Acid-base disequilibrium in the arterial blood of rainbow trout

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

Enhanced hemoglobin-oxygen unloading in migratory salmonids

Recent findings indicate that some teleost fishes may be able to greatly enhance hemoglobin-oxygen (Hb-O₂) unloading at the tissues under conditions that result in catecholamine release. The putative mechanism relies on the high pH sensitivity of teleost hemoglobin (Hb), intracellular red blood cell (RBC) pH regulation via β-adrenergic Na⁺/H⁺ exchanger (β-NHE) activity, and plasma-accessible carbonic anhydrase at the tissues that short-circuits RBC pH regulation. Previous studies have shown that in rainbow trout, this system may double Hb-O₂ unloading to red muscle compared to a situation without short-circuiting. The present study determined that: (1) in rainbow trout this system may be functional even at low concentrations of circulating catecholamines, as shown by conducting a dose-response analysis; (2) Atlantic and coho salmon also possess β-NHE activity, as shown by changes in hematocrit in adrenergically stimulated cells; and (3) with β-NHE short-circuiting, Atlantic and coho salmon may be able to increase Hb-O₂ unloading by up to 74 and 159%, respectively, as determined by modeling based on O₂ equilibrium curves. Together, these results indicate that a system to enhance Hb-O₂ unloading may be common among salmonids and may be operational even under routine conditions. In view of the life histories of Atlantic and coho salmon, a system to enhance Hb-O₂ unloading during exercise may help determine a successful spawning migration and thus reproductive success.

Phosphorylation increases the catalytic activity of rainbow trout gill cytosolic carbonic anhydrase

Cytoplasmic carbonic anhydrase (CAc) in the gill of teleost fish contributes to ionic regulation and acid–base balance by catalyzing the reversible reaction of CO2 and water, CO2 + H2O ↔ H(+) + HCO3(-). Regulation of CAc abundance and activity therefore is expected to fine-tune responses to ionic or acid–base challenges. The present study investigated the potential for gill CAc of rainbow trout, Oncorhynchus mykiss (tCAc), to undergo reversible phosphorylation. The activity of tCAc was approximately doubled by phosphorylation achieved through in vitro stimulation of endogenous protein kinases; kinase stimulation doubled phospho-threonine content from that observed in tCAc isolated under conditions where both kinases and protein phosphatases were inhibited. In vitro incubation to preferentially stimulate specific kinases implicated protein kinase G (PKG) in mediating the increase in tCAc activity. The kinetic parameters of turnover number (k cat) and substrate affinity (K m) were similarly affected by stimulation of either kinase or phosphatase action. However, phosphorylation via kinase stimulation significantly increased the efficiency of tCAc (V max /K m), and this factor may have contributed to the elevation of tCAc activity. In addition, phosphorylation of tCAc by kinase stimulation significantly increased the inhibition constant (K i) for acetazolamide. These results demonstrate that tCAc is subject to reversible phosphorylation; future work should focus on identifying the physiological situation(s) in which phosphorylation of trout branchial CAc occurs.

A unique mode of tissue oxygenation and the adaptive radiation of teleost fishes

Teleost fishes constitute 95% of extant aquatic vertebrates, and we suggest that this is related in part to their unique mode of tissue oxygenation. We propose the following sequence of events in the evolution of their oxygen delivery system. First, loss of plasma-accessible carbonic anhydrase (CA) in the gill and venous circulations slowed the Jacobs-Stewart cycle and the transfer of acid between the plasma and the red blood cells (RBCs). This ameliorated the effects of a generalised acidosis (associated with an increased capacity for burst swimming) on haemoglobin (Hb)-O2 binding. Because RBC pH was uncoupled from plasma pH, the importance of Hb as a buffer was reduced. The decrease in buffering was mediated by a reduction in the number of histidine residues on the Hb molecule and resulted in enhanced coupling of O2 and CO2 transfer through the RBCs. In the absence of plasma CA, nearly all plasma bicarbonate ultimately dehydrated to CO2 occurred via the RBCs, and chloride/bicarbonate exchange was the rate-limiting step in CO2 excretion. This pattern of CO2 excretion across the gills resulted in disequilibrium states for CO2 hydration/dehydration reactions and thus elevated arterial and venous plasma bicarbonate levels. Plasma-accessible CA embedded in arterial endothelia was retained, which eliminated the localized bicarbonate disequilibrium forming CO2 that then moved into the RBCs. Consequently, RBC pH decreased which, in conjunction with pH-sensitive Bohr/Root Hbs, elevated arterial oxygen tensions and thus enhanced tissue oxygenation. Counter-current arrangement of capillaries (retia) at the eye and later the swim bladder evolved along with the gas gland at the swim bladder. Both arrangements enhanced and magnified CO2 and acid production and, therefore, oxygen secretion to those specialised tissues. The evolution of β-adrenergically stimulated RBC Na(+)/H(+) exchange protected gill O2 uptake during stress and further augmented plasma disequilibrium states for CO2 hydration/dehydration. Finally, RBC organophosphates (e.g. NTP) could be reduced during hypoxia to further increase Hb-O2 affinity without compromising tissue O2 delivery because high-affinity Hbs could still adequately deliver O2 to the tissues via Bohr/Root shifts. We suggest that the evolution of this unique mode of tissue O2 transfer evolved in the Triassic/Jurassic Period, when O2 levels were low, ultimately giving rise to the most extensive adaptive radiation of extant vertebrates, the teleost fishes.

Carbonic anhydrase and acid–base regulation in fish

Carbonic anhydrase (CA) is the zinc metalloenzyme that catalyses the reversible reactions of CO2 with water. CA plays a crucial role in systemic acid–base regulation in fish by providing acid–base equivalents for exchange with the environment. Unlike air-breathing vertebrates, which frequently utilize alterations of breathing (respiratory compensation) to regulate acid–base status, acid–base balance in fish relies almost entirely upon the direct exchange of acid–base equivalents with the environment (metabolic compensation). The gill is the critical site of metabolic compensation, with the kidney playing a supporting role. At the gill, cytosolic CA catalyses the hydration of CO2 to H+ and HCO3– for export to the water. In the kidney, cytosolic and membrane-bound CA isoforms have been implicated in HCO3– reabsorption and urine acidification. In this review, the CA isoforms that have been identified to date in fish will be discussed together with their tissue localizations and roles in systemic acid–base regulation.

The role of branchial carbonic anhydrase in acid-base regulation in rainbow trout (Oncorhynchus mykiss)

The objective of the present study was to examine the branchial distribution of the recently identified rainbow trout cytoplasmic carbonic anhydrase isoform (tCAc) and to investigate its role in the regulation of acid-base disturbances in rainbow trout (Oncorhynchus mykiss). In situ hybridization using an oligonucleotide probe specific to tCAc revealed tCAc mRNA expression in both pavement cells and mitochondria-rich cells (chloride cells). Similarly, using a homologous polyclonal antibody,tCAc immunoreactivity was localized to pavement cells and mitochondria-rich cells in the interlamellar region and along the lamellae of the gills. Exposure of rainbow trout to hypercarbia (∼0.8% CO2) for 24 h resulted in significant increases in tCAc mRNA expression (∼20-fold;quantified by real-time PCR) and protein levels (∼1.3-fold; quantified by western analysis) but not enzyme activity (assessed on crude gill homogenates using the delta-pH CA assay). Inhibition of branchial CA activity in vivo using acetazolamide reduced branchial net acid excretion significantly by 20%. This effect was enhanced to a 36% reduction in branchial net acid excretion by subjecting the trout to hypercarbia (∼0.8%CO2) for 10 h prior to acetazolamide injection, an exposure that significantly increased branchial net acid excretion. The results of the present study support the widely held premise that branchial intracellular CA activity (tCAc) plays a key role in regulating acid-base balance in freshwater teleost fish.

Inhibitory effects of ammonia and urea on gill carbonic anhydrase enzyme activity of rainbow trout (Oncorhynchus mykiss)

The effects of ammonia and urea on branchial carbonic anhydrase (CA) enzyme which plays a key role in ionoregulation, osmoregulation and acid-base balance of rainbow trout (Oncorhynchus mykiss) were investigated. CA activity of the control group for ammonia and urea was determined as 1285.7 ± 67.9 and 1261.7 ± 60.8EU/mg protein, respectively. The CA enzyme activities of the other groups were measured at 1, 2 and 3h after ammonia and urea applications. The corresponding activities of ammonia were 774.9 ± 68.8, 732.1 ± 48.6 and 768.1 ± 59.5EU/mg protein, respectively and that of urea were 769.3 ± 58.9, 638.2 ± 47.7 and 1108.1 ± 61.1EU/mg protein, respectively. The differences between the initial CA activities for the controls was not significantly (P > 0.01). The CA activities were significantly (P < 0.01) inhibited both in ammonia and urea group. However, the ammonia inhibited more than urea since there was significant differences between final values of gill CA activities.

Branchial membrane-associated carbonic anhydrase activity maintains CO2excretion in severely anemic dogfish

Plasma CO2reactions in Pacific spiny dogfish ( Squalus acanthias) have access to plasma and gill membrane-associated carbonic anhydrase (CA). Acute severe experimental anemia and selective CA inhibitors were used to investigate the role of extracellular CA in CO2excretion. Anemia was induced by blood withdrawal coupled to volume replacement with saline. Lowering hematocrit from 14.2 ± 0.4% (mean ± SE; N = 31) to 5.2 ± 0.1% ( N = 31) had no significant impact on arterial or venous CO2tensions (PaCO2and PvCO2, respectively) over the subsequent 2 h. Pco2was maintained despite the reduction in red cell number and a significant 32% increase in cardiac output (V̇b), both of which have been found to cause PaCO2increases in teleost fish. By contrast, treatment of anemic dogfish with the CA inhibitors benzolamide (1.3 mg/kg) or F3500 (50 mg/kg), to selectively inhibit extracellular CA, elicited rapid and significant increases in PaCO2of 0.68 ± 0.17 Torr ( N = 6) and 0.53 ± 0.11 Torr ( N = 7), respectively, by 30 min after treatment. These findings provide a functional context in which extracellular CA in dogfish contributes substantially to CO2excretion. Additionally, the apparent lack of effect of V̇bchanges on Pco2suggests that, in contrast to teleost fish, CO2excretion in dogfish does not behave as a diffusion-limited system.

Buffering limits plasma HCO3− dehydration when red blood cell anion exchange is inhibited

Theory suggests that HCO3- dehydration in the plasma of rainbow trout is limited by both the absence of carbonic anhydrase (CA) activity and the low non-bicarbonate buffer capacity of the plasma (betaplasma). The potential for betaplasma to limit plasma HCO3- dehydration was assessed in rainbow trout in which HCO3- dehydration via the red blood cell (RBC) was inhibited using the anion exchange blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). DIDS administration reduced the rate of RBC HCO3- dehydration by 68-80% for at least 6h, resulting in the elevation of arterial CO2 tension (PaCO2) by 3.07 +/- 0.45 Torr (N = 6). Addition of bovine CA to the circulation of DIDS-treated trout caused PaCO2 to decrease significantly. This effect was increased significantly in rainbow trout in which betaplasma was elevated experimentally by intravascular injection of N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES), supporting the hypothesis that CA-catalysed HCO3- dehydration in the plasma of rainbow trout is limited by proton availability.

Sensing and transfer of respiratory gases at the fish gill

The gill is both a site of gas transfer and an important location of chemoreception or gas sensing in fish. While often considered separately, these two processes are clearly intricately related because the gases that are transferred between the ventilatory water and blood at the gill are simultaneously sensed by chemoreceptors on, and within, the gill. Modulation of chemoreceptor discharge in response to changes in O(2) and CO(2) levels, in turn, is believed to initiate a series of coordinated cardiorespiratory reflexes aimed at optimising branchial gas transfer. The past decade has yielded numerous advances in terms of our understanding of gas transfer and gas sensing at the fish gill, particularly concerning the transfer and sensing of carbon dioxide. In addition, recent research has moved from striving to construct a single model that covers all fish species, to recognition of the considerable inter-specific variation that exists with respect to the mechanics of gas transfer and the cardiorespiratory responses of fish to changes in O(2) and CO(2) levels. The following review attempts to integrate gas transfer and gas sensing at the fish gill by exploring recent advances in these areas.

The buffering power of plasma in brown bullhead (Ameiurus nebulosus)

Brown bullhead (Ameiurus nebulosus) blood plasma was found to exhibit an unusually high non-bicarbonate buffer capacity (beta) in relation to that of other teleost fish. In brown bullhead, the non-bicarbonate buffer capacity of plasma (beta(plasma)), at -5.72 +/- 0.34 mmol l(-1) pH unit(-1) (mean +/- S.E.M., N=30), constituted 37% of whole blood beta and was 2.5 times higher than beta(plasma) in rainbow trout (-2.33 +/- 0.42 mmol l(+/-1) pH unit(-1); N=7). The strong buffering power of bullhead plasma was not the result of unusually high plasma protein levels. Size separation chromatography in conjunction with a spectrophotometric assay for buffering capacity were used to isolate a plasma fraction of high buffering power. SDS-polyacrylamide gel electrophoresis revealed that this fraction contained four proteins, but was dominated by a protein of approximately 68-70 kDa molecular mass. On the basis of the amino acid composition of this fraction, the dominant protein was identified as albumin. In comparison to other fish albumins, bullhead albumin appears to be histidine-rich (6.7%). Thus, the unusually high non-bicarbonate buffer capacity of bullhead plasma appears to stem from the presence in the plasma of a histidine-rich albumin.

Sensitivity of CO2 excretion to blood flow changes in trout is determined by carbonic anhydrase availability

The blood transit time through the gills of rainbow trout (Oncorhynchus mykiss) was modified by manipulation of cardiac output (Vb). The experiments tested the hypothesis that efficiency of CO2 excretion is sensitive to changes in blood flow owing to chemical equilibrium limitations. An extracorporeal blood shunt was used to continuously monitor blood gases in fish in which Vb was elevated (by 13.3 +/- 2.4 ml x min(-1) x kg(-1)) by intravascular saline injection or reduced (by 10.8 +/- 1.8 ml x min(-1) x kg(-1)) by removal of plasma. The arterial partial pressure of CO2 (Pa(CO2); an index of CO2 excretion efficiency) was increased with elevated Vb and was decreased with reduced Vb such that the changes in Pa(CO2) exhibited a significant positive sigmoidal relationship with the changes in Vb (r2 =0.75; P < 0.05). In contrast, there was no significant relationship between changes in the arterial partial pressure of O2 (Pa(O2); an index of O2 uptake efficiency) and changes in Vb (r2 = 0.07; P > 0.05). The intravenous administration of carbonic anhydrase (CA; 10 mg/kg) before vascular volume loading eliminated the increase in Pa(CO2) with increased Vb that was observed in control fish.

The CO2/pH ventilatory drive in fish

That ventilation in fish is driven by O2 has long been accepted. The O2 ventilatory drive reflects the much lower capacitance of water for O2 than for CO2, and is mediated by O2 receptors that are distributed throughout the gill arches and that monitor both internal and external O2 levels. In recent years, however, evidence has amassed in support of the existence of a ventilatory drive in fish that is keyed to CO2 and/or pH. While ventilatory responses to CO2/pH may be mediated in part by the O2 drive through CO2/pH-induced changes in blood O2 status, CO2/pH also appear to stimulate ventilation directly. The receptors involved in this pathway are as yet unknown, but the experimental evidence available to date supports the involvement of branchial CO2-sensitive chemoreceptors with an external orientation. Internally-oriented CO2-sensitive chemoreceptors may also be involved, although evidence on this point remains equivocal. In the present paper, the evidence for a CO2/pH-keyed ventilatory drive in fish will be reviewed.

Extracellular carbonic anhydrase in the dogfish, Squalus acanthias: a role in CO2 excretion

In Pacific spiny dogfish (Squalus acanthias), plasma CO(2) reactions have access to plasma carbonic anhydrase (CA) and gill membrane-associated CA. The objectives of this study were to characterise the gill membrane-bound CA and investigate whether extracellular CA contributes significantly to CO(2) excretion in dogfish. A subcellular fraction containing membrane-associated CA activity was isolated from dogfish gills and incubated with phosphatidylinositol-specific phospholipase C. This treatment caused significant release of CA activity from its membrane association, a result consistent with identification of the dogfish gill membrane-bound CA as a type IV isozyme. Inhibition constants (K(i)) against acetazolamide and benzolamide were 4.2 and 3.5 nmol L(-1), respectively. Use of a low dose (1.3 mg kg(-1) or 13 micromol L(-1)) of benzolamide to selectively inhibit extracellular CA in vivo caused a significant 30%-60% reduction in the arterial-venous total CO(2) concentration difference, a significant increase in Pco(2) and an acidosis, without affecting blood flow or ventilation. No effect of benzolamide on any measure of CO(2) excretion was detected in rainbow trout (Oncorhynchus mykiss). These results indicate that extracellular CA contributes substantially to CO(2) excretion in the dogfish, an elasmobranch, and confirm that CA is not available to plasma CO(2) reactions in rainbow trout, a teleost.

Cardiovascular effects of hypercarbia in rainbow trout (Oncorhynchus mykiss): a role for externally oriented chemoreceptors

In situ and in vivo experiments were performed on rainbow trout (Oncorhynchus mykiss) to examine (i) the direct effect of CO(2) on the systemic vasculature and (ii) the influence of internal versus external hypercapnic acidosis on cardiovascular variables including blood pressure, cardiac output and systemic vascular resistance. Results from in situ saline-perfused trunk preparations indicated that CO(2) (0.6, 1.0 or 2.0% CO(2)) elicited a significant vasodilation, but only in the presence of pre-existing humoral adrenergic tone. In the absence of pre-existing vascular tone, CO(2) was without effect on systemic resistance. In contrast, hypercarbia in vivo triggered a statistically significant increase in systemic resistance (approximately 70 %) that was associated with elevated ventral aortic (approximately 42 %) and dorsal aortic (approximately 43 %) blood pressures and with a significant bradycardia (approximately 12 %); cardiac output was not significantly affected. To determine the potential roles of internal versus external chemoreceptors in mediating the cardiovascular responses to hypercarbia, experiments were performed to elevate the endogenous arterial partial pressure of CO(2) (Pa(CO2)) without an accompanying increase in external P(CO2) (Pw(CO2)). In one series, trout were given a bolus injection of the carbonic anhydrase inhibitor acetazolamide (30 mg kg(−1)) to inhibit CO(2) excretion, and thus raise Pa(CO2), 5–7 h prior to being exposed to an acute increase in Pw(CO2) (maximum Pw(CO2)=6.3+/−0.4 mmHg; 1 mmHg=0.133 kPa). Despite a marked increase in Pa(CO2) (approximately 7 mmHg) after injection of acetazolamide, there was no increase in dorsal aortic blood pressure (P(DA)) or systemic resistance (R(S)). The ensuing exposure to hypercarbia, however, significantly increased P(DA) (by approximately 20 %) and R(S) (by approximately 35 %). A second series of experiments used a 5–7 h period of exposure to hyperoxia (Pw(O2)=643+/−16 mmHg) to establish a new, elevated baseline Pa(CO2) (7.8+/−1.1 mmHg) without any change in Pw(CO2). Despite a steadily increasing Pa(CO2) during the 5–7 h of hyperoxia, there was no associated increase in P(DA) or R(S). Ensuing exposure to hypercarbia, however, significantly increased P(DA) (by approximately 20 %) and R(S) (by approximately 150 %). Plasma adrenaline levels were increased significantly during exposure to hypercarbia and, therefore, probably contributed to the accompanying cardiovascular effects. These findings demonstrate that the cardiovascular effects associated with hypercarbia in rainbow trout are unrelated to any direct constrictory effects of CO(2) on the systemic vasculature and are unlikely to be triggered by activation of internally oriented receptors. Instead, the data suggest that the cardiovascular responses associated with hypercarbia are mediated exclusively by externally oriented chemoreceptors.

The distribution and physiological significance of carbonic anhydrase in vertebrate gas exchange organs

The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration/dehydration of CO(2) and water, maintaining a near-instantaneous equilibrium among all chemical species involved in the reaction. CA is found in association with all tissue and organ systems involved in the transport and excretion of CO(2), from the site of CO(2) production, metabolically active tissue such as muscle, to circulating red blood cells in the vasculature, to the various organs of gas exchange, the lungs and gills. The presence of the enzyme in every fluid compartment along the pathway of CO(2) transport appears necessary in order to allow the dehydration of HCO(3)(-) to keep pace with the rapid diffusion of CO(2) across biological membranes. Within the actual organ of gas exchange, CA is compartmentalized in multiple subcellular fractions, with the specific subcellular localization determining the enzyme's physiological function.

CO2 transport and excretion in rainbow trout (Oncorhynchus mykiss) during graded sustained exercise

A quantitative analysis of CO2 transport and excretion was conducted in seawater acclimated rainbow trout (Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO2 excretion increased linearly with cardiac output during exercise but arterial P(CO2) (Pa(CO2)) and total CO2 levels also increased indicating a diffusion limitation to CO2 excretion. The elevated Pa(CO2) was not accompanied by a decrease in pH, indicating that the acid-base compensation was rapid. Mixed-venous P(CO2) increased to a greater extent than Pa(CO2) resulting in a large increase in the venous arterial difference in P(CO2) (Pv(CO2) - Pa(CO2)). The Pv(CO2) - Pa(CO2) difference was used to calculate the proportion of total CO2 excreted comprised of dissolved CO2 which accounted for less than 1% of total CO2 excreted in fish swimming at 11 cm sec(-1) but increased to about 9% at the greatest swimming velocity indicating that the pattern of CO2 excretion changes during exercise. There was no effect of exercise on the proportion of CO2 excreted which was dependent upon HCO3-/Cl- exchange (54%) or that which was dependent upon the dehydration of HCO3- that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO2 excreted that was dependent upon HCO3-/Cl- exchange is significant because this is thought to be the rate limiting step in CO2 excretion.

Interactions between ion and gas transfer in freshwater teleost fish

Carbonic anhydrase and proton ATPase are co-distributed, being restricted to the apical regions of the gill epithelium of freshwater teleosts. Carbonic anhydrase supplies protons to the apical proton ATPase. Carbonic anhydrase is absent from the basal regions of the gill epithelium. Plasma flowing through the gills has no available carbonic anhydrase activity and plasma CO2/bicarbonate reactions are uncatalyzed. Thus, bicarbonate dehydration in plasma is negligible, and catalyzed bicarbonate dehydration occurs in erythrocytes in blood flowing through the gills. This results in tight coupling of carbon dioxide excretion to oxygen uptake and the evolution of hemoglobins with large Haldane effects but low buffering capacities, typical of many freshwater teleosts. Tight coupling of carbon dioxide and oxygen transfer in these fish also ensures that the Root shift does not impair oxygen uptake at the gills. Under these conditions, there is a selective advantage for hemoglobins with a Root shift. The presence of a Root shift augments oxygen transfer to the tissues in general and the eye and swimbladder in particular.

The disequilibrium pH: a tool for the localization of carbonic anhydrase

The disequilibrium pH is defined as any discrepancy between the measured pH and the pH which would exist if CO2-HCO3-H+ reactions were at equilibrium. Measurement of the disequilibrium pH can be used to assess the status of CO2-HCO3--H+ reactions and, in combination with carbonic anhydrase (CA) or CA inhibitor treatments, may also be used to localize CA. Renal physiologists have used disequilibrium experiments to determine that HCO3- reabsorption in the kidney tubule occurs via proton secretion, and that CA activity is available to ultrafiltrate CO2-HCO3-H+ reactions in the proximal convoluted tubule, but not the distal tubule. Disequilibrium experiments were also used in investigating the availability of CA to CO2-HCO3--H+ reactions in water at the fish gill; the opposing results obtained in two studies have not yet been resolved. Respiratory physiologists have used the disequilibrium technique in vivo and with saline-perfused preparations to assess the availability of CA to plasma CO2-HCO3--H+ reactions following gas exchange. Saline-perfused preparations enable direct localization of CA activity, while in vivo measurements encompass the numerous factors affecting CO2-HCO3--H+ equilibration in a multi-phase solution. Given the many organs in which membrane-bound CA activity has now been identified, the usefulness of the disequilibrium pH technique has increased beyond its original applications in renal and pulmonary physiology.