Lactate Efflux Research Articles

Changes in extracellular acid-base homeostasis in cerebral ischemia.

The purpose of this study was to examine the changes in extracellular CO32- and lactate concentration produced by ischemia, especially in relation to the occurrence of anoxic depolarization, and how some of these changes are altered by the inhibition of organic acid transport systems with probenecid. These data demonstrate that (i) the transmembrane mechanisms contributing to intracellular acid-base regulation (Na+/H+ and HCO3-/Cl- exchanges, and lactate/H+ cotransport) are markedly activated during ischemia; (ii) the efficacy of these mechanisms is abolished as the cellular membrane permeability to ions, including H+ and pH-changing anions, suddenly increases with anoxic depolarization; and (iii) efflux of intracellular lactate during ischemia, and its reuptake with reperfusion, mainly occur via a transporter. These findings imply that residual cellular acid-base homeostasis persists as long as cell depolarization does not occur, and strengthen the concept that anoxic depolarization is a critical event for cell survival during ischemia.

Alpha 2-adrenergic agonists increase cellular lactate efflux.

We reported previously that genetic polymorphisms of the alpha 2-adrenergic receptor are associated with hyperinsulinemia, diabetes mellitus, and hypertension in blacks. The evolutionary driving force for maintaining such deleterious mutations in the black population is unknown. Recognizing that vascular alpha 2-adrenergic receptors mediate cold-induced vasoconstriction and that temperature maintenance is a primary thrust of cellular metabolism, we postulated that vascular alpha 2-adrenergic receptors contribute significantly to metabolic heat generation in homeotherms such as humans. Using aerobic lactate production as an indicator of thermogenesis, we measured metabolic heat production in HT29 cells that expressed the gene encoding human vascular alpha 2-adrenergic receptors. Epinephrine, an alpha 2-adrenergic receptor agonist, increased net lactate efflux from 226 +/- 20 to 280 +/- 20 nmol/min (mean +/- SE) (P = .06). Clonidine, a more specific alpha 2-adrenergic agonist, increased lactate efflux from 110 +/- 6 to 156 +/- 8 nmol/min (P < .01). Similarly, in the presence of physiological concentrations of glucose (5.5 mmol/L), insulin increased lactate production from 123 +/- 6 to 175 +/- 10 nmol/min (P < .01). Because differences in aerobic glycolysis may also explain the heat intolerance and abnormal fuel homeostasis found in genetically hypertensive rats, we also measured lactate production in cultured vascular smooth muscle cells isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive control Wistar-Kyoto rats (WKY). Vascular smooth muscle cells from SHRSP had significantly greater lactate efflux compared with cells from normotensive WKY (296 +/- 4 versus 172 +/- 2 nmol/min, P < .001). These differences were not due to abnormalities in glucose uptake, as lactate efflux was greater in SHRSP cells compared with WKY cells when dextrose was replaced with equimolar concentrations of fructose (230 +/- 6 versus 138 +/- 2 nmol/min, P < .001). alpha 2-Adrenergic agonists increase lactate efflux in HT29 cells, and abnormalities in vascular smooth muscle lactate metabolism in genetically hypertensive rats is independent of altered glucose uptake. These data provide support for our hypothesis that balanced polymorphisms of the alpha 2-adrenergic receptor could offer protection against cold stress by increasing the thermogenic response associated with aerobic lactate production.

EFFECT OF ELEVATED [FFA] ON GLUCOSE UPTAKE AND LACTATE EFFLUX DURING SUBMAXIMAL EXERCISE IN MAN 9

The present investigation examined the effect of enhanced plasma free fatty acid availability on leg glucose uptake and lactate efflux during prolonged submaximal cycling exercise. Seven males cycled for 70 min (10 min at 40% and 60 min at 65% VO2max) while infused with either Intralipid (100 ml) and heparin (2000 U) (INTRA) or saline (CON). At the onset of exercise, plasma arterial [FFA] was 0.70 ± 0.03 mM (INTRA) vs. 0.22 ± 0.01 mM(CON). Arterial and femoral venous catheters were inserted and blood samples and expired gases were collected simultaneously at rest and at 8, 18, 34, 50 and 68 min of exercise. Blood flow was estimated from pulmonary O2 uptake and the leg a-v (O2) difference. No difference existed in VO2 or blood flow between trials, while average pulmonary RER was higher in CON (0.94 ± 0.01) compared to INTRA (0.91 ± 0.01)(p<0.05). Mean leg FFA uptake was higher in INTRA (0.16 ± 0.03 mmol/min) vs. CON (0.04 ± 0.01 mmol/min) during exercise (p<0.01). There was a significant decrease in lactate efflux during INTRA (1.55 ± 0.36 mmol/min) compared to CON (3.07 ± 0.47 mmol/min) (p<0.05). Leg glucose uptake was not different at any time point between trials. In conclusion, enhanced FFA availability leads to a decrease in lactate efflux and no change in glucose uptake. Therefore, if carbohydrate sparing occurs at these workloads, it must be a function of reduced glycogenolysis.

EFFECT OF CONTRACTION DURATION ON ENERGY COST AND FATIGUE IN CONTRACTING MUSCLE 836

The purpose of this study was to investigate the extent to which altering contraction duration (and presumably ion pump ATPase activity) can affect energy cost and fatigue in isolated contracting muscle. Canine gastrocnemius muscle (n=6) was isolated and 3 contractile periods (3 min of isometric, tetanic contractions) were conducted by each muscle in a balanced order design. Each of the 3 contractile periods had stimulation patterns that resulted in a 1:1 work-to-rest ratio, with the difference in the 3 contractile periods being in the contraction duration (short duration = 0.25 sec stimulation/0.25 sec rest; moderate duration = 0.50 sec stimulation/0.50 sec rest; and long duration = 1 sec stimulation/1 sec rest) with the total time of stimulation and number of stimulation pulses being the same over each 3 min contraction period. Muscle O2 uptake, the O2 cost of developed force, and the fall in developed force (fatigue) was significantly greater(p<0.05) with contractions of short duration (stimulation = 0.25 sec). Muscle [ATP] did not fall significantly from resting values during any contraction period and was not different among any of the 3 contraction periods. Lactate efflux from the working muscle and muscle [lactate] were significantly greater with contractions of short duration (stimulation = 0.25 sec). These results suggest that contraction duration, through the demand of cellular ion pump ATPases, can significantly affect the energy cost and fatigue (possibly related to increased glycolytic flux) of contracting muscle.

Intracellular pH and intrinsic H+ buffering capacity in normal and hypertrophied right ventricle of ferret heart.

To answer the questions: (a) What is the effect of hypertrophy on the intracellular pH (pHi) and buffering power of cardiac muscle, and (b) How does hypertrophy affect the ability of cardiac muscle to recover from intracellular acidosis induced by hypoxia. In nominally HCO3(-)-free, HEPES-buffered Tyrode solution (35 degrees C), pHi and the intrinsic buffering power (beta i, measured in the presence of amiloride) was investigated using pH-sensitive microelectrodes. beta i was similar in both preparations (25 mM/pH unit at pHi 7.04). beta i was inversely related to pHi but the relationship was not significantly modified by hypertrophy. In the absence of amiloride, the time constant of pHi recovery (tau r) on removal of NH+4, was similar in normal (4.0 +/- 0.2 min, n = 5) and in hypertrophied muscles (4.3 +/- 0.3 min, n = 4; n.s.). In both preparations, net acid extrusion (JH) was similarly increased at lower values of pHi. Lowering temperature from 35 degrees to 22 degrees caused an alkalinization (0.15 pH units) of pHi. At 22 degrees C the mean values of pHi, beta i, tau r and JH were similar in normal and in hypertrophied muscles. At both temperatures and in both groups of preparations, recovery of pHi following hypoxia is approximately exponential. The time constant of recovery of pHi following hypoxia (tau rh) at 22 degrees C was not significantly different in hypertrophied muscles (7.2 +/- 0.9 min, n = 8) compared to controls (10.6 +/- 1.8 min, n = 13). However, at 35 degrees C, there was a significant difference in the mean values of tau rh which was smaller for hypertrophied muscles (3.9 +/- 0.3 min, n = 7) than for normal (7.1 +/- 1.1 min, n = 4, P < 0.005). For pHi 6.8-7.0, net acid extrusion in hypertrophied preparations was increased by a factor of 4 compared to normal. The intracellular buffering capacity and the pHi regulating capacity via Na+/H+ exchange are not significantly modified by right ventricular hypertrophy in ferret heart. The faster pHi recovery from hypoxia-induced acidification can be interpreted in terms of the role of lactate efflux in pHi control. The possible role of energy compartmentalization, its influence on the Na+ gradient and thus on pHi control after hypoxia, is discussed.

Intracellular pH and intrinsic H+ buffering capacity in normal and hypertrophied right ventricle of ferret heart

To answer the questions: (a) What is the effect of hypertrophy on the intracellular pH (pHi) and buffering power of cardiac muscle, and (b) How does hypertrophy affect the ability of cardiac muscle to recover from intracellular acidosis induced by hypoxia.In nominally HCO3(-)-free, HEPES-buffered Tyrode solution (35 degrees C), pHi and the intrinsic buffering power (beta i, measured in the presence of amiloride) was investigated using pH-sensitive microelectrodes.beta i was similar in both preparations (25 mM/pH unit at pHi 7.04). beta i was inversely related to pHi but the relationship was not significantly modified by hypertrophy. In the absence of amiloride, the time constant of pHi recovery (tau r) on removal of NH+4, was similar in normal (4.0 +/- 0.2 min, n = 5) and in hypertrophied muscles (4.3 +/- 0.3 min, n = 4; n.s.). In both preparations, net acid extrusion (JH) was similarly increased at lower values of pHi. Lowering temperature from 35 degrees to 22 degrees caused an alkalinization (0.15 pH units) of pHi. At 22 degrees C the mean values of pHi, beta i, tau r and JH were similar in normal and in hypertrophied muscles. At both temperatures and in both groups of preparations, recovery of pHi following hypoxia is approximately exponential. The time constant of recovery of pHi following hypoxia (tau rh) at 22 degrees C was not significantly different in hypertrophied muscles (7.2 +/- 0.9 min, n = 8) compared to controls (10.6 +/- 1.8 min, n = 13). However, at 35 degrees C, there was a significant difference in the mean values of tau rh which was smaller for hypertrophied muscles (3.9 +/- 0.3 min, n = 7) than for normal (7.1 +/- 1.1 min, n = 4, P < 0.005). For pHi 6.8-7.0, net acid extrusion in hypertrophied preparations was increased by a factor of 4 compared to normal.The intracellular buffering capacity and the pHi regulating capacity via Na+/H+ exchange are not significantly modified by right ventricular hypertrophy in ferret heart. The faster pHi recovery from hypoxia-induced acidification can be interpreted in terms of the role of lactate efflux in pHi control. The possible role of energy compartmentalization, its influence on the Na+ gradient and thus on pHi control after hypoxia, is discussed.

Effect of high FFA on glycogenolysis in oxidative rat hindlimb muscles during twitch stimulation.

The effect of elevated free fatty acids (FFA) on carbohydrate (CHO) utilization in the oxidative muscles of the isolated hindlimb was determined using twitch contraction paradigms evoking a wide range of O2 uptakes and glycogenolysis. The hindlimb was perfused with either 0 or 1.8 mM FFA for 10 min at rest and then subjected to 20 min of stimulation at 0.4, 0.7, 1, 2, 3, or 4 Hz. Soleus (Sol), plantaris (Pl), and red gastrocnemius (RG) were sampled after rest perfusion or stimulation. FFA had little effect on glycogenolysis during stimulation, although glycogen sparing occurred with one of the lesser intensity protocols in each muscle (Sol, 0.4 Hz; RG, 0.7 Hz; Pl, 1 Hz). Muscle citrate and acetyl-CoA were elevated in Sol during several stimulation protocols with high FFA, but this effect was inconsistent in Pl and RG. The sparing of glycogen, when it did occur, was generally unrelated to increases in either citrate or acetyl-CoA content. Furthermore, protocols in which citrate or acetyl-CoA were elevated in the presence of elevated FFA did not demonstrate glycogen sparing. Hindlimb lactate efflux at rest was reduced with FFA but unaffected during stimulation. Glucose uptake was unaffected by FFA at rest and during stimulation protocols, except 3 Hz. The present study does not support the classically proposed roles of citrate and acetyl-CoA in the FFA-induced downregulation of CHO utilization in electrically stimulated rat skeletal muscle.

Relation between energy metabolism, glycolysis, noradrenaline release and duration of ischemia.

We studied the effect of 12-36 min of global ischemia followed by 36 min of reperfusion in Langendorff perfused rabbit hearts (n = 26). Metabolism was determined in terms of peak and total release of purines (adenosine, inosine, hypoxanthine), lactate and noradrenaline during reperfusion; and myocardial content of nucleotides (ATP, ADP, AMP), glycogen and noradrenaline at the end of reperfusion. An inverse relationship (r = -0.79) existed between duration of ischemia and developed pressure post-ischemia. Early during reperfusion, after 12 min of ischemia, the purine concentration (peak release) increased 100x (p < 0.01), that of lactate and noradrenaline 10x (p < 0.05). Total purine release rose with progression of the ischemic period (30x after 36 min of ischemia; p < 0.01), concomitant with a reduction in nucleotide content. Lactate release was independent from the duration of ischemia, although glycogen had declined by 30% (p < 0.01) after 36 min of ischemia. The acid insoluble glycogen fraction, which presumably contains proglycogen, increased substantially during short-term ischemia. Peak noradrenaline increased 100x, and 200x, (p < 0.05) after 24 and 36 min of ischemia, respectively. Total noradrenaline release due to various periods of ischemia mirrored its peak release. Function recovery was inversely related to total purine and noradrenaline efflux (both r = -0.81); it correlated with tissue nucleotide content (r = 0.84). In conclusion, larger amounts of noradrenaline are released only after a substantial drop in myocardial ATP. During severe ischemia ATP consumption more than limited ATP production by anaerobic glycolysis, is a key factor affecting recovery on subsequent reperfusion. In contrast to lactate efflux, purine and noradrenaline release are useful markers of ischemic and reperfusion damage.

Lactic acid excretion by Streptococcus mutans.

Lactic acid is the major end-product of glycolysis by Streptococcus mutans under conditions of sugar excess or low environmental pH. However, the mechanism of lactic acid excretion by S. mutans is unknown. To characterize lactic acid efflux in S. mutans the transmembrane movement of radiolabelled lactate was monitored in de-energized cells. Lactate was found to equilibrate across the membrane in accordance with artificially imposed transmembrane pH gradient (Δψ). The imposition of a transmembrane electrical potential (Δψ) upon de-energized cells did not cause an accumulation of lactate within the cell. The efflux of lactate from lactate-loaded, deenergized cells created a ΔpH, but did not create a Δψ, indicating that lactate crosses the cell membrane in an electroneutral process, as lactic acid. ΔpH and Δψ were determined by the transmembrane equilibration of [14C]benzoic acid and [14C]tetraphenylphosphonium ion (TPP), respectively. The presence of a membrane carrier for lactic acid in S. mutans was suggested by counterflow. Enzymic determination of the intra- and extracellular lactate concentrations of S. mutans cells glycolysing at pHo 6.8 and 5.5 showed that lactate distributed across the cell membrane in accordance with the equation ΔpH = log[lact]i/[lact]o. The addition of high extracellular concentrations of lactate to glycolysing S. mutans at acidic pH resulted in a fall in ΔpH and a subsequent decrease in glycolysis. The fall in ΔpH was attributed to the F1F0 ATPase being unable to raise the pHi back to its initial level due to the build up of lactate anion within the cell creating a large Δψ. The increase in Δψ resulted in the overall proton motive force remaining constant at about -110 mV. The results demonstrate that lactate is transported across the cell membrane of S. mutans as lactic acid in an electroneutral process that is independent of metabolic energy and as such has important bioenergetic implications for the cell.

Cellular mechanisms of brain energy metabolism. Relevance to functional brain imaging and to neurodegenerative disorders.

Astrocyte end-feet surround intraparenchymal microvessels and represent therefore the first cellular barrier for glucose entering the brain. As such, they are a likely site of prevalent glucose uptake. Astrocytic processes are also wrapped around synaptic contacts, implying that they are ideally positioned to sense and be functionally coupled to increased synaptic activity. We have recently demonstrated that glutamate, the main excitatory neurotransmitter, stimulates in a concentration-dependent manner 2-DG uptake and phosphorylation by astrocytes in primary culture. The effect is not receptor-mediated but rather proceeds via one of the recently cloned glutamate transporter. The mechanism involves an activation of the Na+/K+ ATPase. Concomitant to the stimulation of glucose uptake, glutamate causes a concentration-dependent increase in lactate efflux. These observations suggest that glutamate uptake is coupled to aerobic glycolysis in astrocytes. In addition, since glutamate release occurs following the modality-specific activation of a brain region, the glutamate-evoked uptake of glucose into astrocytes provides a simple mechanism to couple neuronal activity to energy metabolism. These data also suggest that modality-specific activation visualized using 2DG-based autoradiography or PET may primarily reflect glutamate-mediated uptake of 2DG into astrocytes.

Effect of blood flow on muscle lactate release studied in perfused rat hindlimb

The influence of blood flow on muscle lactate and H+ release as well as muscle glyconeogenesis was studied in the perfused rat hindlimb. After 2 min of supramaximal stimulation the perfusate flow rate was 7 (F7), 12 (F12), or 18 (F18) ml/min for 30 min. Perfusate samples were drawn frequently and muscle samples were obtained before stimulation, immediately after stimulation, and at 3, 10, and 30 min of recovery from soleus, white gastrocnemius (WG) and red gastrocnemius. During the first 5 min of recovery lactate release was 35-39% lower (P < 0.05) in F7 than in F12 and F18 but with no differences in total release during recovery. In F7 the concentration of lactate was higher (P < 0.05) in soleus after 10 min (18-20%) and in WG after 30 min (63-67%) than in F12 and F18. During the first 2 min of recovery H+ release was 23-34% lower (P < 0.05) in F7 than in F12 and F18. The difference between H+ and lactate release was larger (P < 0.05) in F7 than in F12 and F18 from 3 to 10 min and from 5 to 10 min of recovery, respectively. Muscle glycogen concentrations after 30 min of recovery were independent of flow in each of the muscles. The present data suggest that 1) in the range of blood flow rates from 0.61 to 0.92 ml.min-1.g-1, lactate and H+ release are independent of the flow rate, whereas at a lower flow rate (0.36 ml.min-1.g-1) release of these substances is decreased; 2) low blood flow influences lactate efflux more than H+ release; and 3) muscle glyconeogenesis from lactate is of minor importance.

Effects of hypertonic saline on myocardial function and metabolism in nonischemic and ischemic isolated working rat hearts

To study the direct effects of hypertonic saline on the function of non-ischemic and ischemic myocardium by the use of an isolated working rat heart model. A prospective, randomized, controlled study. Animal laboratory at a university medical center. Adult Wistar rats (n = 32) of both sexes. The heart was excised via thoracotomy in anesthetized rats and prepared for antegrade perfusion at a constant heart rate in an antegrade perfusion apparatus at predetermined preloads and afterloads. Hearts were exposed to ischemia, ischemia followed by repeated hypertonic saline treatment, or repeated hypertonic saline without preceding ischemia. Myocardial ischemia was induced by decreasing the mean aortic pressure to 25 mm Hg. Variables that were measured or calculated included the following: left atrial pressure; mean aortic pressure; heart rate; coronary flow; aortic flow; cardiac output; stroke volume; PO2, electrolyte content, osmolality, and lactate concentration of the perfusate and/or venous effluent; myocardial oxygen extraction; myocardial oxygen consumption; and myocardial lactate efflux. Ischemia resulted in pronounced impairment of myocardial function and metabolism. Hypertonic saline administration during ischemia induced an additional transient myocardial depression. In the nonischemic heart, a transient myocardial-depressant effect after hypertonic saline administration was also seen. The present results from an isolated working heart preparation show that hypertonic saline exerts myocardial-depressive effects in the ischemic as well as in the nonischemic heart. Systemic rather than direct myocardial effects may therefore be responsible for the previously reported beneficial hemodynamic effects of hypertonic saline in shock treatment.

Hindleg muscle energy and substrate balances in cold-exposed rats.

Rats chronically cannulated in the carotid artery and the muscular branch of the femoral vein were subjected to a cold (4 degrees C) environment for up to 2 h. The changes in blood flow (measured with 46Sc microspheres) and arterio-venous differences in the concentrations of glucose, lactate, triacylglycerols and amino acids allowed the estimation of substrate (and energy) balances across the hindleg. Mean glucose uptake was 0.28 mumol min-1, mean lactate release was 0.33 mumol min-1 and the free fatty acid basal release of 0.31 mumol min-1 was practically zero upon exposure to the cold; the initial uptake of triacylglycerols gave place to a massive release following exposure. The measurement of PO2, PCO2 and pH also allowed the estimation of oxygen, CO2 and bicarbonate balances and respiratory quotient changes across the hindleg. The contribution of amino acids to the energy balance of the hindleg was assumed to be low. These data were used to determine the sources of energy used to maintain muscle shivering with time. Three distinct phases were observed in hindleg substrate utilization. (1) The onset of shivering, with the use of glucose/glycogen and an increase in lactate efflux. Lipid oxidation was practically zero (respiratory quotient near 1), but the uptake of triacylglycerols from the blood remained unchanged. (2) A substrate-energy shift, with drastically decreased use of glucose/glycogen, and of lactate efflux; utilization of triacylglycerol as practically the sole source of energy (respiratory quotient approximately 0.7); decreasing uptake of triacylglycerol and increased tissue lipid mobilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoconstrictors alter oxygen, lactate, and glycerol metabolism in the perfused hindlimb of a rat kangaroo.

The Tasmanian bettong (Bettongia gaimardi) is a small marsupial rat kangaroo without detectable brown adipose tissue (BAT). The hindlimb was perfused with constant flow at 25 degrees C after cannulation under anesthesia of the femoral artery and vein to one hindlimb. Norepinephrine (NE, 25 nM-2.5 microM) and vasopressin (VP, 10 nM-0.1 microM) each increased perfusion pressure, oxygen consumption (VO2), and lactate and glycerol efflux of the perfused hindlimb. NE-mediated increases in VO2 and the efflux of lactate and glycerol were unaffected by propranolol (10 microM) but were completely blocked by the further addition of phentolamine (10 microM). In contrast, serotonin (5-HT; 0.1-2.5 microM) inhibited VO2 and inhibited lactate efflux. The changes induced by NE, VP, and 5-HT were all rapidly reversed by nitroprusside. These results suggest that resting thermogenesis in bettong hindlimb can be differentially controlled by the vasculature, which may also contribute to the induced VO2. This vascular control of skeletal muscle VO2 appears widespread in homeotherm evolution.

Persistent Resetting of the Cerebral Oxygen/Glucose Uptake Ratio by Brain Activation: Evidence Obtained with the Kety—Schmidt Technique

Global cerebral blood flow (CBF), global cerebral metabolic rates for oxygen (CMRO2), and for glucose (CMRglc), and lactate efflux were measured during rest and during cerebral activation induced by the Wisconsin card sorting test. Measurements were performed in healthy volunteers using the Kety-Schmidt technique. Global CMRO2 was unchanged during cerebral activation, whereas global CBF and global CMRglc both increased by 12%, reducing the molar ratio of oxygen to glucose consumption from 6.0 during baseline conditions to 5.4 during activation. Data obtained in the period following cerebral activation showed that the activation-induced resetting of the relation between CMRglc and CMRO2 persisted virtually unaltered for > or = 40 min after the mental activation task was terminated. The activation-induced increase in cerebral lactate efflux measured over the same time period accounted for only a small fraction of the activation-induced excess glucose uptake. These data confirm earlier reports that brain activation can induce resetting of the cerebral oxygen/glucose consumption ratio, and indicate that the resetting persists for a long period after cerebral activation has been terminated and physiologic stress indicators returned to baseline values. Activation-induced resetting of the cerebral oxygen/glucose uptake ratio is not necessarily accounted for by increased lactate production from nonoxidative glucose metabolism.

Skeletal muscle perfusion in electrically induced dynamic exercise in humans

Leg blood flow, blood pressure and metabolic responses were evaluated in six men during incremental one-legged dynamic knee extension exercise tests (no load exercise-40 W); one performed with voluntary contractions (VOL) and one with electrically induced contractions (EMS). Pulmonary oxygen uptake was the same in both exercise modes, but the ventilatory coefficient was 2-5 L per L O2 higher in EMS than VOL (P < 0.05). Heart rate and mean arterial pressure were slightly higher with EMS than VOL at all exercise intensities reaching 138 (EMS) and 126 bpm (VOL), as well as 148 (EMS) and 137 mmHg (VOL) at 40 W, respectively (P < 0.05). Leg blood flow, oxygen uptake and conductance were similar in the two exercise modes. At 40 W, mean muscle blood flow was close to 200 (range: 165-220) mL 100 g-1 min-1, mean peak muscle oxygen uptake reached 230 mL kg-1 min-1, and mean conductance became as high as around 45 mL min-1 mmHg-1, and normalized for muscle size and arterial pressure it approached 100 mL min-1 100 g-1 100 mmHg-1. Lactate and ammonia efflux from the leg were higher with EMS than with VOL and the difference became larger with increasing exercise intensity (P < 0.05). Muscle glucose uptake was the same in each exercise mode. Femoral venous K+ concentration increased with exercise intensity and was higher with EMS than with VOL, reaching 5.1 (EMS) and 4.7 mmol L-1 (VOL) at 40 W (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of intracellular pH regulation during postischemic reperfusion of diabetic rat hearts.

A marked decrease in the activity of the amiloride-sensitive Na+/H+ exchanger has been demonstrated in hearts from streptozotocin (STZ)-induced diabetic rats. The aim of this study was to investigate the contribution of other specific sarcolemmal transport mechanisms to intracellular pH (pHi) recovery upon reperfusion in STZ-induced diabetic rat hearts and their relation to recovery of ventricular function. Isovolumic rat hearts were submitted to a zero-flow ischemic period of 28 min at 37 degrees C and then reperfused for 28 min. The time course of pHi decline during ischemia and of recovery on reperfusion was followed by means of 31P-labeled NMR. The perfusion buffers used were either HEPES or CO2/HCO3-. An HCO3(-)-dependent (amiloride-insensitive) mechanism contributed to pHi recovery after ischemia in the diabetic rat hearts. Even when the Na+/H+ exchanger was blocked by amiloride in nominally HCO3(-)-free solution, a rapid rise in pHi occurred during the first 3 min of reperfusion. The early rise in pHi was reduced by external lactate and inhibited by alpha-cyano-4-hydroxycinnamate. This suggested that a coupled H(+)-lactate efflux may be a major mechanism for acid extrusion in the initial stage of reperfusion. The observation of a higher functional recovery on reperfusion in diabetic hearts is in accordance with previous studies using HCO3- buffer. However, this study shows that a good recovery of function occurred even more rapidly in diabetic hearts receiving HEPES-buffered solution than in those receiving HCO3(-)-buffered solution. This suggests that the HCO3(-)-dependent mechanism of regulation may be depressed in diabetic rat hearts.

Lactate efflux from fatigued fast-twitch muscle fibres of Xenopus laevis under various extracellular conditions.

1. Isolated, fast-twitch, low-oxidative muscle fibres from the iliofibularis muscle of Xenopus laevis were fatigued by intermittent tetanic stimulation at 20 degrees C in different Ringer solutions and the amount of lactate released was determined. 2. The rate of lactate efflux was constant during 10 min of intermittent stimulation while lactate in the fibres accumulated, and lactate efflux was not hampered by an unstirred layer surrounding the isolated muscle fibre. 3. The rate of lactate efflux at extracellular pH 7.2 was the same as that at pH 7.8, but depended on the type of buffer used; the highest efflux rate (mean +/- S.E.M., 7.4 +/- 2.2 mumol min-1 (g dry weight)-1, n = 8) was observed in bicarbonate-buffered Ringer solution. This rate was about 2.5 times higher than the rate in phosphate-buffered Ringer solution (2.9 +/- 1.3 mumol min-1 (g dry weight)-1, n = 8), indicating that lactate-bicarbonate exchange is the most important route for lactate extrusion in vivo. 4. The highest rate of lactate efflux corresponds to a rate of glycolytic ATP production which is only about 30% of the oxidative rate of ATP production (calculated from the maximum rate of oxygen consumption determined previously). 5. In the presence of 5 mM alpha-cyano-4-hydroxycinnamate (CHC) the lowest lactate efflux rate (1.5 +/- 0.6 mumol min-1 (g dry weight)-1, n = 16) was found. This rate was independent of the composition of the Ringer solution. Assuming that 5 mM CHC completely inhibits lactate transporters in the sarcolemma, the rate of lactate efflux in the presence of 5 mM CHC can be explained by passive diffusion, but only if most lactate is extruded via the T-tubules.

Control of intracellular pH during regulatory volume decrease in HL-60 cells.

Intracellular pH (pHi) homeostasis was investigated in human promyelocytic leukemic HL-60 cells as they undergo regulatory volume decrease (RVD) in hypotonic media to determine how well pHi is regulated and which transport systems are involved. Cells suspended in hypotonic (50-60% of isotonic) media undergo a small (< 0.2 pH units), but significant (P < 0.05), intracellular acidification within 5 min. However, after 30 min of RVD, pHi is not significantly different from the initial pHi in 20 mM HCO3- medium and is significantly higher in HCO3(-)-free medium. Experiments performed in media with or without 150 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and HCO3- demonstrate that the anion exchanger (AE) mediates a net Cl- influx, with compensating HCO3- efflux, during RVD. To determine which transport systems are involved in counteracting this tendency toward acidification, we measured transport rates and examined the effect of transport system inhibitors on pHi. We found that inhibition of Na+/H+ exchange (NHE) with 12.5 microM ethylisoproplamiloride (EIPA) causes pHi to fall significantly by the end of 30 min of RVD. As assessed by EIPA-sensitive 22Na+ uptake measurements, NHE, largely dormant under resting isotonic conditions, becomes significantly activated by the end of 30 min of RVD, despite recovery of pHi and cell volume to near-normal levels. Thus a shift in the normal pHi dependence and/or volume dependence of NHE activity must occur during RVD under hypotonic conditions. In contrast, H(+)-monocarboxylate cotransport appears to play only a supportive role in pH regulation during RVD, as indicated by lack of stimulation of [14C]lactate efflux during RVD.

Muscle lactate metabolism in recovery from intense exhaustive exercise: impact of light exercise

This study examined the effect of low-intensity exercise on lactate metabolism during the first 10 min of recovery from high-intensity exercise. Subjects exercised (61.0 +/- 5.4 W) one leg to exhaustion (approximately 3.5 min), and after 1 h of rest they performed the same exhaustive exercise with the other leg. For one leg the intense exercise was followed by rest [passive (P) leg], and for the other leg the exercise was followed by a 10-min period with low-intensity exercise at a work rate of 10 W [active (A) leg]. The muscle lactate concentration after the intense exercise was the same in the P and A legs, but after 10 min of recovery, the lactate concentration and the arterial blood lactate level were higher for the P leg than for the A leg (both P < 0.05). During the recovery, the mean blood flow was lower for the P leg than for the A leg (P < 0.05), whereas the mean lactate efflux was not significantly different. During the 10 min of recovery, lactate release accounted for approximately 60% of the change in muscle lactate for either leg. The leg excess postexercise O2 consumption during 10 min of recovery was 440 and 750 ml for the P and A legs, respectively. The present data suggest that a lowered blood lactate level during active recovery is due to an elevated muscle lactate metabolism and is not caused by a transient higher release of lactate from the exercising muscles coupled with greater uptake in other tissues.(ABSTRACT TRUNCATED AT 250 WORDS)