Respiratory and renal roles of carbonic anhydrase in gas exchange and acid-base regulation

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.

The importance of a single amino acid substitution in reduced red blood cell carbonic anhydrase function of early-diverging fish

In most vertebrates, red blood cell carbonic anhydrase (RBC CA) plays a critical role in carbon dioxide (CO₂) transport and excretion across epithelial tissues. Many early-diverging fishes (e.g., hagfish and chondrichthyans) are unique in possessing plasma-accessible membrane-bound CA-IV in the gills, allowing some CO₂ excretion to occur without involvement from the RBCs. However, implications of this on RBC CA function are unclear. Through homology cloning techniques, we identified the putative protein sequences for RBC CA from nine early-diverging species. In all cases, these sequences contained a modification of the proton shuttle residue His-64, and activity measurements from three early-diverging fish demonstrated significantly reduced CA activity. Site-directed mutagenesis was used to restore the His-64 proton shuttle, which significantly increased RBC CA activity, clearly illustrating the functional significance of His-64 in fish red blood cell CA activity. Bayesian analyses of 55 vertebrate cytoplasmic CA isozymes suggested that independent evolutionary events led to the modification of His-64 and thus reduced CA activity in hagfish and chondrichthyans. Additionally, in early-diverging fish that possess branchial CA-IV, there is an absence of His-64 in RBC CAs and the absence of the Root effect [where a reduction in pH reduces hemoglobin's capacity to bind with oxygen (O₂)]. Taken together, these data indicate that low-activity RBC CA may be present in all fish with branchial CA-IV, and that the high-activity RBC CA seen in most teleosts may have evolved in conjunction with enhanced hemoglobin pH sensitivity.

Role of Carbonic Anhydrases and Inhibitors in Acid-Base Physiology: Insights from Mathematical Modeling

Carbonic anhydrases (CAs) catalyze a reaction fundamental for life: the bidirectional conversion of carbon dioxide (CO2) and water (H2O) into bicarbonate (HCO3-) and protons (H+). These enzymes impact numerous physiological processes that occur within and across the many compartments in the body. Within compartments, CAs promote rapid H+ buffering and thus the stability of pH-sensitive processes. Between compartments, CAs promote movements of H+, CO2, HCO3-, and related species. This traffic is central to respiration, digestion, and whole-body/cellular pH regulation. Here, we focus on the role of mathematical modeling in understanding how CA enhances buffering as well as gradients that drive fluxes of CO2 and other solutes (facilitated diffusion). We also examine urinary acid secretion and the carriage of CO2 by the respiratory system. We propose that the broad physiological impact of CAs stem from three fundamental actions: promoting H+ buffering, enhancing H+ exchange between buffer systems, and facilitating diffusion. Mathematical modeling can be a powerful tool for: (1) clarifying the complex interdependencies among reaction, diffusion, and protein-mediated components of physiological processes; (2) formulating hypotheses and making predictions to be tested in wet-lab experiments; and (3) inferring data that are impossible to measure.

Carbonic Anhydrases: Role in pH Control and Cancer

The pH of the tumor microenvironment drives the metastatic phenotype and chemotherapeutic resistance of tumors. Understanding the mechanisms underlying this pH-dependent phenomenon will lead to improved drug delivery and allow the identification of new therapeutic targets. This includes an understanding of the role pH plays in primary tumor cells, and the regulatory factors that permit cancer cells to thrive. Over the last decade, carbonic anhydrases (CAs) have been shown to be important mediators of tumor cell pH by modulating the bicarbonate and proton concentrations for cell survival and proliferation. This has prompted an effort to inhibit specific CA isoforms, as an anti-cancer therapeutic strategy. Of the 12 active CA isoforms, two, CA IX and XII, have been considered anti-cancer targets. However, other CA isoforms also show similar activity and tissue distribution in cancers and have not been considered as therapeutic targets for cancer treatment. In this review, we consider all the CA isoforms and their possible role in tumors and their potential as targets for cancer therapy.

Beyond just hemoglobin: Red blood cell potentiation of hemoglobin-oxygen unloading in fish

Teleosts comprise 95% of fish species, almost one-half of all vertebrate species, and represent one of the most successful adaptive radiation events among vertebrates. This is thought to be in part because of their unique oxygen (O2) transport system. In salmonids, recent in vitro and in vivo studies indicate that hemoglobin-oxygen (Hb-O2) unloading to tissues may be doubled or even tripled under some conditions without changes in perfusion. This is accomplished through the short circuiting of red blood cell (RBC) pH regulation, resulting in a large arterial-venous pH difference within the RBC and induced reduction in Hb-O2 affinity. This system has three prerequisites: 1) highly pH-sensitive hemoglobin, 2) rapid RBC pH regulation, and 3) a heterogeneous distribution of plasma-accessible CA in the cardiovascular system (presence in the tissues and absence at the gills). Although data are limited, these attributes may be general characteristics of teleosts. Although this system is not likely operational to the same degree in other vertebrates, some of these prerequisites do exist, and the generation and elimination of pH disequilibrium states at the RBC will likely enhance Hb-O2 unloading to some degree. In human disease states, there are conditions that may partly satisfy those for enhanced Hb-O2 unloading, tentatively an avenue for future work that may improve treatment efficacy.

Carbonic Anhydrase in Mammalian Vascular Smooth Muscle

Carbonic anhydrase (CA) is ubiquitously expressed and plays a pivotal role in acid-base balance, ion transport, and gas exchange. Limited observations by others, derived from functional, pharmacological, and histochemical studies, suggest that CA is present in vascular smooth muscle and is involved in vasoregulation. The present study, using measurements of bioactivity, inhibition characteristics, and immunohistochemical analysis, was undertaken to more fully evaluate CA in vascular smooth muscle. In isolated bovine aortic smooth muscle, which is devoid of erythrocytes, CA is present in low concentrations with a CO2 hydration activity (at 0C) of 3.5 ± 2.7 U/g. The I50 for acetazolamide inhibition is 0.07 ± 0.01 μM. Results with dorzolamide and bromopyruvate, selective inhibitors of the CA II and I isozymes, respectively, show that roughly 75% of the CA activity is accounted for by CA I, with 20% due to CA II. These results accord qualitatively with immunocytochemical staining with specific CA I and II antibodies, showing that both isozymes are present and that their staining co-localizes with cells positive for smooth muscle α-actin. These data establish the activity, inhibition, and isozyme pattern of carbonic anhydrase expression in mammalian vascular smooth muscle.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.

Probing the surface of human carbonic anhydrase for clues towards the design of isoform specific inhibitors

The alpha carbonic anhydrases (α-CAs) are a group of structurally related zinc metalloenzymes that catalyze the reversible hydration of CO₂ to HCO₃⁻. Humans have 15 different α-CAs with numerous physiological roles and expression patterns. Of these, 12 are catalytically active, and abnormal expression and activities are linked with various diseases, including glaucoma and cancer. Hence there is a need for CA isoform specific inhibitors to avoid off-target CA inhibition, but due to the high amino acid conservation of the active site and surrounding regions between each enzyme, this has proven difficult. However, residues towards the exit of the active site are variable and can be exploited to design isoform selective inhibitors. Here we discuss and characterize this region of “selective drug targetability” and how these observations can be utilized to develop isoform selective CA inhibitors.

Safety of carbonic anhydrase inhibitors

INTRODUCTION

Carbonic anhydrase (CA) inhibitors have an impressive safety record despite the multiple functions that CA isozymes serve because they are not fully inhibited with most dosing. While reducing the targeted CA-dependent process sufficiently for disease control, residual activity and uncatalyzed rates in combination with compensations are adequate to avoid lethal consequences. Some drugs have in vitro selectivity differences against the 13 active isozymes, but none are convincingly selective in vivo or clinically. Efforts to synthesize selective inhibitors should result in safer drugs with fewer side effects.

AREAS COVERED

This review will focus on approved drugs with CA-inhibiting activity, whether used directly for this purpose or others. Side effects are discussed in relation to various organ systems and the disease being treated. Causes of side effects are considered, and strategies for symptom reduction are given.

EXPERT OPINION

Common side effects of paresthesias, dyspepsia, lassitude and fatigue in 30 - 40% of patients are generally tolerable or abate, but if not can be partially relieved by bicarbonate supplementation. The most important safety concerns are severe acidosis, respiratory failure and encephalopathy in patients with renal, pulmonary and hepatic disease where caution is critical, as is also the case in persons with sulfa drug allergies.

Voltage-gated proton channels: molecular biology, physiology, and pathophysiology of the H(V) family

Voltage-gated proton channels (H(V)) are unique, in part because the ion they conduct is unique. H(V) channels are perfectly selective for protons and have a very small unitary conductance, both arguably manifestations of the extremely low H(+) concentration in physiological solutions. They open with membrane depolarization, but their voltage dependence is strongly regulated by the pH gradient across the membrane (ΔpH), with the result that in most species they normally conduct only outward current. The H(V) channel protein is strikingly similar to the voltage-sensing domain (VSD, the first four membrane-spanning segments) of voltage-gated K(+) and Na(+) channels. In higher species, H(V) channels exist as dimers in which each protomer has its own conduction pathway, yet gating is cooperative. H(V) channels are phylogenetically diverse, distributed from humans to unicellular marine life, and perhaps even plants. Correspondingly, H(V) functions vary widely as well, from promoting calcification in coccolithophores and triggering bioluminescent flashes in dinoflagellates to facilitating killing bacteria, airway pH regulation, basophil histamine release, sperm maturation, and B lymphocyte responses in humans. Recent evidence that hH(V)1 may exacerbate breast cancer metastasis and cerebral damage from ischemic stroke highlights the rapidly expanding recognition of the clinical importance of hH(V)1.

Effects of chronic acetazolamide administration on gas exchange and acid-base control in pulmonary circulation in exercising horses

REASONS FOR PERFORMING STUDY

Carbonic anhydrase (CA) catalyses the hydration/dehydration reaction of CO(2) and increases the rate of Cl(-) and HCO(3)(-) exchange between the erythrocytes and plasma. Therefore, chronic inhibition of CA has a potential to attenuate CO(2) output and induce greater metabolic and respiratory acidosis in exercising horses.

OBJECTIVES

To determine the effects of Carbonic anhydrase inhibition on CO(2) output and ionic exchange between erythrocytes and plasma and their influence on acid-base balance in the pulmonary circulation (across the lung) in exercising horses with and without CA inhibition.

METHODS

Six horses were exercised to exhaustion on a treadmill without (Con) and with CA inhibition (AczTr). CA inhibition was achieved with administration of acetazolamide (10 mg/kg bwt t.i.d. for 3 days and 30 mg/kg bwt before exercise). Arterial, mixed venous blood and CO(2) output were sampled at rest and during exercise. An integrated physicochemical systems approach was used to describe acid base changes.

RESULTS

AczTr decreased the duration of exercise by 45% (P < 0.0001). During the transition from rest to exercise CO(2) output was lower in AczTr (P < 0.0001). Arterial PCO(2) (P < 0.0001; mean ± s.e. 71 ± 2 mmHg AczTr, 46 ± 2 mmHg Con) was higher, whereas hydrogen ion (P = 0.01; 12.8 ± 0.6 nEq/l AczTr, 15.5 ± 0.6 nEq/l Con) and bicarbonate (P = 0.007; 5.5 ± 0.7 mEq/l AczTr, 10.1 ± 1.3 mEq/l Con) differences across the lung were lower in AczTr compared to Con. No difference was observed in weak electrolytes across the lung. Strong ion difference across the lung was lower in AczTr (P = 0.0003; 4.9 ± 0.8 mEq AczTr, 7.5 ± 1.2 mEq Con), which was affected by strong ion changes across the lung with exception of lactate.

CONCLUSIONS

CO(2) and chloride changes in erythrocytes across the lung seem to be the major contributors to acid-base and ions balance in pulmonary circulation in exercising horses.

Roles of cortisol and carbonic anhydrase in acid-base compensation in rainbow trout, Oncorhynchus mykiss

Fish compensate for acid-base disturbances primarily by modulating the branchial excretion of acid-base equivalents, with a supporting role played by adjustment of urinary acid excretion. The present study used metabolic acid-base disturbances in rainbow trout, Oncorhynchus mykiss, to evaluate the role played by cortisol in stimulating compensatory responses. Trout infused with acid (an iso-osmotic solution of 70 mmol L(-1) HCl), base (140 mmol L(-1) NaHCO(3)) or saline (140 mmol L(-1) NaCl) for 24 h exhibited significant elevation of circulating cortisol concentrations. Acid infusion significantly increased both branchial (by 328 μmol kg(-1) h(-1)) and urinary (by 5.9 μmol kg(-1) h(-1)) net acid excretion, compensatory responses that were eliminated by pre-treatment of trout with the cortisol synthesis inhibitor metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone). The significant decrease in net acid excretion (equivalent to enhanced base excretion) of 203 μmol kg(-1) h(-1) detected in base-infused trout was unaffected by metyrapone treatment. Acid- and base-infusions also were associated with significant changes in the relative mRNA expression of branchial and renal cytosolic carbonic anhydrase (tCAc) and renal membrane-linked CA IV (tCA IV). Cortisol treatment caused changes in CA gene expression that tended to parallel those observed with acid but not base infusion. For example, significant increases in renal relative tCA IV mRNA expression were detected in both acid-infused (~2x) and cortisol-treated (~10x) trout, whereas tCA IV mRNA expression was significantly reduced (~5x) in base-infused fish. Despite changes in CA gene expression in acid- or base-infused fish, neither acid nor base infusion affected CAc protein levels in the gill, but both caused significant increases in branchial CA activity. Cortisol treatment similarly increased branchial CA activity in the absence of an effect on branchial CAc protein expression. Taken together, these findings provide support for the hypothesis that in rainbow trout, cortisol is involved in mediating acid-base compensatory responses to a metabolic acidosis, and that cortisol exerts its effects at least in part through modulation of CA.

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.

Molecular pharmacodynamics, clinical therapeutics, and pharmacokinetics of topiramate

Topiramate (TPM; TOPAMAX) is a broad-spectrum antiepileptic drug (AED) that is approved in many world markets for preventing or reducing the frequency of epileptic seizures (as monotherapy or adjunctive therapy), and for the prophylaxis of migraine. TPM, a sulfamate derivative of the naturally occurring sugar D-fructose, possesses several pharmacodynamic properties that may contribute to its clinically useful attributes, and to its observed adverse effects. The sulfamate moiety is essential, but not sufficient, for its pharmacodynamic properties. In this review, we discuss the known pharmacodynamic and pharmacokinetic properties of TPM, as well as its various clinically beneficial and adverse effects.

Acetazolamide protects against posthypoxic unstable breathing in the C57BL/6J mouse

Acetazolamide (Acz), a carbonic anhydrase inhibitor, is used to manage periodic breathing associated with altitude and with heart failure. We examined whether Acz would alter posthypoxic ventilatory behavior in the C57BL/6J (B6) mouse model of recurrent central apnea. Experiments were performed with unanesthetized, awake adult male B6 mice (n = 9), ventilatory behavior was measured using flow-through whole body plethysmography. Mice were given an intraperitoneal injection of either vehicle or Acz (40 mg/kg), and 1 h later they were exposed to 1 min of 8% O(2)-balance N(2) (poikilocapnic hypoxia) or 12% O(2)-3% CO(2)-balance N(2) (isocapnic hypoxia) followed by rapid reoxygenation (100% O(2)). Hypercapnic response (8% CO(2)-balance O(2)) was examined in six mice. With Acz, ventilation, including respiratory frequency, tidal volume, and minute ventilation, in room air was significantly higher and hyperoxic hypercapnic ventilatory responsiveness was generally lower compared with vehicle. Poikilocapnic and isocapnic hypoxic ventilatory responsiveness were similar among treatments. One minute after reoxygenation, animals given Acz exhibited posthypoxic frequency decline, a lower coefficient of variability for frequency, and no tendency toward periodic breathing, compared with vehicle treatment. We conclude that Acz improves unstable breathing in the B6 model, without altering hypoxic response or producing short-term potentiation, but with some blunting of hypercapnic responsiveness.

Mechanisms of acid–base regulation in the African lungfishProtopterus annectens

African lungfish Protopterus annectens utilized both respiratory and metabolic compensation to restore arterial pH to control levels following the imposition of a metabolic acidosis or alkalosis. Acid infusion (3 mmol kg–1 NH4Cl) to lower arterial pH by 0.24 units increased both pulmonary (by 1.8-fold) and branchial (by 1.7-fold) ventilation frequencies significantly, contributing to 4.8-fold and 1.9-fold increases in,respectively, aerial and aquatic CO2 excretion. This respiratory compensation appeared to be the main mechanism behind the restoration of arterial pH, because even though net acid excretion(JnetH+) increased following acid infusion in 7 of 11 fish, the mean increase in net acid excretion, 184.5±118.5μmol H+ kg–1 h–1 (mean± s.e.m., N=11), was not significantly different from zero. Base infusion (3 mmol kg–1 NaHCO3) to increase arterial pH by 0.29 units halved branchial ventilation frequency, although pulmonary ventilation frequency was unaffected. Correspondingly, aquatic CO2 excretion also fell significantly (by 3.7-fold) while aerial CO2 excretion was unaffected. Metabolic compensation consisting of negative net acid excretion (net base excretion) accompanied this respiratory compensation, with JnetH+ decreasing from 88.5±75.6 to –337.9±199.4 μmol H+kg–1 h–1 (N=8). Partitioning of net acid excretion into renal and extra-renal (assumed to be branchial and/or cutaneous) components revealed that under control conditions, net acid excretion occurred primarily by extra-renal routes. Finally, several genes that are involved in the exchange of acid–base equivalents between the animal and its environment (carbonic anhydrase, V-type H+-ATPase and Na+/HCO –3 cotransporter) were cloned, and their branchial and renal mRNA expressions were examined prior to and following acid or base infusion. In no case was mRNA expression significantly altered by metabolic acid–base disturbance. These findings suggest that lungfish, like tetrapods, alter ventilation to compensate for metabolic acid–base disturbances, a mechanism that is not employed by water-breathing fish. Like fish and amphibians, however, extra-renal routes play a key role in metabolic compensation.

Effects of chronic acetazolamide administration on fluid flux from the pulmonary vasculature at rest and during exercise in horses

REASONS FOR PERFORMING STUDY

Horses develop high pulmonary pressures during exercise, which force fluid out of pulmonary capillaries. Specific airway diseases in horses, especially those associated with hypoxaemia, hypercapnoea and acidosis may influence pulmonary haemodynamics and pulmonary interstitial fluid equilibrium.

OBJECTIVES

This study was designed to determine fluid flux (J(V-A) l/min) across the lung in exercising horses treated chronically with acetazolamide.

METHODS

Six horses were exercised on a treadmill until fatigue without (Con) and with chronic carbonic anhydrase (CA) inhibition (AczTr) and associated hypercapnoea and acidosis. Carbonic anhydrase inhibition was achieved with administration of acetazolamide (Acz). Arterial and mixed venous blood were sampled, and VCO2 and VO2 measured. Blood volume changes across the lung (deltaBV%) were calculated from changes in plasma protein, haemoglobin and packed cell volume (PCV). Cardiac output (Q) was calculated using Fick principle. J(V-A) across the alveolar-capillary barrier was then quantified based on Q and deltaBV. Variables were analysed using 2-way repeated-measures ANOVA (P<0.05). A significant F ratio was further analysed using Tukey post hoc analysis.

RESULTS

Treatment had a significant effect on J(V-A) (P = 0.002). At rest there was no J(V-A) in Con (0.63 +/- 0.6 l/min) and AczTr (0.84 +/- 0.3 l/min). During exercise Con fluid moved from the pulmonary circulation into the pulmonary interstitium (mean +/- s.e. J(V-A) 9.4 +/- 2.4 l/min). This was different from AczTr (mean +/- s.e. J(V-A) 1.8 +/- 1.9 l/min), where no transvascular fluxes from pulmonary circulation were present during exercise (P = 0.008).

CONCLUSIONS

Chronic Acz treatment with associated hypercapnoea and acidosis affects J(V-A) in lungs of exercising horses. Lung fluid dynamics adapted to hypercapnoea and acidosis with reduction of fluid flow from the pulmonary circulation.

POTENTIAL RELEVANCE

The current data provide comprehensive evidence of in vivo fluid homeostasis in lungs of exercising horses without and with CA inhibition.

Examination of two independent kinetic assays for determining the inhibition of carbonic anhydrases I and II: structure-activity comparison of sulfamates and sulfamides

Enzyme inhibition assays often require deviations from physiological conditions. For carbonic anhydrases, procedures involving native CO(2) and non-native substrates have been used. We compared a native and a non-native substrate in the context of inhibition of human carbonic anhydrases I and II by examining various sulfamate and sulfamide compounds in two kinetic assays: hydration of CO(2) and hydrolysis of 4-nitrophenylacetate. For carbonic anhydrase II, the two assays consistently generated similar K(i) values, with the relative difference between the assays never exceeding 2.5-fold. However, for carbonic anhydrase I there was more variability between the two assays, with K(i) values for three compounds differing by more than 2.5-fold, up to eightfold. In the CO(2) hydration assay, some sulfamates and sulfamides exhibited mixed kinetics or partial inhibition. Our results indicate that K(i) or K(d) values from carbonic anhydrase assays involving non-native substrates should be confirmed by assays that use CO(2) (or HCO), to establish pharmacological relevance. From structure-activity comparisons, the sulfamate is more effective than the sulfamide in inhibiting carbonic anhydrase I and II, but the sulfamate does not confer selectivity. In contrast, the sulfonamide confers selectivity for carbonic anhydrase I (10- to 30-fold). Selectivity for carbonic anhydrase II occurred with the substituted fructose moiety, especially the d-enantiomer (>100-fold).

Type IV carbonic anhydrase is present in the gills of spiny dogfish (Squalus acanthias)

Physiological and biochemical studies have provided indirect evidence for a membrane-associated carbonic anhydrase (CA) isoform, similar to mammalian type IV CA, in the gills of dogfish (Squalus acanthias). This CA isoform is linked to the plasma membrane of gill epithelial cells by a glycosylphosphatidylinositol anchor and oriented toward the plasma, such that it can catalyze the dehydration of plasma HCO(3)(-) ions. The present study directly tested the hypothesis that CA IV is present in dogfish gills in a location amenable to catalyzing plasma HCO(3)(-) dehydration. Homology cloning techniques were used to assemble a 1,127 base pair cDNA that coded for a deduced protein of 306 amino acids. Phylogenetic analysis suggested that this protein was a type IV CA. For purposes of comparison, a second cDNA (1,107 base pairs) was cloned from dogfish blood; it encoded a deduced protein of 260 amino acids that was identified as a cytosolic CA through phylogenetic analysis. Using real-time PCR and in situ hybridization, mRNA expression for the dogfish type IV CA was detected in gill tissue and specifically localized to pillar cells and branchial epithelial cells that flanked the pillar cells. Immunohistochemistry using a polyclonal antibody raised against rainbow trout type IV CA revealed a similar pattern of CA IV immunoreactivity and demonstrated a limited degree of colocalization with Na(+)-K(+)-ATPase immunoreactivity. The presence and localization of a type IV CA isoform in the gills of dogfish is consistent with the hypothesis that branchial membrane-bound CA with an extracellular orientation contributes to CO(2) excretion in dogfish by catalyzing the dehydration of plasma HCO(3)(-) ions.

Roles of cytosolic and membrane-bound carbonic anhydrase in renal control of acid-base balance in rainbow trout, Oncorhynchus mykiss

We tested the hypothesis that cytosolic and membrane-associated carbonic anhydrase (CA IV) are involved in renal urinary acidification and bicarbonate reabsorption in rainbow trout. With the use of homological cloning techniques, a 1,137-bp cDNA was assembled that included an open reading frame encoding for a deduced protein of 297 amino acids. Phylogenetic analysis revealed that this protein was likely a CA IV isoform. With the use of this sequence and a previously described trout cytosolic isoform [tCAc (13)], tools were developed to quantify and localize mRNA and protein for the two CA isoforms. Unlike tCAc, which displayed a broad tissue distribution, trout CA IV mRNA (and to a lesser extent protein) was highly and preferentially expressed in the posterior kidney. The results of in situ hybridization, immunocytochemistry, and standard histological procedures demonstrated that CA IV was likely confined to epithelial cells of the proximal tubule with the protein being expressed on both apical and basolateral membranes. The CA IV-containing tubule cells were enriched with Na(+)-K(+)-ATPase. Similar results were obtained for tCAc except that it appeared to be present in both proximal and distal tubules. The levels of mRNA and protein for tCAc increased significantly during respiratory acidosis (hypercapnia). Although tCA IV mRNA was elevated after 24 h of hypercapnia, tCA IV protein levels were unaltered. By using F3500, a membrane-impermeant (yet filtered) inhibitor of CA, in concert with blood and urine analyses, we demonstrated that CA IV (and possibly other membrane-associated CA isoforms) plays a role in urinary acidification and renal bicarbonate reabsorption.

Cytoplasmic carbonic anhydrase isozymes in rainbow trout Oncorhynchus mykiss: comparative physiology and molecular evolution

It is well established that the gills of teleost fish contain substantial levels of cytoplasmic carbonic anhydrase (CA), but it is unclear which CA isozyme(s) might be responsible for this activity. The objective of the current study was to determine if branchial CA activity in rainbow trout was the result of a general cytoplasmic CA isozyme, with kinetic properties, tissue distribution and physiological functions distinct from those of the red blood cell (rbc)-specific CA isozyme. Isolation and sequencing of a second trout cytoplasmic CA yielded a 780 bp coding region that was 76% identical with the trout rbc CA (TCAb), although the active sites differed by only 1 amino acid. Interestingly, phylogenetic analyses did not group these two isozymes closely together, suggesting that more fish species may have multiple cytoplasmic CA isozymes. In contrast to TCAb, the second cytoplasmic CA isozyme had a wide tissue distribution with high expression in the gills and brain, and lower expression in many tissues, including the red blood cells. Thus, unlike TCAb, the second isozyme lacks tissue specificity and may be expressed in the cytoplasm of all cells. For this reason, it is referred to hereafter as TCAc (trout cytoplasmic CA). The inhibitor properties of both cytoplasmic isozymes were similar (Ki acetazolamide 1.21+/-0.18 nmol l(-1) and 1.34+/-0.10 nmol l(-1) for TCAc and TCAb, respectively). However, the turnover of TCAb was over three times greater than that of TCAc (30.3+/-5.83 vs 8.90+/-1.95 e4 s(-1), respectively), indicating that the rbc-specific CA isoform was significantly faster than the general cytoplasmic isoform. Induction of anaemia revealed differential expression of the two isozymes in the red blood cell; whereas TCAc mRNA expression was unaffected, TCAb mRNA expression was significantly increased by 30- to 60-fold in anaemic trout.

Pharmacokinetic interactions of topiramate

Topiramate is a new antiepileptic drug (AED) that has been approved worldwide (in more than 80 countries) for the treatment of various kinds of epilepsy. It is currently being evaluated for its effect in various neurological and psychiatric disorders. The pharmacokinetics of topiramate are characterised by linear pharmacokinetics over the dose range 100-800 mg, low oral clearance (22-36 mL/min), which, in monotherapy, is predominantly through renal excretion (renal clearance 10-20 mL/min), and a long half-life (19-25 hours), which is reduced when coadministered with inducing AEDs such as phenytoin, phenobarbital and carbamazepine. The absolute bioavailability, or oral availability, of topiramate is 81-95% and is not affected by food. Although topiramate is not extensively metabolised when administered in monotherapy (fraction metabolised approximately 20%), its metabolism is induced during polytherapy with carbamazepine and phenytoin, and, consequently, its fraction metabolised increases. During concomitant treatment with topiramate and carbamazepine or phenytoin, the (oral) clearance of topiramate increases 2-fold and its half-life becomes shorter by approximately 50%, which may require topiramate dosage adjustment when phenytoin or carbamazepine therapy is added or discontinued. From a pharmacokinetic standpoint, topiramate is a unique example of a drug that, because of its major renal elimination component, is not subject to drug interaction due to enzyme inhibition, but nevertheless is susceptible to clinically relevant drug interactions due to induction of its metabolism. Unlike old AEDs such as phenytoin and carbamazepine, topiramate is a mild inducer and, currently, the only interaction observed as a result of induction by topiramate is that with ethinylestradiol. Topiramate only increases the oral clearance of ethinylestradiol in an oral contraceptive at high dosages (>200 mg/day). Because of this dose-dependency, possible interactions between topiramate and oral contraceptives should be assessed according to the topiramate dosage utilised. This paper provides a critical review of the pharmacokinetic interactions of topiramate with old and new AEDs, an oral contraceptive, and the CNS-active drugs lithium, haloperidol, amitriptyline, risperidone, sumatriptan, propranolol and dihydroergotamine. At a daily dosage of 200 mg, topiramate exhibited no or little (with lithium, propranolol and the amitriptyline metabolite nortriptyline) pharmacokinetic interactions with these drugs. The results of many of these drug interaction studies with topiramate have not been published before, and are presented and discussed for the first time in this article.

Carbonic anhydrase is essential for growth of Ralstonia eutropha at ambient CO(2) concentrations

Mutant strain 25-1 of the facultative chemoautotroph Ralstonia eutropha H16 had previously been shown to exhibit an obligately high-CO(2)-requiring (HCR) phenotype. Although the requirement varied with the carbon and energy sources utilized, none of these conditions allowed growth at the air concentration of CO(2). In the present study, a gene designated can and encoding a beta-carbonic anhydrase (CA) was identified as the site altered in strain 25-1. The mutation caused a replacement of the highly conserved glycine residue 98 by aspartate in Can. A can deletion introduced into wild-type strain H16 generated mutant HB1, which showed the same HCR phenotype as mutant 25-1. Overexpression of can in Escherichia coli and mass spectrometric determination of CA activity demonstrated that can encodes a functional CA. The enzyme is inhibited by ethoxyzolamide and requires 40 mM MgSO(4) for maximal activity. Low but significant CA activities were detected in wild-type H16 but not in mutant HB1, strongly suggesting that the CA activity of Can is essential for growth of the wild type in the presence of low CO(2) concentrations. The HCR phenotype of HB1 was overcome by complementation with heterologous CA genes, indicating that growth of the organism at low CO(2) concentrations requires sufficient CA activity rather than the specific function of Can. The metabolic function(s) depending on CA activity remains to be identified.