Lactate production under fully aerobic conditions: the lactate shuttle during rest and exercise

O2 insufficiency and other factors increase the rate of lactate production. Significant quantities of lactate are produced under postabsorptive as well as postprandial conditions in resting individuals. In humans during postabsorptive rest, 25-50% of the total carbohydrate combusted appears to pass through the lactate pool. During sustained submaximal (in terms of VO2max) exercise, the rates of lactate production (Ri) and oxidation (Rox) are greatly elevated as compared to rest. However, lactate production and oxidation increase relatively less than O2 consumption during moderate-intensity exercise. Because the lactate production index (RiI = Ri/VO2) decreases during submaximal, moderate-intensity exercise compared to rest, it is concluded that skeletal muscle and other sites of lactate production are effectively oxygenated. Alterations in the levels of circulating catecholamines can affect levels and turnover rates of glucose and lactate. In pure red dog gracilis muscle in situ and in the healthy and myocardium in vivo, contraction results in glycolysis and lactate production. This production of lactate occurs despite an apparent abundance of O2. Similarly, glucose catabolism in the human brain results in lactate production. The formation of lactate under fully aerobic conditions of rest and exercise represents an important mechanism by which different tissues share a carbon source (lactate) for oxidation and other processes such as gluconeogenesis. This mechanism has been termed the lactate shuttle.     

Glucose, glutamine and ketone body utilisation by resting and concanavalin A activated rat splenic lymphocytes

The utilisation of glucose, glutamine, acetoacetate and D-3-hydroxybutyrate were investigated over 72 h of incubation of rat splenic lymphocytes, with and without concanavalin A. Lymphocytes consumed both ketone bodies; acetoacetate was consumed preferentially. The ketone bodies reduced glucose consumption by 30-50%, but had little effect on lactate production. Glutamine uptake was concentration dependent up to 4 mM, and consumption was increased in the presence of concanavalin. Glutamine stimulated glucose consumption and lactate production in both resting and activated cells. Complete oxidation contributed 65% of glucose-derived ATP, but less than 40% of glutamine-derived ATP. Glutamine metabolism makes only a minor contribution to lymphocyte ATP generation.     

Energy metabolism of swimming trout (Salmo gairdneri)

Summary The oxidation of 1-¹⁴C palmitate, 2-¹⁴C glucose, 1-¹⁴C lactate, 1-¹⁴C alanine, 1-¹⁴C leucine and 1-¹⁴C glutamate, injected via a cannula into the dorsal aorta, was determined in trout, either at rest, or during swimming at 80% of the maximum sustained speed. The oxygen consumption and the excretion rates of¹⁴CO₂ as well as CO₂ were measured.While the oxygen consumption of swimming trout was about twice as high as of resting trout, the oxidation rates of the injected tracers increased by up to 9 time. Despite the increased importance of blood borne substrates, the estimated contribution to total CO₂ production is about 6% for the resting and 17% for the active trout. The majority of the oxidisable substrates must therefore be endogenous.The mobilization and oxidation rates of lactate, palmitate and leucine were particularly increased during swimming. During rest, palmitate and leucine oxidation rates are low. While oxidation rates of alanine and glutamate are intermediate, those of glucose were found to be extremely low, both at rest and during swimming. The measured RQ values for resting and swimming trout were 0.91 and 0.96 respectively, indicating that protein is the major fuel, since glucose oxidation seems of minor importance.Abbreviations and symbols SA specific activity - tt transit time - decay (time) constant - flux (in % of injected dose per hour) - U_crit critical velocity of sustained swimming - TCO₂ total CO₂     

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.     

Potentially conflicting metabolic demands of diving and exercise in seals

Metabolic replacement rates (Ra) for glucose and free fatty acids (FFA) were determined during rest, exercise, and diving conditions in the gray seal using bolus injections of radiotracers. In the exercise experiments the seal swam at a metabolic rate elevated twofold over resting Ra for glucose and FFA while resting were similar to values found in terrestrial mammals and other marine mammal species. During exercise periods glucose turnover increased slightly while FFA turnover changes were variable. However, the energetic demands of exercise could not be met by the increase in the replacement rates of glucose or FFA even if both were completely oxidized. Under diving conditions the tracer pool displayed radically different specific activity curves indicative of the changes in perfusion and metabolic rate associated with a strong dive response. Since the radiotracer curves during exercise and diving differed qualitatively and quantitatively, it is possible that similar studies on freely diving animals can be used to assess the role of the diving response during underwater swimming in nature.     

Leg and arm lactate and substrate kinetics during exercise

To study the role of muscle mass and muscle activity on lactate and energy kinetics during exercise, whole body and limb lactate, glucose, and fatty acid fluxes were determined in six elite cross-country skiers during roller-skiing for 40 min with the diagonal stride (Continuous Arm + Leg) followed by 10 min of double poling and diagonal stride at 72-76% maximal O(2) uptake. A high lactate appearance rate (R(a), 184 +/- 17 micromol x kg(-1) x min(-1)) but a low arterial lactate concentration ( approximately 2.5 mmol/l) were observed during Continuous Arm + Leg despite a substantial net lactate release by the arm of approximately 2.1 mmol/min, which was balanced by a similar net lactate uptake by the leg. Whole body and limb lactate oxidation during Continuous Arm + Leg was approximately 45% at rest and approximately 95% of disappearance rate and limb lactate uptake, respectively. Limb lactate kinetics changed multiple times when exercise mode was changed. Whole body glucose and glycerol turnover was unchanged during the different skiing modes; however, limb net glucose uptake changed severalfold. In conclusion, the arterial lactate concentration can be maintained at a relatively low level despite high lactate R(a) during exercise with a large muscle mass because of the large capacity of active skeletal muscle to take up lactate, which is tightly correlated with lactate delivery. The limb lactate uptake during exercise is oxidized at rates far above resting oxygen consumption, implying that lactate uptake and subsequent oxidation are also dependent on an elevated metabolic rate. The relative contribution of whole body and limb lactate oxidation is between 20 and 30% of total carbohydrate oxidation at rest and during exercise under the various conditions. Skeletal muscle can change its limb net glucose uptake severalfold within minutes, causing a redistribution of the available glucose because whole body glucose turnover was unchanged.     

Effect of elevated FFA on carbohydrate and lipid oxidation during prolonged exercise in humans

Increased availability of circulating free fatty acids (FFA) inhibits the rate of glycolysis in heart and resting skeletal muscle (Randle effect). Whether elevated FFA may play a role in decreasing carbohydrate oxidation during prolonged exercise in humans is more controversial. Using respiratory exchange measurements, we measured substrate utilization during 2.5 h of exercise at approximately 44 +/- 1% maximal O2 uptake (VO2 max) in the presence or absence of elevated FFA levels. After 30 min of base-line determinations, 1,000 U heparin was given intravenously and a 3-h constant infusion of Intralipid 10% (150 g/h) and heparin (500 U/h) was started. After an additional 30 min of rest, subjects exercised for 2.5 h (study 1, n = 6). In another five subjects (study 2) 100 g glucose was ingested after 30 min of exercise. The same protocols (studies 1 and 2) were also performed during a 0.9%-saline infusion. During exercise, without glucose ingestion, higher FFA concentrations prevailed during the Intralipid infusion (1,122 +/- 40 vs. 782 +/- 65 mumol/l), but the relative contributions of carbohydrate (49 +/- 4 vs. 50 +/- 4%) or lipid (49 +/- 4 vs. 47 +/- 6%) oxidation to the total energy expenditure were different only during the first 30 min of exercise. Similarly, higher FFA levels (1,032 +/- 62 vs. 568 +/- 46 mumol/l) did not alter the relative contributions of carbohydrate (62 +/- 4 vs. 69 +/- 2%) or lipid (36 +/- 4 vs. 29 +/- 2%) oxidation to the total energy expenditure after glucose feeding.(ABSTRACT TRUNCATED AT 250 WORDS)     

溶氧水平对鳊鱼、中华倒刺鲃幼鱼游泳能力的影响

在(25 1)℃条件下, 以鳊鱼(Parabramis pekinensis)体重(4.70 0.11)g, n=32、中华倒刺鲃(Spinibarbus sinensis)体重(3.26 0.06)g, n=32幼鱼为研究对象, 采用鱼类游泳代谢测定仪在水体溶氧为8、4、2、1 mgO2/L条件下分别测定其临界游泳速度(Ucrit)和游泳代谢率(MO2), 并计算出静止代谢率(MO2rest)、最大游泳代谢率(MO2max)、代谢范围(MS)及单位位移能耗(COT)等相关参数。结果显示, 随着溶氧水平的下降, 鳊鱼、中华倒刺鲃幼鱼的Ucrit均逐渐下降, 除中华倒刺鲃幼鱼的值在4与8 mgO2/L下没有显著性差异外, 其他各组均差异显著(P0.05); 在同一溶氧水平下的中华倒刺鲃Ucrit显著大于鳊鱼(P0.05)。两种鱼的MO2max和MS均随DO的下降而显著下降, 但MO2rest在溶氧水平低于1 mgO2/L才显著下降(P0.05)。研究还发现鳊鱼、中华倒刺鲃幼鱼的MO2在同一游泳速度下随溶氧水平下降而降低, 而在相同溶氧水平下随游泳速度的上升而显著升高(P0.05); COT随游泳速度上升而显著降低(P0.05), 但在高游泳速度下相对稳定, 在同一游泳速度下随着DO的下降有所减小。中华倒刺鲃的COT整体上小于鳊鱼, 且在低游泳速度下差异更大。87.5%(1-8 mgO2/L)溶氧水平的下降导致两种鱼类相似的Ucrit变化(53% vs. 50%), 但溶氧水平由8降到4 mgO2/L时, 鳊鱼32%MO2max的下降导致Ucrit下降13%, 但同样的溶氧水平下降虽然导致中华倒刺鲃的MO2max下降20%, 但由于MO2rest和COT的下降, 其Ucrit并没有显著的变化。实验结果表明: 不同溶氧水平对不同鱼种游泳能力的影响存在差异, 这种差异与其代谢对策密切相关。       

The effect of submergence on heart rate and oxygen consumption of swimming seals and sea lions

Respiratory, metabolic, and cardiovascular responses to swimming were examined in two species of pinniped, the harbor seal (Phoca vitulina) and the California sea lion (Zalophus californianus). 1. Harbor seals remained submerged for 82-92% of the time at swimming speeds below 1.2 m.s-1. At higher speeds, including simulated speeds above 1.4 m.s-1, the percentage of time spent submerged decreased, and was inversely related to body weight. In contrast, the percentage of time spent submerged did not change with speed for sea lions swimming from 0.5 m.s-1 to 4.0 m.s-1. 2. During swimming, harbor seals showed a distinct breathhold bradycardia and ventilatory tachycardia that were independent of swimming speed. Average heart rate was 137 beats.min-1 when swimming on the water surface and 50 beats.min-1 when submerged. A bimodal pattern of heart rate also occurred in sea lions, but was not as pronounced as in the seals. 3. The weighted average heart rate (WAHR), calculated from measured heart rate and the percentage time spent on the water surface or submerged, increased linearly with swimming speed for both species. The graded increase in heart rate with exercise load is similar to the response observed for terrestrial mammals. 4. The rate of oxygen consumption increased exponentially with swimming speed in both seals and sea lions. The minimum cost of transport calculated from these rates ranged from 2.3 to 3.6 J.m-1.kg-1, and was 2.5-4.0 times the level predicted for similarly-sized salmonids. Despite different modes of propulsion and physiological responses to swimming, these pinnipeds demonstrate similar transport costs.     

Nucleotide turnover rate measured in fully relaxed rabbit skeletal muscle myofibrils

Steady state measurements of the ATP turnover rate of myosin crossbridges in relaxed living mammalian muscle or in in vitro systems are complicated by other more rapid ATPase activities. To surmount these problems we have developed a technique to measure the nucleotide turnover rate of fully relaxed myosin heads in myofibrils using a fluorescent analogue of ATP (mant-ATP). Rabbit myofibrils, relaxed in 1.6 mM ATP, were rapidly mixed with an equal volume of solution containing 80 microM mant-ATP and injected into a fluorimeter. As bound ADP is released, a fraction of the myosin active sites bind mant-ATP and fluorescence emission rises exponentially, defining a rate of nucleotide turnover of 0.03 +/- 0.001 s-1 at 25 degrees C (n = 17). This rate was approximately equal to one half that of purified myosin. The turnover rates for myosin and myofibrils increased between 5 degrees and 42 degrees C, reaching 0.16 +/- 0.04 s-1 and 0.06 +/- 0.005 s-1, respectively, at 39 degrees C, the body temperature of the rabbit. If the rate observed for purified myosin occurred in vivo, it would generate more heat than is observed for resting living muscle. When myosin is incorporated into the myofilament lattice, its ATPase activity is inhibited, providing at least a partial explanation for the low rate of heat production by living resting muscle.     

Lactate flux and gluconeogenesis in fasting, weaned northern elephant seals (Mirounga angustirostris)

Elephant seals maintain rates of endogenous glucose production (EGP) typical of post-absorptive mammals despite enduring prolonged periods of food deprivation concurrent with low rates of glucose oxidation. These high rates of EGP suggest extensive glucose recycling during fasting. We investigated lactate metabolism in fasting elephant seals to assess its role in glucose recycling. Whole-animal glucose and lactate fluxes were measured as the rates of appearance of glucose and lactate (Ra _gluc and Ra _lac, respectively) using a primed constant infusion of [U-¹⁴C] lactate and [6-³H] glucose, and we calculated the minimum contribution of lactate to gluconeogenesis (GNG _lac). Ra _lac was high compared to resting values in other species (3.21 ± 0.71 mmol min^?1* kg^?1), did not change between 14 ± 1 and 31 ± 8 days of fasting and varied directly with Ra _glu. The minimum GNG _lac was 44.6 ± 6.0 % of EGP, varied directly with plasma lactate levels, and did not change over the fast. Ra _lac and Ra _glu both varied directly with plasma insulin concentrations. These data suggest that lactate is the predominant gluconeogenic precursor in fasting elephant seals and that high rates of glucose recycling through Cori cycle activity contribute to the maintenance of EGP during fasting. High levels of Cori cycle activity and EGP may be important components of metabolic adaptations that maintain glucose production while avoiding ketosis during extended fasting or are related to sustained metabolic alterations associated with extended breath-holds in elephant seals.     

Oxygen uptake during swimming in a hypobaric hypoxic environment

The purpose of this study was to determine oxygen uptake (VO2) at various water flow rates and maximal oxygen uptake (VO2max) during swimming in a hypobaric hypoxic environment. Seven trained swimmers swam in normal [N; 751 mmHg (100.1 kPa)] and hypobaric hypoxic [H; 601 mmHg (80.27 kPa)] environments in a chamber where atmospheric pressure could be regulated. Water flow rate started at 0.80 m.s-1 and was increased by 0.05 m.s-1 every 2 min up to 1.00 m.s-1 and then by 0.05 m.s-1 every minute until exhaustion. At submaximal water flow rates, carbon dioxide production (VCO2), pulmonary ventilation (VE) and tidal volume (VT) were significantly greater in H than in N. There were no significant differences in the response of submaximal VO2, heart rate (fc) or respiratory frequency (fR) between N and H. Maximal VE, fR, VT, fc, blood lactate concentration and water flow rate were not significantly different between N and H. However, VO2max under H [3.65 (SD 0.11) l.min-1] was significantly lower by 12.0% (SD 3.4)% than that in N [4.15 (SD 0.18) l.min-1]. This decrease agrees well with previous investigations that have studied centrally limited exercise, such as running and cycling, under similar levels of hypoxia.     

On-line estimation of viable cells in a hybridoma culture at various DO levels using ATP balancing and redox potential measurement

Two on-line methods for the estimation of viable cell number in hybridoma cultivation were investigated. One used an empirical correlation between redox potential and animal cell density. The other was based on an ATP balance with ATP steady-state assumption. Oxygen uptake rate measurement provided the amount of ATP which was produced by oxidation of NADH. Oxygen uptake rate was measured either by stationary liquid phase balance with surface aeration or by gas balance during bubble aeration with headspace flushing with an inert gas. The amount of ATP produced through the glycolysis was estimated based on the amount of lactate produced. In cultures, in which pH was controlled via manipulation of the gas phase composition, the flow of CO(2) was linearly correlated with the lactate concentration. At constant dissolved oxygen levels, the viable cell density was proportional to the estimated ATP production rate, during exponential growth and during later phases. The estimated specific ATP production rate, however, varied from 2.2 pmol cell(-1) h(-1) at 10% air saturation to 4.5 pmol cell(-1) h(-1) at 100% air saturation. Specific rates of glutamine, glucose, and lactate followed the shape of the specific ATP production rate, whereas the specific oxygen uptake rate was minimal at around 50% air saturation. (c) 1996 John Wiley & Sons, Inc.     

EFFECT OF CYANIDE AND ELECTRICAL STIMULATION ON PHOSPHOINOSITIDE METABOLISM IN LOBSTER NERVES

The contents of phosphoinositides, ATP, glucose and lactate in leg and claw nerves of the lobster were determined. Nerves were also analysed after cyanide poisoning, after electrical stimulation, and 1 h after removing the leg from the lobster. Cyanide poisoning decreased the levels of ATP and glucose and increased the content of lactate but did not alter the levels of phosphoinositides. Nerves left in situ for 1 h after disconnection from the central nervous system exhibited a decrease in the content of tri-phosphoinositides (TPI) of 50 per cent, without changes in ATP, glucose or lactate. The TPI change was reversed after incubation for 1 h in oxygenated seawater. Nerves labelled in vivo with ³²P were removed and stimulated at 50 Hz for 5 min. The turnover of TPI phosphorus increased on stimulation in both normal and cyanide-poisoned nerves. In contrast, turnover of ATP increased after stimulation in normal nerves but not in cyanide-treated nerves. We sought to determine whether polyphosphoinositides play a greater role in resting metabolism of the nerve or in the conducting mechanisms. Our results make more likely the involvement of TPI in permeability changes of neural membranes during excitation.     

Comparison of the effects of pre-exercise feeding of glucose, glycerol and placebo on endurance and fuel homeostasis in man

Six men were studied during exercise to exhaustion on a cycle ergometer at 73% of VO2max following ingestion of glycerol, glucose or placebo. Five of the subjects exercised for longer on the glucose trial compared to the placebo trial (p less than 0.1; 108.8 vs 95.9 min). Exercise time to exhaustion on the glucose trial was longer (p less than 0.01) than on the glycerol trial (86.0 min). No difference in performance was found between the glycerol and placebo trials. The ingestion of glucose (lg X kg-1 body weight) 45 min before exercise produced a 50% rise in blood glucose and a 3-fold rise in plasma insulin at zero min of exercise. Total carbohydrate oxidation was increased by 26% compared to placebo and none of the subjects exhibited a fall in blood glucose below 4 mmol X 1-1 during the exercise. The ingestion of glycerol (lg X kg-1 body weight) 45 min before exercise produced a 340-fold increase in blood glycerol concentration at zero min of exercise, but did not affect resting blood glucose or plasma insulin levels; blood glucose levels were up to 14% higher (p less than 0.05) in the later stages of exercise and at exhaustion compared to the placebo or glucose trials. Both glycerol and glucose feedings lowered the magnitude of the rise in plasma FFA during exercise compared to placebo. Levels of blood lactate and alanine during exercise were not different on the 3 dietary treatments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glycogen phosphorylase and pyruvate dehydrogenase transformation in white muscle of trout during high-intensity exercise

We examined the regulation of glycogen phosphorylase (Phos) and pyruvate dehydrogenase (PDH) in white muscle of rainbow trout during a continuous bout of high-intensity exercise that led to exhaustion in 52 s. The first 10 s of exercise were supported by creatine phosphate hydrolysis and glycolytic flux from an elevated glycogenolytic flux and yielded a total ATP turnover of 3.7 micromol x g wet tissue(-1) x s(-1). The high glycolytic flux was achieved by a large transformation of Phos into its active form. Exercise performed from 10 s to exhaustion was at a lower ATP turnover rate (0.5 to 1.2 micromol x g wet tissue(-1) x s(-1)) and therefore at a lower power output. The lower ATP turnover was supported primarily by glycolysis and was reduced because of posttransformational inhibition of Phos by glucose 6-phosphate accumulation. During exercise, there was a gradual activation of PDH, which was fully transformed into its active form by 30 s of exercise. Oxidative phosphorylation, from PDH activation, only contributed 2% to the total ATP turnover, and there was no significant activation of lipid oxidation. The time course of PDH activation was closely associated with an increase in estimated mitochondrial redox (NAD(+)-to-NADH concentration ratio), suggesting that O2 was not limiting during high-intensity exercise. Thus anaerobiosis may not be responsible for lactate production in trout white muscle during high-intensity exercise.     

Metabolic activities of Listeria monocytogenes in the presence of sodium propionate, acetate, lactate and citrate

The effects of sodium propionate, acetate, lactate and citrate on cell proliferation, glucose and oxygen consumption, and ATP production in Listeria monocytogenes were investigated in growing and resting cells. Media pH was 6.7-6.8. Growth inhibition increased while glucose consumption continued in the presence of ≥ 1% propionate, ≥ 3% acetate and ≥ 5% lactate in broth during incubation at 35°C, indicating that glucose consumption was uncoupled from cell proliferation. Acetate and propionate were the most effective antilisterials, whereas citrate (5%) was only slightly inhibitory. Of the four salts, only lactate supported growth, oxygen consumption and ATP production. While concentrations of 1 and 5% propionate, acetate and citrate did not have an effect on oxygen consumption, they inhibited ATP production. ATP production in the presence of the four salts was consistently lower at pH 6.0 than at neutral pH. Lactate served as an alternative energy source for L. monocytogenes in the absence of glucose but became toxic to the organism in the presence of the carbohydrate.     

Disposal of blood [1-13C]lactate in humans during rest and exercise

Lactate irreversible disposal (RiLa) and oxidation (RoxLa) rates were studied in six male subjects during rest (Re), easy exercise [EE, 140 min of cycling at 50% of maximum O2 consumption (VO2max)] and hard exercise (HE, 65 min at 75% VO2max). Twenty minutes into each condition, subjects received a Na+-L(+)-[1-13C]lactate intravenous bolus injection. Blood was sampled intermittently from the contralateral arm for metabolite levels, acid-base status, and enrichment of 13C in lactate. Expired air was monitored continuously for determination of respiratory parameters, and aliquots were collected for determination of 13C enrichment in CO2. Steady-rate values for O2 consumption (VO2) were 0.33 +/- 0.01, 2.11 +/- 0.03, and 3.10 +/- 0.03 l/min for Re, EE, and HE, respectively. Corresponding values of blood lactate levels were 0.84 +/- 0.01, 1.33 +/- 0.05, and 4.75 +/- 0.28 mM in the three conditions. Blood lactate disposal rates were significantly correlated to VO2 (r = 0.78), averaging 123.4 +/- 20.7, 245.5 +/- 40.3, and 316.2 +/- 53.7 mg X kg-1 X h-1 during Re, EE, and HE, respectively. Lactate oxidation rate was also linearly related to VO2 (r = 0.81), and the percentage of RiLa oxidized increased from 49.3% at rest to 87.0% during exercise. A curvilinear relationship was found between RiLa and blood lactate concentration. It was concluded that, in humans, 1) lactate disposal (turnover) rate is directly related to the metabolic rate, 2) oxidation is the major fate of lactate removal during exercise, and 3) blood lactate concentration is not an accurate indicator of lactate disposal and oxidation.     

Fatty acid metabolism in fasting elephant seal pups

Summary The turnover of two plasma free fatty acids (FFA) were measured in 5 northern elephant seal pups (Mirounga angustirostris) after approximately 2 months of post-weaning fasting. Turnover was determined using simultaneous bolus injections of¹⁴C-palmitate,³H-oleate and Evans blue (EB) administered via an indwelling extradural intravertebral catheter. At this time in their natural fast, the seals exhibited plasma FFA levels and turnover values higher than reported for other marine mammals and most terrestrial carnivores. There was no consistent difference between plasma FFA turnover measured by palmitate or oleate tracers. The results imply that FFA metabolism is the primary source of energy during fasting. This is interesting in light of previous observations of minimal ketoacid accumulation and low levels of glucose and protein energy production during fasting in these juvenile seals.     

Adenine nucleotide metabolism of blood platelets. IX. Time course of secretion and changes in energy metabolism in thrombin-treated platelets.

Changes in the energy metabolism of washed human platelets were compared with the kinetics of secretion induced by thrombin (5 units/ml). A 50% decrease in the level of metabolic ATP (3H-labelled), which was essentially complete in 30s, was matched in rate by adenine nucleotide secretion from storage in dense granules. Incubation of platelets with antimycin before thrombin addition increased the rate of fall in metabolic ATP, but did not affect the rate of adenine nucleotide secretion. beta-N-Acetylglucosaminidase secretion, which was slower than adenine nucleotide secretion in control platelets, was noticeably inhibited by antimycin, confirming previous reports that different regulatory mechanisms exist for dense and alpha-granule secretion. The rates of rephosphorylation of metabolic ADP to ATP via glycolysis and oxidative phosphorylation were estimated by measuring lactate production and O2 consumption in resting and thrombin-stimulated platelets and compared to the level of metabolic ATP (9-10 nmol/mg of platelet protein in the resting state). The rate of ATP production was stimulated at least two fold from 12 nmol to 24 nmol/min/mg within seconds of thrombin addition. This increased rate was maintained over the observed period of 5 min although the level of metabolic ATP had decreased to 4-5 nmol/mg within 30 s; the turnover of the remaining metabolic ATP thus increased four fold over the resting state although the actual stimulation of energy production was only two fold.