Alanine and taurine transport by the gill epithelium of a marine bivalve: Effect of sodium on influx

Abstract

Summary. Marine mussels can accumulate amino acids from seawater into the epithelial cells of the gill against chemical gradients in excess of 5 x 106 to 1. Uptake of both alanine and taurine into gill tissue isolated from Mytilus californianus was found to be dependent upon Na § in the external solution. Uptake of these amino acids was described by Michaelis-Menten kinetics, and a reduction in external [Na § (from 425 to 213 mM) increased the apparent Michaelis constants (alanine, from 8 to 17/xM; taurine, from 4 to 39 /ZM) without a significant influence on the Jmax'S of these processes. Five mM harmaline, an inhibitor of Na-cotransport processes in many systems, reduced both alanine and taurine uptake by more than 95%; this inhibition appeared to be competitive in nature, with an apparent Ki of 43 /.LM for the interaction with alanine uptake. Increasing the external [Na § from 0 to 510 mM produced a sigmoid activation of alanine and taurine uptake with gNa'S of approximately 325 mM. The apparent Hill coefficients for this activation were 7.3 and 7.4 for alanine and taurine, respectively. These data are consistent with uptake mechanisms which require comparatively high concentrations of Na + to activate transport, and which couple several Na § ions to the transport of each amino acid. These characteristics, in conjunction with the previously demonstrated low passive permeability of the apical membrane to amino acids, result in systems capable of i) accumulating amino acids from seawater to help meet the nutritional needs of this animal, and ii) maintaining the high intracellular amino-acid concentrations associated with volume regulation in the gill. Key Wordsepithelia 9 integument 9 coupled transport 9 amino-acid transport 9 taurine -alanine -marine bivalve -Mytilus californianus

l-Alanine uptake in brush border membrane vesicles from the gill of a marine bivalve

“…However, interpretation of ion replacement studies with intact tissues must be done with care. Brief exposure (5 min) of isolated gills to Na+-free seawater causes an inhibition of alanine uptake that takes up to an hour to recover upon reexposure to normal seawater (Wright, 1987b). Coincident with this inhibition of uptake is a 35% reduction in 02 consumption that is also slow to recover.…”

“…Furthermore, the Na + concentration used in the studies reported here (300 mM) was considerably less than that of fullstrength seawater (~-440 raM). Ambient Na + concentrations do influence the kinetics of amino acid uptake in intact gills (Wright, 1987b). Consequently, the kinetic parameters for alanine uptake in BBMV must be considered tentative until the effects of Na + are studied in greater depth.…”

Uptake of lysine and prolinevia separate α-neutral amino acid transport pathways inMytilus gill brush border membranes

“…Studies with intact mussel gills indicate that at least three Na + ions are involved in the transport of alanine and taurine (Wright, 1987). Furthermore, the results of the Na+-activation experiments in gill BBMV (Fig.…”

“…In contrast, the transmembrane cationic amino acid gradient is likely to be much smaller than that of a-neutral amino acids or proline. Seawater concentrations of total cationic amino acids are around 0.2 /.~M (Braven, Evens & Butler, 1984;Siebers & Winkler, 1984), but free cationic amino acids are not concentrated in the tissues to any great extent (Wright et al, 1987). Nevertheless, the coupling of more than one Na + ion to the transport of amino acids across the apical membrane of integumental cells is probably requisite for the concentrative transport of these substrates.…”

“…These vertebrate transporters, however, do not normally move substrates against the extreme concentration gradients that exist across the brush border membrane of the mussel integument. Nevertheless, the mechanism of amino acid transport in marine mussels may resemble that in vertebrates, as amino acid uptake in intact mussel gills is sensitive to the presence of Na + in seawater (Wright, 1987).…”