Effect of air breathing on acid-base and ion regulation after exhaustive exercise and during low pH exposure in the bowfin, Amia calva

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.

Learning to Air-Breathe: The First Steps

Air-breathing in vertebrates has evolved many times among the bony fish while in water. Its appearance has had a fundamental impact on the regulation of ventilation and acid-base status. We review the physico-chemical constraints imposed by water and air, place the extant air-breathing fish into this framework, and show how that the advantages of combining control of ventilation and acid-base status are only available to the most obligate of air-breathing fish, thus highlighting promising avenues for research.

Ionoregulatory and oxidative stress issues associated with the evolution of air-breathing

Aquatic areas frequently face hypoxic conditions. In order to get sufficient oxygen to support aerobic metabolism, a number of freshwater fish resort to aerial respiration to supplement gill respiration especially in situations with reduced oxygen availability in the water. In many species a concomitant reduction in gill surface area or in gill perfusion reduces possible loss of aerially acquired oxygen to the water at the gills, but it also compromises the ion regulatory capacity of gill tissue. In consequence, the reduced gill contact area with water requires appropriate compensation to maintain ion and acid-base homeostasis, often with important ramifications for other organs. Associated modifications in the structure and function of the gills themselves, the skin, the gut, the kidney, and the physiology of water exchange and ion-linked acid-base regulation are discussed. In air-breathing fish, the gut may gain particular importance for the uptake of ions. In addition, tissues frequently exposed to environmental air encounter much higher oxygen partial pressures than typically observed in fish tissues. Physostomous fish using the swimbladder for aerial respiration, for example, will encounter aerial oxygen partial pressure at the swimbladder epithelium when frequently gulping air in hypoxic water. Hyperoxic conditions or rapid changes in oxygen partial pressures result in an increase in the production of reactive oxygen species (ROS). Accordingly, in air-breathing fish, strategies of ionoregulation may be greatly modified, and the ROS defense capacity of air-exposed tissues is improved.

Preferential intracellular pH regulation represents a general pattern of pH homeostasis during acid-base disturbances in the armoured catfish, Pterygoplichthys pardalis

Preferential intracellular pH (pHi) regulation, where pHi is tightly regulated in the face of a blood acidosis, has been observed in a few species of fish, but only during elevated blood PCO2. To determine whether preferential pHi regulation may represent a general pattern for acid-base regulation during other pH disturbances we challenged the armoured catfish, Pterygoplichthys pardalis, with anoxia and exhaustive exercise, to induce a metabolic acidosis, and bicarbonate injections to induce a metabolic alkalosis. Fish were terminally sampled 2-3 h following the respective treatments and extracellular blood pH, pHi of red blood cells (RBC), brain, heart, liver and white muscle, and plasma lactate and total CO2 were measured. All treatments resulted in significant changes in extracellular pH and RBC pHi that likely cover a large portion of the pH tolerance limits of this species (pH 7.15-7.86). In all tissues other than RBC, pHi remained tightly regulated and did not differ significantly from control values, with the exception of a decrease in white muscle pHi after anoxia and an increase in liver pHi following a metabolic alkalosis. Thus preferential pHi regulation appears to be a general pattern for acid-base homeostasis in the armoured catfish and may be a common response in Amazonian fishes.

Acid-base and ion balance in fishes with bimodal respiration

The evolution of air breathing during the Devonian provided early fishes with bimodal respiration with a stable O2 supply from air. This was, however, probably associated with challenges and trade-offs in terms of acid-base balance and ionoregulation due to reduced gill:water interaction and changes in gill morphology associated with air breathing. While many aspects of acid-base and ionoregulation in air-breathing fishes are similar to water breathers, the specific cellular and molecular mechanisms involved remain largely unstudied. In general, reduced ionic permeability appears to be an important adaptation in the few bimodal fishes investigated but it is not known if this is a general characteristic. The kidney appears to play an important role in minimizing ion loss to the freshwater environment in the few species investigated, and while ion uptake across the gut is probably important, it has been largely unexplored. In general, air breathing in facultative air-breathing fishes is associated with an acid-base disturbance, resulting in an increased partial pressure of arterial CO2 and a reduction in extracellular pH (pHE ); however, several fishes appear to be capable of tightly regulating tissue intracellular pH (pHI ), despite a large sustained reduction in pHE , a trait termed preferential pHI regulation. Further studies are needed to determine whether preferential pHI regulation is a general trait among bimodal fishes and if this confers reduced sensitivity to acid-base disturbances, including those induced by hypercarbia, exhaustive exercise and hypoxia or anoxia. Additionally, elucidating the cellular and molecular mechanisms may yield insight into whether preferential pHI regulation is a trait ultimately associated with the early evolution of air breathing in vertebrates.

Swimming in air-breathing fishes

Fishes with bimodal respiration differ in the extent of their reliance on air breathing to support aerobic metabolism, which is reflected in their lifestyles and ecologies. Many freshwater species undertake seasonal and reproductive migrations that presumably involve sustained aerobic exercise. In the six species studied to date, aerobic exercise in swim flumes stimulated air-breathing behaviour, and there is evidence that surfacing frequency and oxygen uptake from air show an exponential increase with increasing swimming speed. In some species, this was associated with an increase in the proportion of aerobic metabolism met by aerial respiration, while in others the proportion remained relatively constant. The ecological significance of anaerobic swimming activities, such as sprinting and fast-start manoeuvres during predator-prey interactions, has been little studied in air-breathing fishes. Some species practise air breathing during recovery itself, while others prefer to increase aquatic respiration, possibly to promote branchial ion exchange to restore acid-base balance, and to remain quiescent and avoid being visible to predators. Overall, the diversity of air-breathing fishes is reflected in their swimming physiology as well, and further research is needed to increase the understanding of the differences and the mechanisms through which air breathing is controlled and used during exercise.

Impact of ontogenetic changes in branchial morphology on gill function in Arapaima gigas

Soon after hatching, the osteoglossid fish Arapaima gigas undergoes a rapid transition from a water breather to an obligate air breather. This is followed by a gradual disappearance of gill lamellae, which leaves smooth filaments with a reduced branchial diffusion capacity due to loss of surface area, and a fourfold increase in diffusion distance. This study evaluated the effects these changes have on gill function by examining two size classes of fish that differ in gill morphology. In comparison to smaller fish (approximately 67.5 g), which still have lamellae, larger fish (approximately 724.2 g) without lamellae took up a slightly greater percentage of O2 across the gills (30.1% vs. 23.9%), which indicates that the morphological changes do not place limitations on O2 uptake in larger fish. Both size groups excreted similar percentages of CO2 across the gills (85%-90%). However, larger fish had higher blood PCO2 (26.51.9 vs. 16.51.5 mmHg) and HCO3(-) (40.2 +/- 2.9 vs. 33.6 +/- 4.5 mmol L(-1)) concentrations and lower blood pH (7.58 +/- 0.01 vs. 7.70 +/- 0.04) than did smaller fish, despite having lower mass-specific metabolisms, suggesting a possible diffusion limitation for CO2 excretion in larger fish. With regard to ion regulation, rates of diffusive Na+ loss were about 3.5 times higher in larger fish than they were in smaller fish, despite the lowered branchial diffusion capacity, and rates of Na+ uptake were higher by about the same amount despite 40% lower activity of branchial Na+/K+-ATPase. Kinetic analysis of Na uptake revealed an extremely low-affinity (K(m) = 587.9 +/- 169.5 micromol L(-1)), low-capacity (J(max) = 265.7 +/- 56.8 nmol g(-1) h(-1)) transport system. These data may reflect a general reduction in the role of the gills in ion balance. Renal Na+/K+-ATPase activity was 5-10 times higher than Na+/K+-ATPase activity in the gills, and urine: plasma ratios for Na+ and Cl(-) were very low (0.001-0.005) relative to that of other fish, which suggested an increased role for dietary salt intake and renal salt retention and which was representative of a more "terrestrial" mode of ion regulation. Such de-emphasis of branchial ion regulation confers greatly reduced sensitivity of diffusive ion loss to low water pH. Ammonia excretion also appeared to be impacted by gill changes. Rates of ammonia excretion in larger fish were one third less than that in smaller fish, despite larger fish having blood ammonia concentrations that were twice as high.

The evolution of Root effect hemoglobins in the absence of intracellular pH protection of the red blood cell: insights from primitive fishes

The Root effect, a reduction in blood oxygen (O(2)) carrying capacity at low pH, is used by many fish species to maximize O(2) delivery to the eye and swimbladder. It is believed to have evolved in the basal actinopterygian lineage of fishes, species that lack the intracellular pH (pH(i)) protection mechanism of more derived species' red blood cells (i.e., adrenergically activated Na(+)/H(+) exchangers; betaNHE). These basal actinopterygians may consequently experience a reduction in blood O(2) carrying capacity, and thus O(2) uptake at the gills, during hypoxia- and exercise-induced generalized blood acidoses. We analyzed the hemoglobins (Hbs) of seven species within this group [American paddlefish (Polyodon spathula), white sturgeon (Acipenser transmontanus), spotted gar (Lepisosteus oculatus), alligator gar (Atractosteus spatula), bowfin (Amia calva), mooneye (Hiodon tergisus), and pirarucu (Arapaima gigas)] for their Root effect characteristics so as to test the hypothesis of the Root effect onset pH value being lower than those pH values expected during a generalized acidosis in vivo. Analysis of the haemolysates revealed that, although each of the seven species displayed Root effects (ranging from 7.3 to 40.5% desaturation of Hb with O(2), i.e., Hb O(2) desaturation), the Root effect onset pH values of all species are considerably lower (ranging from pH 5.94 to 7.04) than the maximum blood acidoses that would be expected following hypoxia or exercise (pH(i) 7.15-7.3). Thus, although these primitive fishes possess Hbs with large Root effects and lack any significant red blood cell betaNHE activity, it is unlikely that the possession of a Root effect would impair O(2) uptake at the gills following a generalized acidosis of the blood. As well, it was shown that both maximal Root effect and Root effect onset pH values increased significantly in bowfin over those of the more basal species, toward values of similar magnitude to those of most of the more derived teleosts studied to date. This is paralleled by the initial appearance of the choroid rete in bowfin, as well as a significant decrease in Hb buffer value and an increase in Bohr/Haldane effects, together suggesting bowfin as the most basal species capable of utilizing its Root effect to maximize O(2) delivery to the eye.

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.

Acid-base balance and CO2 excretion in fish: unanswered questions and emerging models

Carbon dioxide (CO(2)) excretion and acid-base regulation in fish are linked, as in other animals, though the reversible reactions of CO(2) and the acid-base equivalents H(+) and HCO(3)(-): CO(2)+H(2)O<-->H(+)+HCO(3)(-). These relationships offer two potential routes through which acid-base disturbances may be regulated. Respiratory compensation involves manipulation of ventilation so as to retain CO(2) or enhance CO(2) loss, with the concomitant readjustment of the CO(2) reaction equilibrium and the resultant changes in H(+) levels. In metabolic compensation, rates of direct H(+) and HCO(3)(-) exchange with the environment are manipulated to achieve the required regulation of pH; in this case, hydration of CO(2) yields the necessary H(+) and HCO(3)(-) for exchange. Because ventilation in fish is keyed primarily to the demands of extracting O(2) from a medium of low O(2) content, the capacity to utilize respiratory compensation of acid-base disturbances is limited and metabolic compensation across the gill is the primary mechanism for re-establishing pH balance. The contribution of branchial acid-base exchanges to pH compensation is widely recognized, but the molecular mechanisms underlying these exchanges remain unclear. The relatively recent application of molecular approaches to this question is generating data, sometimes conflicting, from which models of branchial acid-base exchange are gradually emerging. The critical importance of the gill in acid-base compensation in fish, however, has made it easy to overlook other potential contributors. Recently, attention has been focused on the role of the kidney and particularly the molecular mechanisms responsible for HCO(3)(-) reabsorption. It is becoming apparent that, at least in freshwater fish, the responses of the kidney are both flexible and essential to complement the role of the gill in metabolic compensation. Finally, while respiratory compensation in fish is usually discounted, the few studies that have thoroughly characterized ventilatory responses during acid-base disturbances in fish suggest that breathing may, in fact, be adjusted in response to pH imbalances. How this is accomplished and the role it plays in re-establishing acid-base balance are questions that remain to be answered.

Evolutionary determinants of normal arterial plasma pH in ectothermic vertebrates

Mean values of normal arterial pH in different species of fish, amphibians and reptiles at 15 and 25°C, taken from the literature, are negatively correlated with arterial PCO2 and plasma [Na+]. At either temperature, the data accord with the hypothesis that extracellular acid–base homeostasis evolved to maintain an optimal pH at particular cell-surface sites that are similar in all species. These hypothetical sites bear fixed negative charges that attract H+, but which are partially screened by Na+; for the surface pH to be constant, the bulk interstitial pH should then vary inversely with [Na+], as is the case. At the same time, the bulk interstitial fluid must be more acid than arterial plasma by an amount that increases with decreasing arterial PCO2. With allowance made for additional screening by Ca2+ and Mg2+, the relevant cell-surface pH is probably approximately 6.2.