L-alanine uptake in brush border membrane vesicles from the gill of a marine bivalve

Effects of seawater salinity fluctuations on primary tissue culture from the mussel Mytilus galloprovincialis Potential application to the detection of seawater genotoxicity

The present results were obtained in the context of an attempt to develop an in vitro test intended to assess the genotoxicity of seawater. It is based on a short-term culture method of mussel mantle explants that gives mitotic cells in which DNA damage is likely to be detected. Its principle consists of the incorporation of the seawater to be tested in the culture medium. Two factors that influence cell proliferation were studied: (1) salinity of seawater and (2) basic composition of the culture medium i.e. replacement of Eagle's Basal Medium (BME) by Eagle's Minimum Essential Medium (MEM). When culture medium contained BME, a salinity change from 35 per thousand to 28 per thousand (that is the case for coastal or estuarine waters) resulted in a significant reduction of cell proliferation. In contrast, when culture medium was prepared with MEM, the same decrease of salinity did not change significantly the number of metaphase cells obtained. This suggested medium prepared with MEM allowed tissues in culture to better withstand the hypoosmotic shock generated by 28 per thousand seawater. This has been attributed to amino acids the concentration of which being higher in MEM than in BME. Direct and indirect mechanisms of osmoregulation are suggested to explain the observations. These results show that the test proposed could be used to assess the genotoxicity of coastal or estuarine seawater with salinity comprised from 28 per thousand to 35 per thousand, provided that MEM is used.

Involvement of H(+)-ATPase and carbonic anhydrase in inorganic carbon uptake for endosymbiont photosynthesis

Symbiotic cnidarians absorb inorganic carbon from seawater to supply intracellular dinoflagellates with CO(2) for their photosynthesis. To determine the mechanism of inorganic carbon transport by animal cells, we used plasma membrane vesicles prepared from ectodermal cells isolated from tentacles of the sea anemone, Anemonia viridis. H(14)CO(-)(3) uptake in the presence of an outward NaCl gradient or inward H(+) gradient, showed no evidence for a Cl(-)- or H(+)- driven HCO(-)(3) transport. H(14)CO(-)(3) and (36)Cl(-) uptakes were stimulated by a positive inside-membrane diffusion potential, suggesting the presence of HCO(-)(3) and Cl(-) conductances. A carbonic anhydrase (CA) activity was measured on plasma membrane (4%) and in the cytoplasm of the ectodermal cells (96%) and was sensitive to acetazolamide (IC(50) = 20 nM) and ethoxyzolamide (IC(50) = 2.5 nM). A strong DIDS-sensitive H(+)-ATPase activity was observed (IC(50) = 14 microM). This activity was also highly sensitive to vanadate and allyl isothiocyanate, two inhibitors of P-type H(+)-ATPases. Present data suggest that HCO(-)(3) absorption by ectodermal cells is carried out by H(+) secretion by H(+)-ATPase, resulting in the formation of carbonic acid in the surrounding seawater, which is quickly dehydrated into CO(2) by a membrane-bound CA. CO(2) then diffuses passively into the cell where it is hydrated in HCO(-)(3) by a cytosolic CA.

High-affinity phlorizin binding in Mytilus gill

The gill of the marine mussel, Mytilus, contains a high affinity, Na-dependent D-glucose transporter capable of accumulating glucose directly from sea water. We examined the ability of the beta-glucoside, phlorizin, to act as a high-affinity ligand of this process in intact gills and isolated brush border membrane vesicles (BBMV). The time course of association of nanomolar [3H]phlorizin to gills and BBMV was slow, with t50 values between 10 and 30 min, and a half-time for dissociation of approx. 30 min. 1 mM D-glucose reduced equilibrium binding of 1 nM phlorizin by 90-95%, indicating that there was little non-specific binding of this ligand to the gill. In addition, there was little, if any, hydrolysis by the gill of phlorizin to its constituents, glucose and phloretin. Phlorizin binding to gills and BBMV was significantly inhibited by the addition of 50 microM concentrations of D-glucose and alpha-methyl-D-glucose, and unaffected by the addition of L-glucose and fructose. Binding to gills and BBMV was reduced by greater than 90% when Na+ was replaced by K+. Replacement of Na+ by Li+ effectively blocked binding to the intact gill, although Li+ did support a limited amount of glucose-specific phlorizin binding in BBMV. The Kd values for glucose-specific phlorizin binding in intact gills and BBMV were 0.5 nM and 6 nM, respectively. We conclude that phlorizin binds with extremely high affinity to the Na-dependent glucose transporter of Mytilus gill, which may be useful in future efforts to isolate and purify the protein(s) involved in integumental glucose transport.

Uptake of lysine and proline via separate alpha-neutral amino acid transport pathways in Mytilus gill brush border membranes

Brush border membrane vesicles (BBMV) were prepared from the gills of the marine mussel, Mytilus edulis. These membranes contained two distinct pathways for cotransport of Na+ and alpha-neutral amino acids. The major pathway in mussel gill BBMV was the alanine-lysine (AK) pathway, which had a high affinity for alanine and for the cationic amino acid, lysine. The AK pathway was inhibited by nonpolar alpha-neutral amino acids and cationic amino acids, but was not affected by beta-neutral amino acids or imino acids. The kinetics of lysine transport were consistent with a single saturable process, with a Jmax of 550 pmol/mg-min and a Kt of 5 microM. The AK pathway did not have a strict requirement for Na+, and concentrative transport of lysine was seen in the presence of inwardly directed gradients of Li+ and K+, as well as Na+. Harmaline inhibited the transport of lysine in solutions containing either Na+ or K+. The alanine-proline (AP) pathway transported both alanine and proline in mussel gill BBMV. The AP pathway was strongly inhibited by nonpolar alpha-neutral amino acids, proline, and alpha-(methylamino)isobutyric acid (Me-AIB). The kinetics of proline transport were described by a single saturable process, with a Jmax of 180 pmol/mg-min and Kt of 4 microM. In contrast to the AK pathway, the AP pathway appeared to have a strict requirement for Na+. Na+-activation experiments with lysine and proline revealed sigmoid kinetics, indicating that multiple Na+ ions are involved in the transport of these substrates. The transport of both lysine and proline was affected by membrane potential in a manner consistent with electrogenic transport.

Sodium D-glucose cotransport in the gill of marine mussels: studies with intact tissue and brush-border membrane vesicles

Glucose transport was studied in marine mussels of the genus Mytilus. Initial observations, with intact animals and isolated gills, indicated that net uptake of glucose occurred in mussels by a carrier-mediated, Na+-sensitive process. Subsequent studies included use of brush-border membrane vesicles (BBMV) in order to characterize this transport in greater detail. The highest activity of Na+-dependent glucose transport was found in the brush-border membrane fractions used in this study, while basal-lateral membrane fractions contained the highest specific binding of ouabain. Glucose uptake into BBMV showed specificity for Na+, and concentrative glucose transport was observed in the presence of an inwardly directed Na+ gradient. There was a single saturable pathway for glucose uptake, with an apparent Kt of 3 microM in BBMV and 9 microM in intact gills. The kinetics of Na+ activation of glucose uptake were sigmoidal, with apparent Hill coefficients of 1.5 in BBMV and 1.2 in isolated gills, indicating that more than one Na+ may be involved in the transport of each glucose. Harmaline inhibited glucose transport in mussel BBMV with a Ki of 44 microM. The uptake of glucose was electrogenic and stimulated by an inside-negative membrane potential. The substrate specificity in intact gills and BBMV resembled that of Na+-glucose cotransporters in other systems; D-glucose and alpha-methyl glucopyranoside were the most effective inhibitors of Na+-glucose transport, D-galactose was intermediate in its inhibition, and there was little or no effect of L-glucose, D-fructose, 2-deoxy-glucose, or 3-O-methyl glucose. Phlorizin was an effective inhibitor of Na+-glucose uptake, with an apparent Ki of 154 nM in BBMV and 21 nM in intact gills. While the qualitative characteristics of glucose transport in the mussel gill were similar to those in other epithelia, the quantitative characteristics of this process reflect adaptation to the seawater environment of this animal.

Amino acid transport in the gill epithelium of a marine bivalve

1. Amino acid transport across the intestine of bivalve molluscs is reviewed. 2. Transport of alanine or taurine is dependent on the sodium gradient across the intestine. 3. The time course of the uptake of alanine into the brush border membrane vesicles is also sodium dependent.