Water balance and renal function in two species of African lungfish Protopterus dolloi and Protopterus annectens

Renal acid excretion contributes to acid-base regulation during hypercapnia in air-exposed swamp eel (Monopterus albus)

The swamp eel (Monopterus albus) uses its buccal cavity to air breathe, while the gills are strongly reduced. It burrows into mud during the dry season, is highly tolerant of air exposure, and experiences severe hypoxia both in its natural habitat and in aquaculture. To study the ability of M. albus to compensate for respiratory acidosis, we implanted catheters to sample both arterial blood and urine during hypercapnia (4% CO₂) in either water or air, or during whole-animal air exposure. These hypercapnic challenges caused an immediate reduction in arterial pH, followed by progressive compensation through a marked elevation of plasma HCO₃ ^- over the course of 72 h. There was no appreciable rise in urinary acid excretion in fish exposed to hypercapnia in water, although urine pH was reduced and ammonia excretion did increase. In the air-exposed fish, however, hypercapnia was attended by a large elevation of ammonia in the urine and a large rise in titratable acid excretion. The time course of the increased renal acid excretion overlapped with the time period required to elevate plasma HCO₃ ^-, and we estimate that the renal compensation contributed significantly to whole-body acid-base compensation.

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.

Plasticity of skin water permeability and skin thickness in the amphibious mangrove rivulus Kryptolebias marmoratus

The skin of amphibious fishes is a multipurpose organ, important for gas and ion exchange and nitrogen excretion when fish are out of water (emersed). We tested the hypothesis that skin permeability is altered to maintain water balance through changes in water permeability and skin thickness during salinity acclimation and/or when fish emerse, using the euryhaline, amphibious fish Kryptolebias marmoratus as a model. We first recorded the behaviour of fish out of water to determine which part of the cutaneous surface was in contact with the substrate. Fish spent about 70% of their time on their ventral surface when out of water. Osmotic permeability of the skin was assessed in fish acclimated to 0.3 or 45‰ using ³H₂O fluxes in an in vitro micro-Ussing chamber setup. In freshwater-acclimated fish, ³H₂O influx across the skin was significantly higher compared to hypersaline-acclimated fish, with no significant changes in efflux. Prolonged emersion (7 days) resulted in an increase in skin ³H₂O influx, but not efflux in fish acclimated to a moist 45‰ substrate. In a separate experiment, dorsal epidermal skin thickness increased while the ventral dermis thickness decreased in fish emersed for over a week. However, there was no link between regional skin thickness and water flux in our experiments. Taken together, these findings suggest that K. marmoratus alter skin permeability to maximize water uptake while emersed in hypersaline conditions, adjustments that probably help them survive months of emersion during the dry season when drinking to replace water loss is not possible.

Amphibious fishes: evolution and phenotypic plasticity

Amphibious fishes spend part of their life in terrestrial habitats. The ability to tolerate life on land has evolved independently many times, with more than 200 extant species of amphibious fishes spanning 17 orders now reported. Many adaptations for life out of water have been described in the literature, and adaptive phenotypic plasticity may play an equally important role in promoting favourable matches between the terrestrial habitat and behavioural, physiological, biochemical and morphological characteristics. Amphibious fishes living at the interface of two very different environments must respond to issues relating to buoyancy/gravity, hydration/desiccation, low/high O2 availability, low/high CO2 accumulation and high/low NH3 solubility each time they traverse the air-water interface. Here, we review the literature for examples of plastic traits associated with the response to each of these challenges. Because there is evidence that phenotypic plasticity can facilitate the evolution of fixed traits in general, we summarize the types of investigations needed to more fully determine whether plasticity in extant amphibious fishes can provide indications of the strategies used during the evolution of terrestriality in tetrapods.

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.

Protective effect of cerium ion against ultraviolet B radiation-induced water stress in soybean seedlings

Effects of cerium ion (Ce(III)) on water relations of soybean seedlings (Glycine max L.) under ultraviolet B radiation (UV-B, 280-320 nm) stress were investigated under laboratory conditions. UV-B radiation not only affected the contents of two osmolytes (proline, soluble sugar) in soybean seedlings, but also inhibited the transpiration in soybean seedlings by decreasing the stomatal density and conductance. The two effects caused the inhibition in the osmotic and metabolic absorption of water, which decreased the water content and the free water/bound water ratio. Obviously, UV-B radiation led to water stress, causing the decrease in the photosynthesis in soybean seedlings. The pretreatment with 20 mg L(-1) Ce(III) could alleviate UV-B-induced water stress by regulating the osmotic and metabolic absorption of water in soybean seedlings. The alleviated effect caused the increase in the photosynthesis and the growth of soybean seedlings. It is one of the protective effect mechanisms of Ce(III) against the UV-B radiation-induced damage to plants.

Vasotocin/V2-type receptor/aquaporin axis exists in African lungfish kidney but is functional only in terrestrial condition

The vasopressin/vasotocin (VT)-V2-type receptor (V2R)-aquaporin (AQP)-2 axis plays a pivotal role in renal water reabsorption in tetrapods. It is widely thought that this axis evolved with the emergence of the tetrapods, reflecting a requirement of water retention in terrestrial environment. Here we report that lungfish, the closest living relatives of tetrapods, already possess a system similar to the VT-V2R-AQP2 axis in the kidney, but the system is functional only in the terrestrial estivating condition. We cloned a novel AQP paralogous to AQP0. The water permeability of Xenopus oocytes was increased by injection with the AQP cRNA and was further facilitated by preincubation with cAMP. In the kidney of estivating lungfish, the AQP protein was localized on the apical plasma membrane of the late distal tubule and was colocalized with basolateral V2R. By contrast, we found only little expression of the AQP mRNA and protein in the kidney of lungfish in aquatic condition. The expression levels of mRNA and protein were dramatically increased during estivation and decreased again by reacclimation of estivating lungfish to water. The AQP mRNA levels positively correlated with the VT mRNA levels in the hypothalamus, suggesting that the AQP exerts tubular antidiuretic action under control of VT. Because the tetrapod AQP2/AQP5 lineage is considered to be evolved from duplication of an AQP0 gene, the paralogous AQP0 in the lungfish probably represents ancestral molecule for tetrapod AQP2.

Ionoregulatory physiology of two species of African lungfishes Protopterus dolloi and Protopterus annectens

Basic ionoregulatory physiology was characterized in two species of African lungfish, slender African lungfish Protopterus dolloi and West African lungfish Protopterus annectens, largely under aquatic conditions. There were no substantive differences between the two species. Plasma [Na], [Cl] and [Ca] were only 60-80% of those typical of freshwater teleosts, and plasma Ca activity was particularly low. Unidirectional Na and Cl influx rates from water were also very low, only c. 10% of teleost values, whereas unidirectional Ca influx rates were comparable with teleost rates. Protopterus spp. were fed a 3% ration of bloodworms every 48 h. The bloodworm diet provided similar amounts of Na and Ca as uptake from water, but almost no Cl. Efflux rates of Na and Cl through the urine were greater than via the faeces, whereas the opposite was true for Ca. Net ion flux measurements and ionic balance sheet calculations indicated that (1) both water and dietary uptake routes are important for Na and Ca acquisition; (2) the waterborne route predominates for Cl uptake; (3) unidirectional ion effluxes across the body surface (gills and skin) rather than urine and faeces are the major routes of loss for Na, Cl and Ca. Tissues (muscle, liver, lung, kidney, intestine and heart) and plasma ions were also examined in P. dolloi'terrestrialized' in air for up to 5 months, during which plasma ion concentrations (Na, Cl, Ca and Mg) did not change and there were only a few alterations in tissue ions, that is, increased [Na] in intestine, decreased [Cl] in kidney and increased [Ca] in liver and kidney.

Regulation of gill transcellular permeability and renal function during acute hypoxia in the Amazonian oscar (Astronotus ocellatus): new angles to the osmorespiratory compromise

Earlier studies demonstrated that oscars, endemic to ion-poor Amazonian waters, are extremely hypoxia tolerant, and exhibit a marked reduction in active unidirectional Na+ uptake rate (measured directly) but unchanged net Na+ balance during acute exposure to low PO2, indicating a comparable reduction in whole body Na+ efflux rate. However, branchial O2 transfer factor does not fall. The present study focused on the nature of the efflux reduction in the face of maintained gill O2 permeability. Direct measurements of 22Na appearance in the water from bladder-catheterized fish confirmed a rapid 55% fall in unidirectional Na+ efflux rate across the gills upon acute exposure to hypoxia(PO2=10–20 torr; 1 torr=133.3 Pa), which was quickly reversed upon return to normoxia. An exchange diffusion mechanism for Na+ is not present, so the reduction in efflux was not directly linked to the reduction in Na+ influx. A quickly developing bradycardia occurred during hypoxia. Transepithelial potential, which was sensitive to water [Ca2+], became markedly less negative during hypoxia and was restored upon return to normoxia. Ammonia excretion, net K+ loss rates, and 3H2O exchange rates(diffusive water efflux rates) across the gills fell by 55–75% during hypoxia, with recovery during normoxia. Osmotic permeability to water also declined, but the fall (30%) was less than that in diffusive water permeability (70%). In total, these observations indicate a reduction in gill transcellular permeability during hypoxia, a conclusion supported by unchanged branchial efflux rates of the paracellular marker [3H]PEG-4000 during hypoxia and normoxic recovery. At the kidney, glomerular filtration rate, urine flow rate, and tubular Na+ reabsorption rate fell in parallel by 70% during hypoxia, facilitating additional reductions in costs and in urinary Na+, K+ and ammonia excretion rates. Scanning electron microscopy of the gill epithelium revealed no remodelling at a macro-level, but pronounced changes in surface morphology. Under normoxia,mitochondria-rich cells were exposed only through small apical crypts, and these decreased in number by 47% and in individual area by 65% during 3 h hypoxia. We suggest that a rapid closure of transcellular channels, perhaps effected by pavement cell coverage of the crypts, allows conservation of ions and reduction of ionoregulatory costs without compromise of O2exchange capacity during acute hypoxia, a response very different from the traditional osmorespiratory compromise.