruvate, glycogen formation, mitochondrial oxidative phosphorylation and ATP-synthesis. We have established that effects of Ach on oxygen consumption depend on its concentration. When used at a concentration of 10(-7) M, Ach exerts maximum stimulatory effect, while its infusion at 10(-6) M causes a decrease of oxygen consumption by the liver. Moreover, when used at a concentration of 10(-6) M or 10(-7) M, Ach increases rates of glucose production from the gluconeogenic substrates lactate and pyruvate, leading to enhanced glycogen content in perfused liver. It was also shown that Ach possesses a stimulating effect on alanine and aspartate aminotransferases. As detected by 31P NMR spectroscopy, continuous liver perfusion with pyruvate and lactate in the presence of Ach leads to a significant decrease of ATP level, implying enhanced energy requirements for gluconeogenesis under these conditions. Elimination of the described effects of Ach by atropine, the antagonist of muscarinic receptors, and identification of the type 3 muscarinic receptors (m3) in isolated hepatocytes as well as in whole liver, imply that Ach may exert its effect on liver metabolism through m3 receptors.     

Studies on bile secretion with the aid of the isolated perfused rat liver. II. The effect of two further pentacyclic triterpenes,asiatic acid and 22β-angeloyloxyoleanolic acid

The effect of 2 triterpene acids, 22β-angelolyoxyoleanolic acid and asiatic acid on the isolated perfused rat liver has been studied. From their structures it was predicted that 22β-angeloyloxyoleanolic acid would possess cholestatic properties while asiatic acid would be inactive. It was shown that 2mg of 22β-angeloyloxyoleanolic acid dissolved in ethanol produced cholestasis within 45 min while 10 mg of asiatic acid dissolved in ethanol had little effect on the bile flow rate. Changes in the rate of bile salt excretion closely followed those of the bile flow rate.At the end of the perfusion or when bile flow had ceased the livers were fixed for electron microscopic examination. The cholestasis produced was characterised by alterations of the canalicular membrane while cytoplasmic organisation remained unaffected. In the presence of asiatic acid the canalicular region appeared relatively normal and only minor changes to the cytoplasmic organelles were observed.The presence of 22β-angeloyloxyoleanolic acid had little effect in the changes in lactate and pyruvate levels caused by the metabolism of ethanol. Asiatic acid depressed the pyruvate level which led to an elevated lactate to pyruvate ratio but this had returned to control levels after 2 hours.These results suggest that the cholestatic triterpene acids exert their effect at the canalicular membrane, but the exact mechanism remains unclear.     

The role of the cytoplasmic redox potential in the control of fatty acid synthesis from glucose, pyruvate and lactate in white adipose tissue

The metabolism of lactate, pyruvate and glucose was studied in epididymal adipose tissue of starved, normally fed and starved-re-fed rats. Lactate conversion into fatty acid occurred at an appreciable rate only in the adipocyte of starved-re-fed animals. NNN'N'-Tetramethyl-p-phenylenediamine, an agent that transports reducing power from the cytoplasm to the mitochondria, caused large increments of fatty acid synthesis from lactate and a smaller one from glucose but a decrease in that from pyruvate. Glucose (1.0mm) increased fatty acid synthesis from lactate 4.3-fold but only 1.67-fold from pyruvate in adipocytes from normally fed animals. 2-Deoxyglucose decreased fatty acid synthesis from lactate to a greater degree (threefold) compared to that from pyruvate in adipocytes from starved-re-fed animals. l-Glycerol 3-phosphate contents were approximately equal in epididymal fat-pads, incubated in the presence of lactate or pyruvate, from normally fed animals, whereas the addition of 1mm-glucose resulted in a tenfold increase in l-glycerol 3-phosphate content only in the presence of lactate. The l-glycerol 3-phosphate content was tenfold higher in adipose tissue from starved-re-fed animals incubated in the presence of lactate than in the presence of pyruvate. 2-Deoxyglucose caused these values to be slightly lowered in the presence of lactate. We suggest that lactate metabolism is limited by the rate of NADH removal from the cytoplasm. In the starved-re-fed state, this occurs by reduction of dihydroxyacetone phosphate formed from glycogen to produce l-glycerol 3-phosphate, thus permitting lactate conversion into fatty acid. When glucose is the substrate, and rates of transport are not limiting, the rate of removal of cytoplasmic NADH limits glucose conversion into fatty acid.     

Glucose and pyruvate metabolism in severe chronic obstructive pulmonary disease

The mechanisms leading to weight loss in patients with chronic obstructive pulmonary disease (COPD) are poorly understood but may involve alterations in macronutrient metabolism. Changes in muscle oxidative capacity and lactate production during exercise suggest glucose metabolism may be altered in COPD subjects. The objective of this study was to determine differences in the rates of glucose production and clearance, the rate of glycolysis (pyruvate production), and oxidative and nonoxidative pyruvate disposal in subjects with severe COPD compared with healthy controls. The in vivo rates of glucose production and clearance were measured in 14 stable outpatients with severe COPD (seven with low and seven with preserved body mass indexes) and 7 healthy controls using an intravenous infusion of [(2)H(2)]glucose. Additionally, pyruvate production and oxidative and non-oxidative pyruvate disposal were measured using intravenous infusions of [(13)C]bicarbonate and [(13)C]pyruvate. Endogenous glucose flux and glucose clearance were significantly faster in the combined COPD subjects (P = 0.002 and P < 0.001, respectively). This difference remained significant when COPD subjects were separated by body mass index. Pyruvate flux and oxidation were significantly higher in the combined COPD subjects than controls (P = 0.02 for both), but there was no difference in nonoxidative pyruvate disposal or plasma lactate concentrations between the two groups. In subjects with severe COPD, there are alterations in glucose metabolism leading to increased glucose production and faster glucose metabolism by glycolysis and oxidation compared with controls. However, no difference in glucose conversion to lactate via pyruvate reduction is observed.     

Comparative metabolic analysis of lactate for CHO cells in glucose and galactose

t-PA producing CHO cells have been shown to undergo a metabolic shift when the culture medium is supplemented with a mixture of glucose and galactose. This metabolic change is characterized by the reincorporation of lactate and its use as an additional carbon source. The aim of this work is to understand lactate metabolism. To do so, Chinese hamster ovary cells were grown in batch cultures in four different conditions consisting in different combinations of glucose and galactose. In experiments supplemented with glucose, only lactate production was observed. Cultures with glucose and galactose consumed glucose first and produced lactate at the same time, after glucose depletion galactose consumption began and lactate uptake was observed. Comparison of the metabolic state of cells with and without the shift by metabolic flux analysis show that the metabolic fluxes distribution changes mostly in the reactions involving pyruvate metabolism. When not enough pyruvate is being produced for cells to support their energy requirements, lactate dehydrogenase complex changes the direction of the reaction yielding pyruvate to feed the TCA cycle. The slow change from high fluxes during glucose consumption to low fluxes in galactose consumption generates intracellular conditions that allow the influx of lactate. Lactate consumption is possible in cell cultures supplemented with glucose and galactose due to the low rates at which galactose is consumed. Evidence suggests that an excessive production and accumulation of pyruvate during glucose consumption leads to lactate production and accumulation inside the cell. Other internal conditions such as a decrease in internal pH, forces the flow of lactate outside the cell. After metabolic shift the intracellular pool of pyruvate, lactate and H⁺ drops permitting the reversal of the monocarboxylate transporter direction, therefore leading to lactate uptake. Metabolic analysis comparing glucose and galactose consumption indicates that after metabolic shift not enough pyruvate is produced to supply energy metabolism and lactate is used for pyruvate synthesis. In addition, MFA indicates that most carbon consumed during low carbon flux is directed towards maintaining energy metabolism.     

Metabolic effects of L-carnitine on prepubertal rat Sertoli cells.

The role of carnitine on Sertoli cell metabolism was investigated. Carnitine effects on Sertoli cell lipid metabolism were evaluated by measuring the intracellular levels of non-esterified fatty acids (NEFA) and ketone bodies. The concentration of NEFA in Sertoli cell cultured in the presence of carnitine is significantly reduced as compared to control, while, no significant changes were observed in the concentration of ketone bodies. The functional parameters evaluated to assess the influence of carnitine on Sertoli cell carbohydrate metabolism, i.e., lactate and pyruvate production, lactate dehydrogenase activity and hexose transport, were all significantly increased following carnitine in vitro supplementation. Thus, carnitine appears to drive Sertoli cell intermediary metabolism in an intimately interrelated way, stimulating both fatty acid breakdown and glycolysis. Our results indicate that Sertoli cells are a possible target for a widespread metabolic action of carnitine and strongly support the involvement of carnitine in the regulation of Sertoli cell functions which are related with germ cell "nutrition", convincingly suggesting a direct influence of the compound at testis level.