Uptake of L‐ and D‐alanine by a brackish‐water Bivalve, Corbicula japonica, with special reference to their transport pathways and the salinity effect

The uptake of L‐ and D‐alanine by intact animals or isolated gills of a brackish‐water bivalve, Corbicula japonica , was investigated in relation to their transport pathways and the possible physiological roles. The animals took up L‐alanine at concentrations as low as 10 μM. 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 14 C‐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 14 C‐L‐ and 14 C‐D‐alanine into isolated gills was inhibited by α‐neutral amino acids, but not by α‐acidic or β‐amino acids; all α‐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(s), 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.