Intracellular buffering of heart and skeletal muscles during the onset of hypercapnia

Contractile properties of skeletal muscle fibre bundles from mice deficient in carbonic anhydrase II

The function of cytosolic carbonic anhydrase (CA) isozyme II is largely unknown in skeletal muscle. Because of this, we compared the in vitro contractile properties of extensor digitorum longus (EDL) and soleus (SOL) fibre bundles from mice deficient in CA II (CAD) to litter mate controls (LM). Twitch rise, 1/2 relaxation time and peak twitch force at 22 degrees C of fibre bundles from CAD EDL [28.4+/-1.4 ms, 31.2+/-2.3 ms, 6.2+/-1.0 Newton/cm(2) (N/cm(2)), respectively] and CAD SOL (54.2+/-7.5 ms, 75.7+/-13.8 ms, 2.9+/-0.5 N/cm(2), respectively) were significantly higher compared to LM EDL (20.5+/-2.2 ms, 21.9+/-3.7 ms, 4.5+/-0.2 N/cm(2)) and LM SOL (42.8+/-3.5 ms, 51.4+/-2.4 ms, 2.1+/-0.4 N/cm(2)). However, in acidic Krebs-Henseleit solution, mimicking the pH, PCO(2), and HCO(3) (-) of arterial blood from CAD mice, twitch rise, 1/2 relaxation time, and peak twitch force of fibre bundles from CAD EDL (19.3+/-0.7 ms, 19.7+/-2.3 ms, 4.8+/-0.8 N/cm(2)) and CAD SOL (41.4+/-3.6 ms, 51.9+/-5.5 ms, 2.2+/-0.7 N/cm(2)) were not significantly different from LM fibre bundles in normal Krebs-Henseleit solution (EDL: 19.7+/-1.1 ms, 21.6+/-0.6 ms, 4.7+/-0.2 N/cm(2); SOL: 42.5+/-3.1 ms, 51.8+/-2.6 ms, 1.8+/-0.3 N/cm(2)). A higher pH(i) during exposure to acidic bathing solution was maintained by CAD EDL (7.37+/-0.02) and CAD SOL (7.33+/-0.05) compared to LM EDL (7.28+/-0.04) and LM SOL (7.22+/-0.02). This suggests that the skeletal muscle of CAD mice possesses an improved defense of pH(i) against elevated pCO(2). In support of this, apparent non-bicarbonate buffer capacity (in mequiv H(+) (pH unit)(-1) (kg cell H(2)O)(-1)) as determined by pH microelectrode was markedly increased in CAD EDL (75.7+/-4.1) and CAD SOL (85.9+/-3.3) compared to LM EDL (39.3+/-4.7) and LM SOL (37.5+/-3.8). Both latter phenomena may be related to the slowed rate of intracellular acidification seen in CAD SOL in comparison with LM SOL upon an increase in PCO(2) of the bath. In conclusion, skeletal muscle from mice deficient in CA II exhibits altered handling of acid-base challenges and shows normal contractile behavior at normal intracellular pH.

Use of base in the treatment of severe acidemic states

Severe acidemia (blood pH < 7.1 to 7.2) suppresses myocardial contractility, predisposes to cardiac arrhythmias, causes venoconstriction, and can decrease total peripheral vascular resistance and blood pressure, reduce hepatic blood flow, and impair oxygen delivery. These alterations in organ function can contribute to increased morbidity and mortality. Although it seemed logical to administer sodium bicarbonate to attenuate acidemia and therefore lessen the impact on cardiac function, the routine use of bicarbonate in the treatment of the most common causes of severe acidemia, diabetic ketoacidosis, lactic acidosis, and cardiac arrest, has been an issue of great controversy. Studies of animals and patients with these disorders have reported conflicting data on the benefits of bicarbonate, showing both beneficial and detrimental effects. Alternative alkalinizing agents, tris-hydroxymethyl aminomethane and Carbicarb, have shown some promise in studies of animals and humans, and reevaluation of these buffers in the treatment of severe acidemic states seems warranted. The potential value of base therapy in the treatment of severe acidemia remains an important issue, and further studies are required to determine which patients should be administered base therapy and what base should be used.

Extracellular carbonic anhydrase activity facilitates lactic acid transport in rat skeletal muscle fibres

1. In skeletal muscle an extracellular sarcolemmal carbonic anhydrase (CA) has been demonstrated. We speculate that this CA accelerates the interstitial CO2/HCO3- buffer system so that H+ ions can be rapidly delivered or buffered in the interstitial fluid. Because > 80 % of the lactate which crosses the sarcolemmal membrane is transported by the H+-lactate cotransporter, we examined the contributions of extracellular and intracellular CA to lactic acid transport, using ion-selective microelectrodes for measurements of intracellular pH (pHi) and fibre surface pH (pHs) in rat extensor digitorum longus (EDL) and soleus fibres. 2. Muscle fibres were exposed to 20 mM sodium lactate in the absence and presence of the CA inhibitors benzolamide (BZ), acetazolamide (AZ), chlorzolamide (CZ) and ethoxzolamide (EZ). The initial slopes (dpHs/dt, dpHi/dt) and the amplitudes (DeltapHs, DeltapHi) of pH changes were quantified. From dpHi/dt, DeltapHi and the total buffer factor (BFtot) the lactate fluxes (mM min-1) and intracellular lactate concentrations ([lactate]i) were estimated. 3. BFtot was obtained as the sum of the non-HCO3- buffer factor (BFnon-HCO3) and the HCO3- buffer factor (BFHCO3). BFnon-HCO3 was 35 +/- 4 mM pH-1 for the EDL (n = 14) and 86 /- 16 mM pH-1 for the soleus (n = 14). 4. In soleus, 10 mM cinnamate inhibited lactate influx by 44 % and efflux by 30 %; in EDL, it inhibited lactate influx by 37 % and efflux by 20 %. Cinnamate decreased [lactate]i, in soleus by 36 % and in EDL by 45 %. In soleus, 1 mM DIDS reduced lactate influx by 18 % and efflux by 16 %. In EDL, DIDS lowered the influx by 27 % but had almost no effect on efflux. DIDS reduced [lactate]i by 20 % in soleus and by 26 % in EDL. 5. BZ (0.01 mM) and AZ (0.1 mM), which inhibit only the extracellular sarcolemmal CA, led to a significant increase in dpHs/dt and pHs by about 40 %-150 % in soleus and EDL. BZ and AZ inhibited the influx and efflux of lactate by 25 %-50 % and reduced [lactate]i by about 40 %. The membrane-permeable CA inhibitors CZ (0.5 mM) and EZ (0.1 mM), which inhibit the extracellular as well as the intracellular CAs, exerted no greater effects than the poorly permeable inhibitors BZ and AZ did. 6. In soleus, 10 mM cinnamate inhibited the lactate influx by 47 %. Addition of 0.01 mM BZ led to a further inhibition by only 10 %. BZ alone reduced the influx by 37 %. 7. BZ (0.01 mM) had no influence on the Km value of the lactate transport, but led to a decrease in maximal transport rate (Vmax). In EDL, BZ reduced Vmax by 50 % and in soleus by about 25 %. 8. We conclude that the extracellular sarcolemmal CA plays an important role in lactic acid transport, while internal CA has no effect, a difference most likely attributable to the high internal vs. low extracellular BF(non-HCO3). The fact that the effects of cinnamate and BZ are not additive indicates that the two inhibitors act at distinct sites on the same transport pathway for lactic acid.

Myocardial acidosis associated with CO2 production during cardiac arrest and resuscitation

Previous studies from our institution demonstrated significant hypercarbic acidosis in the mixed venous (pulmonary artery) blood in animals and human patients during cardiac arrest and cardiopulmonary resuscitation (CPR). In the present study, the acid-base state of the myocardium during cardiac arrest was investigated. Cardiac arrest was electrically induced in 11 pentobarbital-anesthetized and mechanically ventilated domestic pigs. Precordial compression was begun 3 minutes after onset of ventricular fibrillation and continued for 8 minutes. During CPR, there was rapid onset of profound myocardial acidosis with an increase in intramyocardial [H+] from 54 +/- 5 to 146 +/- 20 nmol/l (7.27 +/- 0.04 to 6.88 +/- 0.20 pH units). Great cardiac vein PCO2 increased from 57 +/- 2 to 158 +/- 12 mm Hg. Profound hypercarbic acidosis in great cardiac vein blood was associated with myocardial lactate production to levels of 8.1 +/- 0.7 mmol/l. Only moderate decreases in cardiac vein bicarbonate concentrations from 31 +/- 1 to 23 +/- 1 mmol/l were observed. These acid-base changes were almost completely reversed over an interval of 60 minutes after the animals were successfully resuscitated by DC countershock. The PCO2 in cardiac vein blood was significantly greater than that of mixed venous blood, demonstrating disproportionate myocardial production of CO2 during CPR. Accordingly, it is CO2 production during ischemia that is implicated as the predominant mechanism accounting for myocardial [H+] increases during cardiac arrest. Important clinical implications for buffer therapy during CPR and, in particular, treatment with bicarbonate emerge from these observations.

Gas exchange analysis of immediate CO2 storage at onset of exercise

The immediate storage of CO2 at the onset of exercise was estimated in 5 subjects as the difference between measured and predicted CO2 production at work rates of 33, 53 and 75% VO2max. Three time varying models of respiratory exchange ratio (RER) were used to obtain an expected range of predicted CO2 production. Calculated CO2 storage in the transition from rest to 33% VO2max ranged from 0.33 to 0.38 ml X kg-1 X torr-1; from 33 to 53% VO2max, corresponding values were -0.04 to 0.14 ml X kg-1 X torr-1; and, from 53 to 75% VO2max, values ranged from -0.25 to 0.16 ml X kg-1 X torr-1. In two models, CO2 storage decreased significantly at the highest work rate. Estimates of CO2 storage by hyperventilation in the steady state of exercise yielded significantly greater storage. It was concluded that immediate CO2 storage at the onset of exercise was less than hyperventilation estimates, and that it tended to decrease with metabolic acidosis.

The effects of hypocapnia on intracellular pH and bicarbonate

The intracellular pH (pHi) and bicarbonate concentration ([HCO3-]icw) of cardiac and skeletal muscles were monitored during respiratory alkalosis in order to further elucidate the homeostatic processes which operate in these tissues to ameliorate deviations from normal acid-base status. Rats were mechanically hyperventilated to induce hypocapnia, pHi was determined by the DMO method, and [HCO-3]icw was calculated from the Henderson-Hasselbalch equation using pHi and the partial pressure of carbon dioxide of vena caval blood. A significant intracellular alkalosis occurred in both cardiac and skeletal muscles during hypocapnia, but the changes in pHi were less in the heart than in skeletal muscle. The decreases in cardiac [HCO-3]icw were greater than those attributable to the physicochemical buffering of the heart. These data are consistent with an intramyocardial source of protons other than physicochemical buffering during respiratory alkalosis. The decreases in skeletal muscle [HCO-3]icw were less than those due to physicochemical buffering. These data are consistent with a net extrusion from skeletal muscles cells of the protons derived from physicochemical buffering during respiratory alkalosis.

The effect of body warming on the ventilatory response to CO2 in the awake dog

Resting awake dogs breathing CO2 lower body temperature and such a change is reported to decrease the ventilatory response to CO2. Therefore, in 6 awake dogs the ventilatory response to CO2, obtained while body temperature was allowed to decrease spontaneously, was compared to the response obtained during warming of their body surface in an attempt to prevent the fall in temperature during hypercapnia. During warming, rectal temperature was maintained 0.5 to 0.7 degrees C higher than when TR was allowed to fall. In warm studies arterial [HCO-3]a was decreased relative to PaCO2 whereas there was a surprisingly large storage of CO2, 17.3 ml CO2/kg . Torr PaCO2. Ventilation was comparable in the dogs during air control of both experiments and before body warming was initiated. Subsequently, there was no consistent effect of warming on either the slope or threshold of the ventilatory response to CO2 with respect to either PaCO2 or pHa. Therefore, producing a higher and more 'normothermic' TR during hypercapnia in awake dogs had no definitive effect on chemical control of breathing.

Effect of short-term CO2-breathing on the CO2 content and intracellular pH in skeletal muscle of man

Subjects inhaled air with 8% CO2 for 10 min. Carbon dioxide tension in arterial blood increased rapidly from about 5 kPa to 7 kPa, whereas venous CO2 tension increased more slowly, reaching the arterial value at the end of the CO2-breathing period. Muscle content of total CO2 (HCO-3, H2CO3, solubilized CO2) did not change during CO2-breathing or the following 15 min of recovery. However, when CO2-breathing was combined with light bicycle exercise, total CO2 increased by 30%. This could be an effect of both increased local circulation and increased endogenous CO2 production in the muscle. It is concluded that 10 min CO2-breathing alone is insufficient to affect CO2 content and intracellular pH in resting skeletal muscle.

A comparison of carbon dioxide titration curves of arterial mixed venous and coronary sinus blood

Two types of titration curve (log PCO2 vs. pH and whole blood [HCO3-] vs. pH) were obtained in anaesthetized dogs by altering FICO2. Both plots reveal a significant progressive decrease in slope from arterial(a) to mixed venous (v) to coronary sinus (cs) blood. Lines drawn between corresponding points on the arterial and mixed venous or coronary sinus curves have slopes determined by the Respiratory Exchange Ratio (R) and are called R lines. Differences in slope between arterial and venous (v or cs) curves may be due to changes in blood flow or of delivery of CO2 to the blood. The former is indicated by constant R values at different CO2 tensions whilst the latter is a reflection of changing R values due to fluctuations in CO2 stores. The myocardium is not an important store for CO2 but CO2 has a profound effect on coronary blood flow and it is this that causes the difference in a-cs slopes. The reverse is true for skeletal muscle where the charging and discharging of CO2 stores brings about the difference in a-v slopes.

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.

Effect of hypocapnia on intracellular pH during metabolic acidosis

Separate and combined effects of acute metabolic acidosis and hypocapnia were determined in skeletal and cardiac muscles of intact rats. Normocapnic metabolic acidosis, imposed by intraperitoneal injection of hydrochloric acid (6 mEq/kg), did not change skeletal muscle intracellular acid--base parameters. Hypocapnia, induced by mechanical hyperventilation, resulted in intracellular alkalosis within skeletal muscle during both respiratory alkalosis and compensated metabolic acidosis; changes of skeletal muscle intracellular bicarbonate concentration per unit change in carbon dioxide tension were identical during these two experimental procedures. These data suggest that processes other than physicochemical buffering neutralize protons taken into skeletal muscle cells during acute metabolic acidosis. The acid--base state of the heart was quite stable during these experimental manipulations; thus, it appears that cardiac muscle has an extraordinary buffering ability. Moreover, our data suggest that processes other than physicochemical buffering maintain cardiac intracellular pH normal during hypocapnia.

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.