Intracellular pH of isolated rat diaphragm muscle with metabolic and respiratory changes of extracellular pH

Effects of Prior Voluntary Hyperventilation on the 3-min All-Out Cycling Test in Men

INTRODUCTION

The ergogenic effects of respiratory alkalosis induced by prior voluntary hyperventilation (VH) are controversial. This study examined the effects of prior VH on derived parameters from the 3-min all-out cycling test (3MT).

METHODS

Eleven men ( = 46 ± 8 mL·kg-1·min-1) performed a 3MT preceded by 15 min of rest (CONT) or VH ( = 38 ± 5 L·min-1) with PETCO2 reduced to 21 ± 1 mm Hg (HYP). End-test power (EP; synonymous with critical power) was calculated as the mean power output over the last 30 s of the 3MT, and the work done above EP (WEP; synonymous with W') was calculated as the power-time integral above EP.

RESULTS

At the start of the 3MT, capillary blood PCO2 and [H+] were lower in HYP (25.2 ± 3.0 mm Hg, 27.1 ± 2.6 nmol·L-1) than CONT (43.2 ± 2.0 mm Hg, 40.0 ± 1.5 nmol·L-1) (P < 0.001). At the end of the 3MT, blood PCO2 was still lower in HYP (35.7 ± 5.4 mm Hg) than CONT (40.6 ± 5.0 mm Hg) (P < 0.001). WEP was 10% higher in HYP (19.4 ± 7.0 kJ) than CONT (17.6 ± 6.4 kJ) (P = 0.006), whereas EP was 5% lower in HYP (246 ± 69 W) than CONT (260 ± 74 W) (P = 0.007). The ΔWEP (J·kg-1) between CONT and HYP correlated positively with the PCO2 immediately before the 3MT in HYP (r = 0.77, P = 0.006).

CONCLUSION

These findings suggest that acid-base changes elicited by prior VH increase WEP but decrease EP during the all-out 3MT.

Effects of acute respiratory and metabolic acidosis on diaphragm muscle obtained from rats

BACKGROUND

Acute respiratory acidosis is associated with alterations in diaphragm performance. The authors compared the effects of respiratory acidosis and metabolic acidosis in the rat diaphragm in vitro.

METHODS

Diaphragmatic strips were stimulated in vitro, and mechanical and energetic variables were measured, cross-bridge kinetics calculated, and the effects of fatigue evaluated. An extracellular pH of 7.00 was obtained by increasing carbon dioxide tension (from 25 to 104 mmHg) in the respiratory acidosis group (n = 12) or lowering bicarbonate concentration (from 24.5 to 5.5 mM) in the metabolic acidosis group (n = 12) and the results compared with a control group (n = 12, pH = 7.40) after 20-min exposure.

RESULTS

Respiratory acidosis induced a significant decrease in maximum shortening velocity (-33%, P < 0.001), active isometric force (-36%, P < 0.001), and peak power output (-59%, P < 0.001), slowed relaxation, and decreased the number of cross-bridges (-35%, P < 0.001) but not the force per cross-bridge, and impaired recovery from fatigue. Respiratory acidosis impaired more relaxation than contraction, as shown by impairment in contraction-relaxation coupling under isotonic (-26%, P < 0.001) or isometric (-44%, P < 0.001) conditions. In contrast, no significant differences in diaphragmatic contraction, relaxation, or contraction-relaxation coupling were observed in the metabolic acidosis group.

CONCLUSIONS

In rat diaphragm, acute (20 min) respiratory acidosis induced a marked decrease in the diaphragm contractility, which was not observed in metabolic acidosis.

The hydrogen ion in normal metabolism: a review

The production of hydrogen ions (H+) by metabolic processes is described, with particular emphasis on glycolysis and ketogenesis. Total metabolic production of H+ is approximately 150 g day-1 but utilization closely balances production, so that intracellular and extracellular H+ production is maintained within narrow limits. H+ is generated at several sites in glycolysis but no net H+ production occurs unless the ATP formed is hydrolysed. The other main source of metabolic H+ production is ketogenesis. Here H+ accumulation depends on both the relative dominance of ketone body production over utilization and the loss of base in urine. The H+ is produced during the synthesis of 3-hydroxy-3-methylglutaryl-CoA and not because of dissociation of acetoacetic acid. Lipolysis and re-esterification of fats are additional major producers of H+, while net H+ production also occurs with pathological accumulation and incomplete combustion of other organic acids. Many metabolic systems are sensitive to the changes in pH. These effects have been examined in vivo using an ammonium chloride acidaemia model in the rat. Severe insulin resistance and impaired glucose metabolism in liver and muscle were found. One mechanism involved inhibition, by H+, of the binding of insulin to its receptors. Further mechanisms include inhibition of key glycolytic enzymes including phosphofructokinase. It is concluded that too little attention is paid to metabolic production of hydrogen ions and to their effects, in turn, on metabolism.

Effects of hyperventilation on phosphocreatine kinetics and muscle deoxygenation during moderate-intensity plantar flexion exercise

The effects of controlled voluntary hyperventilation (Hyp) on phosphocreatine (PCr) kinetics and muscle deoxygenation were examined during moderate-intensity plantar flexion exercise. Male subjects ( n = 7) performed trials consisting of 20-min rest, 6-min exercise, and 10-min recovery in control [Con; end-tidal Pco2(PetCO2) ∼ 33 mmHg] and Hyp (PetCO2∼17 mmHg) conditions. Phosphorus-31 magnetic resonance and near-infrared spectroscopy were used simultaneously to monitor intramuscular acid-base status, high-energy phosphates, and muscle oxygenation. Resting intracellular hydrogen ion concentration ([H+]i) was lower ( P < 0.05) in Hyp [90 nM (SD 3)] than Con [96 nM (SD 4)]; however, at end exercise, [H+]iwas greater ( P < 0.05) in Hyp [128 nM (SD 19)] than Con [120 nM (SD 17)]. At rest, [PCr] was not different between Con [36 mM (SD 2)] and Hyp [36 mM (SD 1)]. The time constant (τ) of PCr breakdown during transition from rest to exercise was greater ( P < 0.05) in Hyp [39 s (SD 22)] than Con [32 s (SD 22)], and the PCr amplitude was greater ( P < 0.05) in Hyp [26% (SD 4)] than Con [22% (SD 6)]. The deoxyhemoglobin and/or deoxymyoglobin (HHb) τ was similar between Hyp [13 s (SD 8)] and Con [10 s (SD 3)]; however, the amplitude was increased ( P < 0.05) in Hyp [40 arbitrary units (au) (SD 23)] compared with Con [26 au (SD 17)]. In conclusion, our results indicate that Hyp-induced hypocapnia enhanced substrate-level phosphorylation during moderate-intensity exercise. In addition, the increased amplitude of the HHb response suggests a reduced local muscle perfusion in Hyp compared with Con.

Extracellular and Intracellular Acid‐Base Status with Regard to the Energy Metabolism in the OysterCrassostrea gigasduring Exposure to Air

The acid-base status of extra- and intracellular fluids was studied in relation to the anaerobic energy metabolism in the adductor muscle, mantle, gills, and heart of the marine bivalve Crassostrea gigas after exposure to air for periods of 2, 4, 8, 12, 24, and 48 h. Such exposure was found to cause a significant reduction in the pH in the hemolymph (pH(e)) within the first 4 h. The decrease in the pHe was accompanied by elevated Pco2 values, causing [HCO3-] to rise (respiratory acidosis). Thereafter, the pHe fell at a lower rate, and this fall was partially compensated for by a further increase in [HCO3-] in the hemolymph. The increase in the [Ca] levels in the hemolymph indicates a mobilization of Ca2+ from CaCO3 and the involvement of bicarbonates in the buffering of pHe. The main anaerobic end-products that accumulated in the tissues during the first stages of anaerobiosis were alanine and succinate, at a ratio of about 2 : 1. Later on, propionate and acetate were also accumulated at significant rates. In contrast to the adductor muscle, gills, and mantle, opine production in the heart was significant after 12-24 h of exposure to air. Determination of intracellular pH (pHi) revealed that there is a close relationship between the rate of anaerobic end-product accumulation and the extent of intracellular acidosis in the adductor muscle, mantle, and gills. On the contrary, accumulation of anaerobic end-products in the heart did not cause any significant change in its pHi. The intracellular nonbicarbonate, nonphosphate buffering value (beta (NB,NPi)) was determined to be higher in the heart than in the other three tissues and thus probably plays a crucial role in stabilizing heart pHi during exposure to air.

Buffering and H+ ion dynamics in muscle tissues

After anaerobic activity with release of large quantities of intermediary metabolic end products, further energy production critically depends on rapid elimination of H+ ions from the muscle tissues to secure key enzymatic activities. The involved processes, interactions and interrelationships of mechanisms have been analyzed on the basis of a physiological model and available experimental data. The H+ elimination from muscle tissue is a multifactorial process primarily governed by the capillary H+ transport capacitance, effected by buffering capabilities of intracellular and capillary fluids compartments, by dynamically interrelated regulation of intracellular and extracellular pH, by the magnitude and quality of convective perfusional transfer and further factors. Model calculations strongly resemble experimental data obtained in isolated perfused muscles and suggest that discrepancies between disparate literature data are attributable to experimental limitations.

Acid-base balance at exercise in normoxia and in chronic hypoxia. Revisiting the "lactate paradox"

Transitions between rest and work, in either direction, and heavy exercise loads are characterized by changes of muscle pH depending on the buffer power and capacity of the tissues and on the metabolic processes involved. Among the latter, in chronological sequence: (1). aerobic glycolysis generates sizeable amounts of lactate and H(+) by way of the recently described, extremely fast (20-100 ms) "glycogen shunt" and of the excess of glycolytic pyruvate supply; (2). hydrolysis of phosphocreatine, tightly coupled with that of ATP in the Lohmann reaction, is known to consume protons, a process undergoing reversal during recovery; (3). anaerobic glycolysis sustaining ATP production in supramaximal exercise as well as in conditions of hypoxia and ischemia, is responsible for the accumulation of large amounts of lactic acid (up to 1 mol for the whole body). The handling of metabolic acids, i.e., acid-base regulation, occurs both in blood and in tissues, mainly in muscles which are the main producers and consumers of lactic acid. The role of both blood and muscle bicarbonate and non-bicarbonate buffers as well as that of lactate/H(+) cotransport mechanisms is analyzed in relation to acid-base homeostasis in the course of exercise. A section of the review deals with the analysis of the acid-base state of humans exposed to chronic hypoxia. Particular emphasis is put on anaerobic glycolysis. In this context, the so-called lactate paradox is revisited and interpreted on the basis of the most recent findings on exercise at altitude.

Effects of respiratory alkalosis on human skeletal muscle metabolism at the onset of submaximal exercise

The purpose of this study was to examine the effects of respiratory alkalosis on human skeletal muscle metabolism at rest and during submaximal exercise. Subjects exercised on two occasions for 15 min at 55 % of their maximal oxygen uptake while either hyperventilating (R-Alk) or breathing normally (Con). Muscle biopsies were taken at rest and after 1 and 15 min of exercise. At rest, no effects on muscle metabolism were observed in response to R-Alk. In the first minute of exercise, there was a delayed activation of pyruvate dehydrogenase (PDH) in R-Alk compared with Con, resulting in a reduced rate of pyruvate oxidation. Also, glycogenolysis was higher in R-Alk compared with Con, which was attributed to a higher availability of the monoprotonated form of inorganic phosphate (P(i)), resulting in an elevated rate of pyruvate production. The mismatch between pyruvate production and its oxidation resulted in net lactate accumulation. These effects were not seen after 15 min of exercise, with no further differences in muscle metabolism between conditions. The results from the present study suggest that respiratory alkalosis may play an important role in lactate accumulation during the transition from rest to exercise in acute hypoxic conditions, but that other factors mediate lactate accumulation during steady-state exercise.

Effect of acute sodium bicarbonate ingestion on excess CO2 output during incremental exercise

The effect of bicarbonate ingestion on total excess volume of CO2 output (CO2 excess), due to bicarbonate buffering of lactic acid in exercise, was studied in eight healthy male volunteers during incremental exercise on a cycle ergometer performed after ingestion (0.3 g.kg-1 body mass) of CaCO3 (control) and NaHCO3 (alkalosis). The resting arterialized venous blood pH (P < 0.05) and bicarbonate concentration ([HCO3-]b; P < 0.01) were significantly higher in acute metabolic alkalosis [AMA; pH, 7.44 (SD 0.03); [HCO3-]b, 29.4 (SD 1.5) mmol.l-1] than in the control [pH, 7.39 (SD 0.03); [HCO3-]b, 25.5 (SD 1.0) mmol.l-1]. The blood lactate concentrations ([la-]b) during exercise below the anaerobic threshold (AT) were not affected by AMA, while significantly higher [la-]b at exhaustion [12.29 (SD 1.87) vs 9.57 (SD 2.14) mmol.l-1, P < 0.05] and at 3 min after exercise [14.41 (SD 1.75) vs 12.26 (SD 1.40) mmol.l-1, P < 0.05] were found in AMA compared with the control. The CO2 excess increased significantly from the control [3177 (SD 506) ml] to AMA [3897 (SD 381) ml; P < 0.05]. The CO2 excess per body mass was found to be significantly correlated with both the increase of [la-]b from rest to 3 min after exercise (delta[la-]b; r = 0.926, P < 0.001) and with the decrease of [HCO3-]b from rest to 3 min after exercise (delta [HCO3-]b; r = 0.872, P < 0.001), indicating that CO2 excess per body mass increased linearly with both delta [la-]b and delta [HCO3-]b.(ABSTRACT TRUNCATED AT 250 WORDS)

The roles of intracellular buffers and bone mineral in the regulation of acid-base balance in mammals

1. Regulation of intracellular and extracellular pH may conflict in their requirements for movement of acid or base. 2. Cells make a positive or a negative contribution to 'tissue buffering' of extracellular fluid (ECF), depending on their internal buffer value, on the tightness of their internal pH control by membrane mechanisms, and on the nature of the acid-base disturbance. 3. A role is suggested for electrogenic Na-HCO3 co-transport in some of the ion shifts that occur in acid-base disturbances. 4. The time course of 'tissue buffering' in nephrectomized mammals in hypercapnia is variable, and it is far from clear in intact, unanaesthetized mammals. 5. Buffering of ECF by Ca salts of bone mineral in acidosis can only be substantial if accompanied by Ca excretion; the release of HCO3 with Na and K is more significant. 6. The relative importance of cells and of bone mineral in the buffering of ECF is unclear.

Effect of magnesium depletion and potassium depletion and chlorothiazide on intracellular pH in the rat, studied by 31P NMR

1. Both dietary magnesium depletion and potassium depletion (confirmed by tissue analysis) were induced in rats which were then compared with rats treated with chlorothiazide (250 mg/kg diet) and rats on a control synthetic diet. 2. Brain and muscle intracellular pH was measured by using a surface coil and [31P]-NMR to measure the chemical shift of inorganic phosphate. pH was also measured in isolated perfused hearts from control and magnesium-deficient rats. Intracellular magnesium status was assessed by measuring the chemical shift of beta-ATP in brain. 3. There was no evidence for magnesium deficiency in the chlorothiazide-treated rats on tissue analysis or on chemical shift of beta-ATP in brain. Both magnesium and potassium deficiency, but not chlorothiazide treatment, were associated with an extracellular alkalosis. 4. Magnesium deficiency led to an intracellular alkalosis in brain, muscle and heart. Chlorothiazide treatment led to an alkalosis in brain. Potassium deficiency was associated with a normal intracellular pH in brain and muscle. 5. Magnesium depletion and chlorothiazide treatment produce intracellular alkalosis by unknown mechanism(s).

Physiological adaptations of the intertidal rockpool teleost Blennius pholis L., to aerial exposure

Gas exchange, metabolism, ventilation, circulation and acid-base balance in water and air were investigated in Blennius pholis. The rates of gas exchange in water and air were similar with the RQ remaining around 0.8. Aerial gas exchange was equally divided between the head/gills and the tail region. Ventilatory adaptations involved a reduction in rate in air and the mode of ventilation changed from flow-through to tidal, with closed opercula. A transient bradycardia developed on transition to air before heart frequency (fH) returned to aquatic levels. During aerial exposure PvCO2 rose only by 1 Torr with a concomitant decrease in pH of 0.19 pH-units. At the same time a metabolic acidosis was observed which could not be fully accounted for by the formation of lactic acid in the blood, although tissue lactate levels did not change significantly. No histological evidence was found for the presence of carbonic anhydrase in the epithelial cells of the skin or the oesophagus to aid aerial CO2 excretion. Inhibition of CA activity by addition of methazolamide to blood, however, caused PvCO2 to rise by 3 Torr and pHv to decrease by 0.4 pH unit. It is concluded that B. pholis is physiologically well adapted to aerial exposure through adjustments in ventilation and circulation and that erythrocytic carbonic anhydrase plays a major role in CO2 transfer.

Effects of acidosis on tension development in mammalian skeletal muscle

Effect of carbon dioxide acidosis on the tension development in a rat skeletal muscle (extensor digitorum longus) was examined at different temperatures. Experiments were done in vitro and with direct stimulation, mostly at constant temperatures between 30-35 degrees C and 12-20 degrees C. A decrease of saline pH (8.0 to 6.5) with carbon dioxide increased the twitch and the tetanic tensions and enhanced the tension relaxation in experiments done at high temperatures. At low temperatures the same procedure decreased the tetanic tension and enhanced the tension relaxation. An increased tetanic tension at the high temperatures and a decreased tetanic tension at the low temperatures were also obtained at constant saline pH, with procedures known to decrease intracellular pH. The observations made at higher temperatures are discussed in relation to human muscle performance in exercise.

Effects of reduced frequency breathing on arterial hypoxemia during exercise

It is uncertain that exercise with reduced frequency breathing (RFB) results in arterial hypoxemia. This study was designed to investigate whether RFB during exercise creates a true hypoxic condition in arterial blood by examining arterial oxygen saturation (SaO2) directly. Six subjects performed ten 30 s periods of exercise on a Monark bicycle ergometer at a work rate of 210 W alternating with 30 s rest intervals. The breath was controlled to use 1 s each for inspiration and expiration, and two trials with different breathing patterns were used; a continuous breathing (CB) trial and an RFB trial consisting of four seconds of breath-holding at functional residual capacity (FRC). Alveolar oxygen pressure during exercise showed a slight but significant (p less than 0.05) reduction with RFB as compared to CB. However, a marked increase in alveolar-arterial pressure difference for oxygen (A-aDO2) (p less than 0.05) with RFB over CB resulted in a marked (p less than 0.05) reduction in arterial oxygen pressure. Consequently, SaO2 fell as low as 88.8% on average. Additional examination of RFB with breath-holding at total lung capacity showed no increases in A-aDO2 in spite of the same amount of hypoventilation as compared with that at FRC. These results indicate that RFB during exercise can result in arterial hypoxemia if RFB is performed with breath-holding at FRC, this mechanism being closely related to the mechanical responses due to lung volume restriction.

Arteriovenous carbon dioxide and pH gradients during cardiac arrest

In a porcine preparation of cardiac arrest, we demonstrated that there is a marked paradox of venous acidemia and arterial alkalemia. This paradox is related to decreased clearance of CO2 from the lungs when pulmonary blood flow is critically reduced. Accordingly, increased venous PCO2 rather than metabolic acidosis due to lactic acidosis predominates during the initial 8 min of cardiopulmonary resuscitation. Arterial blood gases fail as indicators of systemic acid-base status and therefore as indicators of tissue acidosis.

Correction of metabolic alkalosis by HCl and acetazolamide: effects on extracellular and intracellular acid-base status in rats in vivo

Extracellular plasma pH (pHe) of nephrectomized male or female Sprague-Dawley rats was changed by infusion of either sodium bicarbonate or HCl to predetermined values in the pH range of 7.53-7.14, and then held constant for 2 h. Intracellular pH (pHi) of the liver, heart, brain, and two skeletal muscle groups as calculated from the distribution of 14C-labelled DMO (5.5-dimethyl-2,4-oxazolidinedione) was compared to corresponding tissues of a control group and rats treated with the carbonic anhydrase inhibitor acetazolamide (Diamox). When compared to control, changes of the extracellular pH in male or female rats were followed by similar effects on pHi in the investigated tissues. At the same extracellular pH there were no statistical differences between pHi values of HCl or acetazolamide treated rats, though the arterial PCO2 following acetazolamide administration was significantly increased when compared to control or the corresponding HCl group. This study shows that administration of acetazolamide or HCl results in a dose-dependent decrease of plasma and tissue pH, and that both agents may be used as a logical and safe therapy during severe metabolic alkalosis in rats.

Effects of acidosis on rat muscle metabolism and performance during heavy exercise

The metabolism and performance of a perfused rat hindquarter preparation was examined during heavy exercise in three conditions: control (C), metabolic acidosis (MA, decreased bicarbonate concentration), and respiratory acidosis (RA, increased CO2 tension). A one-pass system was used to perfuse the hindquarters for 30 min at rest and 20 min during tetanic stimulation via the sciatic nerve. The isometric tension generated by the gastrocnemius-plantaris-soleus muscle group was recorded, and biopsies were taken pre- and postperfusion. Initial isometric tensions were similar in all conditions, but the rate of tension decay was largest in acidosis; the 5-min tensions for C, MA, and RA were 1,835 +/- 63, 1,534 +/- 63, and 1,434 +/- 73 g, respectively. O2 uptake in C was greater than in MA and RA (23.4 +/- 1.3 vs. 17.0 +/- 1.4 and 16.5 +/- 2.3 mumol X min-1), paralleling the tension findings. Hindquarter lactate release was greatest in C, least in MA, and intermediate in RA. Acidosis resulted in less muscle glycogen utilization and lactate accumulation than during control. Muscle creatine phosphate utilization and ATP levels were unaffected by acidosis. Acidosis decreased the muscle's ability to generate isometric tension and depressed both aerobic and anaerobic metabolism. During stimulation in this model lactate left the muscle mainly as a function of the production rate, although a low plasma bicarbonate concentration at pH 7.15 depressed muscle lactate release.

Regulation of intracellular acid-base equilibrium in rats

The extracellular pH (pHe) of unrestrained male Sprague-Dawley rats was altered by either inhalation of CO2 or infusion of sodium bicarbonate and then kept constant for at least 2 h. The intracellular pH (pHi) was then determined by the DMO (5,5-dimethyl-2,4-oxazolidinedione)-method in heart, brain, liver, spleen, and ten different skeletal muscle groups. pHi varied linearly with pHe in the heart and the parenchymal organs. pHi in skeletal muscle remained constant over a pHe range from 7.31 to 7.45 but varied linearly with pHe outside this range.

CONCLUSIONS

1. pHi is influenced by CO2 inhalation or bicarbonate infusion. 2. pHi may be different in different tissues at specified pHe. 3. Skeletal muscle seems to be well protected against mild extracellular acidosis. 4. Slight changes of pHe affect the pHi of the heart, brain, liver, and spleen. 5. Regulation of intra- and extracellular acid-base balance probably occurs by bicarbonate transfer between the intra- and extracellular compartments. 6. The acid-base status of the various intracellular compartments cannot be determined from blood-gas analysis.

Intracellular and extracellular acid-base and electrolyte status of submerged anoxic turtles at 3 degrees C

Specimens of fresh water turtles (Chrysemys picta bellii) were acclimated to 3 degrees C and then submerged in completely anoxic water for time periods of up to 12 weeks. Blood withdrawn via indwelling arterial catheters was analysed for plasma pH, PCO2, bicarbonate concentration, [lactate], [Na+], [K+], [Ca2+] and [Mg2+], and tissue samples of skeletal muscle, liver and cardiac muscle were excised. Samples of skeletal muscle were analysed for intracellular pH (DMO), [lactate], [Na+], [K+], [Ca2+], and [Mg2+], and samples of liver and cardiac muscle for intracellular pH and [lactate] during normoxia and after 1, 2, 4, 8 and 12 weeks of anoxia. Arterial plasma pH fell from 8.0 during normoxia to lower than 7.2 concomitant with a reduction in plasma [HCO3-] after 12 weeks of anoxia due to the production of large amounts of lactic acid. The intracellular pH (pHi) of heart muscle and liver dropped in parallel or even more than plasma pH, whereas pHi in skeletal muscle changed less resulting in a delta pHi/delta pHe value of less than 0.6. Intracellular [lactate] and [Ca2+] increased considerably, but attained concentrations much smaller than those observed in the extracellular compartment. The intracellular concentrations of K+, Na+ and Mg2+ were also significantly affected, the changes, however, were small in comparison with those observed for Ca2+ and lactate concentration. The water distribution between intra- and extracellular compartments remained essentially unaffected by anoxia. It is concluded that the considerable increase in extracellular Mg2+ and Ca2+ cannot be the result of release from muscle cells and has to be attributed to release from skeleton and shell.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-base regulation in response to environmental hypercapnia in two aquatic salamanders, Siren lacertina and Amphiuma means

The partial pressure of CO2 (PCO2) in certain areas of the aquatic habitat of the salamanders Siren lacertina and Amphiuma means frequently rises to values of up to 60 mm Hg. This ambient hypercapnia occurs due to hindrance of gas exchange between water and air caused by dense water-surface vegetation. In order to investigate the acid-base regulation in response to the respiratory acidosis, which wound be expected to result from the high CO2 conductance of the amphibian skin, specimens of both species were subjected to water PCO2 of 47 mm Hg while having free access to normocapnic air in a closed water recirculation system. Arterial PCO2 rose considerably from 12 to 35 mm Hg in Siren and from 17 to 36 mm Hg in Amphiuma. The resultant fall in plasma pH remained uncompensated, whereas intracellular pH of white muscle and heart muscle of Siren were little affected owing to elevated intracellular bicarbonate concentrations. The bicarbonate accumulated in the intracellular compartments was in part produced by intracellular and extracellular nonbicarbonate buffering, and in part gained from the environment in exchange for Cl- ions. Elevated water bicarbonate concentration or bicarbonate infusion into Siren had no effect on the acid-base regulation. These data suggest that the availability of bicarbonate is not a limiting factor for extracellular compensation of increased PCO2, but that the threshold of the bicarbonate-regulating structures is simply not readjusted in hypercapnia. This type of regulation may have evolved as a result of the specific environmental conditions of these animals and may be considered as an energetically efficient way of maintaining a constant milieu for the pH-sensitive intracellular structures.

Comparison of intra-and extracellular buffering of clinically used buffer substances: tris and bicarbonate

A large and important group of acid-base disturbances are the metabolic acidoses. In general, every type of metabolic acidosis can be treated with infusion of base when the underlying cause of the disturbances is removed. In our medical centers, the use of tris and bicarbonate is common. For a long time they were competitive agents and until now it was not possible to decide by available clinical methods which of these substances was more suitable for correction of metabolic acidosis. The intracellular pH of the whole rat (mean lcf-pH) was determined from the distribution of 14C labelled DMO (5,5-dimethyl-2,4-oxazolidinedione) and monitored for 6 h following intravenous application of tris or sodiumbiarbonate in a dose of 10 mmol per kg body mass. Arterial plasma pH and PCO2 were also measured. To determine and compare the effectiveness of the two buffer substances, intra- and extracellular bicarbonate were calculated from the Henderson-Hasselbalch equation. It was found that the buffering following bicarbonate infusion is more effective in both body compartments. Sodiumbicarbonate should be preferred in daily practice.

Effects of fatigue and altered pH on isometric force and velocity of shortening at zero load in frog muscle fibres

temporaria (0.9-2.5degrees C) were stimulated to produce a 1 s isometric tetanus at regular intervals until constant mechanical responses were attained. Various degrees of force depression ("fatigue") were produced by decreasing the contraction interval from 30 or 15 min (control to 120, 60, 30 and 15s, respectively. In this was the steady-state tetanic force could be reversibly reduced to approximately 70% of the control value. The velocity of shortening at zero load, V0, was determined at each level of fatigue using an approach for direct measurement of V0. V0 was not significantly affected as long as the decrease in force was less than 10%. With further reduction of the isometric tension there was a progressive decline of V0 according to the following empirical relationship between percentage depression of force (delta P0) and maximum speed (delta V0) of shortening: delta V0 = 0.006 delta P02.48- 1.0 (correlation coefficient, 0.86). Cine photographic recording of nylon markers on the fibre surface provided evidence that fatigue developed uniformly along the fibre with no sign of failure of excitation in any segment. The change in mechanical performance during fatigue could be reproduced inthe non-fatigued fibre by reducing the pH of the external medium within the range 8.0-6.6 using a bicarbonate-CO2 buffer. A decrease in pH thus reduced both the rate of rise and the total amplitude of isometric force and prolonged the relaxation phase. Furthermore, there was a drop in V0 that was related to the force decline in approximately the same way as observed during fatigue. The results support the idea the fatigue involves both a reduced state of activation of the contractile system and a specific (activation independent) inhibition of crossbridge turnover. Increased intracellular H+ concentration is likely to contribute to the development of both these effects during fatigue.

The role of intracellular buffers in acid-base disturbances: mathematical modelling

Shifts of Na+, K+, Cl- and HCO3- between the cells and extracellular fluid of nephrectomized mammals in acute response to hypercapnia and to HCl or KCl infusion are simulated in steady state models involving just intracellular buffering and a simply defined interdependence of ionic gradients. Such models integrate diverse kinds of data and suggest new interpretations. In respiratory acidosis K+, HCO3- and water leave some cells and move both to where chemical buffering is least (ECF and other cells) and to cells that regulate pH particularly well by active transport. Buffering by erythrocytes is important, but the effects of distinguishing erythrocytes from other cells in a model is mainly just to emphasize Cl- movements. Effects of departures from the mammalian norm of body composition are explored.

Intracellular pH changes during experimental sustained hypercapnia

During various time periods lasting 3--28 days rats were continuously exposed to FICO2 = 0.08 or 0.16 in normoxic conditions, pHi was measured by the 3H-inulin and 14C-DMO method in the erythrocyte, the gastrocnemius and in the whole body. The erythrocyte acid base disturbances were linked to the extracellular acidosis. The muscle and the mean body pHi developments were the same during 9 or 14 days depending on the FICO2. They diverged after 28 days at FICO2 = 0.08 (Tables and Fig. 2). This could be explained as an acid base reaction of the "non-muscular" part of the whole body intracellular compartment which may be different from the acid base development of the muscular mass. A short term (1 h) acute hypercapnia (FICO2 - 0.20--0.22) was superimposed on the sustained hypercapnia (FICO2 = 0.16). Acid base disturbance was greater when the acute hypercapnia was added at the beginning (3rd day) of the CO2 exposure (Fig.1).

Response of intrapulmonary chemoreceptors in the duck to changes in PCO2 and pH

We have estimated the relative importance of changes in blood PCO2 and pH in determining activity of intrapulmonary chemoreceptors (IPC) in the unidirectionally ventilated duck. The response of single unit vagal afferents from IPC to changing lung gas PCO2 was tested before and after changing blood pH by intravenous infusion of NaHCO3. Using multiple linear regression analysis, we calculated how much of the change in IPC activity for a given change in PCO2 was due to the changing PCO2 at constant pH (CO2 sensitivity) or to the change in pH concomitant with the change in PCO2 (H+ sensitivity). For 10 IPC, the CO2 sensitivity was on the average 2.3 times larger than the H+ sensitivity. Changes in pH as well as PCO2 of lung blood should be considered in assessing the role of IPC in control of breathing.

Carbon dioxide and acid base balance in the isolated rat diaphragm

A method for measuring the net acid base exchange in an isolated rat diaphragm preparation is described. Particular attention is paid to monitoring the functional status and maintaining optimal diffusion conditions. A steady net acid efflux of the order of 250 n mole/g-min is found in the resting state. This increases following a series of isometric contractions. In the resting state the total measured lactate + pyruvate efflux was found to be less than the net acid efflux. The net acid efflux increases following a sudden decrease in pCO2 and decreases or reverses following a sudden increase in pCO2 or a decrease in external bicarbonate. The net base loss during a period of 1 h following the exposure to high (20%) CO2 represents a large fraction of the predicted total bicarbonate generated within the fibres by non-bicarbonate buffers. This indicates that the effects of intracellular non-bicarbonate buffers can be transmitted to the external solution following a change in pCO2. The most plausible explanation is that passive bicarbonate ion movements are responsible. Values of the 'apparent PHCO3' have been calculated and vary under different conditions from a value of 1.3 X 10(-7) to 1.9 X 10(-6) cm-s-1.

Comparison of efflux rates of hydrogen and lactate ions from isolated muscles in vitro

Relative rates of efflux of hydrogen and lactate ions from skeletal muscle in vitro were determined on isolated rat diaphragms and frog satorius muscles. After a period of lacate accumulation by stimulation in vitro, muscles were suspended in a small volume of Ringer solution for different time periods lasting up to 60 min. The pH change in the solution was monitored continuously. After the predetermined time period, samples of the muscle and the Ringer solution were analysed for lactate content. Results showed that in both types of muscles the rate of efflux of hydrogen ions exceeded that of lactate ions by factors of about 14 and 50 in the case of diaphragm and sartorius muscles respectively. Because of this difference observed in the efflux kinetics of hydrogen and lactate ions, it is evident that the lactate content of a body compartment does not represent the absolute hydrogen ion load of the same compartment, particularly during the early phase of the efflux process.

The effect of the CO2/HCO3- buffer system on the membrane potential of frog skeletal muscle

1. The membrane potential of frog skeletal muscle was measured in various solutions, in the presence and in the absence of the CO2/HCO3- buffer. 2. The CO2/HCO3- buffer (PCO2 = 38-593 mm Hg; [HCO3-] = 5-25 mM/1) generally induced a reversible depolarization. 3. In the presence of C1-, there was a slowly developing but marked depolarization. 4. In the absence of C1-, there was an early depolarization which increased in high-PCO2 or low-K+ solutions, and decreased in low-PCO2, high-K+ or Na+-free solutions. Changing the HCO3- concentration did not modify the depolarization. 5. The early depolarization and contractions observed in C1--free Ringer persisted in presence of tubocurarine chloride (2.5-10(-5) M/1). 6. Possible mechanisms for the depolarization are discussed.

Blockade of myocardial slow inward current at low pH

The effect of low pH on the slow cationic inward current was studied in isolated perfused embryonic chick ventricles (16-21 days old). In order to study the slow current, the fast Na+ current was inactivated by partial depolarization to about -40 mV by elevation of K+ (25 mM). Subsequent exposure of the tissue to catecholamines or methylxanthines allowed slowly rising overshooting electrical responses (the "slow response") with with accompanying contractions to be elicited by electrical stimulation. These slow responses are insensitive to tetrodotoxin and are Na+- and Ca2+-dependent. It was found that the isoproterenol- and caffeine-induced slow responses were abolished at about pH 6.1; 50% inhibition occurred at about pH 6.5. The rate of rise of the normal action potential, which is dependent on a fast Na+ current, was only slightly affected at these same pH levels; however, electromechanical uncoupling occurred, as expected from inhibition of the slow current. Therefore, the slow current was blocked at an acid pH that did not block the fast Na+ current.

Intracellular pH in cold-blooded vertebrates as a function of body temperature

Intracellular pH (pHi) was measured in vivo in tissue of frogs (Rana catesbeiana) and turtles (Pseudemys scripta) using the DMO technique. Animals were permitted 3-8 days to come to a new steady-state body temperature (Tb) which ranged 5-32 degrees C. Least squares regression equation for pHi data are: frog blood, 8.184-0.0206 Tb; frog striated muscle, 7.275-0.0152 Tb; turtle blood, 8.092-0.0207Tb; turtle muscle, 7.421-0.0186 Tb; turtle heart, 7.452-0.0122 Tb; turtle liver, 7.753-0.0233 Tb; turtle esophageal smooth muscle, 7.513-0.0141 Tb. Only turtle cardiac muscle deltapHi/deltaT was significantly different from deltapH/deltaT of blood. Results have been interpreted in terms of protein charge state alterations; in the physiological pH range, histidine residues of proteins are the principal dissociable groups (HPr+ = H+ + Pr) affected by pHi and Tb changes. Constancy of protein charge state can be assessed by monitoring alpha imidazole, alphaIM = Pr/(HPr+ + Pr). A uniform pKIM of 6.85 (20degreesC) and a deltaHO of 7 kcal/mol are assumed in calculating alphaIM. Intracellular alphaIM is preserved in the tissues studied as body temperature changes. These results indicate that ectotherm acid-base balance, alphastat control, regulates not only extracellular blood proteins, but also intracellular compartment proteins in such a way as to preserve functions dependent upon protein net charge states.

Changes in skeletal muscle cell pH during graded changes in PCO2

Blood perfusing isolated dog gracilis muscles was equilibrated with CO2 tensions ranging from 30 to 120 mm Hg, resulting in venous P CO2 from 35 to 135 mm Hg. Extracellular pH values ranged from 6.96 to 7.41, and muscle cell pH, calculated from DMO distribution, ranged from 6.64 to 6.94. When intracellular pH was plotted as a function of the corresponding extracellular pH, a linear relationship (r = 0.92) was observed throughout the entire pH range. The slope deltapHi/deltapHe was 0.64, without evidence of a difference in slope at different pH values. These results do not support the previous observations in rat diaphragms that cell pH is not affected by PCO2 changes over a certain extracellular pH range. pHe was 0.64, without evidence of a difference in slope at different pH values. Theses results do not support the previous observations in rat diaphragms that cell pH is not affected by PCO2 changes over a certain extracellular pH range.

Extracellular and intracellular pH with changes of temperature in the dogfish Scyliorhinus stellaris

Larger Spotted Dogfish, Scyliorhinus stellaris, were exposed to varied ambient temperature (t) in order to determine the behavior of extracellular pH (pHe), Pco2 and bicarbonate concentration as well as intracellular pH (pHi) in three muscle types. pHe was found to vary with temperature slightly less than expected on the basis of the rule of constant relative alkalinity in juvenile (deltapH/deltat= -0.0148 per degree centigrade) as well as in adult (deltapH/deltat= -0.0136) fish. The absolute pHe values of adult fish were about 0.08 pH units higher than in juvenile fish. Arterial pco2 increased with rising temperature, the increase being much more marked in adult than in juvenile fish. Extracellular bicarbonate concentration (calculated from the pH and Pco2 values measured in arterial blood) was not maintained constant, but diminished in juvenile and increased in adult fish with increasing temperature, indicating that extracellular pH in dogfish is regulated by variations of both Pco2 and bicarbonate concentration. Variations of intracellular pH with temperature (deltapHi/deltat), -0.0178 for white muscle, -0.0334 for red muscle, and -0.0098 for heart muscle, were significantly different from the values of the extracellular compartment and, except for white muscle, significantly different from the condition for constant relative alkalinity (deltapH/deltat= -0.0183). These results are in agreement with the rule of constant relative alkalinity with respect to extracellular pH and possibly also with respect to an overall mean intracellular pH, but the rule is not quantitatively followed by the individual body compartments and tissues.

Cardiac and skeletal muscle acid-base composition during metabolic acidosis in dogs

Nephrectomized, open chested dogs were infused with 25-30 ml.kg(-1) body weight of 0.15 M NaCl (group I), 0.15 MHCl (Group II) or 0.3 M lactic acid (Group) III). Pulmonary ventilation was maintained constant in the three groups. Intracellular pH was calculated with the CO2 method. No significant intracellular or extracellular acid-base changes were produced in Group I. A similar degree of extracellular acidosis was achieved in Groups II and III. In spite of constant arterial PCO2, the PCO2 of mixed, coronary sinus and femoral vanous blood increased moderately after the infusion in Groups II and III. It was calculated that less than half of the HCl acid infused remained in the extracellular space. However, no significant changes were observed in the acid-base composition of skeletal muscle in either Group II or III. Comparison of the cardiac muscle cell acid-base composition of Group I with that of Groups II and III whows that metabolic acidosis of the degree and duration produced in these experiments does not produce appreciable myocardial acidosis.