FREE AMINO-ACIDS AND CELL-VOLUME REGULATION IN THE SIPUNCULID PHASCOLOSOMA-ARCUATUM

The ability of Phascolosoma arcuatum to use free amino acids (FAAs) to regulate cell volume was studied. After a downward transfer from 50% to 30% seawater (SW), there was possibly a release of FAAs from the body tissues of P. arcuatum into the coelomic plasma, as indicated by the increase in the total FAA concentration in the latter compartment. When being confronted with an upward transfer from 50% to 100% SW, P. arcuatum exhibited a small uptake of FAAs from the coelomic plasma by the body tissues during the initial phase of the transfer. The significant increase in the total FAA content in the tissues of the worms exposed to 100% SW was mainly due to an increase in protein carabolism as reflected by a significantly smaller water-soluble protein content in the tissues of these worms compared to that of the worms exposed to 30% SW Some of the FAAs released through increased proteolysis in P. arcuatum exposed to 100% SW might undergo partial catabolism and be converted into alanine and glycine, leading to a lower level of NH4+ in the tissues of these worms. This hypothesis is further supported by differences in the activities of glutamate dehydrogenase and alanine aminotransferase, and differences in kinetic properties of the former enzyme, between worms exposed to 100% SW and 30% SW.