Copper transport by lobster (Homarus americanus) hepatopancreatic mitochondria

Integrated comparative transcriptome and weighted gene co-expression network analysis provide valuable insights into the response mechanisms of crayfish (Procambarus clarkii) to copper stress

One of the significant limitations of aquaculture worldwide is the prevalence of divalent copper (Cu). Crayfish (Procambarus clarkii) are economically important freshwater species adapted to a variety of environmental stimuli, including heavy metal stresses; however, large-scale transcriptomic data of the hepatopancreas of crayfish in response to Cu stress are still scarce. Here, integrated comparative transcriptome and weighted gene co-expression network analyses were initially applied to investigate gene expression profiles of the hepatopancreas of crayfish subjected to Cu stress for different periods. As a result, 4662 significant differentially expressed genes (DEGs) were identified following Cu stress. Bioinformatics analyses revealed that the "focal adhesion" pathway was one of the most significantly upregulated response pathways following Cu stress, and seven DEGs mapped to this pathway were identified as hub genes. Furthermore, the seven hub genes were examined by quantitative PCR, and each was found to have a substantial increase in transcript abundance, suggesting a critical role of the "focal adhesion" pathway in the response of crayfish to Cu stress. Our transcriptomic data can be a good resource for the functional transcriptomics of crayfish, and these results may provide valuable insights into the molecular response mechanisms underlying crayfish to Cu stress.

Copper promoting oyster larval growth and settlement: Molecular insights from RNA-seq

As a cofactor of key enzymes, Cu is required in living organisms, although Cu levels in the natural environment are typically low. In this study, the promotion of growth and settlement on the larvae of oyster Crassostrea angulata was observed at an environmentally relevant concentration (10 μg/L Cu). Interestingly, Cu accumulation in the soft tissue of oyster larvae increased during the larval development and exhibited a sharp increase at the late pelagic stage. With the help of RNA-seq, we constructed a high-quality transcriptional database of the oyster C. angulata larvae (24,257 genes with an average length of 1594 bp) via de novo assembly, which provided the basic molecular changes during the larval development. Network analysis of five early developmental stages and differential expression under Cu exposure were integrated to examine the roles of Cu in oyster larvae. Our molecular analysis demonstrated that both ion channels and organic transporters contributed to Cu internalization from the external environment, including zinc transporters and amino acid transporters. The followed distribution of Cu across cells was achieved by ATP7A, the circulatory system, and the Cu transporters (CTRs). Cu exposure enhanced the ribosome and the calcium binding proteins with a higher rate of translation and shell formation, giving rise to faster growth of oyster larvae. Furthermore, Cu facilitated the settling process by upregulating the chitin binding genes and then promoting the formation of the proteinaceous matrix between larvae and substrate. Our study presents the molecular basis for Cu promotion (i.e., hormesis) effects on oyster larval growth and settlement.

A role for divalent metal transporter (DMT1) in mitochondrial uptake of iron and manganese

Much of iron and manganese metabolism occurs in mitochondria. Uptake of redox-active iron must be tightly controlled, but little is known about how metal ions enter mitochondria. Recently, we established that the divalent metal transporter 1 (DMT1) is present in the outer mitochondrial membrane (OMM). Therefore we asked if it mediates Fe²⁺ and Mn²⁺ influx. Mitochondria were isolated from HEK293 cells permanently transfected with inducible rat DMT1 isoform 1 A/+IRE (HEK293-rDMT1). Fe²⁺-induced quenching of the dye PhenGreen™SK (PGSK) occurred in two phases, one of which reflected OMM DMT1 with stronger Fe²⁺ uptake after DMT1 overexpression. DMT1-specific quenching showed an apparent affinity of ~1.5 µM for Fe²⁺and was blocked by the DMT1 inhibitor CISMBI. Fe²⁺ influx reflected an imposed proton gradient, a response that was also observed in purified rat kidney cortex (rKC) mitochondria. Non-heme Fe accumulation assayed by ICPOES and stable ⁵⁷Fe isotope incorporation by ICPMS were increased in HEK293-rDMT1 mitochondria. HEK293-rDMT1 mitochondria displayed higher ⁵⁹Fe²⁺ and ⁵⁴Mn²⁺ uptake relative to controls with ⁵⁴Mn²⁺ uptake blocked by the DMT1 inhibitor XEN602. Such transport was defective in rKC mitochondria with the Belgrade (G185R) mutation. Thus, these results support a role for DMT1 in mitochondrial Fe²⁺ and Mn²⁺ acquisition.

Cytochemical characterization of gill and hepatopancreatic cells of the crab Ucides cordatus (Crustacea, Brachyura) validated by cell metal transport

Ucides cordatus (Linnaeus, 1763) is a hypo-hyper-regulating mangrove crab possessing gills for respiratory and osmoregulatory processes, separated in anterior and posterior sections. They also have hepatopancreas, which is responsible for digestion and absorption of nutrients and detoxification of toxic metals. Each of these organs has specific cells that are important for in vitro studies in cell biology, ion and toxic metals transport. In order to study and characterize cells from gills and hepatopancreas, both were separated using a Sucrose Gradient (SG) from 10 to 40% and cells in each gradient were characterized using the vital mitochondrial dye DASPEI (2-(4-dimethylaminostyryl)-N- ethylpyridinium iodide) and Trichrome Mallory's stain. Both in 20 and 40% SG for gill cells and 30% SG for hepatopancreatic cells, a greater number of cells were colored with DASPEI, indicating a larger number of mitochondria in these cells. It is concluded that the gill cells present in 20% and 40% SG are Thin cells, responsible for respiratory processes and Ionocytes responsible for ion transport, respectively. For hepatopancreatic cells, the 30% SG is composed of Fibrillar cells that possess larger number of membrane ion and nutrient transporters. Moreover, the transport of toxic metal cadmium (Cd) by isolated hepatopancreatic cells was performed as a way of following cell physiological integrity after cell separation and to study differences in transport among the cells. All hepatopancreatic cells were able to transport Cd. These findings are the first step for further work on isolated cells of these important exchange epithelia of crabs, using a simple separation method and to further develop successful in vitro cell culture in crabs.

Copper alters the effect of temperature on mitochondrial bioenergetics in rainbow trout, Oncorhynchus mykiss

We investigated the interaction of temperature and copper (Cu) on mitochondrial bioenergetics to gain insight into how temperature fluctuations imposed by natural phenomena or anthropogenic activities would modulate the effects of Cu on cellular energy homeostasis. Mitochondria were isolated from rainbow trout livers and, in the first set of experiments, exposed to Cu (0-2.5 mM) at 5, 11, and 25 °C with measurement of mitochondrial complex II (mtCII)-driven respiration. In the second set of experiments, unenergized mitochondria were incubated for 30 or 60 min with lower concentrations (0-160 μM) of Cu to measure the effects on mtCII enzyme activity. Whereas maximal (state 3) respiration was inhibited by high Cu exposure, low Cu doses stimulated and high Cu doses inhibited resting (state 4) and 4ol (proton leak) respirations. High temperature alone increased mitochondrial respiration in all states. The Q10 values for state 3, state 4, and proton leak respirations suggested active processes with state 4 respiration and proton leak exhibiting greater thermal sensitivity than state 3 respiration. The differential thermal sensitivity of resting relative to phosphorylating mitochondrial state led to uncoupling and limitation of mitochondrial oxidative capacity at both high temperature and doses of Cu. Moreover, exposure to high Cu caused loss of thermal dependence of the mitochondrial bioenergetics culminating in Q10 values well below unity and decreased activation energies (E a) for both maximal and resting respiration rates. In addition, mtCII activity was increased by low and decreased by high doses of Cu indicating that direct effects on this enzyme contribute to Cu-induced mitochondrial dysfunction. Taken together, it appears that the substrate oxidation (electron transport chain and tricarboxylic acid cycle) and proton leak subsystems are targets of the deleterious effects of Cu and increased temperature on mitochondrial bioenergetics. However, mitochondrial effects of Cu and temperature may not be easily distinguished because they are generally qualitatively similar.

Characterization of copper transport in gill cells of a mangrove crab Ucides cordatus

The branchial epithelium of crustaceans is exposed to the environment and is the first site affected by metal pollution. The aim of this work was to characterize copper (Cu) transport using a fluorescent dye, Phen Green, in gill cells of a hypo-hyper-regulator mangrove crab Ucides cordatus. The results showed that added extracellular CuCl2 (0, 0.025, 0.150, 0.275, 0.550 and 1.110 μM) showed typical Michaelis-Menten transport for Cu in anterior and posterior gill cells (Vmax for anterior and posterior gills: 0.41 ± 0.12 and 1.76 ± 0.27 intracellular Cu in μM × 22.10(4)cells(-1)× 300 s(-1) respectively and Km values: 0.44 ± 0.04 and 0.32 ± 0.13 μM, respectively). Intracellular Cu was significantly higher for posterior gill cells compared to anterior gill cells, suggesting differential accumulation for each gill type. Extracellular Ca at 20mM decreased cellular Cu transport for both anterior and posterior gill cells. Nifedipine and verapamil, calcium channel inhibitors from plasma membrane, decreased Cu transport and affected Km for both gills. These results could be due to a competition between Cu and Ca. Amiloride, a Na/Ca exchanger inhibitor, as well as bafilomycin, a proton pump inhibitor, caused a decrease of intracellular Cu compared to control. Ouabain and KB-R 7943, acting on Na homeostasis, similarly decreased intracellular Cu in both gill cells. Besides that, gill cells exposed to ATP and Cu simultaneously, showed an increase in intracellular copper, which was inhibited by vanadate, an inhibitor of P-type ATPase. These results suggest either the presence of a Cu-ATPase in crab gill cells, responsible for Cu influx, or the effect of a change in electrochemical membrane potential that could also drive Cu to the gill cell interior. Caffeine increased intracellular Cu, suggesting that intracellular Ca could be affecting Cu uptake. Overall the results show that copper uptake in gill cells of crabs is regulated by intracellular Ca, Ca channels and by Na exchangers. This is the first report of Cu transport characterization in whole gill cells of crabs.

Interactive effects of pH and metals on mitochondrial functions of intertidal bivalves Crassostrea virginica and Mercenaria mercenaria

Intertidal bivalves experience broad fluctuations of environmental temperature, pH and oxygen content which could change their intracellular pH. They are also exposed to trace metals such as cadmium (Cd) and copper (Cu) that accumulate in their tissues and may negatively affect mitochondrial functions and bioenergetics. We determined the interactive effects of pH and trace metals (25 μM Cd or Cu) on mitochondrial functions (including respiration and membrane potentials in both ADP-stimulated (state 3) and resting (state 4) states) of two common marine bivalves, the hard clams (Mercenaria mercenaria) and eastern oysters (Crassostrea virginica). In the absence of the trace metals, mitochondrial functions of C. virginica and M. mercenaria were insensitive to pH in a broad physiologically relevant range (6.6-7.8). Mitochondrial respiration was generally suppressed by 25 μM Cd or Cu (with the stronger effects observed for ADP-stimulated compared to the resting respiration) while the mitochondrial membrane potential was unaffected. pH modulated the effects of Cu and Cd on mitochondrial respiration of the bivalves. In oysters, Cu suppressed ADP-stimulated mitochondrial respiration at high and low pH values (6.6 and 7.8, respectively), but had no effect in the intermediate pH range (7.0-7.4). In clams, the negative effect of Cu on ADP-stimulated respiration was only observed at extremely high pH (7.8). A decrease in pH was also protective against Cd in mitochondria of clams and oysters. In clams, 25 μM Cd suppressed ADP-stimulated respiration at all pH; however, at low pH (6.6-7.0) this suppression was paralleled by a decrease in the rates of proton leak thereby effectively restoring mitochondrial coupling. In oysters, the inhibitory effects of Cd on ADP-stimulated respiration were fully abolished at low pH (6.6-7.0). This indicates that moderate acidosis (such as occurs during exposure to air, extreme salinities or elevated CO2 levels in the intertidal zone) may have a beneficial side-effect of protecting mitochondria of clams and oysters against the toxic effects of trace metals in polluted estuaries.

Dual control of cytosolic metals by lysosomal transporters in lobster hepatopancreas

This study describes the membrane transport mechanisms used by lobster (Homarus americanus) hepatopancreatic epithelial lysosomes to accumulate and sequester heavy metals from the cytosol, and thereby aid in the regulation of these ions entering the animal from dietary constituents. The present investigation extends previous work describing lysosomal metal uptake by cation exchange with protons and suggests that a second, parallel, lysosomal transport process involving metal—thiol conjugates may work in conjunction with the cation antiporter to control cytoplasmic metal concentrations. Transport of 65Zn2+ by lysosomal membrane vesicles (LMV) incubated in 1 mmol l−1 glutathione (GSH) was not significantly different from metal transport in the absence of the tripeptide. However, preloading LMV with 1 mmol l−1 α-ketoglutarate (AKG), and then incubating in a medium containing 1 mmol l−1 GSH, more than doubled metal uptake, compared with vesicles equilibrated with chloride or possessing an outwardly directed chloride gradient. Kinetic analysis of lysosomal 65Zn2+ influx as a function of zinc concentration, in vesicles containing 1 mmol l−1 AKG and incubated in 1 mmol l−1 GSH, revealed the presence of a sigmoidal, low affinity, high capacity carrier process transporting the metal into the organelle. These data indicated the possible presence of an organic anion exchanger in lobster lysosomal membranes. Western blot analysis of LMV with a rabbit anti-rat OAT1 antibody showed the presence of an orthologous OAT1-like protein (approximate molecular mass of 80 kDa) signal from these membranes. These results, and those published previously, suggest the occurrence of two metal transporters on hepatopancreatic membranes, a high affinity, low capacity cation antiporter and a low affinity, high capacity organic anion exchanger. Together these two systems have the potential to regulate cytoplasmic metals over a wide concentration range.

Heavy metal detoxification in crustacean epithelial lysosomes: role of anions in the compartmentalization process

Crustacean hepatopancreatic lysosomes are organelles of heavy metal sequestration and detoxification. Previous studies have shown that zinc uptake by lysosomal membrane vesicles (LMV) occurred by a vanadate- and thapsigargin-sensitive ATPase that was stimulated by a transmembrane proton gradient established by a co-localized V-ATPase associated with this organelle. In the present study, hepatopancreatic LMV from the American lobster Homarus americanus were prepared by standard centrifugation methods and 65Zn2+, 36Cl-, 35SO(4)2- and 14C-oxalate2- were used to characterize the interactions between the metal and anions during vesicular detoxification events. Vesicles loaded with SO4(2-) or PO(4)3- led to a threefold greater steady-state accumulation of Zn2+ than similar vesicles loaded with mannitol, Cl- or oxalate2-. The stimulation of 65Zn2+ uptake by intravesicular sulfate was SO(4)2- concentration dependent with a maximal enhancement at 500 micromol l(-1). Zinc uptake in the presence of ATP was proton-gradient enhanced and electrogenic, exhibiting an apparent exchange stoichiometry of 1Zn+/3H+. 35SO4(2-) and 14C-oxalate2- uptakes were both enhanced in vesicles loaded with intravesicular Cl- compared to vesicles containing mannitol, suggesting the presence of anion countertransport. 35SO4(2-) influx was a sigmoidal function of external [SO(4)2-] with 25 mmol l(-1) internal [Cl-], or with several intravesicular pH values (e.g. 7.0, 8.0 and 9.0). In all instances Hill coefficients of approximately 2.0 were obtained, suggesting that 2 sulfate ions exchange with single Cl- or OH- ions. 36Cl- influx was a sigmoidal function of external [Cl-] with intravesicular pH of 7.0 and 9.0. A Hill coefficient of 2.0 was also obtained, suggesting the exchange of 2 Cl- for 1 OH-. 14C-oxalate influx was a hyperbolic function of external [oxalate2-] with 25 mmol l(-1) internal [Cl-], suggesting a 1:1 exchange of oxalate2- for Cl-. As a group, these experiments suggest the presence of an anion exchange mechanism exchanging monovalent for polyvalent anions. Polyvalent inorganic anions (SO4(2-) and PO4(3-)) are known to associate with metals inside vesicles and a detoxification model is presented that suggests how these anions may contribute to concretion formation through precipitation with metals at appropriate vesicular pH.

65Zn2+ Transport by lobster hepatopancreatic lysosomal membrane vesicles

In crustaceans, the hepatopancreas is the major organ system responsible for heavy metal detoxification, and within this structure the lysosomes and the endoplasmic reticulum are two organelles that regulate cytoplasmic metal concentrations by selective sequestration processes. This study characterized the transport processes responsible for zinc uptake into hepatopancreatic lysosomal membrane vesicles (LMV) and the interactions between the transport of this metal and those of calcium, copper, and cadmium in the same preparation. Standard centrifugation methods were used to prepare purified hepatopancreatic LMV and a rapid filtration procedure, to quantify 65Zn2+ transfer across this organellar membrane. LMV were osmotically reactive and exhibited a time course of uptake that was linear for 15-30 sec and approached equilibrium by 300 sec. 65Zn2+ influx was a hyperbolic function of external zinc concentration and followed Michaelis-Menten kinetics for carrier transport (Km = 32.3 +/- 10.8 microM; Jmax = 20.7 +/- 2.6 pmol/mg protein x sec). This carrier transport was stimulated by the addition of 1 mM ATP (Km = 35.89 +/- 10.58 microM; Jmax = 31.94+/-3.72 pmol/mg protein/sec) and replaced by an apparent slow diffusional process by the simultaneous presence of 1 mM ATP+250 microM vanadate. Thapsigargin (10 microM) was also a significant inhibitor of zinc influx (Km = 72.87 +/- 42.75 microM; Jmax =22.86 +/- 4.03 pmol/mg protein/sec), but not as effective in this regard as was vanadate. Using Dixon analysis, cadmium and copper were shown to be competitive inhibitors of lysosomal membrane vesicle 65Zn2+ influx by the ATP-dependent transport process (cadmium Ki = 68.1 +/- 3.2 microM; copper Ki = 32.7 +/- 1.9 microM). In the absence of ATP, an outwardly directed H+ gradient stimulated 65Zn2+ uptake, while a proton gradient in the opposite direction inhibited metal influx. The present investigation showed that 65Zn2+ was transported by hepatopancreatic lysosomal vesicles by ATP-dependent, vanadate-, thapsigargin-, and divalent cation-inhibited, carrier processes that illustrated Michaelis-Menten influx kinetics and was stimulated by an outwardly directed proton gradient. These transport properties as a whole suggest that this transporter may be a lysosomal isoform of the ER Sarco-Endoplasmic Reticulum Calcium ATPase.

Ion regulation in invertebrates: molecular and integrative aspects

The subject of ion regulation in invertebrates is discussed, using a variety of invertebrate model species and approaches that range from the whole-organism level to tissue, subcellular, and molecular levels to illustrate the future direction of the field. These organisms inhabit a variety of aquatic, freshwater, and terrestrial environments, showing specific adaptations to each environment. This overview discusses mechanisms of metal detoxification and the presence of Cl-ATPase in marine organisms to avoid excess intracellular Cl(-); Ca(2+) regulation and endocrine aspects of adaptations to transitional (semiterrestrial) environments; adaptations to Ca(2+)-poor freshwater, particularly the reabsorption of Ca(2+) through specific transporters found in the urine; and finally, ionoregulatory mechanisms for life on land, such as Ca(2+) conservation during molting in isopods and the presence of K(+) channels in insect Malpighian tubules. Convergent mechanisms for dealing with similar problems in dissimilar habitats are discussed, taking into consideration that invertebrates will continue to serve as model systems for the evolution of ionoregulation in different habitats.

Physiological characterization of 45Ca2+ and 65Zn2+ transport by lobster hepatopancreatic endoplasmic reticulum

The crustacean hepatopancreas is an epithelial-lined, multifunctional organ that, among other activities, regulates the flow of calcium into and out of the animal's body throughout the life cycle. Transepithelial calcium flow across this epithelial cell layer occurs by the combination of calcium channels and cation exchangers at the apical pole of the cell and by an ATP-dependent, calcium ATPase in conjunction with a calcium channel and an Na+/Ca2+ antiporter in the basolateral cell region. The roles of intracellular organelles such as mitochondria, lysosomes, and endoplasmic reticulum (ER) in transepithelial calcium transport or in transient calcium sequestration are unclear, but may be involved in transferring cytosolic calcium from one cell pole to the other. The ER membrane has a complement of ATP-dependent calcium ATPases (SERCA) and calcium channels that regulate the uptake and possible transfer of calcium through this organelle during periods of intense calcium fluxes across the epithelium as a whole. This investigation characterized the mechanisms of calcium transport by lobster hepatopancreatic ER vesicles and the effects of drugs and heavy metals on them. Kinetic constants for 45Ca2+ influx under control conditions were K(n) (m)=10.38+/-1.01 microM, J(max)=14.75+/-1.27 pmol/mg protein x sec, and n=2.53+/-0.46. The Hill coefficient for 45Ca2+ influx under control conditions, approximating 2, suggests that approximately two calcium ions were transported for each transport cycle in the absence of ATP or the inhibitors. Addition of 1 mM ATP to the incubation medium significantly (P<0.01) elevated the rate of 45Ca2+ influx at all calcium activities used and retained the sigmoidal nature of the transport relationship. The kinetic constants for 45Ca2+ influx in the presence of 1 mM ATP were K(n) (m)=12.76+/-0.91 microM, J(max)=25.46+/-1.45 pmol/mg protein x sec, and n=1.95+/-0.15. Kinetic analyses of ER 65Zn2+ influx resulted in a sigmoidal relationship between transport rate and zinc activity under control conditions (K(n) (m)=38.63+/-0.52 microM, J(max)=19.35+/-0.17 pmol/mg protein x sec, n=1.81+/-0.03). The Addition of 1 mM ATP enhanced 65Zn2+ influx at each zinc activity, but maintained the overall sigmoidal nature of the kinetic relationship. The kinetic constants for zinc influx in the presence of 1 mM ATP were K(n) (m)=34.59+/-2.31 microM, J(max)=26.09+/-1.17 pmol/mg protein x sec, and n=1.96+/-0.17. Both sigmoidal and ATP-dependent calcium and zinc influxes by ER vesicles were reduced in the presence of thapsigargin and vanadate. This investigation found that lobster hepatopancreatic ER exhibited a thapsigargin- and vanadate-inhibited, SERCA-like, calcium ATPase. This transporter displayed cooperative calcium transport kinetics (Hill coefficient, n approximately 2.0) and was inhibited by the heavy metals zinc and copper, suggesting that the metals may reduce the binding and transport of calcium when they are present in the cytosol.

Ultrastructural Studies and Na+,K+-ATPase Immunolocalization in the Antennal Urinary Glands of the LobsterHomarus gammarus(Crustacea, Decapoda)

Unlike in crustacean freshwater species, the structure and ultrastructure of the excretory antennal gland is poorly documented in marine species. The general organization and ultrastructure of the cells and the localization of Na+,K+-ATPase were examined in the antennal gland of the adult lobster Homarus gammarus. Each gland is composed of a centrally located coelomosac surrounded ventrally by a labyrinth divided into two parts (I and II) and dorsally by a voluminous bladder. There is no differentiated nephridal tubule between them. The labyrinth and bladder cells have in common a number of ultrastructural cytological features, including basal membrane infoldings associated with mitochondria, apical microvilli, and cytoplasmic extrusions, and a cytoplasm packed with numerous vacuoles, vesicles, lysosome-like bodies, and swollen mitochondria. Each type of cell also presents distinctive characters. Na+,K+-ATPase was detected through immunofluorescence in the basal part of the cells of the labyrinth and in the bladder cells with an increasing immunostaining from labyrinth I to the bladder. No immunoreactivity was detected in the coelomosac. The cells of the labyrinth and of the bladder present morphological and enzymatic features of ionocytes. The antennal glands of the lobster thus possess active ion exchanges capabilities.

3H-L-histidine and 65Zn(2+) are cotransported by a dipeptide transport system in intestine of lobster Homarus americanus

The tubular intestine of the American lobster Homarus americanus was isolated in vitro and perfused with a physiological saline whose composition was based on hemolymph ion concentrations and contained variable concentrations of (3)H-l-histidine, (3)H-glycyl-sarcosine and (65)Zn(2+). Mucosa to serosa (M-->S) flux of each radiolabelled substrate was measured by the rate of isotope appearance in the physiological saline bathing the tissue on the serosal surface. Addition of 1-50 micromol l(-1) zinc to the luminal solution containing 1-50 micromol l(-1) (3)H-l-histidine significantly (P<0.01) increased M-->S flux of amino acid compared to controls lacking the metal. The kinetics of M-->S (3)H-l-histidine flux in the absence of zinc followed Michaelis-Menten kinetics (K(m)=6.2+/-0.8 micromol l(-1); J(max) =0.09+/-0.004 pmol cm(-2) min(-1)). Addition of 20 micromol l(-1) zinc to the luminal perfusate increased both kinetic constants (K(m)=19+/-3 micromol l(-1); J(max)=0.28+/-0.02 pmol cm(-2) min(-1)). Addition of both 20 micromol l(-1) zinc and 100 micromol l(-1) l-leucine abolished the stimulatory effect of the metal alone (K(m)=4.5+/-1.7 micromol l(-1); J(max)=0.08+/-0.008 pmol cm(-2) min(-1)). In the absence of l-histidine, M-->S flux of (65)Zn(2+) also followed the Michaelis-Menten relationship and addition of l-histidine to the perfusate significantly (P<0.01) increased both kinetic constants. Addition of either 50 micromol l(-1) Cu(+) or Cu(2+) and 20 micromol l(-1) l-histidine simultaneously abolished the stimulatory effect of l-histidine alone on transmural (65)Zn(2+) transport. Zinc-stimulation of M-->S (3)H-l-histidine flux was significantly (P<0.01) reduced by the addition of 100 micromol l(-1) glycyl-sarcosine to the perfusate, as a result of the dipeptide significantly (P<0.01) reducing both l-histidine transport K(m) and J(max). Transmural transport of (3)H-glycyl-sarcosine was unaffected by the presence of either l-histidine or l-leucine when either amino acid was added to the perfusate alone, but at least a 50% reduction in peptide transport was observed when zinc and either of the amino acids were added simultaneously. These results show that (3)H-l-histidine and (65)Zn(2+) are cotransported across the lobster intestine by a dipeptide carrier protein that binds both substrates in a bis-complex (Zn-[His](2)) resembling the normal dipeptide substrate. In addition, the transmural transports of both substrates may also occur by uncharacterized carrier processes that are independent of one another and appear relatively specific to the solutes used in this study.

65Zn2+ transport by lobster hepato-pancreatic baso-lateral membrane vesicles

The lobster (Homarus americanus) hepato-pancreatic epithelial baso-lateral cell membrane possesses three transport proteins that transfer calcium between the cytoplasm and hemolymph: an ATP-dependent calcium ATPase, a sodium-calcium exchanger, and a verapamil-sensitive cation channel. We used standard centrifugation methods to prepare purified hepato-pancreatic baso-lateral membrane vesicles and a rapid filtration procedure to investigate whether (65)Zn(2+) transfer across this epithelial cell border occurs by any of these previously described transporters for calcium. Baso-lateral membrane vesicles were osmotically reactive and exhibited a time course of uptake that was linear for 10-15 s and approached equilibrium by 120 s. In the absence of sodium, (65)Zn(2+) influx was a hyperbolic function of external zinc concentration and followed the Michaelis-Menten equation for carrier transport. This carrier transport was stimulated by the addition of 150 microM ATP (increase in K(m) and J(max)) and inhibited by the simultaneous presence of 150 micromol l(-1) ATP+250 micromol l(-1) vanadate (decrease in both K(m) and J(max)). In the absence of ATP, (65)Zn(2+) influx was a sigmoidal function of preloaded vesicular sodium concentration (0, 5, 10, 20, 30, 45, and 75 mmol l(-1)) and exhibited a Hill Coefficient of 4.03+/-1.14, consistent with the exchange of 3 Na(+)/1Zn(2+). Using Dixon analysis, calcium was shown to be a competitive inhibitor of baso-lateral membrane vesicle (65)Zn(2+) influx by both the ATP-dependent (K(i)=205 nmol l(-1) Ca(2+)) and sodium-dependent (K(i)=2.47 micromol l(-1) Ca(2+)) transport processes. These results suggest that zinc transport across the lobster hepato-pancreatic baso-lateral membrane largely occurred by the ATP-dependent calcium ATPase and sodium-calcium exchanger carrier proteins.

Calcium regulation in crustaceans during the molt cycle: a review and update

Epithelial cells of the gut, gills, antennal glands and integument regulate calcium concentrations in crustaceans during the molt cycle. A cellular calcium transport model has been proposed suggesting the presence of calcium pumps, cation antiporters and calcium channels in transporting epithelial membranes that regulate the movements of this cation across the cell layer. Basolateral calcium transport during postmolt appears mainly regulated by the low affinity NCX antiporter, while calcium regulating 'housekeeping' activities of these cells in intermolt are controlled by the high affinity calcium ATPase (PMCA). A model is proposed for the involvement of the epithelial ER in the massive transepithelial calcium fluxes that occur during premolt and postmolt. This model involves the endoplasmic reticulum SERCA and RyR proteins and proposed cytoplasmic unstirred layers adjacent to apical and basolateral plasma membranes where calcium activities may largely exceed those in the bulk cytoplasmic phase. A result of the proposed transepithelial calcium transport model is that large quantities of calcium can be moved through these cells by these processes without affecting the low, and carefully controlled, bulk cytoplasmic calcium activities.

Copper transport across pea thylakoid membranes

The initial rate of Cu2+ movement across the thylakoid membrane of pea (Pisum sativum) chloroplasts was directly measured by stopped-flow spectrofluorometry using membranes loaded with the Cu(2+)-sensitive fluorophore Phen Green SK. Cu2+ transport was rapid, reaching completion within 0.5 s. The initial rate of uptake was dependent upon Cu2+ concentration and saturated at about 0.6 microm total Cu2+. Cu2+ uptake was maximal at a thylakoid lumen pH of 7.0. Cu2+ transport was inhibited by Zn2+ but was largely unaffected by Mn2+ and Cu+. Zn2+ inhibited Cu2+ transport to a maximum of 60%, indicating that there may be more than one transporter for copper in pea thylakoid membranes.

Cellular localization of calcium, heavy metals, and metallothionein in lobster (Homarus americanus) hepatopancreas

This investigation combines confocal microscopy with the cation-specific fluorescent dyes Fluo-3 and BTC-5N to localize calcium and heavy metals along the length of intact lobster (Homarus americanus) hepatopancreatic tubules and isolated cells. A metallothionein-specific antibody, developed in mollusks with cross-reactivity in crustaceans, showed the tissue-specific occurrence of this metal-binding protein in several organ systems in lobster and in single cell types isolated from lobster hepatopancreas. Individual lobster hepatopancreatic epithelial cell types were separated into pure single cell type suspensions for confocal and antibody experiments. Intact hepatopancreatic tubules showed high concentrations of both calcium and heavy metals at the distal tips of tubules where mitotic stem cells (E-cells) are localized. In addition, a concentrated distribution of calcium signal within isolated single premolt E-cells in solution was disclosed that might suggest an endoplasmic reticulum compartmentation of this cation within these stem cells. Both E- and R-cells showed significantly (P < 0.05) greater intracellular calcium concentrations in premolt than intermolt, suggesting the accumulation of this cation in these cells prior to the molt. Antibody studies with lobster tissues indicated that the hepatopancreas possessed 5-10 times the metallothionein concentration as other lobster organ systems and that isolated E-cells from the hepatopancreas displayed more than twice the binding protein concentrations of other cells of this organ or those of blood cells. These results suggest that crustacean hepatopancreatic stem cells (E-cells) and R-cells play significant roles in calcium and heavy metal homeostasis in this tissue. Interactions between the four hepatopancreatic cell types in this regulatory activity remain to be elucidated.

Copper transport by lobster (Homarus americanus) hepatopancreatic lysosomes

Lysosomes are known centers for sequestration of calcium and a variety of heavy metals in many invertebrate tissues, and as a result of this compartmentalization these organelles perform important detoxification roles in the animals involved. The present investigation uses a centrifugation method to isolate and purify hepatopancreatic lysosomes from the American lobster, Homarus americanus. Purified lysosomal preparations were used to characterize membrane transport mechanisms in these organelles for transferring and sequestering cytoplasmic copper following its absorption across the plasma membrane from dietary constituents. The copper-specific fluorescent dye, Phen Green, was employed to quantify transmembrane fluxes of this metal as has been recently used to investigate copper movements across hepatopancreatic mitochondrial and plasma membranes. Results indicated the presence of a vanadate-sensitive, calcium-stimulated, copper ATPase in the membranes of these organelles that displayed high affinity carrier-mediated transport kinetics and may significantly contribute to organismic copper homeostasis. Together with a putative bafilomycin-sensitive V-ATPase in the membrane of the same organelles, importing hydrogen ions into the organellar interior, this copper ATPase may function as part of a physiological mechanism for precipitate formation between metallic cations and anions. These ionic precipitate complexes may then act as a sink for excess metals and thereby reduce the circulating concentrations of these elements.