UPTAKE OF ORGANIC MATERIAL BY AQUATIC INVERTEBRATES. IV. THE INFLUENCE OF SALINITY ON THE UPTAKE OF AMINO ACIDS BY THE BRITTLE STAR, OPHIACTIS ARENOSA

¹ Department of Organismic Biology, University of California, Irvine, California 92664

1. The brittle star Ophiactis arenosa shows uptake of C¹⁴-labelled glycine, valine, alanine and arginine from dilute solution. The process is linear with time for at least 30 minutes.

2. The bulk of the radioactivity accumulated during a 30-minute exposure to glycine-C¹⁴ remains in alcohol-soluble form. Autoradiography reveals the radioactivity to be associated with glycine.

3. If the animals are allowed to remain in sea water following such an exposure, there is a gradual assimilation of the label into alcohol-insoluble compounds. Some radioactivity appears as C¹⁴O₂, implying oxidation of the amino acid.

4. A double-reciprocal plot of concentration against rate of uptake does not give a straight line. Even at ambient concentrations as high as 10-² M, the accumulation system apparently is not saturated.

5. Common metabolic inhibitors decrease the rate of both accumulation and assimilation.

6. The free amino acid pool of Ophiactis in 100% sea water is of the order of 200 mM/kg. body water. In animals subjected to reduced salinities, there is a decrease in the size of the pool corresponding to the degree of dilution of the medium.

7. One-and two-dimensional chromatograms of alcohol extracts of the animals show several ninhydrin-positive spots, of which glycine, alanine, threonine, and taurine are most prominent.

8. When animals maintained at reduced salinities are exposed to labelled glycine or valine, the response to modest decrease in salinity is a stimulation of uptake. As salinity is decreased further, there is a decrease in the rate of accumulation. In all cases, however, there is a marked increase in the rate of assimilation of the accumulated material into alcohol-insoluble compounds.

9. The response to reduced salinity, with respect to both the size of the free amino acid pool and the incorporation of the label into alcohol-insoluble fraction, is slow, occurring over a period of several days.

10. The significance of the results is discussed in terms of the energy relations of the animals, and the functions of the free amino acid pool.

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