Measurement of Krogh's diffusion constant of CO2 in respiring muscle at various CO2 levels: evidence for facilitated diffusion

How does carbon dioxide permeate cell membranes? A discussion of concepts, results and methods

We review briefly how the thinking about the permeation of gases, especially CO₂, across cell and artificial lipid membranes has evolved during the last 100 years. We then describe how the recent finding of a drastic effect of cholesterol on CO₂ permeability of both biological and artificial membranes fundamentally alters the long-standing idea that CO₂—as well as other gases—permeates all membranes with great ease. This requires revision of the widely accepted paradigm that membranes never offer a serious diffusion resistance to CO₂ or other gases. Earlier observations of “CO₂-impermeable membranes” can now be explained by the high cholesterol content of some membranes. Thus, cholesterol is a membrane component that nature can use to adapt membrane CO₂ permeability to the functional needs of the cell. Since cholesterol serves many other cellular functions, it cannot be reduced indefinitely. We show, however, that cells that possess a high metabolic rate and/or a high rate of O₂ and CO₂ exchange, do require very high CO₂ permeabilities that may not be achievable merely by reduction of membrane cholesterol. The article then discusses the alternative possibility of raising the CO₂ permeability of a membrane by incorporating protein CO₂ channels. The highly controversial issue of gas and CO₂ channels is systematically and critically reviewed. It is concluded that a majority of the results considered to be reliable, is in favor of the concept of existence and functional relevance of protein gas channels. The effect of intracellular carbonic anhydrase, which has recently been proposed as an alternative mechanism to a membrane CO₂ channel, is analysed quantitatively and the idea considered untenable. After a brief review of the knowledge on permeation of O₂ and NO through membranes, we present a summary of the ¹⁸O method used to measure the CO₂ permeability of membranes and discuss quantitatively critical questions that may be addressed to this method.

Extramitochondrial domain rich in carbonic anhydrase activity improves myocardial energetics

CO2 is produced abundantly by cardiac mitochondria. Thus an efficient means for its venting is required to support metabolism. Carbonic anhydrase (CA) enzymes, expressed at various sites in ventricular myocytes, may affect mitochondrial CO2 clearance by catalyzing CO2 hydration (to H(+) and HCO3(-)), thereby changing the gradient for CO2 venting. Using fluorescent dyes to measure changes in pH arising from the intracellular hydration of extracellularly supplied CO2, overall CA activity in the cytoplasm of isolated ventricular myocytes was found to be modest (2.7-fold above spontaneous kinetics). Experiments on ventricular mitochondria demonstrated negligible intramitochondrial CA activity. CA activity was also investigated in intact hearts by (13)C magnetic resonance spectroscopy from the rate of H(13)CO3(-) production from (13)CO2 released specifically from mitochondria by pyruvate dehydrogenase-mediated metabolism of hyperpolarized [1-(13)C]pyruvate. CA activity measured upon [1-(13)C]pyruvate infusion was fourfold higher than the cytoplasm-averaged value. A fluorescent CA ligand colocalized with a mitochondrial marker, indicating that mitochondria are near a CA-rich domain. Based on immunoreactivity, this domain comprises the nominally cytoplasmic CA isoform CAII and sarcoplasmic reticulum-associated CAXIV. Inhibition of extramitochondrial CA activity acidified the matrix (as determined by fluorescence measurements in permeabilized myocytes and isolated mitochondria), impaired cardiac energetics (indexed by the phosphocreatine-to-ATP ratio measured by (31)P magnetic resonance spectroscopy of perfused hearts), and reduced contractility (as measured from the pressure developed in perfused hearts). These data provide evidence for a functional domain of high CA activity around mitochondria to support CO2 venting, particularly during elevated and fluctuating respiratory activity. Aberrant distribution of CA activity therefore may reduce the heart's energetic efficiency.

Carbon dioxide transport and carbonic anhydrase in blood and muscle

CO(2) produced within skeletal muscle has to leave the body finally via ventilation by the lung. To get there, CO(2) diffuses from the intracellular space into the convective transport medium blood with the two compartments, plasma and erythrocytes. Within the body, CO(2) is transported in three different forms: physically dissolved, as HCO(3)(-), or as carbamate. The relative contribution of these three forms to overall transport is changing along this elimination pathway. Thus the kinetics of the interchange have to be considered. Carbonic anhydrase accelerates the hydration/dehydration reaction between CO(2), HCO(3)(-), and H(+). In skeletal muscle, various isozymes of carbonic anhydrase are localized within erythrocytes but are also bound to the capillary wall, thus accessible to plasma; bound to the sarcolemma, thus producing catalytic activity within the interstitial space; and associated with the sarcoplasmic reticulum. In some fiber types, carbonic anhydrase is also present in the sarcoplasm. In exercising skeletal muscle, lactic acid contributes huge amounts of H(+) and by these affects the relative contribution of the three forms of CO(2). With a theoretical model, the complex interdependence of reactions and transport processes involved in CO(2) exchange was analyzed.

Acetazolamide reduces peripheral afferent transmission in humans

Carbonic anhydrase has been localized in skeletal muscle and nerve, thus, inhibition with acetazolamide (ACZ) may alter nerve and/or muscle function in healthy humans. ACZ (3 oral doses 14, 8, and 2 h prior to testing) reduced isometric force (37%) and peak to peak electromyographic (EMG) amplitude (1.38 mV to 0.83 mV), while increasing EMG latency associated with a unilateral Achilles tendon-tap. Reflex recovery profiles, following a contralateral conditioning tap, were similar in both placebo and ACZ experiments. ACZ led to significant changes in Hmax/Mmax ratio (52.19/14.42 to 45.73/15.65) and H-reflex latency (34.18 +/- 2.54 ms to 35.24 +/- 2.74 ms). Motor nerve conduction velocity and maximal voluntary isometric torque (knee extensors) were unaltered by ACZ. These data suggest that inhibition of the tendon-tap reflex and associated isometric force, following ACZ, is related to impairment of synaptic integrity between la fibers of the muscle spindle and the alpha motor neuron and not impairment of the muscle spindle or force-generating capacity.

Leakage of carbonic anhydrase III from normal and denervated rat skeletal muscle following contractile activity

Skeletal muscle extracellular carbonic anhydrase III was investigated in anesthetized rats by a microdialysis technique. A small dialysis probe was inserted into the tibialis anterior (TA) muscle and perfused continuously. Perfusates were collected before and during muscle contraction, induced by electrical stimulation of the muscle or of the sciatic nerve. In the perfusate of resting normal and denervated muscle, the concentration of CA III was 10 to 12 ng/mL, as measured by a radioimmunosorbent technique. During contractile activity, the concentrations of CA III increased markedly in the normal and denervated muscle. A TA muscle suspended in physiological saline behaved similarly, even though the leakage before and during contraction was higher than in vivo. The results show that skeletal muscle leaks CA III both in vivo and in vitro, a leakage which was markedly increased by contractile activity. The microdialysis technique should also be useful in humans to study the efflux of various proteins from different kinds of diseased or fatigued muscles.

Effects of carbonic anhydrase inhibitors on contraction, intracellular pH and energy-rich phosphates of rat skeletal muscle

1. The effects of carbonic anhydrase inhibitors on contractile parameters, intracellular pH (pHi) and energy-rich phosphates were studied in isolated rat soleus and extensor digitorum longus (EDL) muscles. 2. The muscles were incubated either in Ringer solutions (95% O2/5% CO2 = control) or in solutions to which one of the inhibitors, 5 X 10(-4) M-chlorzolamide or 10(-2) M-NaCNO, had been added. Muscles were stimulated directly and contracted under isometric conditions. 3. Compared with control muscles, both inhibitor-treated muscles showed a significantly decreased tetanic force and an increased half-relaxation time of twitches and tetani. Chlorzolamide increased time-to-peak in both muscles. Cyanate decreased isometric twitch force in both muscles. 4. Both inhibitors decreased pHi in both muscles; chlorzolamide by 0.1 unit, cyanate by 0.4 unit in soleus and by 0.8 unit in EDL. 5. Chlorzolamide increased the concentrations of creatine and inorganic phosphate (Pi) in soleus (the effect of chlorzolamide was not studied in EDL). Cyanate caused these same changes in soleus as well as EDL and in addition decreased the concentrations of ATP and phosphocreatine in soleus and EDL. 6. In muscles acidified by either low external HCO3- (2 mM) or by elevated PCO2 (30% CO2 in the gas phase) in the bath, decreases in isometric force and increases in half-relaxation time of tetani were observed. In addition there were increases in muscle Pi. These effects were more pronounced with 30% CO2 than with 2 mM-HCO3-. 7. Neither acidifying solutions prolonged either half-relaxation time or time-to-peak of twitches. 8. We conclude that carbonic anhydrase inhibition exerts its effect (a) on isometric tension at least partly via an elevated Pi (perhaps in combination with lowered pHi); (b) on the half-relaxation time of tetani by means of lowered pHi and elevated concentration of Pi; (c) on relaxation and time-to-peak of twitches by some unknown mechanism, neither directly by a change in pHi nor in Pi.

Nonequilibrium facilitated transport of carbon dioxide in bicarbonate and bovine albumin solutions

The nonlinear diffusion-reaction equations describing the nonequilibrium transport of CO2 through flat layers of complex but homogeneous aqueous media were solved by an approximate analytical method called the "Combined Damköhler Number" (CDN) technique. Unlike other approximate analytical solutions, the CDN technique is valid for the full range of Damköhler numbers, i.e., for any layer thickness. The present theoretical treatment uses as a basis the equilibrium approach of Stroeve, Hoofd, and Kreuzer which accounts for any species in the solution except for possible carbamate formation (binding of CO2 by the protein). The nonequilibrium model developed here for CO2 transport is the most general technique currently available in the literature. Theoretical results were compared to experimental data from the literature for diffusion in bicarbonate and albumin solutions and were generally in good agreement. Results obtained from numerical calculations were also compared and were found to be in excellent agreement with the CDN results.

Carbonic anhydrase in the sarcoplasmic reticulum of rabbit skeletal muscle

Sarcoplasmic reticulum vesicles and mitochondria were prepared from red and white skeletal muscles of the rabbit. The preparations were characterized in terms of their specific activities of citrate synthase, basal (Mg2+-dependent) and Ca2+-dependent ATPase (the latter two in the presence of NaN3 and ouabain), and their specific carbonic anhydrase activities were determined. Skeletal muscle mitochondria had high specific activities of citrate synthase (700-1200 mu. mg protein-1) and low carbonic anhydrase activities (0.1-0.4 u. ml mg protein-1). The latter are likely to be due to a contamination of the preparations with sarcoplasmic reticulum (s.r.) Preparations of s.r. vesicles showed negligible activities of citrate synthase and the expected differing patterns of basal and Ca2+-dependent ATPase in red and white muscles. Specific carbonic anhydrase activities in s.r. from both muscle types were high (2-4 u. ml mg protein-1). The highest carbonic anhydrase activity, 11 u. ml mg protein-1, was found in s.r. from rabbit m. masseter. The inhibition constant of s.r. carbonic anhydrase towards acetazolamide was 4-6 X 10(-8) M and similar but not identical to that of cytosolic carbonic anhydrase II. It appears possible that the carbonic anhydrase II-like enzyme previously found by us in muscle homogenates (Siffert & Gros, 1982) originates from the s.r. Histochemical studies using the dansylsuphonamide method described previously (Dermietzel, Leibstein, Siffert, Zamboglou & Gros, 1985) showed an intracellular pattern of carbonic anhydrase staining compatible with the presence of the enzyme in s.r.: spots homogeneously distributed across the fibre cross-sections in transversely sectioned fibres and thin, longitudinally oriented, bands in longitudinally sectioned fibres. It is estimated that s.r. carbonic anhydrase accelerates CO2 hydration within the s.r. approximately 1000-fold. Thus, CO2 and HCO3- react fast enough to provide a rapid source and sink for protons leaving and entering the s.r. in exchange for Ca2+.

Facilitated diffusion of carbon dioxide in whole blood and hemoglobin solutions

The values of effective permeability (Krogh's diffusion coefficient) for carbon dioxide have been measured in horizontal stationary layers of whole blood and hemoglobin solutions in quasi-steady state, with the goal of understanding the specific nature of facilitated diffusion of carbon dioxide occurring in these media. The average partial pressure of carbon dioxide within the layer ranged from 0.74 kPa (5.6 mm Hg) to 15.7 kPa (118 mm Hg). Facilitation effects were significant in hemolysed blood and in hemoglobin solutions at low pCO2; the facilitation factor was up to 2.3. Facilitation effects were considerably less for intact blood; the facilitation factor of intact blood with hematocrit 45% was 0.3. The presence of the red cell membrane appears to have a negative effect on facilitation of carbon dioxide transport.

Effects of inhibiting carbonic anhydrase on isometric contraction of frog skeletal muscle

Carbonic anhydrase (CA) activity was determined in a homogenate of frog skeletal muscle by measuring the kinetics of CO2 hydration in a pH stopped-flow apparatus. The results suggest that frog skeletal muscle contains a high-activity CA with properties similar to those of the isoenzyme CA II found in white skeletal muscle tissue of the rabbit. In an attempt to assess the functional significance of CA in skeletal muscle, the maximal isometric force of frog gastrocnemius muscle was measured in response to direct or indirect (ischiadic nerve) single-pulse electrical stimulation before (control) and after exposing the muscle to various concentrations of the specific carbonic anhydrase inhibitors, ethoxzolamide, acetazolamide, and methazolamide. In the range of ethoxzolamide concentration between 10(-9) and 10(-6) M, maximal isometric force with indirect supramaximal stimulation declined progressively with inhibitor concentration to less than 10% of the control value. Acetazolamide and methazolamide were less effective in that concentrations of above 10(-4) M were necessary to inhibit maximum isometric force by 50%. Even at the highest ethoxzolamide concentration used (10(-6) M), no effect was observed either on the amplitude of the compound nerve action potential or on the conduction velocity of group I fibres in the ischiadic nerve, suggesting that ethoxzolamide did not affect the mechanisms responsible for spike generation or conduction in the motor fibres. With direct supramaximal stimulation of the gastrocnemius muscle, no effects on maximal isometric force were observed of CA inhibition by any of the inhibitors used. The results suggest that CA acts on the neuromuscular transmission. The exact site and mechanism of action are unknown.

Carbonic anhydrase C in white-skeletal-muscle tissue

We investigated the activity of carbonic anhydrase in blood-free perfused white skeletal muscles of the rabbit. Carbonic anhydrase activities were measured in supernatants and in Triton extracts of the particulate fractions of white-skeletal-muscle homogenate by using a rapid-reaction stopped-flow apparatus equipped with a pH electrode. An average carbonic anhydrase concentration of about 0.5 microM was determined for white skeletal muscle. This concentration is about 1% of that inside the erythrocyte. Some 85% of the muscle enzyme was found in the homogenate supernatant, and only 15% appeared to be associated with membranes and organelles. White-skeletal-muscle carbonic anhydrase was characterized in terms of its Michaelis constant and catalytic-centre activity (turnover number) for CO2 and its inhibition constant towards ethoxzolamide. These properties were identical with those of the rabbit erythrocyte carbonic anhydrase C, suggesting that a type-C enzyme is present in white skeletal muscle. Affinity chromatography of muscle supernatant and of lysed erythrocytes showed that, whereas rabbit erythrocytes contain about equal amounts of carbonic anhydrase isoenzymes B and C, the B isoenzyme is practically absent from white skeletal muscle. Similarly, ethoxzolamide-inhibition curves suggested that white skeletal muscle contains no carbonic anhydrase A. It is concluded that white skeletal muscle contains essentially one carbonic anhydrase isoenzyme, the C form, most of which is probably of cytosolic origin.

Gas exchange in air sacs: contribution to respiratory gas exchange in ducks

Air sac gas exchange was studied in ducks by measuring the rates of inert gas uptake and of O2 and CO2 equilibration in caudal thoracic air sac whose ventilation was prevented by surgival sealing of the ostia. The data were analyzed on a model incorporating three possible routes by which air sac gas could be exchanged with the surrounding tissue: (1) into the blood perfusing the air sac walls; (2) into the adjoining air sac via tissue membranes; (3) into the bronchial system of the lung via diffusion through lung tissue bordering upon the caudal thoracic air sac. Exchange rates of gases via the two latter paths were found to be small as compared with the first route. From application of model parameters to O2 and CO2 exchange in air sacs under physiological conditions the following conclusions were drawn: (1) the caudal thoracic air sac makes the major contribution to total gas exchange between air sacs and blood; (2) this exchange can account for less than 5% of total respiratory gas exchange; (3) the exchange is too small to account for the O2 and CO2 partial pressures in caudal thoracic air sacs of ducks. Other mechanisms like gas exchange in neopulmonic parabronchi, which conduct air to the caudal air sacs during inspiration or re-inspiration of dead space appear to play a more significant role in the deviation of O2 and CO2 partial pressures in the caudal air sacs from those in inspired air.