How to overcome osmotic stress? Marine crabs conquer freshwater. New insights from modern electrophysiology

Modulation by K+ Plus NH4+ of microsomal (Na+, K+)-ATPase activity in selected ontogenetic stages of the diadromous river shrimp Macrobrachium amazonicum (Decapoda, Palaemonidae)

We investigate the synergistic stimulation by K⁺ plus NH₄⁺ of (Na⁺, K⁺)-ATPase activity in microsomal preparations of whole zoea I and decapodid III, and in juvenile and adult river shrimp gills. Modulation of (Na⁺, K⁺)-ATPase activity is ontogenetic stage-specific, and particularly distinct between juveniles and adults. Although both gill enzymes exhibit two different sites for K⁺ and NH₄⁺ binding, in the juvenile enzyme, these two sites are equivalent: binding by both ions results in slightly stimulated activity compared to that of a single ionic species. In the adult enzyme, the sites are not equivalent: when one ion occupies its specific binding site, (Na⁺, K⁺)-ATPase activity is stimulated synergistically by ≈50% on binding of the complementary ion. Immunolocalization reveals the enzyme to be distributed predominantly throughout the intralamellar septum in the gill lamellae of juveniles and adults. Western blot analyses demonstrate a single immunoreactive band, suggesting a single (Na⁺, K⁺)-ATPase α-subunit isoform that is distributed into different density membrane fractions, independently of ontogenetic stage. We propose a model for the modulation by K⁺ and NH₄⁺ of gill (Na⁺, K⁺)-ATPase activity. These findings suggest that the gill enzyme may be regulated by NH₄⁺ during ontogenetic development in M. amazonicum.

Kinetic analysis of gill (Na⁺,K⁺)-ATPase activity in selected ontogenetic stages of the Amazon River shrimp, Macrobrachium amazonicum (Decapoda, Palaemonidae): interactions at ATP- and cation-binding sites

We investigated modulation by ATP, Mg²⁺, Na⁺, K⁺ and NH₄⁺ and inhibition by ouabain of (Na⁺,K⁺)-ATPase activity in microsomal homogenates of whole zoeae I and decapodid III (formerly zoea IX) and whole-body and gill homogenates of juvenile and adult Amazon River shrimps, Macrobrachium amazonicum. (Na⁺,K⁺)-ATPase-specific activity was increased twofold in decapodid III compared to zoea I, juveniles and adults, suggesting an important role in this ontogenetic stage. The apparent affinity for ATP (K(M) = 0.09 ± 0.01 mmol L⁻¹) of the decapodid III (Na⁺,K⁺)-ATPase, about twofold greater than the other stages, further highlights this relevance. Modulation of (Na⁺,K⁺-ATPase activity by K⁺ also revealed a threefold greater affinity for K⁺ (K₀.₅ = 0.91 ± 0.04 mmol L⁻¹) in decapodid III than in other stages; NH₄⁺ had no modulatory effect. The affinity for Na⁺ (K₀.₅ = 13.2 ± 0.6 mmol L⁻¹) of zoea I (Na⁺,K⁺)-ATPase was fourfold less than other stages. Modulation by Na⁺, Mg²⁺ and NH₄⁺ obeyed cooperative kinetics, while K⁺ modulation exhibited Michaelis-Menten behavior. Rates of maximal Mg²⁺ stimulation of ouabain-insensitive ATPase activity differed in each ontogenetic stage, suggesting that Mg²⁺-stimulated ATPases other than (Na⁺,K⁺)-ATPase are present. Ouabain inhibition suggests that, among the various ATPase activities present in the different stages, Na⁺-ATPase may be involved in the ontogeny of osmoregulation in larval M. amazonicum. The NH₄⁺-stimulated, ouabain-insensitive ATPase activity seen in zoea I and decapodid III may reflect a stage-specific means of ammonia excretion since functional gills are absent in the early larval stages.

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.

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.

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.

Ultracytochemical location of Na(+)/K(+)-atpase activity and effect of high salinity acclimation in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii (Crustacea, Decapoda)

Accumulation sites of lead phosphate reaction product consequent to Na(+)/K(+)-ATPase activity in gill and renal epithelia of the freshwater shrimp Macrobrachium olfersii were located ultracytochemically by para-nitrophenyl-phosphate hydrolysis and lead precipitation, and quantified per unit membrane area and cytoplasmic volume. In shrimps in freshwater (<0.5 per thousand S, 20 mOsm/kg H(2)O, 0.7 mEq Na(+)/liter), numerous sites of electron-dense, Na(+)/K(+)-ATPase reaction product accumulation were demonstrated in the membrane invaginations of the mitochondria-rich, intralamellar septal cells (12.5 +/- 1.7 sites/microm(2) membrane, 179 +/- 22 sites/microm(3) cytoplasm, mean+/- SEM, N </= 7) and in the basal region of the medial renal tubules (19.8 +/- 1.8 sites/microm(2) membrane, 437 +/- 53 sites/microm(3) cytoplasm), but not in the pillar cells whose apical flanges form the primary interface with the external medium. A putative, ouabain-insensitive Na(+)- or H(+)-ATPase was found in the apical microvilli of the medial renal tubules (17.4 +/- 1.7 sites/microm(2) membrane, 629 +/- 101 sites/microm(3) cytoplasm). This restricted location of Na(+)/K(+)-ATPase activity within the gill epithelium suggests that during uptake, Na(+) moves across the apical pillar cell membrane, passes through specialized, basolateral coupling junctions into the septal cell cytoplasm and is pumped into the hemolymph via the Na(+)/K(+)-ATPase in the invagination membranes. In shrimps acclimated to a high-salinity medium (21 per thousand S, 630 mOsm/kg H(2)O, 280 mEq Na(+)/liter) for 2 and 5 days, the mean number of sites of para-nitrophenylphosphatase activity/microm(2) membrane and /microm(3) cytoplasm for both epithelia increases markedly by 83 and 163%, respectively. However, after 10 days acclimation, the number of sites declines dramatically, attaining values far below those for shrimps in freshwater. These acclimation-induced alterations in numerical density/microm(3) cytoplasm cannot be accounted for by corresponding changes in membrane surface density (microm(2) membrane/microm(3) cytoplasm) and reflect a real alteration in the number of Na(+)/K(+)-ATPase reaction product sites/unit membrane area. These data suggest that neither the gill nor the renal Na(+)/K(+)-ATPase systems function at maximal activity in shrimps in freshwater, possibly due to the low Na(+) concentration, and are initially stimulated by the increase in external ionic concentration. However, these powerful Na(+) transport systems respond to salt loading by a notable reduction in the number of hydrolysis sites, possibly through the incorporation of the Na(+)/K(+)-ATPase into isolated membrane vesicles in the basal invaginations of the medial renal tubules, together with ultrastructural alterations like the spatial isolation of the mitochondria by multiple membrane stacks in the intralamellar septal cells. J. Exp. Zool. 284:617-628, 1999.