OSMOTIC HOMEOSTASIS IN DICTYOSTELIUM-DISCOIDEUM - EXCRETION OF AMINO-ACIDS AND INGESTED SOLUTES

The response to osmotic stress in axenically cultured Dictyostelium di scoideum was examined. Hypoosmotic buffers elicited two changes in the large (similar to 50 mM) cytosolic pool of amino acids: a) the total size of the pool diminished, while b) about half of the initial pool w as excreted. Hyperosmotic stress had the opposite effect. Among the pr edominant amino acids in the pool were glycine, alanine and proline. P utrescine, the major diamine, was neither excreted nor modulated. Rece ntly ingested radioactive amino acids were excreted in preference to t hose in the cytoplasm, suggesting that the endocytic pathway might be involved in water excretion. Furthermore, hypoosmotic stress stimulate d the selective excretion of small, membrane-impermeable fluorescent d yes which had been ingested into endocytic vacuoles. Caffeine inhibite d the excretion of the fluorophores but not the amino acids. We conclu de that the response of Dictyostelium to osmotic stress is complex and includes both modulation of the cytoplasmic amino acid pool and the e xcretion of amino acids and other small solutes from the endocytic pat hway.