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The uptake of L-and D-alanine by intact animals or isolated gills of a brackishwater bivalve, Corbicula japonica, was investigated in relation to their transport pathways and the possible physiological roles. The animals took up L-alanine a t concentrations as low as 10 FM. Both the net transport and the influx were strongly affected by external salinity; animals acclimated to fresh water showed much lower rates of net transport and influx than did animals acclimated to 250 mOsm. Patterns of accumulation of 14C-L-alanine in different tissues of intact animals revealed that the gill was the principal site for the transport of the amino acid. Influx of both l4C-L-and 14C-D-alanine into isolated gills was inhibited by a-neutral amino acids, but not by a-acidic or p-amino acids; all a-neutral amino acids examined (glycine, L-glutamine, L-leucine, L-valine, and even D-and L-alanine) inhibited the influx of D-alanine more strongly than the influx of L-alanine. Kinetic analysis of the influx indicated that D-and L-alanine were transported via the same carrier(s1, but that the carrier(s) had a higher affinity for L-alanine than for D-alanine. The transport mechanism appears to play a significant role in the net uptake of D-and L-alanine or in the re-uptake of the amino acids diffused from the integument according to the ambient concentrations of the substrates. o 1992 Wiley-Liss, Inc.

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