The influence of extracellular buffer concentration and propionate on lactate efflux from frog muscle

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

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

Bicarbonate attenuates intracellular acidosis

BACKGROUND

This study was prompted by concern that administration of bicarbonate for correction of lactate acidosis aggravates a low intracellular pH (pHi). In healthy subjects we evaluated skeletal muscle pHi using 31P-magnetic resonance spectroscopy during 5-minute rhythmic handgrip to provoke intracellular acidosis.

METHODS

Subjects were randomized to treatment with bicarbonate or saline infused intravenously in a cross-over study design with 1 h between trials.

RESULTS

In response to rhythmic handgrip, muscle venous O(2) hemoglobin saturation decreased from 51 +/- 4% to 36 +/- 2% and lactate increased from 1.0 +/- 0.1 to 4.9 +/- 0.5 mmol/l with a reduction in pH from 7.43 +/- 0.01-7.23 +/- 0.01 (P<0.05). pHi decreased from 7.06 +/- 0.02-6.36 +/- 0.08 (P<0.05). Infusion of bicarbonate increased the arterial blood concentration from 26 +/- 1 to 39 +/- 1 mmol/l (P<0.05). The arterial CO(2) partial pressure decreased from 5.6 +/- 0.2 to 5.2 +/- 0.3 kPa during rhythmic handgrip, whereas it increased to 5.9 +/- 0.2 kPa (P<0.05) during infusion of bicarbonate. Bicarbonate treatment also increased pH of arterial and venous blood (7.55 +/- 0.01 vs. 7.44 +/- 0.02 and 7.31 +/- 0.01 vs. 7.23 +/- 0.02, respectively; P<0.05). In the last min of rhythmic handgrip the decrease in pHi was attenuated by the administration of bicarbonate (6.60 +/- 0.11 vs. 6.40 +/- 0.12; P<0.05).

CONCLUSION

During exercise-induced metabolic acidosis, intravenous administration of bicarbonate increased the buffering capacity of blood and attenuated the decrease in intracellular muscle pH, although there was a small increase in the arterial carbon dioxide pressure.

Accuracy of 1H and 31P MRS analyses of lactate in skeletal muscle

As the end product of anaerobic metabolism and a source of H(+), lactic acid is important in metabolism and pH regulation. Several methods have been introduced to calculate changes in the lactate anion (Lac(-)) concentration in exercising skeletal muscle from information derived from the (31)P spectrum. Alternatively, Lac-may be observed directly with (1)H MRS. Both (1)H and (31)P spectroscopy have potential problems, which could prevent accurate determination of [Lac(-)]. It is demonstrated that quantitatively accurate (1)H MRS measurements of changes in [Lac(-)] due to exercise are possible in isolated muscle. In general, calculation by (31)P MRS overestimates Lac-production. An analysis is presented of possible sources of errors in the (1)H and (31)P MRS methods.

The role of intracellular acidosis in muscle fatigue

Muscle fatigue is often accompanied by an intracellular acidosis of variable size. The variability reflects the involvement of different metabolic pathways, the presence or absence of blood flow and the effectiveness of pH-regulating pathways. Intracellular acidosis affects many aspects of muscle cell function; for instance it reduces maximal Ca(2+)-activated force and Ca2+ sensitivity, slows the maximal shortening velocity and prolongs relaxation. However, acidosis is not the only metabolic change in fatigue which causes each of the above, and there are important aspects of muscle fatigue (e.g., the failure of Ca2+ release) which do not appear to be caused by acidosis.

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.

Lactate transport in mammalian ventricle. General properties and relation to K+ fluxes

Net cellular L-lactate efflux associated with accelerated anaerobic glycolysis has been implicated as a potential cause of the marked cellular K+ loss contributing to lethal cardiac arrhythmias in ischemic heart and to impaired function of fatigued skeletal muscle. To examine the mechanisms of transsarcolemmal L-lactate movement in the heart, isolated guinea pig ventricular myocytes were loaded with the fluorescent H+ or K+ indicators, carboxy SNARF-1 or PBFI, respectively, under whole-cell patch-clamp conditions. With H+ as the only permeable monovalent cation, a rapid increase in extracellular L-lactate concentration ([L-]o) from 0 to 30 mmol/L at constant pHo (7.35) caused an intracellular acidification averaging 0.18 +/- 0.02 pH units in 60 seconds (n = 7), reflecting L-lactate influx in association with H+ influx (or OH- efflux). Under voltage-clamp conditions, no significant electrogenic current was associated with H(+)-coupled L-lactate influx, and membrane potential (-75 to +75 mV) had no effect on the degree of acidification produced by 30 mmol/L [L-]o, indicating that L-lactate influx was predominantly nonelectrogenic. Acidification in response to increased [L-]o was saturable (Km, approximately 5 mmol/L), partially stereospecific for L-lactate over D-lactate, and inhibited by 55 +/- 7% and 82 +/- 7% by the monocarboxylate carrier inhibitors alpha-cyano-4-hydroxycinnamate and mersalyl acid, respectively, consistent with a carrier-mediated transport mechanism. Extracellular K+ inhibited H(+)-coupled L-lactate influx by 36 +/- 2%, suggesting that K+ either inhibited or substituted for H+ in cotransport with L-lactate. However, in myocytes loaded with PBFI, no significant increase in [K+]i was detected during exposure to 30 mmol/L [L-]o, suggesting that only a minor component, if any, of L-lactate influx was cotransported or codiffused with K+.

Lactate transport in rat sarcolemmal vesicles and intact skeletal muscle, and after muscle contraction

To determine whether it was possible to measure lactate transport rates into intact skeletal muscles, the transport of lactate (zero-trans) was determined in soleus muscle strips incubated in vitro and compared with lactate transport in sarcolemmal vesicles. In addition, the effects of muscle contractility on lactate transport were investigated in electrically-stimulated soleus muscle strips. In both the intact muscle and the sarcolemmal preparations the rates of transport were saturable, stereospecific, and inhibitable by monocarboxylates (pyruvate, alpha-cyano-4-hydroxycinnamate) and a protein modifier (N-ethylmaleimide; P < 0.05). The anion exchange inhibitor SITS had no effect on lactate uptake (P > 0.05). In both preparations lactate transport followed an inwardly directed proton gradient. Relative comparisons (%) between the preparations indicated a similar slope of increasing transport rates with increasing lactate concentrations and similar responses to a changing pH environment. These characterizations of L-lactate transport into isolated sarcolemmal vesicles and muscle strips revealed that both preparations yielded similar conclusions regarding the transmembrane movement of L-lactate. By using this more physiological muscle preparation, contractile activity, induced by electrical stimulation, did not increase lactate uptake in skeletal muscle in the post-exercise period whereas under similar conditions a marked increase in 2-deoxy-D-glucose uptake occurred (+ 47%; P < 0.05). These data suggest that the transport of glucose and lactate in contracting muscle is regulated differently. These studies also show that the incubated muscle strip preparation may be useful for studying lactate transport in an intact cell system during physiological experiments.

Muscle lactate transport studied in sarcolemmal giant vesicles

Lactate transport was studied in giant (median diameter 6.3 microns) sarcolemmal vesicles obtained by collagenase treatment of rat skeletal muscle. The lactate transport displayed stereospecificity, had a high temperature coefficient, and could be inhibited up to 90% with known transport inhibitors (PCMBS and cinnamate). In equilibrium exchange experiments, the L-lactate flux demonstrated saturation kinetics with Km = 23.7 mM and Vmax = 108 pmol cm-2 s-1. With lactate present on only one side of the membrane, (zero trans conditions), Vmax was reduced to 48 pmol cm-2 s-1. The flux rate displayed transacceleration. The lactate flux was coupled to a parallel H+ flux. Under equilibrium exchange conditions, the carrier-mediated lactate flux was not pH-dependent. In the zero trans experiments, H+ on the trans side acted as an inhibitor. The loaded form of the carrier reorients faster than the unloaded form, and the protonated form with no lactate bound reorients slowly or is immobile. When compared to intact muscles, the giant sarcolemmal vesicles retain their transport characteristics both qualitatively and quantitatively.

The influence of lactic acid on adenosine release from skeletal muscle in anaesthetized dogs

1. In anaesthetized and artificially ventilated dogs, a gracilis muscle was vascularly isolated and perfused at a constant flow rate of 11.9 +/- 2.2 ml min-1 100 g-1 (mean +/- S.E.M., n = 16; equivalent to 170.2 +/- 21.3% of its resting free flow). 2. Stimulation (3 Hz) of the obturator nerve produced twitch contractions of the gracilis muscle, reduced venous pH from 7.366 +/- 0.027 to 7.250 +/- 0.031 (n = 5), increased oxygen consumption from 0.62 +/- 0.24 to 2.76 +/- 0.46 ml min-1 100 g-1 (n = 5) and increased adenosine release from -0.40 +/- 0.14 (net uptake) to 1.36 +/- 0.50 nmol min-1 100 g-1 (n = 8). 3. Infusion of lactic acid (4.2 mM) into the artery reduced venous pH to 7.281 +/- 0.026 (n = 5) and increased adenosine release to 0.96 +/- 0.40 nmol min-1 100 g-1 (n = 8), but did not significantly alter oxygen consumption (0.80 +/- 0.19 ml min-1 100 g-1; n = 5). Stimulation (3 Hz) in the presence of lactic acid infusion produced no further significant changes in venous pH or adenosine release, but increased oxygen consumption to 2.53 +/- 0.37 ml min-1 100 g-1 (n = 5). 4. Infusion of a range of lactic acid concentrations (> or = 1.83 mM) produced dose-dependent increases in adenosine release. The maximum lactic acid concentration tested (5.95 mM) reduced venous pH to 7.249 +/- 0.023 (n = 5) and increased adenosine release to 2.64 +/- 1.26 nmol min-1 100 g-1 (n = 6). 5. A strong correlation existed between the adenosine release and the venous pH (r = -0.92); points obtained during muscle stimulation and/or lactic acid infusion fell on a single correlation line. 6. The vasoactivity of adenosine administered by close-arterial injection was unaltered by infusion of either lactic acid (7.2 mM) or saline. 7. These results suggest that the release of adenosine from skeletal muscle can be induced by a decrease in pH (probably at an intracellular site), and that this mechanism may contribute to the release of adenosine during muscle contractions.

The effect of lactate on intracellular pH and force recovery of fatigued sartorius muscles of the frog, Rana pipiens

1. The effects of pHo (extracellular pH) and lactic acid on pHi (intracellular pH) and tetanic force were examined in frog sartorius muscle. Ion-selective microelectrodes were used to measure pHi. Tetanic force was elicited by field stimulation. Experiments were performed in HEPES-buffered solution equilibrated with 100% O2. 2. Mean pHi values (+/- S.E.M.) of unfatigued frog sartorius muscles were 7.14 +/- 0.02 and 7.05 +/- 0.09 at pHo 7.2 and 6.4, respectively. 3. A stimulation at a rate of one 100 ms tetanic contraction per second for 3 min reduced pHi to 6.21 +/- 0.09 and 6.20 +/- 0.04 at pHo 7.2 and 6.4, respectively. Meanwhile at pHo 7.2, the tetanic force (defined as the maximum force developed during a tetanus) decreased by 82.9 +/- 2.6%, the maximum rate of relaxation decreased by 92.9 +/- 0.9%, and the rate constant of the relaxation decreased by 88.5 +/- 1.6%. At pHo 6.4, the decrease in tetanic force, maximum rate of relaxation and rate constant were 90.6 +/- 1.8%, 93.8 +/- 0.5 and 87.5 +/- 2.7%, respectively. 4. The maximum rates of recovery of pHi following fatigue were 0.068 +/- 0.05 and 0.025 +/- 0.05 pH units min-1 at pHo 7.2 and 6.4, respectively. Recovery of normal tetanic force and relaxation rate was also slower at acidic pHo than at neutral pHo. 5. In the presence of 40 mmol l-1 L-lactic acid at pHo 7.2, the maximum rate of pHi recovery following fatigue was only 0.027 +/- 0.03 pH units min-1 at pHo 7.2. The presence of lactic acid also reduced the recovery of the relaxation phase, but not the recovery of tetanic force. 6. It is suggested that pHi recovery is not a limiting factor for tetanic force recovery and that the extracellular H+ inhibits tetanic force recovery by acting at a site located on the outer surface of the sarcolemma. The recovery of the relaxation phase is believed to be pHi dependent.

Lactate transport in isolated mouse muscles studied with a tracer technique--kinetics, stereospecificity, pH dependency and maximal capacity

Lactate transport across the sarcolemma of isolated mouse muscles was studied with a 14C tracer technique. The cellular tracer uptake could be inhibited by unlabelled L-lactate (and pyruvate) and to a lesser extent by D-lactase. The stereospecific fraction had a Km of 3.5 mM, and made up 50% of the total transport. The tracer uptake was unaffected by 0.05 mM DIDS and 0.2 mM amiloride, but was inhibited by cinnamate (Ki = 8 mM) and PCMBS (Ki = 0.8 mM). With high concentrations of the latter inhibitor compounds or with high concentrations of unlabelled L-lactate, the tracer uptake was inhibited 80%, which indicates that the main part of the transport involves facilitated diffusion. The remaining fraction (20%) was non-saturable, reduced at high pH, and could not be inhibited; it is probably mediated by diffusion of undissociated lactic acid. Lactate transport was pH-dependent, which is consistent with a lactate-H+ symport. The maximal transport capacity, as calculated from the pH changes measured with pH-sensitive micro-electrodes while the lactate gradient was 30 mM, was 11.8 mmol kg-1 min-1 (pH 6.2).

Intracellular pH of canine subendocardial Purkinje cells surviving in 1-day-old myocardial infarcts

A large reduction of intracellular potassium activity in depolarized subendocardial Purkinje fibers 24 hours after coronary artery ligation is accompanied by a much smaller increase in intracellular sodium activity. Similar intracellular ionic changes also occur during acute ischemia in ventricular muscle and are consistent with mechanisms based on intracellular acidification, which is known to occur in acutely ischemic muscle. To determine if canine subendocardial Purkinje cells 24 hours after myocardial infarction are also acidic, their intracellular pH, surface pH, and maximum diastolic potential (MDP) were measured with double-barrel pH-sensitive microelectrodes and compared with control fibers in noninfarcted hearts. In 12 mM bicarbonate Tyrode's solution (5% CO2-95% O2), the average intracellular pH was not significantly different (p greater than 0.25) for normal tissue (6.83 +/- 0.08, SD, MDP = -83.5 +/- 3.2 mV), for depolarized Purkinje fibers in infarct preparations during the first hour of superfusion (6.88 +/- 0.11, MDP = -47.8 +/- 11.8 mV), and for partially recovered Purkinje fibers in infarcts averaged over the third to sixth hours of superfusion (6.85 +/- 0.12, MDP = -74.5 +/- 9.6 mV). In 24 mM bicarbonate Tyrode's solution, infarct intracellular pH during both the first hour of superfusion (7.08 +/- 0.13, MDP = -57.6 +/- 15.7 mV) and during the third to sixth hours of superfusion (7.06 +/- 0.15, MDP = -76.5 +/- 9.6 mV) was significantly alkaline (p less than 0.0005) compared with average control pH (6.92 +/- 0.12, MDP = 82.1 +/- 3.7 mV). In 24 mM bicarbonate Tyrode's solution, the intracellular pH did vary with MDP (0.0032 pH units/mV). During superfusion of normal Purkinje fibers with hypoxic Tyrode's solution, intracellular pH acidified by 0.22 pH units as they depolarized. Therefore, intracellular acidification does not seem to be a cause of the depolarization of subendocardial Purkinje cells 24 hours after myocardial infarction.

The effect of extracellular weak acids and bases on the intracellular buffering power of snail neurones

1. Intracellular pH (pHi) was measured in snail neurones using pH-sensitive glass microelectrodes. The influence of externally applied weak acids and bases on the total intracellular buffering power (beta T) was investigated by monitoring the pHi changes caused by the intracellular ionophoretic injection of HCl. 2. In the absence of weak acids or bases a reduction in the extracellular HEPES concentration had no effect on pHi or on beta T. It did, however, reduce slightly the rate of pHi recovery following HCl injection. 3. The presence of CO2 greatly increased beta T. However, as predicted for an open buffer system, the contributions to intracellular buffering by CO2 (beta CO2) decreased as pHi decreased. 4. When added to the superfusate, procaine, 4-aminopyridine, trimethylamine and NH4Cl (1-10 mM) all increased steady-state pHi. Procaine was fastest at increasing pHi and 4-aminopyridine the slowest. All four of these weak bases increased beta T. 5. The intracellular buffering action by these weak bases varied. HCl injection in the presence of procaine usually resulted in steady-state pHi changes with no pHi transients. In the presence of the other three weak bases HCl injections resulted in intracellular acidifications which were followed by pHi recovery-like transients. However, these were not blocked by SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) or by CaCl2 and I thus conclude that these transients were as a result of slow or incomplete intracellular buffering by the weak bases. 6. In many cells there was a good correlation between the measured contributions to intracellular buffering by the weak bases (beta base) and those predicted assuming a simple two-compartment open system. In all cases, as predicted, beta base increased as pHi decreased. 7. I found a clear relationship between the concentration of external buffer (HEPES) and the rate at which weak bases, applied to the superfusate, were able to increase pHi. The greater the extracellular buffer concentration the greater was the speed of intracellular alkalinization. 8. Lowering the extracellular buffer concentration reduced the efficiency of intracellular buffering by weak bases in response to an intracellular acid load. HCl injection in the presence of weak base caused a larger initial intracellular acidification if the extracellular HEPES concentration was reduced. 9. In conclusion, both weak acids and weak bases can make very large, pHi-dependent contributions to intracellular buffering by way of open buffer systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of rate-dependent pH changes in the sheep cardiac Purkinje fibre

1. The mechanism of the rate-dependent decrease in intracellular pH (pHi) and its recovery were studied in isolated sheep cardiac Purkinje fibres. Intracellular Na+ activity (aiNa) and pHi were measured using ion-selective microelectrodes. Twitches were elicited by field stimulation or by depolarizing pulses applied using a two-microelectrode voltage clamp. 2. A 3 Hz train of short (50 ms) depolarizing voltage-clamp pulses induced a reversible fall in pHi which was accompanied by a reversible increase in aiNa. A train of longer (200 ms) pulses also produced a fall in pHi which was now paralleled by a decrease in aiNa. These observations indicate that the rate-dependent acidosis is not dependent upon a rise in aiNa. 3. Neither the fall in pHi nor the increase in aiNa seen upon an increase in action potential frequency was inhibited by amiloride (1 mmol l-1) which indicates that Na+-H+ exchange is not involved in the generation of the acidosis. Furthermore, the rate-dependent acidosis was not abolished in Na+-free solution (Li+ or N-methyl glucamine substituted) indicating that other Na+-requiring processes (such as Na+-Ca2+ exchange) are not a necessary requirement. Rate-dependent pHi changes were also unaffected by the stilbene compound DIDS indicating no participation by Cl--HCO-3 exchange. 4. The rate-dependent acidosis was inhibited by the organic calcium antagonist D600 (20 mumol l-1) which also inhibited twitch tension. This suggests that the acidosis is related to the activation by Ca2+ of developed tension. D600 also inhibited the rate-dependent rise in aiNa (field stimulation). 5. The rate-dependent acidosis was not inhibited by cyanide (2 mmol l-1) but it was blocked by iodoacetate (0.5 mmol l-1) and by 2-deoxyglucose (DOG) (10 mmol l-1, applied in glucose-free solution). These results suggest that the acidosis is generated metabolically via stimulation of glycolysis, following an increase in contraction. Contributions from aerobic metabolism are likely to be small. 6. Twitch tension was inhibited by ryanodine (10 mumol l-1) but the drug had little inhibitory effect on the rate-dependent acidosis. A tonic component of tension was observed, however, in the presence of ryanodine. The lack of effect of ryanodine upon the rate-induced acidosis is discussed. 7. The half-time of pHi recovery from the frequency-dependent acidosis was consistently shorter than that from an intracellular acid load induced by adding and then removing external NH4Cl (10 mmol l-1).(ABSTRACT TRUNCATED AT 400 WORDS)

A microelectrode study of the mechanisms of L-lactate entry into and release from frog sartorius muscle

1. Changes in intracellular pH and intracellular anion levels were monitored in frog sartorius muscle fibres during exposure to extracellular L-lactate, using ion-sensitive microelectrodes. 2. Resting intracellular pH (pHi) in 20 mmol l-1 HEPES buffer was 7.18 +/- 0.015 (S.E. of mean, n = 62). Exposure to an extracellular solution at pH 6.5 buffered with 20 mmol l-1 3-(N-morpholino)propanesulphonic acid (MOPS) resulted in a slow intracellular acidification. 3. A reversible decrease in pHi and an increase in intracellular anion levels was observed when L-lactate replaced chloride in equimolar amounts. The increase in intracellular anion level is consistent with intracellular accumulation of L-lactate ion. 4. The rate and steady-state change in pHi and anion level was a function of both extracellular pH and L-lactate concentration, providing evidence for the coupled movement of lactate and proton equivalents. 5. The initial rate of uptake of L-lactate, as measured by the change of pHi, was a non-linear function of the extracellular L-lactate concentration at extracellular pH 6.8 and 7.35. 6. No saturation was observed with concentrations of L-lactate between 5 and 60 mmol l-1 at pH 7.35 and 2.5 and 40 mmol l-1 at pH 6.8. 7. The non-linear relationship between the initial rate of change in pHi and extracellular L-lactate was well fitted by a curve defining uptake as the sum of a carrier process displaying Michaelis-Menten kinetics and a passive diffusion component. The apparent Km of the carrier was 10 mmol l-1 at pHo 7.35 and 4 mmol l-1 at pHo 6.8. 8. The initial rate of change of pHi in the presence of L-lactate was significantly inhibited 39.1 +/- 6.2% by 2-5 mmol l-1 alpha-cyano-4-hydroxycinnamate (n = 9; P less than 0.05, paired t test). 9. alpha-Cyano-4-hydroxycinnamate had no detectable effect on the initial rate of change of pHi induced by propionate exposure. 10. The initial rate of change of pHi induced by L-lactate was not affected by 20-100 mumol l-1 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS). 11. We conclude that L-lactate crosses the membrane of the frog sartorius muscle with proton equivalents via (1) a carrier-mediated process, and (2) passive diffusion of lactic acid. In the physiological range of L-lactate concentrations and pH the transport process dominates.

The relation between force and intracellular pH in fatigued, single Xenopus muscle fibres

Intracellular pH (pHi) has been measured before fatiguing stimulation and during recovery in single toe-muscle fibres of Xenopus at room temperature. Liquid ion-sensor microelectrodes were used for pHi measurements. The pHi measured before fatiguing stimulation was 6.93 +/- 0.11 (mean +/- SD, n = 9) in type 1 fibres and 6.99 +/- 0.10 (n = 4) in type 2 fibres. About 1 min after tension had been suppressed to approximately 40% of the original by repeated tetanic contractions, pHi was measured again; it was then reduced to 6.34 +/- 0.13 (range 6.15-6.50) and 6.71 +/- 0.17 (range 6.50-6.85) in type 1 and type 2 fibres, respectively. The pHi recovered at a rate of about 0.05 pH units min-1 and was always normalized well before tension. Fibres which exhibited post-contractile depression (PCD), a delayed force suppression during the recovery period, had similar pHi normalization rates to those of other fibres. The large variation in pHi values obtained in fibres fatigued to a standard tension level leads us to conclude that an intracellular acidification is not likely to be the major cause of fatigue produced by intermittent tetanic stimulation. However, an important inhibitory effect in the most acidified fibres, cannot be excluded. Furthermore, we conclude that force recovery in our experiments is controlled by factors other than pHi.

Intracellular pH recovery and lactate efflux in mouse soleus muscles stimulated in vitro: the involvement of sodium/proton exchange and a lactate carrier

The intracellular pH recovery after stimulation of mouse soleus muscles in vitro was studied by means of intracellular pH-sensitive microelectrodes. The lactate efflux and the total lactate content were measured by means of an enzymic method. During electrical stimulation for 2 min in a CO2/HCO3- -buffered Ringer's solution, pHi decreased by 0.5 units. The rate of pHi-recovery was independent of external bicarbonate, but dependent on the buffer concentration. The rate of intracellular pH recovery was reduced by the lactate transport inhibitors PCMBS and cinnamate, whereas the inhibitors of inorganic anion-exchange SITS and DIDS had no effect. The Na+/H+ exchange inhibitor amiloride reduced the rate of pHi recovery. The pHi recovery was faster than the lactate efflux, which could be accounted for by an Na+/H+ exchange. A number of inhibitor compounds were used in order to discriminate between the three possible lactate efflux pathways: the monocarboxylate carrier mechanism, the inorganic anion exchange, and the molecular (non-ionic) diffusion of lactic acid. The lactate efflux was partly inhibited by cinnamate, PCMBS and phloretin, but was unaffected by DIDS and tetrathionate. These experiments demonstrate the existence of a lactate carrier in mammalian skeletal muscles. The lactate carrier is responsible for more than half of the lactate efflux after muscle activity. Both the pHi recovery studies and the lactate efflux measurements showed that, under the given conditions, the inorganic anion-exchange mechanism is not essentially involved in the recovery processes after muscle activity.

Conduction of the impulse in the ischemic myocardium--implications for malignant ventricular arrhythmias

Ventricular arrhythmias occurring consequent to regional disturbances of myocardial perfusion are the most frequent cause of sudden cardiac death. They are related to marked changes of impulse propagation in the ischemic region, which consist of circulating excitation with re-entry. Mapping of the impulse during ventricular tachycardias and ventricular fibrillation shows that the circus movements change their shape and localization from beat to beat. Zones of tissue which block the impulse during one beat may conduct the impulse at a fast rate during the next beat. The main cause underlying this behavior is the depression of the ischemic action potential. This depression is caused by the partial inactivation and the prolonged recovery of the rapid sodium inward current. In addition to the decrease in resting potential, other factors, such as acidosis, contribute to the inactivation of the inward currents generating the upstroke of the action potential. An increase of coupling resistance between myocardial cells and/or an increase of extracellular resistance appear to be less important for explaining conduction disturbances in acute ischemia.