A model for measurement of lactate disappearance with isotopic tracers in the steady state

Characterization of circannual patterns of metabolic recovery from activity inRana catesbeianaat 15°C

We characterized carbohydrate metabolism following activity in the American bullfrog, Rana catesbeiana, and compared whole body metabolic profiles between two seasons. Forty-eight adult male Rana catesbeianawere chronically cannulated and injected with[U-14C]l-lactic acid sodium salt in either summer (June)or winter (January) after acclimation for 2 weeks at 15°C with a 12 h:12 h L:D photoperiod. Following injection with [14C]lactate, frogs were either allowed to rest for 240 min (REST), hopped for 2 min on a treadmill and immediately sacrificed (PE), or hopped for 2 min on a treadmill and allowed to recover for 240 min (REC 4). Exercise caused a significant increase in blood lactate level from 2.7±0.1 mmol l–1 at rest to 17.0±2.1 mmol l–1 immediately following exercise. This increase persisted throughout the recovery period, with average blood lactate level only reduced to 13.7±1.1 mmol l–1 after 240 min of recovery, despite complete recovery of intramuscular lactate levels. Lactate levels were not significantly different between seasons in any treatment (REST, PE, REC4), in either gastrocnemius muscle or blood. The vast majority of [14C]lactate was recovered in the muscle, in both winter (86.3%) and summer (87.5%). Season had no effect on total amount of 14C label recovered. [14C]Lactate was measured in the forms of lactate, glucose and glycogen, in the liver and the muscle sampled. The most robust difference found in seasonal metabolism was that both the liver and the gastrocnemius contained significantly higher levels of intracellular free glucose under all treatments in winter. These data suggest that, overall, bullfrogs accumulate and slowly clear lactate in a manner quite similar to findings in fish, other amphibians and lizards. Additionally, our findings indicate that lactate metabolism is not highly influenced by season alone, but that intracellular glucose levels may be sensitive to annual patterns.

The concept of maximal lactate steady state: a bridge between biochemistry, physiology and sport science

The maximal lactate steady state (MLSS) is defined as the highest blood lactate concentration (MLSSc) and work load (MLSSw) that can be maintained over time without a continual blood lactate accumulation. A close relationship between endurance sport performance and MLSSw has been reported and the average velocity over a marathon is just below MLSSw. This work rate delineates the low- to high-intensity exercises at which carbohydrates contribute more than 50% of the total energy need and at which the fuel mix switches (crosses over) from predominantly fat to predominantly carbohydrate. The rate of metabolic adenosine triphosphate (ATP) turnover increases as a direct function of metabolic power output and the blood lactate at MLSS represents the highest point in the equilibrium between lactate appearance and disappearance both being equal to the lactate turnover. However, MLSSc has been reported to demonstrate a great variability between individuals (from 2-8 mmol/L) in capillary blood and not to be related to MLSSw. The fate of enhanced lactate clearance in trained individuals has been attributed primarily to oxidation in active muscle and gluconeogenesis in liver. The transport of lactate into and out of the cells is facilitated by monocarboxylate transporters (MCTs) which are transmembrane proteins and which are significantly improved by training. Endurance training increases the expression of MCT1 with intervariable effects on MCT4. The relationship between the concentration of the two MCTs and the performance parameters (i.e. the maximal distance run in 20 minutes) in elite athletes has not yet been reported. However, lactate exchange and removal indirectly estimated with velocity constants of the individual blood lactate recovery has been reported to be related to time to exhaustion at maximal oxygen uptake.

Effects of active versus passive recovery on power output during repeated bouts of short term, high intensity exercise

ATP repletion following exhaustive exercise is approximated to be 90-95% complete in 3 minutes, and is crucial in the performance of short duration, high intensity work. Few studies appear to have used this 3-minute interval in the investigation of recovery modes, blood lactate accumulation and power output. Thus, our aim was to investigate changes in peak power (PP), average power (AP) and blood lactate during repeated bouts of high intensity, short duration cycling, comprising active and passive recovery modes lasting 3 minutes. Seven male cyclists (age 21.8±3.3 yrs, mass 73.0±3.8kgs, height 177.3±3.4cm) performed both an active (3 min at 80rpm & 1kg resistance) and a passive recovery (no work between bouts) protocol. Following a warm-up, subjects performed six 15-second maximal sprints against a fixed workload of 5.5kg. Mean PP across the six trials was 775±11.2Watts (W) and 772±33.4W for active and passive protocols respectively; whereas mean AP was 671±26.4W and 664±10.0W, respectively. Neither was significantly different. There was a significant difference within trials for both peak power and average power (p<0.05), with both values decreasing over time. However, the decrease was significantly smaller for both PP and AP values during the active recovery protocol (p<0.05). In the current study, variation in power output cannot be explained by lactate values, as values did not differ between the active and passive protocol (p=0.37). Lactate values did differ significantly between trials within protocols (p<0.05). The results of this study suggest that an active recovery of 3 minutes between high intensity, short duration exercise bouts significantly increases PP and AP compared to a passive recovery, irrespective of changes in blood lactate levels.

Endotoxin-induced hyperlactatemia results from decreased lactate clearance in hemodynamically stable rats

OBJECTIVE

To determine whether endotoxin-induced hyperlactatemia in hemodynamically stable animals is due to increased lactate production or decreased lactate clearance by measuring lactate turnover rate in the vascular compartment (LTRvc).

DESIGN

Prospective, controlled trial.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Male Sprague-Dawley rats weighing 275-425 g with chronic vascular catheters.

INTERVENTIONS

Chronically catheterized rats were treated with 6 microg/kg endotoxin or saline. LTRvc was determined from the specific activity of carbon-14 [14C]lactate in aortic blood during a constant infusion of [14C]lactate into the inferior vena cava. The role of the splanchnic organs in lipopolysaccharide-induced alterations in LTRvc was determined from the splanchnic first-pass clearance of [14C]lactate infused into the superior mesenteric artery and direct measurements of blood lactate concentration gradients across the splanchnic organs.

MEASUREMENTS AND MAIN RESULTS

Despite a 260% increase in lactate concentrations after lipopolysaccharide treatment, the specific activity of [14C]lactate and the LTRvc did not change, indicating that lipopolysaccharide-induced hyperlactatemia is caused by decreased lactate clearance from the vascular compartment rather than increased lactate flux into the vascular compartment. In contrast, lactate clearance by the splanchnic system was increased. The specific activity of [14C]lactate in aortic blood decreased 33% after lipopolysaccharide treatment when the [14C]lactate was infused into the superior mesenteric artery, indicating increased first-pass clearance of [14C]lactate by the splanchnic organs. Furthermore, the hepatic venous-aortic concentration gradient of lactate became increasingly negative after lipopolysaccharide treatment, indicating increased vascular extraction of lactate by the splanchnic system (0.07 +/- 0.07 micromol/mL vs. -0.34 +/- 0.14 micromol/mL).

CONCLUSIONS

Lipopolysaccharide-induced hyperlactatemia in hemodynamically stable rats is caused by a net decrease in lactate clearance from the vascular compartment despite the fact that the clearance of lactate by the splanchnic system remains intact.

Reduced synthesis of NO causes marked alterations in myocardial substrate metabolism in conscious dogs

To test whether the acute reduction of nitric oxide (NO) synthesis causes changes in cardiac substrate metabolism and in the activity of key enzymes of fatty acid and glucose oxidation, we blocked NOS by giving N(omega)-nitro-L-arginine methyl ester (L-NAME; 35 mg/kg iv two times) to nine chronically instrumented dogs. [3H]oleate, [14C]glucose, and [13C]lactate were infused to measure the rate of cardiac substrate uptake and oxidation. Glyceraldehyde-3-phosphate dehydrogenase, acetyl-CoA carboxylase, and malonyl-CoA decarboxylase activities were measured in myocardial biopsies. In eight control dogs, ANG II was infused (20-40 ng x kg(-1) x min(-1)) to mimic the hemodynamic effects of L-NAME. After L-NAME, significant changes occurred for fatty acid oxidation (from 9.8 +/- 0.8 to 7.1 +/- 1.2 micromol/min), glucose uptake (from 12.9 +/- 5.5 to 45.0 +/- 14.2 micromol/min), and oxidation (from 4.4 +/- 1.2 to 19.9 +/- 2.3 micromol/min). ANG caused only a significantly lower increase in glucose oxidation. Lactate uptake increased by more than twofold in both groups. The enzyme activities did not differ significantly between the two groups. In conclusion, the acute inhibition of NO synthesis causes marked metabolic alterations that do not involve key rate-controlling enzymes of fatty acid oxidation nor glyceraldehyde-3-phosphate dehydrogenase.

Lactate clamp: a method to measure lactate utilization in vivo

A lactate clamp method has been developed to quantify the whole body lactate utilization in conscious, unstressed rats. Dichloroacetate (DCA), a known lactate utilization enhancer, was used to validate the method. Fasting blood lactate concentrations before the clamps were identical for DCA-treated (1 mmol/kg) and control groups (1.65 +/- 0.37 vs. 1.65 +/- 0.19 mM). The animals received a primed continuous lactate infusion for 90 min at variable rates to clamp the blood lactate concentration at 2 mM. The steady-state (60-90 min) lactate infusion rate, which represents the whole body lactate utilization in DCA-treated animals, was 144% higher than that in the control animals (13.2 +/- 1.0 vs. 5.4 +/- 1.1 mg . kg-1 . min-1; P < 0.001). The markedly increased lactate infusion rate indicates an enhanced lactate flux by DCA. To determine whether the increased lactate infusion by DCA reflected reduced endogenous lactate production, lactate production was measured. The results indicate that endogenous lactate production was not affected by DCA. In conclusion, the lactate clamp provides a sensitive and reliable method to assess lactate utilization in vivo, a dynamic measurement that may not be clearly demonstrated by blood lactate concentrations per se.

Child-adult differences in whole blood lactate responses to incremental treadmill exercise

The aim of this study was to evaluate whether fixed blood lactate reference values of 2.5 and 4.0 mmol.1(-1) corresponded to the equivalent intensity of exercise in prepubertal and teenage boys, and men. Twenty six prepubertal boys (mean(sd) age) 11.1(0.4) years), 26 teenage boys (mean(sd) age 14.1(0.3) years), and 23 men (mean(sd) age 22.4(2.7) years) gave informed consent to participate in the study. Oxygen consumption (VO2) and heart rates (HR) corresponding to the 2.5 and 4.0 mmol.1(-1) fixed blood lactate reference values were used as the criterion measures during incremental treadmill exercise. At the 2.5 mmol.1(-1) level there were no significant differences (P > 0.05) in % peak VO2 between groups. For both prepubertal and teenage boys the 4.0 mmol.1(-1) lactate level represented a higher mean % peak. VO2 than for the mean (P < 0.05). The prepubertal and teenage values were again not significantly different (P > 0.05). Factors other than maturation during puberty influence blood lactate responses to exercise.

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)

Lactate-pyruvate interconversion in blood: implications for in vivo tracer studies

We have evaluated lactate and pyruvate kinetics in whole blood or plasma by the addition of [1-13C]lactate (n = 5) or [1-13C]pyruvate (n = 5) and application of compartmental modeling to the resulting data. Pyruvate and lactate concentrations and tracer-to-tracee ratios were measured at frequent intervals for 45 min. Pyruvate and lactate tracer-to-tracee ratios equilibrated almost completely within 3-4 min in whole blood, whereas there was no isotopic exchange in plasma. The average rate of interconversion between unlabeled lactate and pyruvate was four to five times (pyruvate to lactate) and three to four times (lactate to pyruvate) the net production rate of lactate. We conclude that there is a very rapid interconversion between lactate and pyruvate in blood that has to be considered in the interpretation of in vivo tracer studies.

On the determination of turnover in vivo with tracers

Theoretical and practical aspects of the application of tracer methods for the measurement of turnover of blood-borne compounds are discussed, with special regard to lactate. The validity of the application of the tracer into the aortic arch and sampling from the right atrium (A-V), the administration of tracer into the vena cava and sampling from the aorta (V-A), and sampling to the determination of turnover are examined, using numerical examples. It is shown that the difference between specific activity in arterial and mixed venous blood depends mainly on the cardiac output, the ratio of tracee turnover to the mass of circulating tracee, and the sites of production and utilization of the tracee. Conditions are shown under which the A-V and V-A modes overestimate or underestimate the true rate of turnover. In theory, the A-V mode provides an exact estimate of turnover when the mean specific activity of the tracee in the whole body equals the specific activity of mixed venous blood in the right heart. It is shown that, for compounds with a high turnover rate, the underestimate in the A-V mode is small, and the mode provides a close approximation of true turnover. The underestimate in the V-A mode at high turnover rates is extensive. Experimental evidence indicates that, in several animal species, the specific activity of lactate and several amino acids in several organs and tissues nearly equals that in the venous blood, with the A-V mode providing a close approximation of the true turnover for these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)