Transepithelial potential differences and Na+ flux in isolated perfused gills of the crabChasmagnathus granulatus(Grapsidae) acclimated to hyper- and hypo-salinity

Primary amino acid sequences of decapod (Na+, K+)-ATPase provide evolutionary insights into osmoregulatory mechanisms

Decapod Crustacea exhibit a marine origin, but many taxa have occupied environments ranging from brackish to fresh water and terrestrial habitats, overcoming their inherent osmotic challenges. Osmotic and ionic regulation is achieved by the gill epithelia, driven by two active ATP-hydrolyzing ion transporters, the basal (Na+, K+)-ATPase and the apical V(H+)-ATPase. The kinetic characteristic of gill (Na+, K+)-ATPase and the mRNA expression of its α subunit have been widely studied in various decapod species under different salinity challenges. However, the evolution of the primary structure has not been explored, especially considering the functional modifications associated with decapod phylogeny. Here, we proposed a model for the topology of the decapod α subunit, identifying the sites and motifs involved in its function and regulation, as well as the patterns of its evolution assuming a decapod phylogeny. We also examined both the amino acid substitutions and their functional implications within the context of biochemical and physiological adaptation. The α-subunit of decapod crustaceans shows greater conservation (∼94% identity) compared to the β-subunit (∼40%). While the binding sites for ATP and modulators are conserved in the decapod enzyme, the residues involved in the α-β interaction are only partially conserved. In the phylogenetic context of the complete sequence of (Na+, K+)-ATPase α-subunit, most substitutions appear to be characteristic of the entire group, with specific changes for different subgroups, especially among brachyuran crabs. Interestingly, there was no consistent separation of α-subunit partial sequences related to habitat, suggesting that the convergent evolution for freshwater or terrestrial modes of life is not correlated with similar changes in the enzyme's primary amino acid sequence.

Evolving views of ionic, osmotic and acid-base regulation in aquatic animals

The regulation of ionic, osmotic and acid-base (IOAB) conditions in biological fluids is among the most fundamental functions in all organisms; being surrounded by water uniquely shapes the IOAB regulatory strategies of water-breathing animals. Throughout its centennial history, Journal of Experimental Biology has established itself as a premier venue for publication of comparative, environmental and evolutionary studies on IOAB regulation. This Review provides a synopsis of IOAB regulation in aquatic animals, some of the most significant research milestones in the field, and evolving views about the underlying cellular mechanisms and their evolutionary implications. It also identifies promising areas for future research and proposes ideas for enhancing the impact of aquatic IOAB research.

Pseudobranch mimics gill in expressing Na+K+-ATPase 1 α-subunit and carbonic anhydrase in concert with H+-ATPase in adult hilsa (Tenualosa ilisha) during river migration

In hilsa (Tenualosa ilisha), pseudobranch comprises a row of parallel filaments bear numerous leaf-like lamellae arranged on both sides throughout its length. The purpose of this study was to elucidate involvement of pseudobranchial Na⁺, K⁺-ATPase (NKA) 1 α-subunit, and carbonic anhydrase (CA) in concert with H⁺-ATPase (HAT) compared to their branchial counterparts in freshwater acclimation of hilsa during spawning migration from off-shore of the Bay of Bengal to the Bhagirathi-Hooghly zones of the Ganga river system in India. Adult hilsa fish were collected from seawater (SW), freshwater 1 (FW1), and freshwater 2 (FW2) locations, where the salinity level was 26-28‰, 1-5‰, and 0-0.04‰, respectively. Hilsa migrating through freshwater showed a consistent decrease in the plasma osmolality, sodium (Na⁺) and chloride (Cl^-) ion levels indicates unstable ionic homeostasis. The mRNA expression and activity of NKA 1 α-subunit in pseudobranch as well as in true gills declined with the migration to upstream locations. The pseudobranchial CA activity almost mirrors its branchial counterpart most notably while hilsa entered the freshwater zone, in the upstream river suggesting its diverse role in hypo-osmotic regulatory acclimation. Nevertheless, the H⁺-ATPase activity of both the tissues increased with the freshwater entry and remained similar during up-river movement into the freshwater environment. The results confirm that the pseudobranchial NKA 1 α-subunit mRNA expression and activity mimic its branchial counterpart in the process of ionoregulatory acclimation during migration through salt barriers. Also, the increase in the activities of pseudobranchial and branchial CA in concert with H⁺-ATPase (HAT) during freshwater acclimation of hilsa suggests their critical involvement in ion uptake.

Redox and metabolic strategies developed by anterior and posterior gills of the crab Neohelice granulata after short periods of hypo- or hyper-osmotic stress

The aim of this study was to identify the response pattern of redox balance, Na⁺/K⁺ATPase activity and HSP70 expression in the posterior and anterior gills of the crab Neohelice granulata submitted to hypo- or hyper-osmotic stress for 1 h and 6 h. After 1 h of either type of osmotic stress, there was an increase in catalase activity, but a decrease in GSSG/GSH ratio (oxidized to reduced glutathione ratio) and Na⁺/K⁺ATPase activity in both gill sets. H₂O₂ levels decreased only in the posterior gills. H₂O₂ levels and Na⁺/K⁺ATPase activity remained reduced after 6 h of exposure to either type of osmotic stress in both gill sets. The GSSG/GSH ratio returned to initial levels after 6 h of hyper-osmotic stress, whereas it increased 10 times in both gill sets after hypo-osmotic stress. Furthermore, HSP70 protein expression increased in posterior gills after 6 h of hypo-osmotic stress. H₂O₂ levels in tank water decreased after hypo-osmotic challenge and increased after 6 h of hyper-osmotic stress, indicating increased H₂O₂ excretion. Therefore, N. granulata gills have redox, metabolic and molecular strategies to deal with rapid osmotic challenges, an important environmental parameter that influences juvenile and adult crab distribution and abundance within different populations.

A study of the electrical polarization of Sepia officinalis yolk envelope, a role for Na(+)/K(+)-ATPases in osmoregulation?

The cuttlefish Sepia officinalis mate and spawn in the intertidal zone where eggs are exposed during low tide to osmotic stress. Embryonic outer yolk sac is a putative site for osmoregulation of young S. officinalis embryos. By using electrophysiological recordings and immunostaining we showed, (i) that the chorion is only a passive barrier for ions, since large molecules could not pass through it, (ii) that a complex transepithelial potential difference occurs through the yolk epithelium, (iii) that ionocyte-like cells and Na⁺/K⁺-ATPases were localized in the yolk epithelium and (iv) that ouabain sensitive Na⁺/K⁺-ATPase activity could participate to this yolk polarization. These data warrant further study on the role of ion transport systems of this epithelium in the osmoregulation processes in S. officinalis embryos.

Synchronization modulation increases transepithelial potentials in MDCK monolayers through Na/K pumps

Transepithelial potential (TEP) is the voltage across a polarized epithelium. In epithelia that have active transport functions, the force for transmembrane flux of an ion is dictated by the electrochemical gradient in which TEP plays an essential role. In epithelial injury, disruption of the epithelial barrier collapses the TEP at the wound edge, resulting in the establishment of an endogenous wound electric field (∼100 mV/mm) that is directed towards the center of the wound. This endogenous electric field is implicated to enhance wound healing by guiding cell migration. We thus seek techniques to enhance the TEP, which may increase the wound electric fields and enhance wound healing. We report a novel technique, termed synchronization modulation (SM) using a train of electric pulses to synchronize the Na/K pump activity, and then modulating the pumping cycles to increase the efficiency of the Na/K pumps. Kidney epithelial monolayers (MDCK cells) maintain a stable TEP and transepithelial resistance (TER). SM significantly increased TEP over four fold. Either ouabain or digoxin, which block Na/K pump, abolished SM-induced TEP increases. In addition to the pump activity, basolateral distribution of Na/K pumps is essential for an increase in TEP. Our study for the first time developed an electrical approach to significantly increase the TEP. This technique targeting the Na/K pump may be used to modulate TEP, and may have implication in wound healing and in diseases where TEP needs to be modulated.

Regulation of ion transport by pH and [HCO3−] in isolated gills of the crabNeohelice(Chasmagnathus)granulata

Posterior isolated gills of Neohelice ( Chasmagnathus) granulatus were symmetrically perfused with hemolymph-like saline of varying [HCO3−] and pH. Elevating [HCO3−] in the saline from 2.5 to 12.5 mmol/l (pH 7.75 in both cases) induced a significant increase in the transepithelial potential difference ( Vte), a measure of ion transport. The elevation in [HCO3−] also induced a switch from acid secretion (−43.7 ± 22.5 μequiv·kg−1·h−1) in controls to base secretion (84.7 ± 14.4 μequiv·kg−1·h−1). The HCO3−-induced Vteincrease was inhibited by basolateral acetazolamide (200 μmol/l), amiloride (1 mmol/l), and ouabain (5 mmol/l) but not by bafilomycin (100 nmol/l). The Vteresponse to HCO3−did not take place in Cl−-free conditions; however, it was unaffected by apical SITS (2 mmol/l) or DIDS (1 mmol/l). A decrease in pH from 7.75 to 7.45 pH units in the perfusate also induced a significant increase in Vte, which was matched by a net increase in acid secretion of 67.8 ± 18.4 μequiv kg−1h−1. This stimulation was sensitive to basolateral acetazolamide, bafilomycin, DIDS, and Na+-free conditions, but it still took place in Cl−-free saline. Therefore, the cellular response to low pH is different from the HCO3−-stimulated response. We also report V-H+-ATPase- and Na+-K+-ATPase-like immunoreactivity in gill sections for the first time in this crab. Our results suggest that carbonic anhydrase (CA), basolateral Na+/H+exchangers and Na+-K+-ATPase and apical anion exchangers participate in the HCO3−-stimulated response, while CA, apical V-H+-ATPase and basolateral HCO3−-dependent cotransporters mediate the response to low pH.

A structure-function analysis of ion transport in crustacean gills and excretory organs

Osmotic and ionic regulation in the Crustacea is mostly accomplished by the multifunctional gills, together with the excretory organs. In addition to their role in gas exchange, the gills constitute organs of active, transepithelial, ion transport, an activity of major importance that underlies many essential physiological functions like osmoregulation, calcium homeostasis, ammonium excretion and extracellular pH regulation. This review focuses on structure-function relationships in crustacean gills and excretory effectors, from the organ to molecular levels of organization. We address the diversity of structural architectures encountered in different crustacean gill types, and in constituent cell types, before examining the physiological mechanisms of Na(+), Cl(-), Ca(2+) and NH(4)(+) transport, and of acid-base equivalents, based on findings obtained over the last two decades employing advanced techniques. The antennal and maxillary glands constitute the principal crustacean excretory organs, which have received less attention in functional studies. We examine the diversity present in antennal and maxillary gland architecture, highlighting the structural similarities between both organ types, and we analyze the functions ascribed to each glandular segment. Emphasis is given to volume and osmoregulatory functions, capacity to produce dilute urine in freshwater crustaceans, and the effect of acclimation salinity on urine volume and composition. The microanatomy and diversity of function ascribed to gills and excretory organs are appraised from an evolutionary perspective, and suggestions made as to future avenues of investigation that may elucidate evolutionary and adaptive trends underpinning the invasion and exploitation of novel habitats.

Dopaminergic regulation of ion transport in gills of the euryhaline semiterrestrial crabChasmagnathus granulatus: interaction between D1- and D2-like receptors

The effects of dopamine (DA) and dopaminergic agonists and antagonists on ion transport were studied in isolated perfused gills of the crab Chasmagnathus granulatus. DA applied under steady state conditions(perfusion with hemolymph-like saline) produced a transient increase of the transepithelial potential difference (Vte) from 2.2±0.2 to 4.8±0.3 mV, describing an initial cAMP-dependent stimulating phase followed by an inhibitory phase. Spiperone and domperidone(antagonists of D2-like DA receptors in vertebrates) completely blocked the response to DA, while the D1-like antagonist SCH23390 blocked only the inhibitory phase. Theophylline (phosphodiesterase inhibitor) and okadaic acid(protein phosphatases PP1 and PP2A inhibitor) were also able to block the inhibitory phase, suggesting that it depends on adenylyl cyclase inhibition and on protein phosphatases. When the gills were perfused with hypo-osmotic solution, or with the adenylyl cyclase activator forskolin, Vte was increased several-fold. DA applied under these stimulated conditions partially reversed the Vte increase by 54% and 25%, respectively. Similarly, the D1-like agonist, fenoldopam,produced a 33% reduction in the stimulated Vte. We propose that, in C. granulatus gills, DA stimulates adenylyl cyclase and therefore ion transport through D1-like receptors linked to a Gs protein,although they respond to antagonists that interact with D2-like receptors in vertebrates. The inhibitory phase seems to be mediated by D2-like receptors linked to a Gi/o protein, which inhibits adenylyl cyclase, although these receptors can be activated or blocked by agonists or antagonists that interact with D1-like receptors in vertebrates and insects.

Physiological and behavioral effects of chemoreceptors located in different body parts of the swimming crab Callinectes danae

By perfusing their branchial chambers with filtered seawater, we have developed a preparation that allows us to maintain the swimming crab Callinectes danae outside water without any major effects on its cardiac activity. This in turn allowed us to selectively stimulate chemoreceptors located in different body parts, and specifically to discriminate between the receptors located in the branchial chambers and those located in the oral region (mainly in the mouthparts, antennules and antennae). We show that a taurine solution can evoke bradycardia when applied to the oral region or to a combination of the oral region and the branchial chambers. Although the precise localization of the oral region receptors involved remains to be determined, ablation experiments show that the olfactory organs (i.e., the antennules) are not involved. Finally, we show that although stimulating the pereiopods has no effect on the animals' cardiac activity it causes the animals to move, putatively to try to grasp a piece of food, a reaction not evoked by stimulating the gills or the oral regions. Our results lend support to the idea that chemoreceptors located in different parts of the body play different functional roles in decapod crustaceans.

Induction of branchial ion transporter mRNA expression during acclimation to salinity change in the euryhaline crab Chasmagnathus granulatus

Using quantitative real-time PCR, the expression of mRNAs encoding three transport-related proteins and one putative housekeeping protein was analyzed in anterior and posterior gills of the euryhaline crab Chasmagnathus granulatus following transfer from isosmotic conditions (30 per thousand salinity) to either dilute (2 per thousand) or concentrated (45 per thousand) seawater. Modest changes were observed in the abundance of mRNAs encoding the housekeeping protein arginine kinase and the vacuolar-type H(+)-ATPase B-subunit, both of which were highly expressed under all conditions. By contrast, the expression of Na(+)/K(+)-ATPase alpha-subunit mRNA and Na(+)/K(+)/2Cl(-) cotransporter mRNA was strongly responsive to external salinity. During acclimation to dilute seawater, cotransporter mRNA increased 10-20-fold in posterior gills within the first 24 h while Na(+)/K(+)-ATPase alpha-subunit mRNA increased 35-55-fold. During acclimation to concentrated seawater, cotransporter mRNA increased 60-fold by 96 h and Na(+)/K(+)-ATPase alpha-subunit increased approximately 25-fold in posterior gills. Our results indicate a complex pattern of transcriptional regulation dependent upon the direction of salinity change and the developmental background of the gills.

Possible role of carbonic anhydrase, V–H+–ATPase, and Cl−/HCO3− exchanger in electrogenic ion transport across the gills of the euryhaline crab Chasmagnathus granulatus

We studied the participation of carbonic anhydrase (CA), V-H(+)-ATPase, and Cl(-)/HCO3- exchanger in electrogenic ion absorption through the gills of Chasmagnathus granulatus. CA activity was measured in anterior gills and posterior gills after acclimation to 2 per thousand, 10 per thousand, 30 per thousand (about seawater), and 45 per thousand salinity. The highest CA specific activity was detected in the microsomal fraction in anterior gills, and in the cytosolic fraction, in posterior ones. Both fractions were strongly induced by decreasing salinity only in posterior gills. Perfusion of posterior gills from crabs acclimated to either 2 per thousand or 10 per thousand with acetazolamide inhibited CA activity almost completely. In posterior gills from crabs acclimated to 2 per thousand and perfused with 20 per thousand saline (iso-osmotic for these crabs), acetazolamide reduced transepithelial potential difference (V(te)) by 47%, further addition of ouabain enhanced the effect to 88%. Acetazolamide had no effect in the same gills perfused with 30 per thousand saline (iso-osmotic for seawater acclimated crabs). Bafilomycin A1 and SITS (inhibitors of V-H(+)-ATPase and Cl(-)/HCO3-) reduced V(te) by 15-16% in gills perfused with normal 20 per thousand saline, and by 77% and 45%, respectively when they were applied in Na-free 20 per thousand saline, suggesting the participation of those transporters and cytosolic CA in electrogenic ion absorption.

Ammonia excretion in aquatic and terrestrial crabs

The excretory transport of toxic ammonia across epithelia is not fully understood. This review presents data combined with models of ammonia excretion derived from studies on decapod crabs, with a view to providing new impetus to investigation of this essential issue. The majority of crabs preserve ammonotely regardless of their habitat, which varies from extreme hypersaline to freshwater aquatic environments, and ranges from transient air exposure to obligate air breathing. Important components in the excretory process are the Na+/K+(NH4+)-ATPase and other membrane-bound transport proteins identified in many species, an exocytotic ammonia excretion mechanism thought to function in gills of aquatic crabs such as Carcinus maenas, and gaseous ammonia release found in terrestrial crabs, such as Geograpsus grayi and Ocypode quadrata. In addition, this review presents evidence for a crustacean Rhesus-like protein that shows high homology to the human Rhesus-like ammonia transporter both in its amino acid sequence and in its predicted secondary structure.

Modulation of ion uptake across posterior gills of the crab Chasmagnathus granulatus by dopamine and cAMP

Cyclic AMP (cAMP) and dopamine modulate ion uptake across isolated and perfused posterior gills of Chasmagnathus granulatus acclimated to 10 per thousand salinity. Addition of cAMP agonists, such as cp-cAMP, forskolin, and IBMX, produced a significant increase in the transepithelial potential difference (Vte), which reflects ion transport activity. Dopamine (DA) also had a stimulatory effect on ion uptake, increasing Vte and Na(+) influx, although this effect was transient, since both variables remained elevated for less than 30 min. In addition, the dose-response curve for DA concentration-Vte was biphasic, and the maximum stimulation was obtained with 10 micromol l(-1). When the effects of forskolin and DA on the Na(+)/K(+)-ATPase activity were tested, they correlated well with the Vte and Na(+) influx experiments; the enzyme activity increased significantly after preincubation of gill fragments for 10 min with forskolin or DA (51 and 64%, respectively), but there was no effect after pre-incubation with DA for 20 min. Finally, KT5720, a specific inhibitor of cAMP-dependent protein kinase (PKA), completely abolished the stimulatory effect of DA on Vte, suggesting the involvement of PKA in this mechanism.

The mechanism of sodium chloride uptake in hyperregulating aquatic animals

The emphasis in this review will be on Na+ absorption across the skin and gills of vertebrates and the gills of crustaceans. However, some recent studies of Cl– uptake, especially in crustaceans, will also be described.

Na(+)+K(+)-ATPase in gills of aquatic crustacea

The sodium pump, or Na(+)+K(+)-ATPase, provides at least part of the driving force for transepithelial movement of monovalent ions across the gills and other transporting tissues in many aquatic animals including the Crustacea. The crustacean Na(+)+K(+)-ATPase, like that in all animal cells, is composed of a catalytic alpha-subunit and an accompanying beta-subunit. The amino acid sequence of the crustacean alpha-subunit is 71-74% identical to vertebrate alpha-subunit sequences. In brachyuran Crustacea, the Na(+)+K(+)-ATPase is more highly expressed in posterior gills compared with anterior and is found predominantly in mitochondria-rich cells that are morphologically and biochemically specialized to mediate NaCl uptake from the medium. When the external salinity is lowered from that of normal seawater, producing conditions in which many euryhaline Crustacea hyperosmo regulate their hemolymph, both the enzymatic activity of the Na(+)+K(+)-ATPase and the gene expression of the alpha-subunit are increased in these tissues. Although the precise regulatory mechanism is not known, evidence suggests that crustacean hyperglycemic hormone may be responsible for the induction of Na(+)+K(+)-ATPase activity. Whether it also plays a role in activation of gene transcription is not known. A comparison of a range of aquatic Crustacea suggests that the level of Na(+)+K(+)-ATPase function in transporting tissues may be correlated with their ability to invade estuarine habitats.

Active NaCl absorption across posterior gills of hyperosmoregulating Chasmagnathus granulatus

Split lamellae of posterior gills of Chasmagnathus granulatus adapted to 2.5 per thousand salinity were mounted in a modified Ussing chamber. With NaCl-saline on both sides of the preparation a transepithelial voltage (V(te)) of 4.1+/-0.5 mV (outside positive) was measured. After voltage-clamping, the negative short-circuit current (I(sc)) amounted to -142+/-21 micro A cm(-2) at a conductance (G(te)) of 44+/-5 mS cm(-2). Substitution of either chloride (by nitrate) or sodium (by choline) on both sides of split gill lamellae significantly reduced I(sc) (by 70-80%) and G(te) (by 30-50%). External CsCl (but not BaCl(2) or furosemide) inhibited the negative I(sc) without affecting G(te). Addition of ouabain, BaCl(2) or diphenylamine-2-carboxylate to the internal bath inhibited I(sc) at unchanged G(te). Internal acetazolamide did not affect I(sc) or G(te) across split gill lamellae. Unidirectional Na(+) influx across isolated and perfused posterior gills, however, was reduced by internal acetazolamide by approximately 20% at constant V(te). The results suggest that posterior gills of hyperosmoregulating C. granulatus display a high conductance epithelium that actively absorbs NaCl in a coupled way by an electrogenic mechanism similar to that seen in the thick ascending limb of Henle's loop and, to a minor degree, by an electroneutral mechanism, presumably via apical Na(+)/H(+)- and Cl(-)/HCO(3)(-)-antiports.

Electrophysiology of posterior, NaCl-absorbing gills of Chasmagnathus granulatus: rapid responses to osmotic variations

In the present study, the influence of short-term osmotic variations on some electrophysiological properties related to NaCl absorption across posterior gills of Chasmagnathus granulatus was investigated. The transepithelial potential difference (V(te)) of isolated and perfused gills increased significantly when hyposmotic saline (699 mosmol l(-1)) was used instead of isosmotic solution (1045 mosmol l(-1)). A reduction of the concentration of Na(+) or Cl(-) at constant osmolarity did not produce any change in V(te). Transepithelial short-circuit current (I(sc)) and conductance (G(te)), measured with split gill lamellae mounted in a modified Ussing chamber, also increased after changing to hyposmotic salines (I(sc): from -89.0+/-40.8 microA cm(-2) to -179.3+/-37.0 microA cm(-2); G(te): from 40.5+/-16.9 mS cm(-2) to 47.3+/-15.8 mS cm(-2)). The observed effects of reduced osmolarity were fast, reversible and gradually dependent on the magnitude of the osmotic variation. The activity of the Na(+)/K(+)-ATPase increased significantly after perfusion with hyposmotic saline, from 18.73+/-6.35 micromol P(i) h(-1) mg(-1) to 41.84+/-14.54 micromol P(i) h(-1) mg(-1). Theophylline maintained part of the elevated V(te) induced by hyposmotic saline, suggesting that an increased cellular cyclic AMP level is involved in the response to reduced osmolarity. In summary, the results indicate that the hemolymph osmolarity regulates active transbranchial NaCl absorption by modulating the activity of the basolateral Na(+)/K(+)-ATPase and by changing a conductive pathway, probably at the apical membrane.