UPTAKE OF L-ALANINE 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-mu-M. 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 C-14-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 C-14-L- and C-14-D-alanine into isolated gills was inhibited by alpha-neutral amino acids, but not by alpha-acidic or beta-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(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.