Regulation of net amino acid exchange in sea urchin larvae

Bacteria-free larvae of Strongylocentrotus purpuratus remove each of 14 free amino acids (FAA) from very dilute solution in seawater. When larvae are maintained in seawater, influx of radiolabeled FAA and net removal of substrate as determined by high-performance liquid chromatography (HPLC) occur at the same rate in those substrates examined. Exposure of larvae to high concentrations of glutamate, glycine, or serine (greater than 50 microM) produces large increases in these substrates in the internal pool of FAA. After incubation of larvae in high concentrations of serine, the rate of influx of 14C-labeled serine is decreased compared with unincubated controls. Comparison of influx of radiolabeled serine with net change in ambient serine as determined by HPLC indicates that the apparent change in rate of influx is a consequence of an increase in ambient serine with time and the resulting decrease in the specific activity of labeled serine. Analysis of data indicates that abnormally high internal serine leads to carrier-mediated efflux of serine as well as other neutral amino acids. The rate of such efflux is increased in the presence of increased levels of ambient serine.