Lactate Efflux Research Articles

Trans-Stimulation of Lactate Transport from Rat Sarcolemmal Membrane Vesicles

We examined the trans-stimulation characteristics of L(+)-lactate efflux from rat skeletal muscle sarcolemmal membrane vesicles isolated by sucrose density gradient centrifugation. Vesicles were preloaded with 1 mML(+)-lactate containing 14C tracer lactate. Initial efflux of intravesicular lactate was stimulated above baseline efflux by the presence of 30, 50, or 100 mM L(+)-lactate in the external medium; resulting efflux rates were 120, 138, and 141% of baseline values, respectively. Extravesicular pyruvate (100 mM) stimulated efflux to only 110% of baseline, while unlabeled D(−)-lactate in the external medium did not stimulate lactate efflux. Trans-stimulation of L(+)-lactate efflux in response to extravesicular lactate exhibited saturation kinetics. We conclude that the sarcolemmal lactate transporter is stereospecific for L(+)-lactate; pyruvate can share the transporter, although the affinity of the transporter for pyruvate is less than that for L(+)-lactate. Furthermore, because the presence of lactate on one side of the sarcolemma stimulates transport from the opposite side, the transporter is not likely to be a pore or channel, but rather this carrier probably undergoes a conformational change as it translocates lactate from one membrane side to the other.

Potassium loss during myocardial ischaemia and hypoxia: does lactate efflux play a role?

In heart, the available evidence suggests that transmembrane lactate flux is mediated predominantly by an H(+)-lactate transporter with properties similar, but not identical, to the H(+)-monocarboxylate transporter present in many other tissues. Passive (electro-)diffusion of HL and L- comprise only minor components of total transmembrane lactate flux over the range of lactate concentrations relevant to normal physiological and pathophysiological states. The cardiac H(+)-lactate transporter is non-electrogenic, and transport is partially inhibited by potassium, possibly by competition for the H+ binding site on the carrier. However, K+ is cotransported with lactate very inefficiently, if at all, compared to H+. From these observations, a direct mechanism coupling potassium efflux to lactate efflux, by either an electrogenic or a non-electrogenic mechanism, is unlikely to account for the majority of net potassium loss during myocardial ischaemia or hypoxia, unless the properties of transmembrane lactate flux are markedly altered by the ischaemic and hypoxic environment. Nevertheless, it is intriguing that alterations in pHi, pHo, and transmembrane pH gradients in ischaemic cardiac muscle and fatigued skeletal muscle have effects on net potassium loss that qualitatively parallel the predicted effects on L- efflux. In view of the lack of evidence for a direct link between potassium and lactate efflux in the heart, it is likely that this apparent relationship is either coincidental or indirect, mediated through a series of intermediate transport processes. The nature of these interactions remain to be defined. Further studies are still needed to solve the puzzle of what causes net cellular potassium loss during myocardial ischaemia and hypoxia.

Lactate efflux and intracellular pH during severe hypoxia in rat cerebral cortex in vitro studied by nuclear magnetic resonance spectroscopy

Intracellular pH (pH i) and lactate were monitored in a superfused brain slice preparation using NMR spectrocopy in order to study the role of lactate washout in maintenance of pH i during hypoxia. Data are consistence with a functioning lactate-H + cotransport in the energetically intact cerebral cortex. This pathway is not, however, linked to regulation of pH i during energy failure with external pH of 6.8 and thus appears not to have physiological impact in H + homeostasis during cerebral hypoxia.

High physiological levels of epinephrine do not enhance muscle glycogenolysis during tetanic stimulation

This study examined whether high physiological concentrations of epinephrine (EPI) would enhance muscle glycogenolysis during intense muscular contractions. Muscles of the rat hindlimb were perfused for 12 min at rest and 45 s of tetanic stimulation (1.0-Hz train rate, 100-ms train duration at 80 Hz) without EPI (control) or with 15 or 35 nM EPI. In the EPI groups the muscles were perfused with EPI for the last 2 min of rest perfusion and throughout stimulation. Glycogenolysis in the white gastrocnemius, red gastrocnemius, plantaris, and soleus muscles during stimulation was unaffected by the presence of EPI in the perfusion medium. In addition, muscle lactate and hindlimb lactate efflux were similar in EPI and control groups. It is concluded that EPI is not important for enhancing glycogenolysis in rat muscles composed predominantly of fast-twitch fibers during intense short-term tetanic stimulation.

Facilitated Transport of Lactate by Rabbit Corneal Endothelium

Proton coupled lactate transport across the rabbit corneal endothelium was studied using a pH-sensitive intracellular fluorescent probe. Functional indications that lactate transport is carrier-mediated and coupled to H + at the apical endothelium (aqueous humor facing) that were found were: (1) proton influx was a saturable function of factate concentration; (2) L-lactate produced a faster maximal H + influx and had a higher affinity for the transporter ( V max = 1·6 mM min -1, apparent K1/2 = 31 mM) than its optical isomer, d -Lactate ( V max = 0·9 mM min -1, apparent K 1/2 = 59 mM); (3) the lactate-induced acidification was inhibitable by apical mersalyl acid. These results are consistent with the presence of an apical lac -:H + cotransporter. On basolateral addition of lactate to a de-epithelialized cornea, the endothelial cells transiently acidified by 0·05 units, but then alkalinized by 0·07, 0·02 units over baseline. The basolateral lactate-induced acidification was inhibited by 75% by mersalyl acid, while the net alkalinization observed under control conditions was unaffected. The alkalinization was Na +-dependent yet amiloride-insensitive. Alkalinization on addition of basolateral lactate could also be inhibited by apical mersalyl. These results suggest that the acidification represents lactate influx through basolateral Lac -:H + cotransport while the net alkalinization is due to basolateral Na +-dependent lactate uptake and lactate efflux through apical Lac -:H + cotransport. These facilitated transport mechanisms could provide for the efficient removal of lactate from the highly glycolytic cornea.

Improved Cytoplasmic pH Regulation, Increased Lactate Efflux, and Reduced Cytoplasmic Lactate Levels Are Biochemical Traits Expressed in Root Tips of Whole Maize Seedlings Acclimated to a Low-Oxygen Environment.

We tested the hypothesis (J.-H. Xia and P.H. Saglio [1992] Plant Physiol 100: 40-46) that the enhanced ability of maize (Zea mays) root tips to survive anoxia, elicited by a 4-h exposure to 3% O2 ("acclimation"), is due to less cytoplasmic acidosis early in anoxia. Cytoplasmic pH and fermentation reactions were monitored in excised and intact (attached) maize root tips by simultaneous in vivo 13C- and 31P-NMR spectroscopy. We demonstrate that both excised and intact acclimated root tips have significantly higher cytoplasmic pH values under anoxia. This reduction in cytoplasmic acidosis is greater in intact root tips. Remarkably, cytoplasmic pH does not change when root tips are transferred from 3% O2 to anoxia. The earlier observation of considerable lactate efflux and lowered intracellular lactate in excised, acclimated root tips (ibid.) was extended to intact seedlings. The predominant fermentation end product retained in the cells of acclimated root tips is alanine. We discuss the relationship between cytoplasmic pH and levels of intracellular lactate and alanine in sugar-replete roots, and the role of cytoplasmic pH in determining survival under anoxia.

Evidence for a lactate-anion exchanger in the rat jejunal basolateral membrane

Background/Aims: The mechanism by which lactate, absorbed from the intestinal lumen or generated within the epithelium, crosses the basolateral membrane of the enterocyte and enters the bloodstream has not previously been characterized in detail. Methods:l-lactate uptake into and efflux from isolated jejunal basolateral membrane vesicles was investigated at room temperature using rapid filtration techniques. Results: Furosemide sensitive uptake of l-lactate was unaffected by cis sodium or proton gradients but could be stimulated by a trans gradient of bicarbonate and chloride. Kinetic analysis showed uptake to consist of a saturable component with a Michaelis constant (Km) of 3.2 mmol/L and a maximum velocity (Vmax) of 67 pmol·mg protein−1 · s−1 and a nonsaturable α-4-hydroxy-cinnamic acid insensitive component. Pyruvate, butyrate, acetate, valerate, and proprionate competitively inhibited lactate uptake into the vesicles. Efflux of lactate from preloaded vesicles was furosemide sensitive and accelerated by a trans bicarbonate gradient as well as by 10 mmol/L acetate, butyrate, and pyruvate. Conclusions: It is concluded that there is a short chain-fatty acid carrier system in the intestinal basolateral membrane, which operates as an anion exchanger.

Cerebral metabolism during sustained hypoxemia in preterm fetal sheep

OBJECTIVE: The purpose of this study was to determine the effect of sustained hypoxia with resulting metabolic acidosis on cerebral metabolism in the preterm ovine fetus. STUDY DESIGN: Twelve fetal sheep were studied at 0.75 of gestation during a normoxic control period, after 1 and 8 hours of sustained hypoxemia, and again after a 1-hour recovery period. Cerebral arteriovenous differences were analyzed for oxygen content, blood gases and pH, glucose, and lactate. Cerebral blood flow was measured with the microsphere technique. RESULTS: Induced hypoxemia resulted in a variable degree of fetal acidemia that was entirely metabolic. Although cerebral oxidative metabolism was well maintained throughout the study, cerebral glucose consumption was variably increased when measured after 1 hour of sustained hypoxemia, with a subsequent decrease after 1 hour of recovery. Although lactate was neither consumed nor produced during the control period, by 8 hours of hypoxic study a significant efflux of lactate from the brain was evident, which continued into the recovery period. CONCLUSION: Sustained hypoxemia results in an increase in the anaerobic metabolism of glucose by the preterm fetal brain independent of any change in cerebral oxidative metabolism, which may give rise to an accumulation of lactic acid and contribute to neurologic impairment. (AM J OBSTET GYNECOL 1994;170;939-44.)

Brain metabolism during short-term starvation in humans.

During prolonged starvation, brain energy requirements are covered in part by the metabolism of ketone bodies. It is unknown whether short-term starvation of a few days' duration may lead to reduced brain glucose metabolism due to the change toward ketone body consumption. In the present study we measured the cerebral metabolism of glucose and ketone bodies in nine healthy volunteers before and after 3.5 days of starvation. Regional glucose metabolism was measured by dynamic positron emission tomography using [18F]2-fluoro-2-deoxy-D-glucose. The mean value of K1* in gray and white matter increased by 12% (p < 0.05), whereas k2* and k3* were unchanged compared with control values. Regional glucose metabolism in cortical gray matter was reduced by 26% from 0.294 +/- 0.054 to 0.217 +/- 0.040 mumol g-1 min-1 (p < 0.001). White matter glucose metabolism decreased by 27% (p < 0.02). The decrease was uniform in gray and white matter with regional decreases ranging from 24 to 30%. A determination using Fick's principle confirmed the reduction in glucose metabolism yielding a decrease of 24% from 0.307 +/- 0.050 to 0.233 +/- 0.073 mumol g-1 min-1 (p < 0.05), whereas CBF did not change (0.57 +/- 0.07 vs. 0.57 +/- 0.06 ml g-1 min-1). The global net uptake of beta-hydroxybutyrate increased 13-fold from 0.012 +/- 0.024 to 0.155 +/- 0.140 mumol g-1 min-1 (p < 0.05). Net uptake of acetoacetate and net efflux of lactate and pyruvate did not change significantly during starvation. The present study shows that the human brain adapts to the changes in energy supply as early as 3 days following initiation of starvation, at which time ketone bodies account for approximately one-fourth of the cerebral energy requirements.

Dynamics of myocardial lactate efflux after a step in heart rate in isolated rabbit hearts

We investigated whether a glycolytic burst contributes to the initial adaptation of ATP synthesis to increased cardiac metabolic demand. Six isolated rabbit hearts were perfused with glucose-containing Tyrode solution at 28 degrees C. In venous and arterial samples the lactate concentration was determined with a sensitive enzymatic cycling method. After the heart rate was doubled from 60 to 120 beats/min, lactate efflux increased from 0.23 +/- 0.10 (SE) to 0.45 +/- 0.12 mumol.min-1.g-1 dry weight with a mean response time of 21.3 s but without an overshoot. The transport time for lactate is longer than 15.7 s, suggesting that lactate production adapts with a mean response time of less than 6 s. Because no overshoot in lactate efflux was found, it is unlikely that a glycolytic burst after a step in heart rate contributes to the fast adaptation of ATP synthesis to demand in the isolated rabbit heart, although it might be possible that a change in cytosolic lactate production is not reflected in an increase in lactate efflux. Extrapolation of the results of this study to the in vivo situation should be done with caution.

ATP formation and ATP hydrolysis during fatiguing, intermittent stimulation of different types of single muscle fibres from Xenopus laevis

This report describes changes of the rate of ATP hydrolysis in single, intact muscle fibres during the development of fatigue induced by intermittent tetanic stimulation. High (type 3) and low (type 1) oxidative muscle fibres dissected from the iliofibularis muscle of Xenopus laevis were studied at 20 degrees C. The rate of ATP hydrolysis was calculated during different time intervals from changes in the content of nucleotides, creatine compounds and lactate, as well as lactate efflux and oxygen uptake. During the first phase of intermittent stimulation, phosphocreatine is fully reduced while the rate of oxygen consumption increases to its maximum, the lactate content increases to a maximum level, and a small amount of IMP is formed; the rate of ATP hydrolysis in type 3 fibres is constant while force decreases, whereas the rate decreases approximately in proportion to force in type 1 fibres. After the first phase, the rate of ATP hydrolysis in type 3 fibres decreases slightly and the fibres reach a steady metabolic state in which the rates of ATP formation and hydrolysis are equal; in type 1 fibres a drastic change of the rate of ATP hydrolysis occurs and a steady metabolic state is not reached. On the basis of the time courses of the metabolic changes, it is concluded that the rate of ATP hydrolysis in type 3 fibres is reduced by acidification and/or a reduced calcium efflux from the sarcoplasmic reticulum, whereas in type 1 fibres inorganic phosphate and/or acidification inhibit the rate initially and ADP is a likely candidate to explain the drastic fall of the rate of ATP hydrolysis during late phases of fatiguing stimulation.

Occurrence of intracellular pH transients during spontaneous contractions in rat uterine smooth muscle.

1. Intracellular pH (pHi) and contraction have been measured simultaneously in isolated longitudinal rat myometrium. Tissue was loaded with the pH-sensitive fluorophore carboxy-SNARF and superfused with oxygenated, buffered salt solution at 37 degrees C and pH 7.4. 2. Small (0.04 pH unit) transient intracellular acidifications followed each spontaneous uterine contraction. The pHi transients lagged contraction by 15 s. Removal of external Ca2+ abolished both spontaneous contractions and the pHi changes. 3. Increasing uterine contraction by depolarization (40 mM K+) or agonist (50 microM carbachol) produced a larger acidification (0.18 pH unit). 4. The acidification with contraction does not appear to be due to increased glycolysis as no increase in lactate efflux occurred with high-K+ stimulation. 5. Uterine contraction is known to be sensitive to changes in pHi. Comparable pHi changes to the pHi transients seen during spontaneous contraction were induced by application and withdrawal of weak acids and bases. The resulting pHi changes (0.02-0.08 pH unit) produced effects on contraction frequency and amplitude, alkalinization augmenting and acidification depressing contractile activity, suggesting that the pHi transients with contraction are functionally important.

Short-term biventricular circulatory support in the regionally stunned pig myocardium.

Mechanical circulatory support is only meaningful when reversible myocardial damage exists. Prompt biventricular assistance during reperfusion for six hours was tried in the regionally sublethally injured pig myocardium which has perspective for recovery. Postmortem tissue was examined with a quickly performed histochemical technique for tissue viability. Animals underwent five coronary occlusions (5 min occlusion/10 min reperfusion). Postischemic systolic wall thickening, observed by direct echocardiography, fell about 50% (p < 0.05 vs base line) before treatment which paralleled cardiac inosine and lactate efflux. Biventricular unloading (n = 5) resulted in contractile recovery to 94 +/- 19% (p < 0.05 vs stunning), untreated pigs remained depressed (33 +/- 12%, n = 5). During circulatory support the arterial lactate levels rose three-fold, concomitant with a drop in pH of 0.2 units (p < 0.05). Post-mortem histochemistry showed scattered areas with decreased activities of cytoplasmic lactate dehydrogenase and mitochondrial beta-hydroxybutyrate dehydrogenase, irrespective of treatment. Transmission electron microscopy revealed irreversible damage to mitochondria in depleted areas in contrast to areas where enzyme activity was present. We conclude that prompt volume unloading during reperfusion accelerated contractile recovery of sublethally postischemic myocardium.

ATP‐Sensitive Potassium Channels and Local Energy Demands in the Rat Hippocampus: An In Vivo Study

Microdialysis coupled with an enzyme-based flow injection analysis was used to monitor brain extracellular lactate and glucose in the freely moving rat. Glucose levels reflect the balance between supply from the blood and local utilisation, and lactate efflux indicates the degree of local nonoxidative glucose metabolism. Local application of tolbutamide, a blocker of the ATP-sensitive potassium channel, decreased extracellular glucose and lactate levels in the hippocampus but not in the striatum. The increase in glucose and lactate levels following mild behavioural stimulation was also reduced by tolbutamide in the hippocampus. Similar effects on both basal and stimulated lactate levels were obtained with local application of 10 mM glucose. These results indicate that ATP-sensitive potassium channels are active under physiological conditions in the hippocampus and that the effects of tolbutamide can be mimicked by physiological glucose levels.

Mitochondrial pyruvate transport in working guinea-pig heart. Work-related vs. carrier-mediated control of pyruvate oxidation

Myocardial pyruvate oxidation is work- or calcium-load-related, but control of pyruvate dehydrogenase (PDH) by the specific mitochondrial pyruvate transporter has also been proposed. To test the transport hypothesis distribution of pyruvate across the cell membrane as well as rates of mitochondrial pyruvate net transport plus oxidation were examined in isolated perfused but stable and physiologically working guinea-pig hearts. 150 μM-1.2 mM α-cyanohydroxycinnamate proved to specifically block mitochondrial pyruvate uptake in these hearts. When perfusate glucose as cytosolic pyruvate precursor was supplied in combination with octanoate (0.2 or 0.5 mM) as diffusible alternative fatty acid substrate, α-cyanohydroxycinnamate produced up to 20- and 3-fold increases in pyruvate and lactate efflux, respectively. Cinnamates did not alter myocardial hemodynamics nor sarcolemmal pyruvate and lactate export. In contrast the tested concentrations of cinnamate produced reversible, dose-dependent decreases in 14CO 2 production from [1- 14C]pyruvate or [U- 14C]glucose by inhibiting mitochondrial pyruvate uptake. Linear least-squares estimates of available cinnamate-sensitive total pyruvate transport potential yielded rates close to 110 μmol/min per g dry mass at S 0.5 ≈ 120 μ M , which compared reasonably well with literature values from isolated cardiac mitochondria. This transport potential was severalfold larger than total extractable myocardial PDH activity of ≈ 32 μ mol/min per g dry mass at 37° C . Even when cytosolic pyruvate levels were in the lower physiologic range of about 90 μM, pyruvate oxidation readily kept pace with mitochondrial respiration over a wide range of workload and inotropism. Furthermore, dichloroacetate, a selective activator of PDH, stimulated pyruvate oxidation without affecting myocardial O 2 consumption, regardless of the metabolic or inotropic state of the hearts. Consequently, little or no regulatory function with regard to pyruvate oxidation could be assigned to the native mitochondrial pyruvate carrier of the working heart. Therefore, mitochondrial pyruvate-H + symport was the normal, highly efficient (rather than controlling) mechanism for pyruvate entry into the mitochondria where PDH regulation controlled pyruvate oxidation.

Peptidoleukotrienes increase the efflux of glutathione from perfused rat liver

The effect of peptidoleukotrienes (LT) on the efflux of glutathione (GSH) from the perfused rat liver was investigated. LTD 4, C 4 and E 4 were infused at a final concentration of 20 nM into the portal vein of rat livers perfused with Krebs-Henseleit buffer. Perfusion pressure, efflux of glucose and release of LDH increased during the infusion of LTC 4 and D 4 and returned to baseline upon cessation of the infusion of LT. In contrast, the efflux of GSH did not change during the infusion of LT, but increased from 15 ± 2 to 26 ± 4 nmol/min.g liver 20 min after cessation of the infusion of LTC 4. LTE 4 did not increase the efflux of LDH, glucose, lactate, or GSH. During the LTC 4- and LTD 4-induced rise in perfusion pressure bile-flow decreased transiently by one third. The biliary excretion of GSH, however, decreased by an average of 75% and recovered more slowly than the cholestasis. In the presence of the selective LTD 4 receptor antagonist LY171883 the effects of LTC 4 and LTD 4 were largely abolished. The delayed effects of LT on GSH efflux suggest that LT shift the efflux of GSH from the canalicular towards the sinusoidal side of the hepatocyte independent of other effects of LT on hepatic function. The sustained increase in efflux of GSH resulting from LT will raise the extracellular concentration of this antioxidant, such that more GSH is available at sites of inflammation to detoxify reactive oxygen species released by activated inflammatory cells.

Effects of muscle contraction on cytochrome a,a3 redox state.

The relationships among mitochondrial O2 availability, O2 delivery, and lactate formation in exercising skeletal muscle remain unclear. Some data suggest that muscle O2 provision is sufficient at maximal O2 consumption (VO2max) to challenge the concept of a mitochondrial O2 limitation at VO2max. The relationships among VO2, mitochondrial O2 availability, and net lactate production were studied over a wide range of exercise intensities. Using near-infrared spectroscopy, the oxidation-reduction state of cytochrome a,a3 was monitored in the canine gracilis in vivo. Twenty adult dogs were anesthetized with alpha-chloralose, intubated, and mechanically ventilated on room air. Five-minute stimulation periods at rates of 2, 3, 4, 5, 7, 8, 10, or 12 stimuli/s were performed. VO2max generally was achieved at a stimulation rate of 8 stimuli/s; mean VO2max was 0.12 +/- 0.09 (SE) ml.min-1 x g-1. The concentration of oxidized mitochondrial cytochrome a,a3 decreased at all work loads relative to resting state and demonstrated a near-linear relationship with muscle VO2 (r2 = 0.99). Muscle lactate efflux and the lactate-pyruvate ratio also were correlated positively with cytochrome a,a3 reduction, suggesting a common regulatory mechanism coupling the processes of aerobic glycolysis and oxidative phosphorylation. At VO2max, the corresponding cytochrome oxidation was not significantly different from that observed at death. Thus, in the gracilis maximal exercise leads to near-complete reduction of cytochrome a,a3 secondary to deficient O2 provision. We conclude that VO2max is limited primarily by O2 delivery to this muscle and not by other factors limiting mitochondrial ATP production or substrate oxidation.

Dichloroacetate Inhibits Peripheral Efflux of Pyruvate and Alanine during Hormonally Simulated Catabolic Stress

The purpose of this study was to examine the relationship of peripheral metabolism of glucose, lactate, alanine, and muscle protein catabolism to pyruvate availability during stress. Peripheral catabolic stress was simulated by the infusion of epinephrine, cortisol, and glucagon into the femoral artery of 12 healthy volunteers, 6 of whom received prior treatment with dichloroacetate while 6 served as controls. The catabolic hormone infusion reproduced the peripheral stress response in which glucose consumption increased and the efflux of lactate, alanine, and total amino acid nitrogen (i.e., net muscle protein catabolism) from the leg increased. Dichloroacetate (DCA), which is known to increase pyruvate oxidation, reduced the hormonally stimulated efflux of pyruvate and alanine from the leg and decreased the rate of extremity glucose consumption. DCA had no effect on the rate of lactate efflux and except for alanine had no effect on the stimulated rate of total amino acid nitrogen loss. These results demonstrate the dependance of alanine efflux on pyruvate availability during stress and suggest that the rate of glycolysis within peripheral tissues is a major factor in regulating the quantity of alanine efflux. This study further illustrates that except for alanine, pyruvate kinetics are not salient in the regulation of muscle protein catabolism and elucidates the dichotomy between alanine kinetics and true muscle protein breakdown.

The optimal glucose concentration for intermittent cardioplegia in isolated rat heart when added to St. Thomas’ Hospital cardioplegic solution

The purpose of this study was to determine the mechanisms by which high physiologic concentrations of glucose (11 mmol/L) were protective while even higher concentrations (20 or 50 mmol/L) were harmful when added to St. Thomas' Hospital No. 2 cardioplegic solution. We evaluated the recovery of isolated working rat hearts subjected to 3 hours of hypothermic multidose cardioplegic arrest. The addition of glucose 11 mmol/L was associated with better aortic flow (79.2% +/- 1.3%) than the addition of glucose 1 mmol/L (61.7% +/- 2.7%), 20 mmol/L (73.6% +/- 1.1%), or 50 mmol/L (66.0% +/- 3.2%) (p < 0.01 versus glucose 1 and 50 mmol/L). An increase in glucose concentration from 1 to 50 mmol/L progressively augmented glucose flux from 2.2 +/- 0.33 to 10.4 +/- 0.79 mumol/gm per 3 hours (p < 0.01), but higher glucose concentrations of 20 and 50 mmol/L inhibited glycogenolysis (p < 0.05 versus glucose 1 and 11 mmol/L), so that total glycolysis was decreased and consequently glycolytic adenosine triphosphate production was reduced from 35.9 +/- 0.47 (glucose 11 mmol/L) to 27.5 +/- 1.25 mumol/gm per 3 hours (glucose 50 mmol/L) (p < 0.01). The end products of glycolysis (lactate and protons) did not appear to affect the recovery of the hearts, because both lactate efflux and tissue lactate were highest in the presence of glucose 11 mmol/L and the pH of the cardioplegic effluent was more alkalotic in glucose 11 and 20 mmol/L. Thus a high physiologic concentration of glucose (11 mmol/L) in the cardioplegic solution improved recovery because of an increased glycolytic adenosine triphosphate production during cardioplegic arrest, whereas even higher concentrations of glucose inhibited these effects.

Pharmacological studies of K + loss from ischaemic myocardium in vitro: roles of ATP-dependent K + channels and lactate-coupled efflux

Perfused guinea-pig hearts were rendered ischaemic by 95% reductions in coronary flow. K + and lactate release over the first 6 min of ischaemia were reduced by glibenclamide (described as a K ATP + channel blocker), 2-deoxyglucose (inhibitor of lactate synthesis) and α-cyano-4-hydroxycinnamic acid (inhibitor of lactate transport). Glibenclamide did not selectively reduce K + loss without affecting lactate release, as would be expected for a selective K ATP + channel blocker. During a single 30 min period of ischaemia, a secondary release of K + was observed corresponding to the onset of ventricular fibrillation, with no associated increase in lactate efflux, which appeared sensitive to glibenclamide. In conclusion, glibenclamide failed to reduce K + loss in early ischaemia without reducing lactate release as would be expected for a selective K ATP + channel blocker. Caution should be exercised when using glibenclamide as a specific blocker of K ATP + channels in the absence of measurements of metabolic parameters.