Calcium absorption by fish intestine: the involvement of ATP- and sodium-dependent calcium extrusion mechanisms

A review of reductionist methods in fish gastrointestinal tract physiology

A holistic understanding of a physiological system can be accomplished through the use of multiple methods. Our current understanding of the fish gastrointestinal tract (GIT) and its role in both nutrient handling and osmoregulation is the result of the examination of the GIT using multiple reductionist methods. This review summarizes the following methods: in vivo mass balance studies, and in vitro gut sac preparations, intestinal perfusions, and Ussing chambers. From Homer Smith's initial findings of marine fish intestinal osmoregulation in the 1930s through to today's research, we discuss the methods, their advantages and pitfalls, and ultimately how they have each contributed to our understanding of fish GIT physiology. Although in vivo studies provide substantial information on the intact animal, segment specific functions of the GIT cannot be easily elucidated. Instead, in vitro gut sac preparations, intestinal perfusions, or Ussing chamber experiments can provide considerable information on the function of a specific tissue and permit the delineation of specific transport pathways through the use of pharmacological agents; however, integrative inputs (e.g. hormonal and neuronal) are removed and only a fraction of the organ system can be studied. We conclude with two case studies, i) divalent cation transport in teleosts and ii) nitrogen handling in the elasmobranch GIT, to highlight how the use of multiple reductionist methods contributes to a greater understanding of the organ system as a whole.

Uptake of benzo[a]pyrene, but not of phenanthrene, is inhibited by fatty acids in intestinal brush border membrane vesicles of rainbow trout (Oncorhynchus mykiss)

Partial replacement of fish ingredients with vegetable ingredients has elevated levels of polycyclic aromatic hydrocarbons (PAHs) in Atlantic salmon reared on these feeds. PAH uptake in the intestinal tract is postulated to occur in association with lipid absorption and could well be affected by fatty acid composition. We therefore investigated the effects of a fish oil and vegetable oil fatty acid, eicosapentaenoic acid (EPA; 20:5n-3) and oleic acid (18:1n-9) respectively, on the uptake of benzo[a]pyrene (BaP) and phenanthrene (PHE) across the intestinal brush border membrane in the salmonid species rainbow trout (Oncorhynchus mykiss). BaP and PHE were solubilized in mixed micelles composed of either EPA or oleic acid and administrated to isolated brush border membrane vesicles (BBMV) derived from the pyloric caeca, proximal intestine and distal intestine. In the absence of free fatty acids (FFA) trans-membrane uptake of BaP and PHE was 2-7 times lower than the fraction associated to or in the membrane. In the presence of FFA, trans-membrane BaP uptake had decreased by 80 and 40% at the highest EPA and oleic acid concentration, respectively, whereas PHE uptake was virtually unaffected. In the presence of BaP, but not PHE, trans-membrane EPA uptake in BBMV had decreased. This study obtained evidence for PAH-dependent interactions with FFA uptake. We conclude that intestinal BaP uptake is reduced by luminal FFA contents whereas PHE uptake is not. A large fraction of the administrated BaP and PHE remains associated with the cellular membrane of enterocytes and may interfere with uptake of nutrients.

Toxicological perspective on the osmoregulation and ionoregulation physiology of major ions by freshwater animals: Teleost fish, crustacea, aquatic insects, and Mollusca

Anthropogenic sources increase freshwater salinity and produce differences in constituent ions compared with natural waters. Moreover, ions differ in physiological roles and concentrations in intracellular and extracellular fluids. Four freshwater taxa groups are compared, to investigate similarities and differences in ion transport processes and what ion transport mechanisms suggest about the toxicity of these or other ions in freshwater. Although differences exist, many ion transporters are functionally similar and may belong to evolutionarily conserved protein families. For example, the Na+ /H+ -exchanger in teleost fish differs from the H+ /2Na+ (or Ca2+ )-exchanger in crustaceans. In osmoregulation, Na+ and Cl- predominate. Stenohaline freshwater animals hyperregulate until they are no longer able to maintain hypertonic extracellular Na+ and Cl- concentrations with increasing salinity and become isotonic. Toxic effects of K+ are related to ionoregulation and volume regulation. The ionic balance between intracellular and extracellular fluids is maintained by Na+ /K+ -adenosine triphosphatase (ATPase), but details are lacking on apical K+ transporters. Elevated H+ affects the maintenance of internal Na+ by Na+ /H+ exchange; elevated HCO3- inhibits Cl- uptake. The uptake of Mg2+ occurs by the gills or intestine, but details are lacking on Mg2+ transporters. In unionid gills, SO42- is actively transported, but most epithelia are generally impermeant to SO42- . Transporters of Ca2+ maintain homeostasis of dissolved Ca2+ . More integration of physiology with toxicology is needed to fully understand freshwater ion effects. Environ Toxicol Chem 2017;36:576-600. Published 2016 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Linking physiology and biomineralization processes to ecological inferences on the life history of fishes

Biomineral chemistry is frequently used to infer life history events and habitat use in fishes; however, significant gaps remain in our understanding of the underlying mechanisms. Here we have taken a multidisciplinary approach to review the current understanding of element incorporation into biomineralized structures in fishes. Biominerals are primarily composed of calcium-based derivatives such as calcium carbonate found in otoliths and calcium phosphates found in scales, fins and bones. By focusing on non-essential life elements (strontium and barium) and essential life elements (calcium, zinc and magnesium), we attempt to connect several fields of study to synergise how physiology may influence biomineralization and subsequent inference of life history. Data provided in this review indicate that the presence of non-essential elements in biominerals of fish is driven primarily by hypo- and hyper-calcemic environmental conditions. The uptake kinetics between environmental calcium and its competing mimics define what is ultimately incorporated in the biomineral structure. Conversely, circannual hormonally driven variations likely influence essential life elements like zinc that are known to associate with enzyme function. Environmental temperature and pH as well as uptake kinetics for strontium and barium isotopes demonstrate the role of mass fractionation in isotope selection for uptake into fish bony structures. In consideration of calcium mobilisation, the action of osteoclast-like cells on calcium phosphates of scales, fins and bones likely plays a role in fractionation along with transport kinetics. Additional investigations into calcium mobilisation are warranted to understand differing views of strontium, and barium isotope fractionation between calcium phosphates and calcium carbonate structures in fishes.

Thiamin deficiency induces impaired fish gill immune responses, tight junction protein expression and antioxidant capacity: Roles of the NF-κB, TOR, p38 MAPK and Nrf2 signaling molecules

In this study, we investigate the effects of dietary thiamin deficiency on immune responses, tight junctions, antioxidant capacity and related signaling molecules in the gills of young grass carp (Ctenopharyngodon idella). Fish were fed diets that contained 0.12-2.04 mg thiamin kg(-1) for 8 weeks. We found that dietary thiamin deficiency resulted in reduced complement 3 content, lysozyme and acid phosphatase activities, mRNA levels of hepcidin, liver-expressed antimicrobial peptides 2, transforming growth factor (TGF)-β1, interleukin (IL)-10, inhibitor protein-κBα (IκBα), ribosomal S6 protein kinase 1 and target of rapamycin (TOR) and increased expression of interferon-γ2, tumor necrosis factor-α, TGF-β2, IL-1β, IL-8, IκB kinases (IKKβ and IKKγ) and nuclear factor-κB p65 (NF-κB p65). Our findings showed that thiamin deficiency reduced the immune status of fish gills. Furthermore, thiamin deficiency resulted in reduced mRNA transcript levels of claudin b, claudin 3, claudin 12, zonula occludens 1 (ZO-1) and occludin and increased mRNA transcript levels of claudin 15a, myosin light-chain kinase (MLCK) and p38 mitogen-activated protein kinase (p38 MAPK) in fish gill tissues. These data suggested that thiamin deficiency disrupted tight junction-mediated fish gill barrier function. Additionally, reactive oxygen species, malondialdehyde and protein carbonyl levels and both the activities and expression levels of Cu/Zn superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferases and glutathione reductase, as well as NF-E2-related factor 2 gene expression in fish gills, were lower in fish fed a thiamin-deficient diet. By contrast, thiamin deficiency increased levels of Kelch-like-ECH-associated protein 1a (Keap1a) and Keap1b mRNA transcript expression in fish gills. Taken together, our findings indicated that thiamin deficiency impaired fish gill health by effects on the expression of genes encoding cytokines, tight junction proteins, antioxidant enzymes, NF-κB p65, MLCK and Nrf2.

Species-specific and transgenerational responses to increasing salinity in sympatric freshwater gastropods

Freshwater salinization is a global concern partly attributable to anthropogenic salt contamination. The authors examined the effects of increased salinity (as NaCl, 250-4,000 µS/cm, specific conductance) on two sympatric freshwater gastropods (Helisoma trivolvis and Physa pomillia). Life stage sensitivities were determined by exposing naive eggs or naive juveniles (through adulthood and reproduction). Additionally, progeny eggs from the juvenile-adult exposures were maintained at their respective parental salinities to examine transgenerational effects. Naive H. trivolvis eggs experienced delayed development at specific conductance > 250 µS/cm; reduced survivorship and reproduction were also seen in juvenile H. trivolvis at 4,000 µS/cm. Survival and growth of P. pomilia were not affected by increased salinity following egg or juvenile exposures. Interestingly, the progeny of H. trivolvis exposed to higher salinity may have gained tolerance to increased salinity whereas P. pomilia progeny may have experienced negative transgenerational effects. The present study demonstrates that freshwater snail species vary in their tolerance to salinization and also highlights the importance of multigenerational studies, as stressor impacts may not be readily apparent from shorter term exposures.

In vitro characterization of calcium transport along the gastrointestinal tract of freshwater rainbow trout Oncorhynchus mykiss

Using an in vitro gut-sac technique, this study examined the mechanisms of calcium (Ca) uptake along the gastrointestinal tract (GIT) of rainbow trout Oncorhynchus mykiss. Ca uptake into three different compartments (mucous-bound, mucosal epithelium and blood space) of four distinct GIT segments (stomach, anterior intestine, mid intestine and posterior intestine) was monitored after luminal exposure to 10 mM Ca saline (radiolabelled with (45) Ca). Ca transport was determined to be both time-dependent and concentration-dependent. The concentration-dependent kinetics of Ca uptake was investigated using varying luminal concentrations of Ca (1, 10, 30, 60 and 100 mM). In the blood-space compartment, Ca uptake was saturable at high Ca concentrations in the mid intestine (suggesting mediated transport), while linear uptake was found in the other gut segments. In the mucous-bound and mucosal epithelium compartments, however, saturation kinetics were found for most GIT segments, also suggesting mediated transport. Manipulation of serosal saline osmotic pressure with mannitol demonstrated that Ca uptake was not greatly affected by solvent drag. Elevated mucosal cadmium (Cd) did not appear to inhibit Ca uptake into the blood space in any of the GIT sections, and Ca uptake did not appear to be sodium dependent. Maximum transport capacities for Ca and Cd were found to be comparable between the gills and gut, but affinities were much higher at the gills (up to 3000 times).

Diet influences salinity preference of an estuarine fish, the killifish Fundulus heteroclitus

Understanding the interplay among the external environment, physiology and adaptive behaviour is crucial for understanding how animals survive in their natural environments. The external environment can have wide ranging effects on the physiology of animals, while behaviour determines which environments are encountered. Here, we identified changes in the behavioural selection of external salinity in Fundulus heteroclitus, an estuarine teleost, as a consequence of digesting a meal. Fish that consumed high levels of dietary calcium exhibited a higher preferred salinity compared with unfed fish, an effect that was exaggerated by elevated dietary sodium chloride. The mean swimming speed (calculated as a proxy of activity level) was not affected by consuming a diet of any type. Constraining fish to water of 22 p.p.t. salinity during the digestion of a meal did not alter the amount of calcium that was absorbed across the intestine. However, when denied the capacity to increase their surrounding salinity, the compromised ability to excrete calcium to the water resulted in significantly elevated plasma calcium levels, a potentially hazardous physiological consequence. This study is the first to show that fish behaviourally exploit their surroundings to enhance their ionoregulation during digestion, and to pinpoint the novel role of dietary calcium and sodium in shaping this behaviour. We conclude that in order to resolve physiological disturbances in ion balance created by digestion, fish actively sense and select the environment they inhabit. Ultimately, this may result in transient diet-dependent alteration of the ecological niches occupied by fishes, with broad implications for both physiology and ecology.

The influence of 17β-estradiol on intestinal calcium carbonate precipitation and osmoregulation in seawater-acclimated rainbow trout (Oncorhynchus mykiss)

The intestine of marine teleosts produces carbonate precipitates from ingested calcium as part of their osmoregulatory strategy in seawater. The potential for estrogens to control the production of intestinal calcium carbonate and so influence osmoregulation was investigated in seawater-acclimated rainbow trout following intraperitoneal implantation of 17β-estradiol (E2) at two doses (0.1 and 10 μg E2 g(-1)). Levels of plasma vitellogenin provided an indicator of estrogenic effect, increasing significantly by three and four orders of magnitude at the low and high doses, respectively. Plasma osmolality and muscle water content were unaffected, whereas E2-treated fish maintained lower plasma [Na(+)] and [Cl(-)]. Plasma [Ca(2+)] and [Mg(2+)] and muscle [Ca(2+)] increased with vitellogenin induction, whereas the intestinal excretion of calcium carbonate was reduced. This suggests that elevated levels of circulating E2 may enhance Ca(2+) uptake via the gut and simultaneously reduce CaCO(3) formation, which normally limits intestinal availability of Ca(2+). Increasing E2 caused an elevation of [Na(+)] and [Cl(-)] and a reduction of [HCO(3(-))] in intestinal fluid. We speculate that E2 may influence a number of intestinal ion transport processes that ultimately may influence water absorption: (1) reduced NaCl cotransport, (2) reduced Cl(-) uptake via Cl(-)/HCO(3(-)) exchange and (3) reduced precipitation of Ca(2+) and Mg(2+) carbonates. Despite these effects on intestinal ion and water transport, overall osmoregulatory status was not compromised in E2-treated fish, suggesting the possibility of compensation by other organs.

Cadmium- and calcium-mediated toxicity in rainbow trout (Oncorhynchus mykiss) in vivo: interactions on fitness and mitochondrial endpoints

Rainbow trout were exposed to sublethal waterborne Cd (5 and 10 μg L(-1)) and dietary Ca (60 mg g(-1)), individually and in combination, for 30 d to elucidate the interactive effects and evaluate the toxicological significance of mitochondrial responses to these cations in vivo. Indices of fish condition and mortality were measured and livers, centers of metabolic homeostasis, were harvested to assess mitochondrial function and cation accumulation. All indices of condition assessed (body weight, hepatosomatic index and condition factor) were reduced in all the treatment groups. Mortality occurred in the Cd-exposed groups with dietary Ca partly protecting against and enhancing it in the lower and higher Cd exposure, respectively. State 3 mitochondrial respiration was inhibited by 30%, 35% and 40% in livers of fish exposed to Ca, Cd and Cd+Ca, respectively, suggesting reduced ATP turnover and/or impaired substrate oxidation. While the phosphorylation efficiency was unaffected, state 4 and state 4+ (+ oligomycin) respirations were inhibited by all the exposures. Mitochondrial coupling was reduced and transiently restored denoting partially effective compensatory mechanisms to counteract Cd/Ca toxicity. The respiratory dysfunction was associated with accumulation of both Cd and Ca in the mitochondria. Although fish that survived acute effects of Cd and Ca exposure apparently made adjustments to energy generation such that liver mitochondria functioned more efficiently albeit at reduced capacity, reduced fitness was persistent possibly due to increased demands for maintenance and defense against toxicity. Overall, interactions between Cd and Ca on condition indices and mitochondrial responses were competitive or cooperative depending on exposure concentrations and duration.

Skin healing and scale regeneration in fed and unfed sea bream, Sparus auratus

Background

Fish scales are an important reservoir of calcium and phosphorus and together with the skin function as an integrated barrier against environmental changes and external aggressors. Histological studies have revealed that the skin and scales regenerate rapidly in fish when they are lost or damaged. In the present manuscript the histological and molecular changes underlying skin and scale regeneration in fed and fasted sea bream (Sparus auratus) were studied using a microarray 3 and 7 days after scale removal to provide a comprehensive molecular understanding of the early stages of these processes.

Results

Histological analysis of skin/scales revealed 3 days after scale removal re-epithelisation and formation of the scale pocket had occurred and 53 and 109 genes showed significant up or down-regulation, respectively. Genes significantly up-regulated were involved in cell cycle regulation, cell proliferation and adhesion, immune response and antioxidant activities. 7 days after scale removal a thin regenerated scale was visible and only minor changes in gene expression occurred. In animals that were fasted to deplete mineral availability the expression profiles centred on maintaining energy homeostasis. The utilisation of fasting as a treatment emphasised the competing whole animal physiological requirements with regard to barrier repair, infection control and energy homeostasis.

Conclusions

The identification of numerous genes involved in the mitotic checkpoint and cell proliferation indicate that the experimental procedure may be useful for understanding cell proliferation and control in vertebrates within the context of the whole animal physiology. In response to skin damage genes of immune surveillance were up-regulated along with others involved in tissue regeneration required to rapidly re-establish barrier function. Additionally, candidate fish genes were identified that may be involved in cytoskeletal re-modelling, mineralization and stem cells, which are of potential use in aquaculture and fish husbandry, as they may impact on the ability of the fish to produce structural proteins, such as muscle, efficiently.

The influence of environmental calcium concentrations on calcium flux, compensatory drinking and epithelial calcium channel expression in a freshwater cartilaginous fish

Calcium metabolism and mRNA levels of the epithelial calcium channel (ECaC) were examined in a freshwater cartilaginous fish, the lake sturgeon Acipenser fulvescens. Lake sturgeon were acclimated for ≥2 weeks to 0.1 (low), 0.4 (normal) or 3.3 (high) mmol l(-1) environmental calcium. Whole-body calcium flux was examined using (45)Ca as a radioactive marker. Net calcium flux was inward in all treatment groups; however, calcium influx was greatest in the low calcium environment and lowest in the high calcium environment, whereas efflux had the opposite relationship. A significant difference in the concentration of (45)Ca in the gastrointestinal tract (GIT) of fish in the low calcium environment led to the examination of drinking rate and calcium flux across the anterior-middle (mid) intestine. Drinking rate was not different between treatments; however, calcium influx across the mid-intestine in the low calcium treatment was significantly greater than that in both the normal and high calcium treatments. The lake sturgeon ECaC was 2831 bp in length, with a predicted protein sequence of 683 amino acids that shared a 66% identity with the closest sequenced ECaCs from the vertebrate phyla. ECaC mRNA levels were examined in the gills, kidney, pyloric caeca, mid-intestine and spiral intestine. Expression levels were highest in the gills, then the kidneys, and were orders of magnitude lower in the GIT. Contrary to existing models for calcium uptake in the teleost gill, ECaC expression was greatest in high calcium conditions and kidney ECaC expression was lowest in low calcium conditions, suggesting that cellular transport mechanisms for calcium may be distinctly different in these freshwater cartilaginous fishes.

Parathyroid hormone-related protein-stanniocalcin antagonism in regulation of bicarbonate secretion and calcium precipitation in a marine fish intestine

Bicarbonate secretion in the intestine (duodenum) of marine fish has been suggested to play a major role in regulation of calcium availability for uptake. However, while the end process may lead to carbonate precipitation, regulation of transport of calcium and/or bicarbonate may actually result in fine-tuning of calcium availability for transport. To test this hypothesis, sea bream ( Sparus auratus ) duodenal preparations were mounted in Ussing-type chambers and the effect of parathyroid hormone-related protein (PTHrP) and stanniocalcin 1 (STC 1) on the control of intestinal bicarbonate secretion and calcium transport was analyzed. As expected, PTHrP increased net calcium uptake, as a result of an increase of calcium uptake without changes in calcium efflux. In contrast, purified sea bream STC 1 caused a minor decrease of calcium uptake and a two- to threefold increase in calcium efflux. As a result, STC 1 was able to invert the calcium flux from net calcium uptake to net calcium loss, which is in keeping with its known actions as a hypocalcemic factor. Furthermore, both PTHrP and STC 1 regulate intestinal bicarbonate secretion. PTHrP increased calcium uptake and simultaneously reduced the single factor that induces calcium precipitation, bicarbonate secretion. In contrast, STC 1, while reversing the calcium net flux to make it secretory, promoted intestinal bicarbonate secretion, both actions directed to decrease the calcium gradient across the epithelium and promote immobilization in the form of bicarbonate in the intestinal lumen. Together our results provide robust evidence to support an antagonistic action of PTHrP and STC 1 in the fine control of movements of both calcium and bicarbonate in the intestine of seawater fish.

Gastrointestinal transport of Ca2+ and Mg2+ during the digestion of a single meal in the freshwater rainbow trout

A diet containing an inert marker (ballotini beads, quantified by X-radiography) was used to quantify the transport of two essential minerals, Ca(2+) and Mg(2+) from the diet during the digestion and absorption of a single meal of commercial trout food (3% ration). Initially, net uptake of Ca(2+) was observed in the stomach followed by subsequent Ca(2+) fluxes along the intestine which were variable, but for the most part secretory. This indicated a net secretion of Ca(2+) along the intestinal tract resulting in a net assimilation of dietary Ca(2+) of 28%. Similar handling of Ca(2+) and Mg(2+) was observed along the gastrointestinal tract (GI), although net assimilation differed substantially between the cations, with Mg(2+) assimilation being close to 60%, mostly a result of greater uptake by the stomach. The stomach displayed the highest net uptake rates for both cations (1.5 and 1.3 mmol kg(-1) fish body mass for Ca(2+) and Mg(2+), respectively), occurring within 2 h following ingestion of the meal. Substantial secretions of both Ca(2+) and Mg(2+) were observed in the anterior intestine, which were attributed to bile and other intestinal secretions, while fluxes in the mid and posterior intestine were small and variable. The overall patterns of Ca(2+) and Mg(2+) handling in the GI tract were similar to those observed for Na(+) and K(+) (but not Cl(-)) in a previous study. Overall, these results emphasize the importance of dietary electrolytes in ionoregulatory homeostasis.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

Physiological characterization of the Na(+)/Ca(2+) exchanger (NCX) in hepatopancreatic and antennal gland basolateral membrane vesicles isolated from the freshwater crayfish Procambarus clarkii

The purpose of this study was to physiologically characterize the basolateral Na(+)/Ca(2+) exchanger (NCX) in basolateral membrane vesicles (BLMVs) of hepatopancreas and antennal gland of intermolt crayfish. Conditions were optimized to measure Na(+)-dependent Ca(2+) uptake and retention in the BLMV including use of intravesicular (IV) oxalate and measuring initial uptake rates at 20 s. Na(+)-dependent Ca(2+) uptake rate into BLMV was temperature insensitive. Na(+)-dependent Ca(2+) uptake rate was dependent upon free Ca(2+) with saturable Michaelis-Menten kinetics determined as follows: hepatopancreas, maximal uptake rate (J(max))=2.45 nmol/mg per min, concentration at which carrier operates at half-maximal uptake rate (K(m))=0.69 microM Ca(2+); antennal gland, J(max)=13.2 nmol/mg per min, K(m)=0.59 microM Ca(2+). The two vesicle populations exhibited different sensitivity to putative NCX inhibitors. Benzamil had no effect on Na(+)-dependent Ca(2+) uptake rate in hepatopancreas; in antennal gland it was inhibitory at concentrations up to 30 microM and was stimulatory at higher concentrations. Conversely the inhibitor quinacrine was inhibitory at 10 microM in hepatopancreas and was stimulatory at 1000 microM; meanwhile it was ineffective in antennal gland BLMV. Short circuiting the BLMV had no effect on Na(+)-dependent Ca(2+) uptake rate suggesting that the process may be electroneutral. Compared with another prominent basolateral transporter in hepatopancreas the plasma membrane Ca(2+) ATPase (PMCA), the NCX has 70-fold greater J(max) (at comparable temperature) and a lower affinity. In antennal gland the NCX has 40-fold greater J(max) and a lower affinity. In hepatopancreas and antennal gland BLMV NCX appears to determine the rate of basolateral Ca(2+) efflux in intermolt.

Intestinal iron uptake in the European flounder (Platichthys flesus)

Iron is an essential element because it is a key constituent of the metalloproteins involved in cellular respiration and oxygen transport. There is no known regulated excretory mechanism for iron, and homeostasis is tightly controlled via its uptake from the diet. This study assessed in vivo intestinal iron uptake and in vitro iron absorption in a marine teleost, the European flounder Platichthys flesus. Ferric iron, in the form 59FeCl3, was reduced to Fe2+ by ascorbate, and the bioavailability of Fe3+ and Fe2+ were compared. In vivo Fe2+ uptake was significantly greater than Fe3+ uptake and was reduced by the iron chelator desferrioxamine. Fe2+ was also more bioavailable than Fe3+ in in vitro studies that assessed the temporal pattern and concentration-dependency of iron absorption. The posterior region, when compared with the anterior and mid regions of the intestine, was the preferential site for Fe2+ uptake in vivo. In vitro iron absorption was upregulated in the posterior intestine in response to prior haemoglobin depletion of the fish, and the transport showed a Q10 value of 1.94. Iron absorption in the other segments of the intestine did not correlate with haematocrit, and Q10 values were lower. Manipulation of the luminal pH had no effect on in vitro iron absorption. The present study demonstrates that a marine teleost absorbs Fe2+ preferentially in the posterior intestine. This occurs in spite of extremely high luminal bicarbonate concentrations recorded in vivo, which may be expected to reduce the bioavailability of divalent cations as a result of the precipitation as carbonates (e.g. FeCO3).

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

ATP-dependent calcium uptake into basolateral vesicles from transporting epithelia of intermolt crayfish

ATP-dependent Ca2+ uptake was determined into inside-out basolateral membrane vesicles (BLMV) from intermolt crayfish (Procambarus clarkii) Ca2+-transporting epithelia: gill, hepatopancreas (liver), and antennal gland (kidney). Extravesicular (EV) ATP (5 mM) increased 45Ca2+ uptake (free Ca2+ 5 microM) by fivefold but was abolished by pretreatment with either vanadate or the ionophore A-23187. Addition of A-23187 to Ca2+-loaded vesicles produced 70% efflux. The saturable carrier exhibited a Km for Ca2+ of 0.11-0.27 microM and maximal influx of 20-123 pmol. mg-1. min-1. The Km for ATP was 0.01-0.04 mM. The temperature coefficient ranged from 1.43 to 2.06. EGTA treatment of hepatopancreas and antennal gland vesicles decreased 45Ca2+ uptake by 50-90%; uptake was restorable by calmodulin. However, in gill, 45Ca2+ uptake was unaffected by EGTA treatment and calmodulin decreased uptake in both EGTA-treated and untreated vesicles. Addition of EV Na+ (5 mM) increased ATP-dependent Ca2+ uptake into hepatopancreas and antennal gland BLMV by 60%; in hepatopancreas BLMV, this increase was inhibitable by ouabain. However, ATP-dependent Ca2+ uptake in gill vesicles was Na+ independent. The relative role of each epithelium in whole animal Ca2+ homeostasis has been interpreted based on in vitro characteristics.

Ion transport processes of crustacean epithelial cells

Epithelial cells of the gut, antennal glands, integument, and gills of crustaceans regulate the movements of ions into and across these structures and thereby influence the concentrations of ions in the hemolymph. Specific transport proteins serving cations and anions are found on apical and basolateral cell membranes of epithelia in these tissues. In recent years, a considerable research effort has been directed at elucidating their physiological and molecular properties and relating these characteristics to the overall biology of the organisms. Efforts to describe ion transport in crustaceans have focused on the membrane transfer properties of Na+/H+ exchange, calcium uptake as it relates to the molt cycle, heavy metal sequestration and detoxification, and anion movements into and across epithelial cells. In addition to defining the properties and mechanisms of cation movements across specific cell borders, work over the past 5 yr has also centered on defining the molecular nature of certain transport proteins such as the Na+/H+ exchanger in gill and gut tissues. Monovalent anion transport proteins of the gills and gut have received attention as they relate to osmotic and ionic balance in euryhaline species. Divalent anion secretion events of the gut have been defined relative to potential roles they may have in hyporegulation of the blood and in hepatopancreatic detoxification events involving complexation with cationic metals.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Cadmium ion is a non-competitive inhibitor of red cell Ca(2+)-ATPase activity

In the presence as well as in the absence of calmodulin, Cd2+ inhibits the human erythrocyte plasma membrane Ca(2+)-ATPase activity non-competitively with Ki = 2 nM, whereas ATP-dependent Ca(2+)-transport across the red cell membrane was found to be inhibited competitively by Cd2+ (Verbost, P.M., Flik, G., Pang, P.K.T., Lock, R.A.C. and Wendelaar Bonga, S.E. (1989) J. Biol. Chem. 264, 5613-5615). In this study it will be argued that Cd2+ also inhibits Ca(2+)-transport non-competitively, and that the discrepancy with previous conclusions most probably relies on use of an incorrect computer program that calculates the free concentrations of Ca2+ and Cd2+ at the experimental conditions applied for measurement of Ca2+ uptake.

Actions of cadmium on basolateral plasma membrane proteins involved in calcium uptake by fish intestine

The inhibition of Ca(2+)-ATPase, (Na+ + K+)-ATPase and Na+/Ca2+ exchange by Cd2+ was studied in fish intestinal basolateral plasma membrane preparations. ATP driven 45Ca2+ uptake into inside-out membrane vesicles displayed a Km for Ca2+ of 88 +/- 17 nM, and was extremely sensitive to Cd2+ with an IC50 of 8.2 +/- 3.0 pM Cd2+, indicating an inhibition via the Ca2+ site. (Na+ + K+)-ATPase activity was half-maximally inhibited by micromolar amounts of Cd2+, displaying an IC50 of 2.6 +/- 0.6 microM Cd2+. Cd2+ ions apparently compete for the Mg2+ site of the (Na+ + K+)-ATPase. The Na+/Ca2+ exchanger was inhibited by Cd2+ with an IC50 of 73 +/- 11 nM. Cd2+ is a competitive inhibitor of the exchanger via an interaction with the Ca2+ site (Ki = 11 nM). Bepridil, a Na+ site specific inhibitor of Na+/Ca2+ exchange, induced an additional inhibition, but did not change the Ki of Cd2+. Also, Cd2+ is exchanged against Ca2+, albeit to a lesser extent than Ca2+. The exchanger is only partly blocked by the binding of Cd2+. In vivo cadmium that has entered the enterocyte may be shuttled across the basolateral plasma membrane by the Na+/Ca2+ exchanger. We conclude that intracellular Cd2+ ions will inhibit plasma membrane proteins predominantly via a specific interaction with divalent metal ion sites.

Role of Na+/Ca2+ exchange in transcellular Ca2+ transport across primary cultures of rabbit kidney collecting system

Cells from connecting tubule and cortical collecting duct of rabbit kidney were isolated by immunodissection with mAb R2G9 and cultured on permeable filters. Confluent monolayers developed an amiloride-sensitive transepithelial potential difference of -50 +/- 1 mV (lumen negative) and a transepithelial resistance of 507 +/- 18 omega cm2. Transepithelial Ca2+ transport increased dose-dependently with apical [Ca2+] and, in solutions containing 1 mM Ca2+, the active transcellular Ca2+ transport rate was 92 +/- 2 nmol h-1 cm-2. Transcellular Ca2+ transport was dependent on basolateral Na+ (Nab+). Isoosmotic substitution of Nab+ for N-methylglucamine resulted in a concentration-dependent decrease in Ca2+ absorption, with maximal inhibition of 67 +/- 5%. A Hill plot of the Na(+)-dependence yielded a coefficient of 1.9 +/- 0.4, indicating more than one Na+ site on a Na(+)-dependent Ca2+ transport system. In addition, the absence of Cab2+ resulted in a significant increase in Ca2+ transport both in the presence and absence of Nab+. Added basolaterally, ouabain (0.1 mM) inhibited Ca2+ transport to the same extent as did Na(+)-free solutions, while bepridil (0.1 mM), an inhibitor of Na+/Ca2+ exchange, reduced Ca2+ transport by 32 +/- 6%. Methoxyverapamil, felodipine, flunarizine and diltiazem (10 microM) were without effect. Depolarisation of the basolateral membrane, by raising [K+]b to 60 mM, significantly decreased transcellular Ca2+ transport, which is indicative of electrogenic Na+/Ca2+ exchange. In conclusion, active Ca2+ transport in the collecting system of rabbit kidney is largely driven by basolateral Na+/Ca2+ exchange. However, a residual Ca2+ absorption of about 30% was always observed, suggesting that other Ca2+ transport mechanisms, presumably a Ca(2+)-ATPase, participate as well.

Active Ca2+ transport systems in basolateral membranes from rabbit distal colon

Active Na+ absorption by tight epithelia such as frog skin and distal colon share common features like feedback inhibition of cellular [Na+] on Na+ influx through amiloride-sensitive Na+ channels. It is postulated that the negative feedback of increasing cell [Na+] is mediated via a rise in cell [Ca2+]. In this model, cell [Na+] is coupled to cell [Ca2+] via a Na+/Ca2+ exchange system in the basolateral membrane. In the present study, the Ca2+ transporting systems in rabbit distal colon basolateral membranes were characterized. ATP-dependent Ca2+ uptake could be demonstrated in membrane vesicles from surface cells with the following kinetic parameters: Km = 0.09 microM Ca2+ and Vm = 3.8 nmol Ca2+ mg-1 protein min-1. The ATP-dependent Ca2+ transport was not responsive to ruthenium red and oxalate, suggesting a plasmalemmal origin. The addition of 75 mM Na+ to the uptake medium, 10 min after addition of ATP, did not release Ca2+ from the vesicles in significant amounts. In the absence of ATP, outwardly directed Na+ gradients were incapable of stimulating Ca2+ uptake. This study demonstrates that rabbit distal colon epithelium lacks a well-defined Na+/Ca2+ exchange system, and (Ca2+, Mg2+)-ATPase appears to be the sole Ca2+ extrusion system. Alternatives for the coupling of cell [Na+] to cell [Ca2+] are discussed.