Inhibition of lactate removal by ketone bodies in rat liver. Evidence for a quantitatively important role of the plasma membrane lactate transporter in lactate metabolism

A Ketone Monoester with Carbohydrate Improves Cognitive Measures Postexercise, but Not Performance in Trained Females

PURPOSE

The acute ingestion of a ketone monoester with the coingestion of a carbohydrate (KME + CHO) compared with carbohydrate (CHO) was investigated on cycling performance and cognitive performance in trained females.

METHODS

Using a two condition, placebo-controlled, double-blinded and crossover design, 12 trained females (mean ± SD: age, 23 ± 3 yr; height, 1.64 ± 0.08 m; mass, 65.2 ± 12.7 kg) completed a baseline assessment of cognitive performance (psychomotor vigilance testing (PVT), task switching, and incongruent flanker), followed by 6 × 5-min intervals at 40%, 45%, 50%, 55%, 60%, and 65% of their maximal power output (W max ) and then a 10-km time trial, concluding with the same assessments of cognitive performance. Participants consumed either 375 mg·kg -1 body mass of KME with a 6% CHO solution (1 g·min -1 of exercise) or CHO alone, across three boluses (50:25:25).

RESULTS

Blood β-hydroxybutyrate concentrations averaged 1.80 ± 0.07 and 0.13 ± 0.01 mM during exercise in KME + CHO and CHO, respectively. Blood glucose decreased after drink 1 of KME + CHO (~15%; P = 0.01) but not CHO, and lactate concentrations were lower in KME + CHO at 50%, 55%, 60%, and 65% W max (all P < 0.05) compared with CHO. Despite these changes, no differences were found between conditions for time trial finishing times (KME + CHO, 29.7 ± 5.7 min; CHO, 29.6 ± 5.7 min; P = 0.92). However, only KME + CHO resulted in increases in psychomotor vigilance testing speed (~4%; P = 0.01) and faster reaction times (~14%; P < 0.01), speed (~15%; P < 0.01), and correct responses (~13%; P = 0.03) in the incongruent flanker during posttesting compared with CHO.

CONCLUSIONS

The acute ingestion of a KME + CHO elevated blood β-hydroxybutyrate and lowered glucose and lactate across multiple time points during exercise compared with CHO. Although these changes did not affect physical performance, several markers of cognitive performance were improved by the addition of a KME in trained females.

Arterial Blood Gases and Acid-Base Regulation

Disorders of acid-base status are common in the critically ill and prompt recognition is central to clinical decision making. The bicarbonate/carbon dioxide buffer system plays a pivotal role in maintaining acid-base homeostasis, and measurements of pH, PCO2, and HCO3 - are routinely used in the estimation of metabolic and respiratory disturbance severity. Hypoventilation and hyperventilation cause primary respiratory acidosis and primary respiratory alkalosis, respectively. Metabolic acidosis and metabolic alkalosis have numerous origins, that include alterations in acid or base intake, body fluid losses, abnormalities of intermediary metabolism, and renal, hepatic, and gastrointestinal dysfunction. The concept of the anion gap is used to categorize metabolic acidoses, and urine chloride excretion helps define metabolic alkaloses. Both the lungs and kidneys employ compensatory mechanisms to minimize changes in pH caused by various physiologic and disease disturbances. Treatment of acid-base disorders should focus primarily on correcting the underlying cause and the hemodynamic and electrolyte derangements that ensue. Specific therapies under certain conditions include renal replacement therapy, mechanical ventilation, respiratory stimulants or depressants, and inhibition of specific enzymes in intermediary metabolism disorders.

Exogenous ketosis increases blood and muscle oxygenation but not performance during exercise in hypoxia

Available evidence indicates that elevated blood ketones are associated with improved hypoxic tolerance in rodents. From this perspective, we hypothesized that exogenous ketosis by oral intake of the ketone ester (R)-3-hydroxybutyl (R)-3-hydroxybutyrate (KE) may induce beneficial physiological effects during prolonged exercise in acute hypoxia. As we recently demonstrated KE to deplete blood bicarbonate, which per se may alter the physiological response to hypoxia, we evaluated the effect of KE both in the presence and absence of bicarbonate intake (BIC). Fourteen highly trained male cyclists performed a simulated cycling race (RACE) consisting of 3-h intermittent cycling (IMT_180') followed by a 15-min time-trial (TT_15') and an all-out sprint at 175% of lactate threshold (SPRINT). During RACE, fraction of inspired oxygen ([Formula: see text]) was gradually decreased from 18.6% to 14.5%. Before and during RACE, participants received either 1) 75 g of ketone ester (KE), 2) 300 mg/kg body mass bicarbonate (BIC), 3) KE + BIC, or 4) a control drink in addition to 60 g of carbohydrates/h in a randomized, crossover design. KE counteracted the hypoxia-induced drop in blood ([Formula: see text]) and muscle oxygenation by ∼3%. In contrast, BIC decreased [Formula: see text] by ∼2% without impacting muscle oxygenation. Performance during TT_15' and SPRINT were similar between all conditions. In conclusion, KE slightly elevated the degree of blood and muscle oxygenation during prolonged exercise in moderate hypoxia without impacting exercise performance. Our data warrant to further investigate the potential of exogenous ketosis to improve muscular and cerebral oxygenation status, and exercise tolerance in extreme hypoxia.

Exogenous Ketosis Impairs 30-min Time-Trial Performance Independent of Bicarbonate Supplementation

PURPOSE

We recently demonstrated that coingestion of NaHCO3 to counteract ketoacidosis resulting from oral ketone ester (KE) intake improves mean power output during a 15-min time trial (TT) at the end of a 3-h cycling race by ~5%. This ergogenic effect occurred at a time when blood ketone levels were low, as ketosis was only induced during the initial ~2 h of the race. Therefore, in the current study, we investigated whether performance also increases if blood ketone levels are increased in the absence of ketoacidosis during high-intensity exercise.

METHODS

In a double-blind crossover design, 14 well-trained male cyclists completed a 30-min TT (TT30') followed by an all-out sprint at 175% of lactate threshold (SPRINT). Subjects were randomized to receive (i) 50 g KE, (ii) 180 mg·kg-1 body weight NaHCO3 (BIC), (iii) KE + BIC, or (iv) a control drink (CON).

RESULTS

KE ingestion increased blood d-ß-hydroxybutyrate to ~3-4 mM during the TT30' and SPRINT (P < 0.001 vs CON). In KE, blood pH and bicarbonate concomitantly dropped, causing 0.05 units lower pH and 2.6 mM lower bicarbonate in KE compared with CON during the TT30' and SPRINT (P < 0.001 vs CON). BIC coingestion resulted in 0.9 mM higher blood d-ß-hydroxybutyrate (P < 0.001 vs KE) and completely counteracted ketoacidosis during exercise (P > 0.05 vs CON). Mean power output during TT30' was similar between CON and BIC at 281 W, but was 1.5% lower in the KE conditions (main effect of KE: P = 0.03). Time to exhaustion in the SPRINT was ~64 s in CON and KE and increased by ~8% in the BIC conditions (main effect of BIC: P < 0.01).

DISCUSSION

Neutralization of acid-base disturbance by BIC coingestion is insufficient to counteract the slightly negative effect of KE intake during high-intensity exercise.

Serum metabolic profiles of septic shock patients based upon co-morbidities and other underlying conditions

Diagnosis and management of patients with septic shock is still a significant challenge for clinicians with its high mortality amongst hospitalized patients. Septic shock is a heterogeneous condition and is usually accompanied by various underlying disease conditions. Dissecting the specific metabolic changes induced by these underlying disease conditions through metabolomics has shown the potential to improve our understanding of the disease's relevant pathophysiological mechanisms, leading to improved treatment. This study has shown the metabolic alterations caused due to co-morbid conditions like diabetes, hypertension, CAD, and CKD in septic shock. It has also shown the distinct metabolic profiles of septic shock patients with underlying respiratory illnesses and encephalopathy. Metabolic profiling of sera obtained from 50 septic shock patients and 20 healthy controls was performed using high-resolution 1D ¹H CPMG and diffusion-edited NMR spectra. Univariate and multivariate statistical analyses were performed to identify the potential molecular biomarkers. Noted dysregulations in amino acids, carbohydrates, and lipid metabolism were observed in septic shock patients. Further stratification within the septic shock patients based on co-morbid conditions and primary diagnosis has shown their role in causing metabolic alterations. Evaluation of these compounds during treatment will help design a personalized treatment protocol for the patients, improving therapeutics.

Importance of pH homeostasis in metabolic health and diseases: crucial role of membrane proton transport

Protons dissociated from organic acids in cells are partly buffered. If not, they are transported to the extracellular fluid through the plasma membrane and buffered in circulation or excreted in urine and expiration gas. Several transporters including monocarboxylate transporters and Na(+)/H(+) exchanger play an important role in uptake and output of protons across plasma membranes in cells of metabolic tissues including skeletal muscle and the liver. They also contribute to maintenance of the physiological pH of body fluid. Therefore, impairment of these transporters causes dysfunction of cells, diseases, and a decrease in physical performance associated with abnormal pH. Additionally, it is known that fluid pH in the interstitial space of metabolic tissues is easily changed due to little pH buffering capacitance in interstitial fluids and a reduction in the interstitial fluid pH may mediate the onset of insulin resistance unlike blood containing pH buffers such as Hb (hemoglobin) and albumin. In contrast, habitual exercise and dietary intervention regulate expression/activity of transporters and maintain body fluid pH, which could partly explain the positive effect of healthy lifestyle on disease prognosis.

Mechanistic modeling of monocarboxylate transporter-mediated toxicokinetic/toxicodynamic interactions between γ-hydroxybutyrate and L-lactate

Overdose of γ-hydroxybutyrate (GHB) can result in severe respiratory depression. Monocarboxylate transporter (MCT) inhibitors, including L-lactate, increase GHB clearance and represent a potential treatment for GHB intoxication. GHB can also affect L-lactate clearance, and L-lactate has been reported to affect respiration. In this research, we characterize these toxicokinetic/toxicodynamic interactions between GHB and L-lactate using mechanistic modeling. Plasma, urine, and respiration data were taken from our previous study in which GHB and sodium L-lactate were administered alone and concomitantly in rats. A model incorporating active renal reabsorption for both agents fit GHB and L-lactate toxicokinetic data. The Km for renal reabsorption of GHB (650 μg/mL) was close to its Km for the proton-dependent MCT1 and that for L-lactate (13.5 μg/mL) close to its Km for the sodium-dependent SMCT1. Inhibition of reabsorption by both agents was necessary to model concomitant drug administration. The metabolic Km for L-lactate closely resembled that for MCT-mediated hepatic uptake in vitro, and GHB inhibited this process. L-lactate significantly inhibited respiration at a high dose, and an indirect response model was used to fit these data. GHB toxicodynamics was modeled as a direct effect delayed by nonlinear transport into the brain extracellular fluid, with a Km value of 1,865 μg/mL for brain uptake which is similar to the in vitro Km value determined in rat brain endothelial cells. This model was useful for characterizing multiple MCT-mediated interactions. Incorporation of many parameters that can be determined in vitro may allow for clinical translation of these interactions.

Ketone bodies promote a rapid rise in glutamate efflux from the isolated perfused rat liver without altering the rate of glutamine production

Livers of starved (48 hr) male Wistar rats were perfused in a non recirculating manner with a near physiological mix of ammonium, lactate, ornithine and pyruvate in Krebs buffer. The addition of ketone bodies (3-DL-hydroxybutyrate [B OHB] 2-30 mM or lithium-acetoacetate (15 mM) to the perfusate resulted in a rapid rise in the efflux of glutamate from the liver (five times above basal). This was not seen with control solutions (sodium chloride or lithium chloride). The increased efflux was sustained for the duration of the addition of the ketone bodies (7 min), was rapidly reversible and dose dependant. Glutamine export rates were not altered, suggesting that either the glutamate originated from cells not responsible for glutamine synthesis or that this glutamate was superfulous to the requirement of glutamine synthesis. There was no evidence that the lactate transporter was involved in the entry of lactate into perivenous hepatocytes for glutamine synthesis; lactate presumably entering the hepatocyte by an alternative pathway, probably nonionic diffusion.

γ-Hydroxybutyrate (GHB)-induced respiratory depression: combined receptor-transporter inhibition therapy for treatment in GHB overdose

Overdose of γ-hydroxybutyrate (GHB) frequently causes respiratory depression, occasionally resulting in death; however, little is known about the dose-response relationship or effects of potential overdose treatment strategies on GHB-induced respiratory depression. In these studies, the parameters of respiratory rate, tidal volume, and minute volume were measured using whole-body plethysmography in rats administered GHB. Intravenous doses of 200, 600, and 1500 mg/kg were administered to assess the dose-dependent effects of GHB on respiration. To determine the receptors involved in GHB-induced respiratory depression, a specific GABA(B) receptor antagonist, (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH50911), and a specific GABA(A) receptor antagonist, bicuculline, were administered before GHB. The potential therapeutic strategies of receptor inhibition and monocarboxylate transporter (MCT) inhibition were assessed by inhibitor administration 5 min after GHB. The primary effect of GHB on respiration was a dose-dependent decrease in respiratory rate, accompanied by an increase in tidal volume, resulting in little change in minute volume. Pretreatment with 150 mg/kg SCH50911 completely prevented the decrease in respiratory rate, indicating agonism at GABA(B) receptors to be primarily responsible for GHB-induced respiratory depression. Administration of 50 mg/kg SCH50911 after GHB completely reversed the decrease in respiratory rate; lower doses had partial effects. Administration of the MCT inhibitor l-lactate increased GHB renal and total clearance, also improving respiratory rate. Administration of 5 mg/kg SCH50911 plus l-lactate further improved respiratory rate compared with the same dose of either agent alone, indicating that GABA(B) and MCT inhibitors, alone and in combination, represent potential treatment options for GHB-induced respiratory depression.

Diabetes, metformin and lactic acidosis

OBJECTIVE

Metformin has long been thought to cause lactic acidosis (LA) but evidence from various sources has led researchers to question a direct causative relationship. We assessed the relationship of metformin prescription and other factors to the incidence of LA.

METHODS

All cases of LA at a single hospital were identified from laboratory lactate measurements. We compared patients classified as Cohen and Woods class A and B, patients with and without diabetes, and those taking metformin or not.

RESULTS

LA was more common than in published analyses based on hospital coding of diagnoses. The incidence of LA was greater in diabetes than in the nondiabetic population but with no further increase in patients taking metformin. Lactate levels were no greater in patients on metformin than in patients with type 2 diabetes not on metformin even if patients with acute cardiorespiratory disturbance (Cohen and Woods class A) were excluded. Acidosis was greater in diabetes (hydrogen ion 94·9 ± 4·6 vs 83·2 ± 2·3 10(-9) m, P = 0·027) but factors besides lactate contributed. Acute cardiorespiratory illness, acute renal impairment and sepsis were the most common of the recognized precipitating factors. Age (P = 0·01), acute renal failure (P = 0·015) and sepsis (P = 0·005) were associated with mortality.

CONCLUSIONS

Diabetes rather than metformin therapy is the major risk factor for the development of LA. Lactic acidosis occurs in association with acute illness particularly in diabetes. Current guidance for the prevention of lactic acidosis may overemphasize the role of metformin.

Differential Diagnosis of Metabolic Acidosis

Metabolic acidosis is defined as an acidemia created by one of three mechanisms: increased production of acids, decreased excretion of acids, or loss of alkali. This article addresses the identification and correct diagnosis of metabolic acidosis by reviewing important historical factors, pathophysiological principles, clinical presentation,and laboratory findings accompanying common high and normal anion gap metabolic acidoses in emergency department patients.

Hepatic and gastrointestinal oxygen and lactate metabolism during low cardiac output in lambs

We previously observed young lambs to be more tolerant of hypoxia; compared with older lambs, they accumulate lactate at a slower rate during comparable reduction in cardiac output, and have a greater percent decrease in cardiac output before onset of systemic lactate accumulation. To determine the mechanism of lactic acidosis and the cause for this "tolerance," we reduced cardiac output progressively in seven chronically catheterized conscious lambs (16.4 + 5.1 d) and measured hepatic and gastrointestinal (GI) blood flow (radioactive microspheres) and delivery, uptake, and extraction of lactate and O2. Hepatic O2 consumption declined proportionately below a critical hepatic O2 delivery (approximately 2 mL O2/min/kg), corresponding to the systemic O2 delivery associated with the onset of systemic lactate accumulation. As hepatic O2 delivery decreased below the critical value, there was initially net hepatic lactate uptake and then a change to net production when the O2 delivery decreased below approximately 1 mL O2/min kg. The GI tract had net lactate production at rest, but surprisingly switched to lactate uptake as cardiac output decreased. The mechanism of lactic acidosis was failure of hepatic lactate uptake to increase despite increased hepatic lactate delivery, as reported in adults subjects. However, in contrast, there was "true" hepatic dysfunction and lactate production only at the lowest levels of cardiac output, after onset of systemic lactate accumulation. Moreover, we speculate that tolerance of young lambs to hypoxia is at least due to two factors: 1) hepatic lactate uptake is maintained beyond the "critical" O2 delivery and fall in hepatic O2 consumption, and 2) there is a switch to lactate uptake by the GI tract serving to buffer the lactate.

The kinetics, substrate, and inhibitor specificity of the monocarboxylate (lactate) transporter of rat liver cells determined using the fluorescent intracellular pH indicator, 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein

The kinetics of transport of L-lactate, pyruvate, ketone bodies, and other monocarboxylates into isolated hepatocytes from starved rats were measured at 25 degrees C using the intracellular pH-sensitive dye, 2',7'-bis(carboxyethyl)- 5(6)-carboxyfluorescein, to detect the associated proton influx. Transport kinetics were similar, but not identical, to those determined using the same technique for the monocarboxylate transporter (MCT) of Ehrlich Lettré tumor cells (MCT1) (Carpenter, L., and Halestrap, A. P. (1994) Biochem. J. 304, 751-760). Km values for L-lactate (4.7 mM), D-lactate (27 mM), D,L-2-hydroxybutyrate (3.3 mM), L-3-hydroxybutyrate (12.7 mM), and acetoacetate (6.1 mM) were very similar in both cell types, whereas in hepatocytes the Km values were higher than MCT1 for pyruvate (1.3 mM, cf. 0.72 mM), D-3-hydroxybutyrate (24.7 mM, cf. 10.1 mM), D-2-chloropropionate (1.3 mM, cf. 0.8 mM), 4-hydroxybutyrate (18.1 mM, cf. 7.7 mM), and acetate (5.4 mM, cf. 3.7 mM). In contrast, the hepatocyte carrier had lower Km values than MCT1 for glycolate, chloroacetate, dichloroacetate, and 2-hydroxy-2-methylpropionate. Differences in stereoselectivity were also detected; both carriers showed a lower Km for L-lactate than D-lactate, while hepatocyte MCT exhibited a lower Km for D- than L-2-chloropropionate and for L- than D-3-hydroxybutyrate; this is not the case for MCT1. A range of inhibitors of MCT1, including alpha-cyanocinnamate derivatives, phloretin, and niflumic acid, inhibited hepatocyte MCT with K0.5 values significantly higher than for tumor cell MCT1, while stilbene disulfonate derivatives and p-chloromercuribenzene sulfonate had similar K0.5 values in both cell types. The branched chain ketoacids alpha-ketoisocaproate and alpha-ketoisovalerate were also potent inhibitors of hepatocyte MCT with K0.5 values of 270 and 340 microM, respectively. The activation energy of L-lactate transport into hepatocytes was 58 kJ mol-1, and measured rates of transport at 37 degrees C were considerably greater than those required for maximal rates of gluconeogenesis. The properties of the hepatocyte monocarboxylate transporter are consistent with the presence of a distinct isoform of MCT in liver cells as suggested by the cloning and sequencing of MCT2 from hamster liver (Garcia, C. K., Brown, M. S., Pathak, R. K., and Goldstein, J. L. (1995) J. Biol. Chem. 270, 1843-1849).

The effects of fasting on the thermogenic, metabolic and cardiovascular responses to infused adrenaline

The effects of fasting on the thermogenic, lipolytic and cardiovascular responses to adrenaline were examined in nine normal, young, non-obese subjects. Each subject attended for study after 12, 36 and 72 h fasting. After basal measurements adrenaline was infused at 25 ng/min per kg ideal body weight for 90 min. Fasting increased the thermogenic effect of the adrenaline (mean 14.6 (SE 1.7), 16.6 (SE 1.8), 22.6 (SE 1.6) J/min per kg fat-free mass after 12, 36 and 72 h fasting respectively; P < 0.001, ANOVA). Basal plasma palmitate turnover increased with duration of fasting (1.48 (SE 0.22), 1.95 (SE 0.34) and 2.26 (SE 0.33) mumol/min per kg body weight; P < 0.001, ANOVA), but the response to adrenaline was unaffected by fasting. The percentage values for basal plasma palmitate turnover oxidized were 44 (SE 2; 12 h), 46 (SE 5; 36 h) and 42 (SE 4)% (72 h). In response to adrenaline this percentage fell, suggesting that adrenaline infusion may favour intra-tissue lipid oxidation.

Lactate and pyruvate isotopic enrichments in plasma and tissues of postabsorptive and starved rats

It has been proposed that plasma pyruvate isotopic enrichment (IE) during infusion of labeled lactate could be used to estimate the intracellular IE of lactate and pyruvate and thus to calculate their turnover rate. We determined the relations of plasma and tissue IE of lactate and pyruvate in anesthetized rats infused with [3-13C]lactate in an artery and sampled from a vein (A-V mode) or infused in a vein and sampled from an artery (V-A mode). In both groups of rats, the ratio of tissue to plasma lactate IE was < 1 with large differences between tissues: the highest ratio was observed in heart and the lowest in soleus. With the exception of liver, this ratio was higher in the A-V than in the V-A mode. Pyruvate IE was lower than lactate IE in tissues, with a few exceptions, and in plasma. This ratio of pyruvate to lactate IE was approximately 0.70 in plasma in A-V and V-A modes. Moreover pyruvate IE was also always higher in plasma than in tissues. This seemingly surprising result could be explained by the production of labeled pyruvate from labeled lactate inside the circulation by erythrocytes, because we observed a rapid isotopic equilibrium between lactate and pyruvate in blood "in vitro." Apparent lactate turnover was higher in the A-V than in the V-A mode when it was calculated using lactate as well as pyruvate IE. Therefore plasma pyruvate IE cannot be used in rats to estimate tissue IE and did not reconcile turnover rates measured using the A-V or V-A mode.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Acid/base may be more variable than previously thought

Organ systems in the human body may not be homogeneous in regard to pH. Possibly specific organ systems under certain conditions maintain a pH level significantly different than the systemic pH.

A basolateral lactate/H+ co-transporter in Madin-Darby Canine Kidney (MDCK) cells

Monolayers of Madin-Darby Canine Kidney (MDCK) cells grown on permeable filters generated lactate aerobically and accumulated it preferentially in the basolateral compartment, suggesting the presence of a lactate carrier. The mechanism of lactate transport across apical and basolateral membranes was examined by determining intracellular pH (pHi) microspectrofluorimetrically after addition of lactate to the extracellular solutions and by measuring uptake of [14C]lactate. Addition of 20 mM lactate to the apical compartment produced no change in pHi, whereas lactate added to the basolateral compartment rapidly and reversibly lowered pHi. Pyruvate produced similar results. Inhibitors of lactate/H+ co-transporters, alpha-cyano-4-hydroxycinnamate (CnCN) and quercetin, partially inhibited the fall in pHi produced by basolateral lactate. In contrast, the disulphonic stilbene. DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid) produced no inhibition at 0.5 mM. Kinetic analysis was performed by applying basolateral lactate at various concentrations and measuring the rate of entry (delta pHi/min) in the presence and absence of CnCN. Lactate flux was shown to occur by both non-ionic diffusion and a alpha-cyano-4-hydroxycinnamate-sensitive component (carrier). The latter has a Km of approximately 7 mM for the lactate anion. Propionate, but not formate, lowered pHi to the same degree as did equimolar lactate, but the propionate effect was not inhibited by CnCN. Influx of [14C]lactate was substantially greater across the basolateral membrane than across the apical membrane and occurred in the absence of Na+. We conclude that MDCK cells grown on permeable filters generate lactate aerobically and transport it across the basolateral membrane by way of a lactate/H+ cotransporter.

Effects of different lipid substrates on glucose metabolism in normal postabsorptive humans

We investigated the effects on glucose metabolism of the infusion of either long-chain triglycerides (LCT), a mixture of long-chain and medium-chain triglycerides (MCT/LCT), D-beta-hydroxybutyrate (D-beta-OHB), or saline in normal postabsorptive subjects. Plasma insulin, C-peptide, and glucagon concentrations were unchanged in all groups. LCT and MCT/LCT infusions increased levels of plasma free fatty acids (FFA) compared with those of the saline group, whereas D-beta-OHB decreased them. Plasma ketone body concentrations were higher during the D-beta-OHB and triglyceride infusions than during the saline test. Glucose concentrations and appearance (Ra) and disappearance (Rd) rates were not modified during saline infusion. Glucose levels decreased only in the D-beta-OHB and MCT/LCT groups (P < .05), whereas they were unchanged during LCT infusion. Glucose Ra decreased slightly by 15% to 17% in LCT, MCT/LCT, and D-beta-OHB groups (P < .05 v saline). Glucose Rd decreased by 14% to 16% in each lipid-infusion group (P < .05 v saline). Glucose clearance rates decreased by 14% only in the LCT group (P < .001). Glucose oxidation rates did not change significantly during the lipid substrate infusions compared with saline infusion. In conclusion, (1) the effects of fatty acids on glucose metabolism appear to depend on the fatty acid chain length, since only LCT infusion significantly impaired glucose utilization; and (2) in subjects with normal endocrine pancreas function, we found no adverse effects of a short-term increase in lipid substrate availability on glucose production rate and concentration.

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.

L-lactate uptake by rat liver. Effect of food deprivation and substrate availability

We have studied the role of substrate availability on net L-lactate uptake by liver of anaesthetized fed and 24 h-fasted rats. L-Lactate was infused through a mesenteric vein at infusion rates equivalent to 0, 0.125, 0.25 and 0.5 times the basal turnover rate (Rt). By these means we were able to increase L-lactate portal concentrations up to 5.5 mM, without significant changes in portal pH. In the basal state (0 Rt), a net L-lactate uptake by liver was found in 24 h-fasted animals. No net balance was observed in fed rats. Infusion of L-lactate in fed animals failed to induce a net hepatic uptake, except when L-lactate levels in portal vein were raised above 5 mM. In fasted animals, net L-lactate uptake by liver increased linearly (r = 0.99) as a function of L-lactate concentration in the portal vein, even beyond the saturation of its specific carrier. It is concluded that, first, the L-lactate carrier does not limit net L-lactate uptake, and second, that substrate availability is an important factor modulating net L-lactate uptake by liver.

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.

Plasma glucose, ketone bodies, insulin, glucagon and enteroglucagon in cows: diurnal variations related to ketone levels before feeding and to the ketogenic effects of feeds

Ingestions of a moderately ketogenic silage twice daily were followed by transient increments in plasma insulin and ketone bodies and decreases in plasma glucose. Ketone bodies and glucose were negatively correlated throughout the day, but the insulin elevations culminated before the maximal effects on ketone bodies and glucose were established. Cows with varying glucose levels before morning feeding reacted to a highly ketogenic silage by decreasing their glucose level uniformly to about 3 mmol/l, in spite of a widely varying feeding-induced insulin increment. Hay-feeding caused insulin increments of the same magnitude as silage-feeding, but the glucose decrease and the ketone increment was much smaller. The results indicate some direct action of ketone bodies on blood sugar regulation, in addition to effects mediated by insulin. The role of ketone bodies as the insulinotropic factor was not confirmed. The insulin level after feeding seems to be determined by the carbohydrate status of the animal before feeding. No significant changes in plasma glucagon were observed after feeding, and no consistent differences in plasma levels of this hormone were found when non-ketonemic, ketonemic, and clinically ketotic cows were compared. The plasma level of enteroglucagon (GLI) was positively correlated to the relative amount of concentrates consumed, but no relation to plasma glucose was found.

Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles

To study the kinetics of lactate transport in an isolated, nonmetabolizing system, skeletal muscle sarcolemmal membrane vesicles were purified from 22 female Sprague-Dawley rats. L(+)-[U-14C] Lactate at 10 concentrations demonstrated saturation kinetics with a Vmax of 139.4 nmol/mg/min, and an apparent Km of 40.1 mM. Threefold higher initial rates of L(+)-lactate uptake were seen at 37 degrees C than at 25 degrees C, indicating temperature sensitivity. Transport was stereospecific for the L(+) isomer: isotopic D(-) uptake rates remained linear at concentrations from 1 to 200 mM, and 1 mM D(-) remained 6-fold lower in net uptake after 60 min than the L(+) isomer. Furthermore, unlabeled 10 mM D(-)-lactate in the external medium could only inhibit 1 mM isotopic (L(+) uptake by 12%, whereas unlabeled 10 mM L(+)-lactate and pyruvate inhibited 82 and 71%, respectively. Additionally, 10 mM beta-hydroxybutyrate and acetoacetate could moderately inhibit (27 and 32%, respectively) 1 mM L(+)-lactate transport, but the unsubstituted aliphatic monocarboxylates (formate, acetate, propionate), tricarboxylic acid cycle intermediates (malate, succinate, oxaloacetate, alpha-ketoglutyrate, citrate), amino acids (alanine, aspartate, glutamate), and palmitate or adenosine in 10-fold excess could not effectively inhibit 1 mM L(+)lactate uptake under cis-transport conditions. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid could inhibit L(+)-lactate transport by only 13%, so that lactate transport does not appear to be affected directly by Cl- or HCO3- fluxes. It was demonstrated that KCl could not evoke a membrane potential-induced overshoot of lactate uptake in the presence or absence of valinomycin. Moreover, gluconate could substitute for Cl-, indicating that Cl- flux does not contribute to a membrane potential-dependent component of the transport mechanism, suggesting an electroneutral translocation process. Protein-modifying reagents significantly inhibited 1 mM L(+)-lactate transport during pH-stimulated conditions (p-chloromercuriphenyl-sulfonic acid, 83%; N-ethylmaleimide, 86%; HgCl2, 56%; mersalyl, 63% inhibition). We conclude that the skeletal muscle lactate transporter is a membrane-bound protein, specifically associated with the sarcolemma, that demonstrates saturation kinetics, competition, stereospecificity, and sensitivity to temperature as well as various ionic cis-inhibitors. The lactate transporter is a potentially important regulator of lactate flux across skeletal muscle, and may help to regulate intracellular pH and intermediary metabolism during lactic acidosis.

Plasma glucose, ketone bodies, insulin, glucagon and enteroglucagon in cows: diurnal variations related to ketone levels before feeding and to the ketogenic effects of feeds

Ingestions of a moderately ketogenic silage twice daily were followed by transient increments in plasma insulin and ketone bodies and decreases in plasma glucose. Ketone bodies and glucose were negatively correlated throughout the day, but the insulin elevations culminated before the maximal effects on ketone bodies and glucose were established. Cows with varying glucose levels before morning feeding reacted to a highly ketogenic silage by decreasing their glucose level uniformly to about 3 mmol/l, in spite of a widely varying feeding-induced insulin increment. Hay-feeding caused insulin increments of the same magnitude as silage-feeding, but the glucose decrease and the ketone increment was much smaller. The results indicate some direct action of ketone bodies on blood sugar regulation, in addition to effects mediated by insulin. The role of ketone bodies as the insulinotropic factor was not confirmed. The insulin level after feeding seems to be determined by the carbohydrate status of the animal before feeding. No significant changes in plasma glucagon were observed after feeding, and no consistent differences in plasma levels of this hormone were found when non-ketonemic, ketonemic, and clinically ketotic cows were compared. The plasma level of enteroglucagon (GLI) was positively correlated to the relative amount of concentrates consumed, but no relation to plasma glucose was found.

Gluconeogenesis and the protection of hepatic intracellular pH during diabetic ketoacidosis in rats

Studies were made of the mechanism whereby hepatic gluconeogenesis is increased in diabetic ketoacidosis (DKA) despite evidence in vitro of inhibition of gluconeogenesis by systemic acidosis. In perfused livers taken from normal rats, marked inhibition of lactate uptake and glucose output was achieved by simulation of metabolic acidosis in the perfusate. In perfused livers obtained from animals with DKA, lactate uptake and glucose output were greater than in normal perfused liver at all values of perfusate pH, and it was not possible to demonstrate significant inhibition of gluconeogenesis from lactate by perfusate acidosis. Varying severity of acidosis was induced in rats by (a) HCl infusion, (b) NH4Cl ingestion or (c) experimental DKA. Hepatic intracellular pH (pHi) was measured in vivo by 31P-n.m.r. spectroscopy. Whereas at the severer degrees of systemic acidosis marked falls in hepatic pHi were seen in the HCl- and NH4Cl-treated animals, little fall was seen in rats with DKA. The protection of hepatic pHi in rats with DKA was not due to differences in respiratory compensation compared with the other groups. It is suggested that this protection of hepatic pHi in DKA may be responsible for the failure of acidotic inhibition of gluconeogenesis from lactate. Possible reasons for pHi protection in DKA are considered. There is no difference in hepatic energy status as assessed in vivo by ATP/Pi ratios between control, DKA and NH4Cl-treated rats. DKA rats show a striking decrease in hepatic glycerophosphoethanolamine content.

Carrier-mediated uptake of L-(+)-lactate in plasma membrane vesicles from rat liver

Plasma membrane vesicles from rat liver transported L-lactate into the inner vesicular space. Kinetic analysis of L-lactate uptake gave a Km value of approx. 2.9 mM. Selective inhibition was found in a similar pattern to that described for the hepatic lactate carrier. L-Lactate transport was enhanced when a pH gradient was created across the plasma membrane. Vesicles obtained from fasted rats showed a higher uptake of L-lactate than those from fed rats, when incubated with physiological concentrations of L-lactate.

Blood lactate accumulation in intermittent supramaximal exercise

Blood lactate accumulation rate and oxygen consumption have been studied in six trained male runners, aged 20 to 30 years. Subjects ran on a treadmill at a rate representing 172 +/- 5% VO2max for four 45 s sessions, separated by 9 min rest periods. Oxygen consumption was measured throughout. Blood lactate was determined in samples taken from the ear and VO2 was measured at the end of each exercise session, and two, five and nine minutes later. After the fourth exercise session, the same measurements were made every five min for 30 min. 4 subjects repeated a single exercise of the same type, duration and intensity and the same measurements were taken. With repetitive intermittent exercise, gradual increases in blood lactate concentration [( LA]b) occurred, whereas its rate of accumulation (delta[LA]b) decreased. The amount of oxygen consumed during each 45 s exercise session remained unchanged for a given subject. After cessation of intermittent exercise, the half-time of blood lactate was 26 min, whereas it was only 15 min after a single exercise session. VO2 values, on the other hand, returned to normal after 15 to 20 min. All other conditions being equal, the gradual decrease in delta[LA]b during intermittent exercise could be explained if the lactate produced during the first exercise session is used during the second period, and/or if the diffusion space of lactate increases. The diffusion space seems to be multi-compartmental on the basis of half-time values noted for [LA]b after intermittent exercise, compared with those noted after a single exercise session.(ABSTRACT TRUNCATED AT 250 WORDS)