Enhanced carrier-mediated lactate entry into isolated hepatocytes from starved and diabetic rats

Brain Endothelial Cells: Metabolic Flux and Energy Metabolism

The neurovascular unit (NVU) consists of multiple cell types including brain endothelial cells, pericytes, astrocytes, and neurons that function collectively to maintain homeostasis within the CNS microenvironment. As the principal barrier-forming component of the NVU, the endothelial cells perform an array of complex functions that require substantial energy resources. The principal metabolic pathways for producing ATP are glycolysis and mitochondrial oxidative phosphorylation. While previous studies have demonstrated that glycolysis is a primary pathway for most endothelial cells, details about the energy producing pathways of brain endothelial cells are not fully characterized. The contributions of glycolysis and mitochondrial respiration to energy metabolism are quantifiable using metabolic flux analysis that measures cellular oxygen consumption and acidification (proton production) in a closed microtiter plate format. ATP production rates are then calculated. The bioenergetics of the human brain microvascular endothelial cell line, hCMEC/D3, indicate that these cells exhibit relatively elevated rates of glycolytic flux and glycolytic ATP production, thus confirming their glycolytic nature even in the presence of abundant oxygen. Furthermore, energy producing pathways involving mitochondrial respiration are relatively low, although contributing significantly to total ATP production. Interestingly, the bioenergetics of the hCMEC/D3 cells are relatively similar to those of human primary brain microvascular endothelial cells (hBVECs). These findings allow a quantitative understanding of the bioenergetics of brain endothelial cells in a cultured and proliferative state and also provide a platform for comparative studies of disease states and conditions involving exposures to drugs or metabolic disruptors.

Effects of insulin on the metabolic control of hepatic gluconeogenesis in vivo

OBJECTIVE

Insulin represses the expression of gluconeogenic genes at the mRNA level, but the hormone appears to have only weak inhibitory effects in vivo. The aims of this study were 1) to determine the maximal physiologic effect of insulin, 2) to determine the relative importance of its effects on gluconeogenic regulatory sites, and 3) to correlate those changes with alterations at the cellular level.

RESEARCH DESIGN AND METHODS

Conscious 60-h fasted canines were studied at three insulin levels (near basal, 4x, or 16x) during a 5-h euglycemic clamp. Pancreatic hormones were controlled using somatostatin with portal insulin and glucagon infusions. Glucose metabolism was assessed using the arteriovenous difference technique, and molecular signals were assessed.

RESULTS

Insulin reduced gluconeogenic flux to glucose-6-phosphate (G6P) but only at the near-maximal physiological level (16x basal). The effect was modest compared with its inhibitory effect on net hepatic glycogenolysis, occurred within 30 min, and was associated with a marked decrease in hepatic fat oxidation, increased liver fructose 2,6-bisphosphate level, and reductions in lactate, glycerol, and amino acid extraction. No further diminution in gluconeogenic flux to G6P occurred over the remaining 4.5 h of the study, despite a marked decrease in PEPCK content, suggesting poor control strength for this enzyme in gluconeogenic regulation in canines.

CONCLUSIONS

Gluconeogenic flux can be rapidly inhibited by high insulin levels in canines. Initially decreased hepatic lactate extraction is important, and later reduced gluconeogenic precursor availability plays a role. Changes in PEPCK appear to have little or no acute effect on gluconeogenic flux.

Effect of weight loss on lactate transporter expression in skeletal muscle of obese subjects

The effects of weight loss on skeletal muscle lactate transporter [monocarboxylate transporter (MCT)] expression in obese subjects were investigated to better understand how lactate transporter metabolism is regulated in insulin-resistant states. Ten obese subjects underwent non-macronutrient-specific energy restriction for 15 wk. Anthropometric measurements and a needle biopsy of the vastus lateralis muscle before and after the weight loss program were performed. Enzymatic activity, fiber type distribution, and skeletal muscle MCT protein expression were measured. Muscle from nonobese control subjects was used for comparison of MCT levels. The program induced a weight loss of 9.2 ± 1.6 kg. Compared with controls, muscle from obese subjects showed a strong tendency ( P = 0.06) for elevated MCT4 expression (+69%) before the weight loss program. MCT4 expression decreased (−7%) following weight loss to reach levels that were not statistically different from control levels. There were no differences in MCT1 expression between controls and obese subjects before and after weight loss. A highly predictive regression model ( R2= 0.93), including waist circumference, citrate synthase activity, and percentage of type 1 fibers, was found to explain the highly variable MCT1 response to weight loss in the obese group. Therefore, in obesity, MCT1 expression appears linked both to changes in oxidative parameters and to changes in visceral adipose tissue content. The strong tendency for elevated expression of muscle MCT4 could reflect the need to release greater amounts of muscle lactate in the obese state, a situation that would be normalized with weight loss as indicated by decreased MCT4 levels.

Molecular features, regulation, and function of monocarboxylate transporters: implications for drug delivery

The diffusion of monocarboxylates such as lactate and pyruvate across the plasma membrane of mammalian cells is facilitated by a family of integral membrane transport proteins, the monocarboxylate transporters (MCTs). Currently, at least eight unique members of the MCT family have been discovered and orthologs to each have been identified in a variety of species. Four MCTs (MCT1-MCT4) have been functionally characterized. Each isoform possesses unique biochemical properties such as kinetic constants and sensitivity to known MCT inhibitors. Several fold changes in the expression of MCTs may be evoked by altered physiological conditions, yet the molecular mechanisms underlying the regulation of MCTs are poorly understood. Post-translational regulation of MCT1 and MCT4 occurs, in part, by interaction with CD147, an accessory protein that is necessary for trafficking, localization, and functional expression of these transporters. Because of the physiological importance of monocarboxylates to the overall maintenance of metabolic homeostasis, the function of MCTs is significant to several pathologies that occur with disease, such as ischemic stroke and cancer. Finally, the expression of MCT1 in the epithelium of the small intestine and colon and in the blood-brain barrier may provide routes for the intestinal and blood to brain transfer of carboxylated pharmaceutical agents and other exogenous monocarboxylates.

Exercise training alleviates MCT1 and MCT4 reductions in heart and skeletal muscles of STZ-induced diabetic rats

We compared the changes in monocarboxylate transporter 1 (MCT1) and 4 (MCT4) proteins in heart and skeletal muscles in sedentary control and streptozotocin (STZ)-induced diabetic rats (3 wk) and in trained (3 wk) control and STZ-induced diabetic animals. In nondiabetic animals, training increased MCT1 in the plantaris (+51%; P < 0.01) but not in the soleus (+9%) or the heart (+14%). MCT4 was increased in the plantaris (+48%; P < 0.01) but not in the soleus muscles of trained nondiabetic animals. In sedentary diabetic animals, MCT1 was reduced in the heart (-30%), and in the plantaris (-31%; P < 0.01) and soleus (-26%) muscles. MCT4 content was also reduced in sedentary diabetic animals in the plantaris (-52%; P < 0.01) and soleus (-25%) muscles. In contrast, in trained diabetic animals, MCT1 and MCT4 in heart and/or muscle were similar to those of sedentary, nondiabetic animals (P > 0.05) but were markedly greater than in the sedentary diabetic animals [MCT1: plantaris +63%, soleus +51%, heart +51% (P > 0.05); MCT4: plantaris +107%, soleus +17% (P > 0.05)]. These studies have shown that 1) with STZ-induced diabetes, MCT1 and MCT4 are reduced in skeletal muscle and/or the heart and 2) exercise training alleviated these diabetes-induced reductions.

Effect of food restriction on lactate sarcolemmal transport

The objective of this study was to investigate the effects of 6 weeks of food restriction (FR) on sarcolemmal lactate transport in rats. The daily food consumption of rats was monitored for 10 days, after which they were assigned to either a control group (CTL, n = 7) that consumed food ad libitum or an FR group (n = 7) that received a daily ration equal to 60% of their predetermined baseline food intake. After the 6-week period, we observed in red gastrocnemius (RG) a fall of 48% in glycogen content (P <.01) and a reduction in glutathione peroxidase activity (P <.05), confirming that the FR program was well executed. FR resulted in a reduction in muscle lactate (P <.05) and liver glycogen contents (P <.01). Moreover, hyperlactatemia was noted in the FR group: 1.77 +/- 0.24 versus 2.67 +/- 0.29 mmol/L (P <.05). Lactate transport capacity was significantly increased (P <.05) in FR rats, although monocarboxylate transporter isoforms (MCT1 and MCT4) did not change significantly. We conclude that FR alters sarcolemmal lactate transport activity without affecting MCT1 and MCT4 expression.

Effect of Echinococcus multilocularis on the origin of acetyl-coA entering the tricarboxylic acid cycle in host liver

Carbon-13 nuclear magnetic resonance (NMR) spectroscopy was employed to investigate alterations in hepatic carbohydrate metabolism in Meriones unguiculatus infected with Echinococcus multilocularis. Following portal vein injections of an equimolar mixture of [1,2-13C2]acetate and [3-13C]lactate, perchloric acid extracts of the livers were prepared and NMR spectra obtained. Isotopomer analysis using glutamate resonances in these spectra showed that the relative contributions of endogenous and exogenous substrates to the acetyl-CoA entering the tricarboxylic acid cycle differed significantly between infected and control groups. The mole fraction of acetyl-CoA that was derived from endogenous, unlabelled sources (F(U)) was 0.50 +/- 0.10 in controls compared to 0.34 +/- 0.04 in infected animals. However, the fraction of acetyl-CoA derived from [3-13C]lactate (FLL) was larger in livers of infected animals than those from controls with values of 0.27 +/- 0.04 and 0.18 +/- 0.04, respectively. Similarly, the fraction of acetyl-CoA derived from [1,2-13C2]acetate (FLA) was larger in livers of infected animals compared to those in controls; the fractions were 0.38 +/- 0.01 and 0.32 +/- 0.07, respectively. The ratio of FLA:FLL was significantly smaller in the infected group with a value of 1.42 +/- 0.18 compared to 1.74 +/- 0.09 for the controls. These results indicate that alveolar hydatid disease has a pronounced effect on the partitioning of substrates within the pathways of carbohydrate metabolism in the host liver.

Further observations on the uptake and effects of phosphonates in perfused rat liver studied by (31)P-NMR

We examined the route of uptake of 2-aminoethylphosphonate (NEthPo) and of phenylphosphonate (PhePo; 10 mM each) in perfused liver by (31)P-NMR. Uptake of NEthPo was concentrative. The rate of uptake was reduced to 21 +/- 2% (n = 3; all percentages refer to control rates) by substituting choline for Na(+), and to 21 +/- 4% (n = 3), 32 +/- 6% (n = 5) and 70 +/- 5% (n = 3) by replacing Cl(-) by gluconate, SO(4)(2-) or NO(3)(-), respectively. Taurine (20 mM) reduced NEthPo uptake to 38 +/- 6% (n = 3). The data are consistent with uptake of NEthPo by the Na(+)-coupled Cl(-)-dependent beta-amino acid transporter. A small fraction of NEthPo was incorporated into phospholipid. PhePo uptake evolved over 1 h towards levels of the membrane-permeant volume marker dimethyl methylphosphonate. Uptake depended on H(+), and was inhibited by 4, 4'-diisothiocyanato-stilbene-2,2'-disulphonic acid (100 microM), bumetanide and furosemide (1 mM each) and alpha-cyano-4-OH-cinnamic acid (5 mM) to 31 +/- 4% (n = 4), 28 +/- 4% (n = 4), 27 +/- 5% (n = 6) and 40 +/- 7% (n = 4), respectively. These characteristics of PhePo uptake are reminiscent of H(+)-coupled monocarboxylate transport. The monocarboxylates, lactate and acetate (20 mM), and the substrate analogue, phenylalanine (20 mM), were not inhibitory, while benzoic acid (20 mM) slightly inhibited (to 82 +/- 5%; n = 4) PhePo uptake. The tested phosphonates (10 mM) did not significantly affect hepatic extraction of [(3)H]-cholate or [(3)H]-taurocholate (25 microM each; 1:3 bile salt:albumin). The monocarboxylate analogue, PhePo (10 mM), did not significantly interfere with disposal of lactate (0.3-5 mM).

Lactate transport in L6 skeletal muscle cells and vesicles: allosteric or multisite mechanism and functional membrane marker of differentiation

Membrane lactate transport was studied in skeletal muscle cells and membrane vesicles from the L6 line in relation to in vitro myogenesis. In myoblasts, lactate was transported by simple diffusion and insensitive to classical inhibitors: a positive correlation between onset of creatine kinase activity and lactate transport in differentiated myotubes was observed and could be considered to be a functional marker of cell differentiation. In myotubes, complete analysis of the velocity curves (direct coordinates, Eadie-Scatchard plots, Hill plots) gave parameters showing that lactate was carried by an allosteric or multisite system. This was confirmed by using sarcolemmal vesicles and specific inhibitors. In whole cells, alpha-cyano-4-hydroxycinnamic acid (CIN) and parachloromercuribenzylsulphonic acid (pCMBS) inhibited the maximal velocity without modifying the global cooperativity of the system. The weak effect of 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), which has a low affinity constant (Ki = 22.5 microM), implicated the monocarboxylate system rather than the anionic exchanger as a carrier system in muscle cells. CIN and DIDS exhibited one type of interaction with lactate carriers, and the curvilinear shape of the lactate Hill plot with or without inhibitors suggested that inhibitors were active at the same family of interaction sites and had a common range of affinities. The apparent competitive inhibition of pyruvate (Ki = 3.2 mM) did not modify the transport pathway of lactate in L6 myotubes. In conclusion, kinetic analysis of lactate transport in the presence or absence of inhibitors gave evidence for a multisite lactate carrier activity in myotubes composed of two systems at least, related to two or three isoforms of lactate carriers.

Carrier-mediated lactate entry into isolated hepatocytes from fed and starved rats: zonal distribution and temperature dependence

We examined the possibility of quantitative differences in lactate entry into periportal and perivenous hepatocytes under different nutritional states. The rate of 14C-L(+)-lactate uptake was determined after 15-second incubations with freshly isolated zonally separated hepatocytes using a centrifuge stop technique at 37 degrees C and 4 degrees C, in the presence or absence of either differing amounts of unlabelled lactate or of a hepatocyte lactate transport inhibitor, alpha-cyano-3-hydroxycinnamate. Total entry as well as carrier mediated entry of 14C-L(+)-lactate into the isolated cell populations was found to be similar in periportal and perivenous hepatocytes, irrespective of the nutritional state of the animal. Periportal and perivenous hepatocytes showed a greater tendency to transport lactate when isolated from starved animals, in agreement with previously reported data from non-zonally separated isolated hepatocytes. The activity of the hepatocyte plasma-membrane lactate transporter was diminished between fourfold and eightfold in transport studies conducted at 4 degrees C; similar results were obtained in unseparated and zonally separated suspensions. Temperature dependence of the hepatocyte transporter is markedly less than that reported for the erythrocyte transporter.

L(+)-lactate binding to a protein in rat skeletal muscle plasma membranes

Biochemical studies were conducted to determine the location of a putative lactate transport protein in rat skeletal muscle plasma membranes (PM). PM (50-100 micrograms protein) were incubated with [U-14C] L(+)-lactate, in the presence or absence of unlabeled monocarboxylates or potential inhibitors, after which proteins were separated by SDS-PAGE. Gel slices (2 mm) were cut and analyzed for 14C. [U-14C] L(+)-lactate was bound to plasma membranes in the 30 to 40 kDa molecular mass range. Binding of [U-14C] L(+)-lactate was inhibited by N-ethylmaleimide, unlabeled L-lactate and pyruvate, and in a dose dependent manner by alpha-cyano-4-hydroxycinnamate (r = 0.995), but not by cytochalasin-B. The inhibition of [U-14C] L(+)-lactate binding was similar to the inhibition of lactate transport. Therefore the transport of L(+)-lactate across skeletal muscle plasma membranes involves a polypeptide of 30 to 40 kDa.

Effects of D-3-hydroxybutyrate and acetoacetate on lactate removal in isolated perfused livers from starved and fed rats

We examined the influence of nutritional state on the role of the hepatic plasma membrane lactate transporter in determining overall hepatic lactate disposal. The effects of infusion of sodium D-3-hydroxybutyrate (DOHB) on lactate uptake were studied in perfused livers from fed or starved rats. In livers from starved rats, DOHB (15 to 20 mmol/L) inhibited lactate removal by approximately 45%. This effect was associated with a decrease in intracellular lactate concentration, with cell pH remaining unchanged. Inhibition was maximal when perfusate lactate was less than 1.6 mmol/L, and was undetectable at concentrations exceeding 2.5 mmol/L. A similar degree of inhibition was observed with infusion of acetoacetate. These observations add to the evidence that the inhibition of lactate removal by DOHB seen in livers from starved animals is mediated through an effect on the hepatocyte lactate transporter. At similar low levels of perfusate lactate, DOHB infusion produced a decrease in output of lactate from livers obtained from fed animals. When such livers were subjected to prolonged preperfusion, lactate removal, rather than output, was observed; in these livers DOHB stimulated lactate removal, an effect directionally opposite to that observed in livers from starved animals. These data confirm that hepatic lactate transport is a limiting factor for lactate utilization in intact livers from starved rats; in contrast, lactate utilization in livers from fed animals is limited at a step subsequent to plasma membrane transport, ie, possibly pyruvate transport into mitochondria.

Hormonal modulation of hepatic plasma membrane lactate transport in cultured rat hepatocytes

Hormonal modulation of hepatic plasma membrane lactate transport was studied in primary cultures of isolated hepatocytes from fed rats to examine the mechanism for the known enhancement of lactate transport in starvation and diabetes. Total cellular lactate entry was increased by 14% in the presence of dexamethasone; this was accounted for by an approximately 40% increase in the carrier-mediated component of entry with no effect on diffusion. A trend of similar magnitude was evident with glucagon. The effects of dexamethasone and glucagon on lactate transport constitute an additional potential mechanism for enhancement of gluconeogenesis by these hormones.