Electrospray identification of new polyoxochromate species

Electrospray ionization mass spectrometry (ESI MS) has been conducted on the ammonium and alkali metal (A=Li⁺, Na⁺ and K⁺) dichromate systems. A large number of previously unknown polyoxochromate species have been characterized. Major series that have been identified include [A_x+1H_xCr^VI_xO_4x]⁺ (Li⁺, x=1-5; Na⁺, x=1-7; K⁺, x=1-4) and [A_2x−1Cr^VI_xO_4x−1]⁺ (Li⁺, x=2, 3; Na⁺, x=2-4; K⁺, x=2, 3) in the alkali metal dichromate systems, and [HCr^VI_xO_3x+1]⁻ (x=1-5) in the ammonium dichromate system. Several series also contain mixed oxidation state species, ranging from Cr(V) to Cr(II) in conjunction with Cr(VI), which is consistent with the ease of reduction of Cr(VI). Negative ion ESI MS spectra clearly demonstrate the existence of [HCrO₄]⁻ as the most abundant ion at −20 V, suggesting that its existence in solution is not just hypothetical, as was previously thought. The polymerization units for the series observed include {AHCrO₄}, {A₂CrO₄} and {CrO₃}, with the latter prominent in the alkali metal systems. This presumably arises from the fragmentation of dichromate, A₂Cr₂O₇→{A₂CrO₄}+{CrO₃}. Moreover, the ESI MS of the dichromate compounds have illustrated that the preservation of tetrahedral stereochemistry is of paramount importance for these systems, which leads to only limited polymerization compared to the related molybdate and tungstate systems.     

Linkage of Mutations Affecting minus Flagellar Membrane Agglutinability to the mt Mating-Type Locus of Chlamydomonas

Two independently isolated mutant strains, imp-10 and imp-12, were obtained by UV irradiation of wild-type mating-type minus (wt^-). Each fails to agglutinate sexually with gametes of either mating type, but mating and zygote formation can be elicited by agglutinating either strain to wt⁺ gametes by means of anti-flagellar antiserum. Tetrad analysis of the resultant zygotes shows that both imp-10 and imp-12 are very closely linked to mt^-, with no recombinants observed. Diploid strains constructed between imp-10 or imp-12 and wt⁺ gametes are wt^-, that is, they agglutinate and fuse like normal minus cells. Tetrad analysis of triploids from imp-10 diploid x wt⁺ haploid crosses shows that only imp-10 and wt⁺ products are recovered. A model is proposed to account for these results.     

Inactivation of Bacteriophage T4 by Ethyl Methanesulfonate: Influence of Host and Viral Genotypes

Inactivation of bacteriophage T4 by ethyl methanesulfonate (EMS) is a complex process which depends critically upon the conditions of treatment and upon both the viral and the host genotypes. EMS-inactivated particles are capable of multiplicity and cross-reactivation, indicating the need for caution in using EMS in certain types of mutation studies. The pyrimidine dimer excision systems of the phage and the host do not affect the EMS sensitivity of T4, but the T4x⁺y⁺ system does. Mutational defects in the deoxyribonucleic acid (DNA) ligase and the DNA polymerase systems both of the virus and of its host also affect viral EMS sensitivity.     

Riverine nitrogen export in Swedish catchments dominated by atmospheric inputs

We present the first estimates of net anthropogenic nitrogen input (NANI) in European boreal catchments. In Swedish catchments, nitrogen (N) deposition is a major N input (31–94%). Hence, we used two different N deposition inputs to calculate NANI for 36 major Swedish catchments. The relationship between riverine N export and NANI was strongest when using only oxidized deposition (NO_y) as atmospheric input (r² = 0.70) rather than total deposition (i.e., both oxidized and reduced nitrogen, NO_y + NH_x deposition, r² = 0.62). The y-intercept (NANI = 0) for the NANI calculated with NO_y is significantly different from zero (p = 0.0042*) and indicates a background flux from the catchment of some 100 kg N km^?2 year^?1 in addition to anthropogenic inputs. This agrees with similar results from North American boreal catchments. The slope of the linear regressions was 0.25 for both N deposition inputs (NO_y and NO_y + NH_x), suggesting that on average, 25% of the anthropogenic N inputs is exported by rivers to the Baltic Sea. Agricultural catchments in central and southern Sweden have increased their riverine N export up to tenfold compared to the inferred background flux. Although the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition, these catchments do not show significantly elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention may be saturating as loading rates increase. In northern and western Sweden the export of nitrogen is largely controlled by the hydraulic load, i.e., the riverine discharge normalized by water surface area, which has units of distance time^?1. Besides hydraulic load the percent total forest cover also affects the nitrogen export primarily in the northern and western catchments.     

A kinetic comparison between E2P and the E2P-like state induced by a beryllium fluoride complex in the Na,K-ATPase. Interactions with Rb+

Metal-fluoride complexes have been used to induce E2P-like states with the aim of studying the events that occur during E2P hydrolysis in P-type ATPases. In the present work, we compared the E2P-like state induced by a beryllium fluoride complex (BeF_x) with the actual E2P state formed through backdoor phosphorylation of the Na,K-ATPase. Formation of E2P and E2P-like states were investigated employing the styryl dye RH421. We found that BeF_x is the only fluorinated phosphate analog that, like Pi, increases the RH421 fluorescence. The observed rate constant, k_obs, for the formation of E2P decreases with [Pi] whereas that of E2BeF_x increases with [BeF_x]. This might wrongly be taken as evidence of a mechanism where the binding of BeF_x induces a conformational transition. Here, we rather propose that, like for Pi, binding of BeF_x follows a conformational-selection mechanism, i.e. it binds to the E2 conformer forming a complex that is much more stable than E2P, as seen from its impaired capacity to return to E1 upon addition of Na⁺. Although E2P and E2BeF_x are able to form states with 2 occluded Rb⁺, both enzyme complexes differ in that the affinity for the binding and occlusion of the second Rb⁺ is much lower in E2BeF_x than in E2P. The higher rates of Rb⁺ occlusion and deocclusion observed for E2BeF_x, as compared to those observed for other E2P-like transition and product states suggest a more open access to the cation transport sites, supporting the idea that E2BeF_x mimics the E2P ground state.     

Inhibitory efficacy of bufadienolides on Na+,K+-pump activity versus cell proliferation

Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na⁺,K⁺pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na⁺,K⁺-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na⁺,K⁺pumps expressed in Xenopus oocytes.All tested bufadienolides inhibited cell proliferation and Na⁺,K⁺pump activity. An activity coefficient A=100xIC₅₀^{Na,K pump}/IC₅₀^{proliferation} was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na⁺,K⁺pump isoforms may be a promising strategy for development of anticancer drugs.     

Trapping of a transcription complex using a new nucleotide analogue: AMP aluminium fluoride

Mechanochemical proteins rely on ATP hydrolysis to establish the different functional states required for their biological output. Studying the transient functional intermediate states these proteins adopt as they progress through the ATP hydrolysis cycle is key to understanding the molecular basis of their mechanism. Many of these intermediates have been successfully ‘trapped’ and functionally characterised using ATP analogues. Here, we present a new nucleotide analogue, AMP-AlF_x, which traps PspF, a bacterial enhancer binding protein, in a stable complex with the σ⁵⁴-RNA polymerase holoenzyme. The crystal structure of AMP-AlF_x•PspF_1-275 provides new information on protein-nucleotide interactions and suggests that the β and γ phosphates are more important than the α phosphate in terms of sensing nucleotide bound states. In addition, functional data obtained with AMP-AlF_x establish distinct roles for the conserved catalytic AAA⁺ (ATPases associated with various cellular activities) residues, suggesting that AMP-AlF_x is a powerful new tool to study AAA⁺ protein family members and, more generally, Walker motif ATPases.     

Dihydropyridine Action on Voltage-dependent Potassium Channels Expressed in Xenopus Oocytes

Dihydropyridines (DHPs) are well known for their effects on L-type voltage-dependent Ca²⁺ channels. However, these drugs also affect other voltage-dependent ion channels, including Shaker K⁺ channels. We examined the effects of DHPs on the Shaker K⁺ channels expressed in Xenopus oocytes. Intracellular applications of DHPs quickly and reversibly induced apparent inactivation in the Shaker K⁺ mutant channels with disrupted N- and C-type inactivation. We found that DHPs interact with the open state of the channel as evidenced by the decreased mean open time. The DHPs effects are voltage-dependent, becoming more effective with hyperpolarization. A model which involves binding of two DHP molecules to the channel is consistent with the results obtained in our experiments.     

A neural network study on the dynamic identification of a fermentation system

The objective of this paper is to propose neural networks for the study of dynamic identification and prediction of a fermentation system which produces mainly 2,3-butanediol (2,3-BDL). The metabolic products of the fermentation, acetic acid, acetoin, ethanol, and 2,3-BDL were measured on-line via a mass spectrometer modified by the insertion of a dimethylvinylsilicone membrane probe. The measured data at different sampling times were included as the input and output nodes, at different learning batches, of the network. A fermentation system is usually nonlinear and dynamic in nature. Measured fermentation data obtained from the complex metabolic pathways are often difficult to be entirely included in a static process model, therefore, a dynamic model was suggested instead. In this work, neural networks were provided by a dynamic learning and prediction process that moved along the time sequence batchwise. In other words, a scheme of two-dimensional moving window (number of input nodes by the number of training data) was proposed for reading in new data while forgetting part of the old data. Proper size of the network including proper number of input/output nodes were determined by trained with the real-time fermentation data. Different number of hidden nodes under the consideration of both learning performance and computation efficiency were tested. The data size for each learning batch was determined. The performance of the learning factors such as the learning coefficient η and the momentum term coefficient α were also discussed. The effect of different dynamic learning intervals, with different starting points and the same ending point, both on the learning and prediction performance were studied. On the other hand, the effect of different dynamic learning intervals, with the same starting point and different ending points, was also investigated. The size of data sampling interval was also discussed. The performance from four different types of transfer functions, x/(1+|x|), sgn(x)·x ²/(1+x ²), 2/(1+e ^{? x} )?1, and 1/(1+e ^{? x} ) was compared. A scaling factor b was added to the transfer function and the effect of this factor on the learning was also evaluated. The prediction results from the time-delayed neural networks were also studied.     

Channel Properties of Nax Expressed in Neurons

Na_x is a sodium-concentration ([Na⁺])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na⁺] ([Na⁺]_o) in vitro. We previously reported that Na_x was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na⁺] sensing for the control of salt-intake behavior. Although Na_x was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Na_x have not yet been adequately characterized in neurons. We herein verified that Na_x was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Na_x. To investigate the channel properties of Na_x expressed in neurons, we established an inducible cell line of Na_x using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Na_x. Functional analyses of this cell line revealed that the [Na⁺]-sensitivity of Na_x in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Na_x channel in cations was revealed to be Na⁺ ≈ Li⁺ > Rb⁺ > Cs⁺ for the first time. Furthermore, we demonstrated that Na_x bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Na_x and PSD95 may be involved in promoting the surface expression of Na_x channels because the depletion of endogenous PSD95 resulted in a decrease in Na_x at the plasma membrane. These results indicated, for the first time, that Na_x functions as a [Na⁺]-sensitive Na channel in neurons as well as in glial cells.     

Electrophysiology of phagocytic membranes: intracellular K+ activity and K+ equilibrium potential in macrophage polykaryons

The role of K⁺ as current carrier during the slow membrane hyperpolarizations (SH) elicited by iontophoretic Ca²⁺ injections into macrophage polykaryons is studied. The intracellular K⁺ activity (a_K) and the K⁺ equilibrium potential (E_K) are measured using ion-sensitive microelectrodes. The mean value of a_K is 84 ± 5 mM in a culture medium containing 5.3 mM K⁺, but increases to 100 ± 8 mM when the extracellular K⁺ concentration is raised to 30.3 mM. Under the same conditions the values of E_K obtained from the Nernst equation are −81 ± 2 mV and −40 ± 2 mV, respectively. The reversal potentials (E_R) of the SH are calculated from changes observed in transmembrane potential and input resistance, according to an equivalent model based only on passive ionic fluxes. The mean E_R values obtained are −74 ± 8 mV in the presence of low K⁺ concentration and −37 ± 3 mV for the high K⁺ medium. These values are significantly smaller than the estimated E_K for the corresponding situations. Evidence for the existence of an electrogenic (Na⁺ + K⁺)-ATPase activity is also presented. The evidence indicates that an increase in the membrane potassium permeability can account for about 90% of the total permeability change occurring during the SH.     

Transport ATPase of erythrocyte membrane: sensitivities of Na plus, K, plus-ATPase and Kplus-phosphatase activities to ouabain.

Cardiac glycosides are inhibitors of Na⁺,K⁺-ATPase, and K⁺-phosphatase activities of the transport enzyme. Previous studies have shown that when the sensitivities of these two activities to ouabain are compared by the addition of varying concentrations of the drug to the assay media, the K⁺-phosphatase is significantly less sensitive than Na⁺,K⁺-ATPase. This work was done to seek an explanation for this phenomenon. 3-O-Methyl-fluorescein phosphate was used as substrate for the continuous fluorimetric assay of K⁺-phosphatase obtained from human red cells. When ouabain was added to the assay medium, a time-dependent inhibition of K⁺-phosphatase was observed. The rate of inhibition was also influenced by the order of additions of K⁺ and ouabain. In view of these results, several enzyme samples exposed to ouabain for varying lengths of time were prepared, and their Na⁺,K⁺-ATPase and K⁺-phosphatase activities were then determined. A good correlation between the extent of inhibition of the two activities was obtained. These results prove that the previously observed discrepancies between the sensitivities of Na⁺,K⁺-ATPase and K⁺-phosphatase to ouabain are due to the different kinetics of drug interaction with the enzyme under the different conditions of the two assays and that once a certain level of ouabain binding to the enzyme is achieved, both activities are equally inhibited.     

Gust J, a salt-insensitive mutant ofDrosophila melanogaster

A set of 340 P insert lines on the X chromosome and the autosomes were screened for altered responses of the labellar chemoreceptors to salts and sucrose. A mutant linegustJ was isolated in which the electrophysiological response of the salt-sensitive neuron to Na⁺ in the sensilla of the proboscis is reduced. The responses to KC1 and sucrose are unaffected. In feeding tests,gustJ flies have Na⁺-specific defects. Heterozygotes ofgustJ with two other salt mutantsgustE andBE1323 are normal. Multiple alleles ofgustJ have been obtained by excision of the original P element. All mutants have defects in Na⁺ sensing specifically, thus defining a new gene that affects Na⁺ response of the fly.     

Oligomerization of the Cu(II) and Ni(II) tetraazaannulene complexes: A pulse radiolysis study of the associated reaction mechanism

The oligomerization of [Cu^II(H_x(tmdnTAA))]^x+ (x = 0, 1, 2 and (tmdnTAA))²⁻ is 2,4,9,11-tetramethyl-dinaphto[14]-2,4,6,9,11,13-hexaeneN₄) was initiated in homogeneous solution via the reaction of this Cu(II) complex with pulse radiolytically generated radicals. The reaction produces Cu(III) intermediates which are rapidly converted to Cu(II) ligand-radical species. In contrast to the mechanism proposed for the electrochemical oligomerization, where the local concentration of radicals is probably high, the reaction kinetics in homogeneous solution is propagated by a process where the Cu(II) ligand-radical precursors react with [Cu^II(H_x(tmdnTAA))]^x+.     

Active-Site Monovalent Cations Revealed in a 1.55-Å-Resolution Hammerhead Ribozyme Structure

We have obtained a 1.55-Å crystal structure of a hammerhead ribozyme derived from Schistosoma mansoni under conditions that permit detailed observations of Na⁺ ion binding in the ribozyme's active site. At least two such Na⁺ ions are observed. The first Na⁺ ion binds to the N7 of G10.1 and the adjacent A9 phosphate in a manner identical with that previously observed for divalent cations. A second Na⁺ ion binds to the Hoogsteen face of G12, the general base in the hammerhead cleavage reaction, thereby potentially dissipating the negative charge of the catalytically active enolate form of the nucleotide base. A potential but more ambiguous third site bridges the A9 and scissile phosphates in a manner consistent with that of previous predictions. Hammerhead ribozymes have been observed to be active in the presence of high concentrations of monovalent cations, including Na⁺, but the mechanism by which monovalent cations substitute for divalent cations in hammerhead catalysis remains unclear. Our results enable us to suggest that Na⁺ directly and specifically substitutes for divalent cations in the hammerhead active site. The detailed geometry of the pre-catalytic active-site complex is also revealed with a new level of precision, thanks to the quality of the electron density maps obtained from what is currently the highest-resolution ribozyme structure in the Protein Data Bank.     

Mechanism for phenanthridines synthesis by nitrogenation of 2-acetylbiphenyls in acidic solution: a DFT study

The mechanism of phenanthridines synthesis from the nitrogenation of 2-acetylbiphenyls (1) by TMSN₃ in TFA has been studied by DFT calculations. Results at the B3LYP/6-311G(d) level showed that: 1) reaction of TMSN₃/HN₃ with the protonated form of 1 (1H⁺), which generates the key intermediate C ^x+ by removal of TMSOH/H₂O, is the rate determining step, and TMSN₃ as the nitrogen source is certainly preferred over HN₃. 2) from C ^x+, the two pathways leading to 2 ^x H⁺ and 3 ^x H⁺ are both thermodynamically and kinetically feasible and competitive to each other. 3) The high barriers of the reverse reactions suggest that the ratio of the final products 2 ^x :3 ^x is determined by the branching ratio of reaction rates of C ^x+ to intermediates D ^x+ in pass_I and E ^x+ in pass_II.

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Graphical Abstract DFT results indicate that the replacement of -OH by -N₃ which generates C ^x+ controls the consumption rate of 1 ^x H⁺, and the ratio of C ^x+ transforms to D ^x+ and C ^x+ transforms to E ^x+ (k:k') determines the final ratio of products 2x:3x.

     

Na/H and k/h antiport in root membrane vesicles isolated from the halophyte atriplex and the glycophyte cotton

Proton fluxes have been followed into and out of membrane vesicles isolated from the roots of the halophyte Atriplex nummularia and the glycophyte Gossypium hirsutum, with the aid of the ΔpH probe [¹⁴C]methylamine. Evidence is presented for the operation of Na⁺/H⁺ and K⁺/H⁺ antiporters in the membranes of both plants. Cation supply after a pH gradient has been set up across the vesicle membrane (either as a result of providing ATP to the H⁺-ATPase or by imposing an artificial pH gradient) brings about dissipation of the ΔpH, but does not depolarize the membrane potential as observed in similar experiments, but in the absence of Cl⁻, using the ΔΨ probe SCN⁻. Cation/H⁺ exchange is thus indicated. This exchange is not due to nonspecific electric coupling, nor to competition for anionic adsorption sites on the membrane, nor to inhibition of the H⁺-ATPase; coupling of the opposed cation and H⁺ fluxes by a membrane component is the most likely explanation. Saturation kinetics have been observed for both Na⁺/H⁺ and K⁺/H⁺ antiport in Atriplex. Moreover, additive effects are obtained when Na⁺ is supplied together with saturating concentrations of K⁺, and vice versa, suggesting that separate antiporters for Na⁺ and for K⁺ may be operating. In the case of both Atriplex and Gossypium evidence was obtained suggesting the presence of antiporters in both plasmalemma and tonoplast.     

Transport ATPase: further studies on the properties of the thimerosal-treated enzyme.

Our previous studies showed that when ethylmercurithiosalicylate (thimerosal) interacts with the transport ATPase of the guinea pig kidney under specified conditions, the Na⁺ + K⁺-dependent ATPase activity is inhibited, while the Na⁺-dependent ATPase, the Na⁺ + ATP-dependent phosphorylation of the enzyme, and the K⁺-dependent discharge of the phosphoenzyme seem to be unaffected. Here we describe other properties of the thimerosal-treated enzyme: Na⁺-dependent ADP-ATP exchange, Na⁺-dependent UTPase, and K⁺-dependent p-nitrophenylphosphatase activities of the modified enzyme are not inhibited. Kinetics of the Na⁺ effect on the UTPase activities of the native and the modified enzyme are the same. However, K⁺ has a greater inhibitory effect on the Na⁺-UTPase of the modified enzyme than on the Na⁺-UTPase of the native enzyme. The increase in the apparent affinity of the thimerosal-treated enzyme for K⁺ is also evident from the kinetics of the K⁺ effect on p-nitrophenylphosphatase. Neither the native enzyme nor the modified enzyme catalyzes a P₁-ATP exchange. The uninhibited activities of the thimerosal-treated enzyme are sensitive to ouabain. These data provide further support for those reaction mechanisms in which the existence of two ATP sites within the enzyme is assumed.     

Integration, at HAG or Elsewhere, of H2 (Phase-2 Flagellin) Genes Transduced from Salmonella to ESCHERICHIA COLI

A fla mutant of E. coli K12 was given fla⁺ and H1-i by phage P1kc cotransduction from S. typhimurium, then made Fla^- by transduction of ah1 from S. typhimurium. Motile clones expressing a Salmonella phase-2 antigen, e,n,x or 1,2, were obtained from the K12 i ah1 (therefore Fla^-) line by P1kc transduction of flagellin-specifying genes, H2-e,n,x or H2–1,2, from Salmonella donors. Of eighteen such transductants sixteen failed to show phase variation, and on transduction back to Salmonella each structural gene for a phase-2 flagellin (or at least for its antigenically determinant part) now behaved as an allele of H1, presumably in consequence of incorporation in the hag region of the K12 recipient, in place of H1-i ah1 . The e,n,x- and 1,2-specifying genes were shown to have been integrated in the K12 chromosome without the linked H1-repressor gene or the adjacent vh2 gene (controlling rate of phase-variation) and they responded to the repressing activity of an H2 allele elsewhere in the cell, in this respect resembling H1 alleles of Salmonella or hag alleles of E. coli. Two K12 e,n,x transductants had flagellin-specifying genes which when transduced back to Salmonella were integrated at H2; they are inferred to have resulted from integration of H2-e,n,x in the K12 chromosome elsewhere than the hag region. These two clones showed phase variation, between a Fla⁺ phase, with antigen e,n,x, and a Fla^- phase (with e,n,x determinant in the nonactive state and the determinant of antigen i inactivated by ah1). The two integrated e,n,x genes when in the "active" state retained the ability to repress expression of exogenote H1 alleles, which indicates that the closely linked H1-repressor gene also was integrated. One of the two exceptional transductants derived its e,n,x gene from a Salmonella donor with the linked vh2 ^- gene, which in Salmonella almost entirely prevents change of phase, and transduction of this e,n,x gene back to Salmonella recipients proved that vh2^- had been incorporated into the E. coli chromosome along with the e,n,x determinant and the H1-repressor gene. The high frequency of change of phase (Fla⁺↔Fla ^-) in the K12 e,n,x vh2^- transductant concerned suggests that vh2^- fails to prevent frequent change of state of the phase-determinant part of H2 when vh2 ^- and H2 are incorporated in the E. coli chromosome.     

Response to environmental salinity of Na-KATPase activity in individual gills of the euryhaline crab Cyrtograpsus angulatus

The occurrence, localization and response to environmental salinity changes of Na⁺-K⁺ATPase activity were studied in each of the individual gills 4-8 of the euryhaline crab Cyrtograpsus angulatus from Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina). Na⁺-K⁺ATPase activity appeared to be differentially sensitive to environmental salinity among gills. Upon an abrupt change to low salinity, a differential response of Na⁺-K⁺ATPase activity occurred in each individual gill which could suggest a differential role of this enzyme in ion transport process in the different gills of C. angulatus. With the exception of gill 8, a short-term increase of Na⁺-K⁺ATPase specific activity was observed in posterior gills, which is similar to adaptative variations of this activity described in other euryhaline crabs. However, and conversely to that described in other hyperregulating crabs, the highest increase of activity occurred in anterior gills 4 by 1 day after the change to dilute media which could suggest also a role for these gills in ion transport processes in C. angulatus. The fact that variations of Na⁺-K⁺ATPase activity in anterior and posterior gills were concomitant with the transition to hyperregulation indicate that this enzyme could be a component of the branchial ionoregulatory mechanisms at the biochemical level in this crab. The results suggest a differential participation of branchial Na⁺-K⁺ATPase activity in ionoregulatory mechanisms of C. angulatus. The possible existence of functional differences as well as distinct regulation mechanisms operating in individual gills is discussed.