Amino acid transport in Myxicola giant axon: stability of the amino acid pool, taurine efflux, and trans effect of sodium

Abstract

1. The giant axon of Myxicola infundibulum was assessed for its suitability as a model preparation for study of amino acid transport mechanisms.2. The amino acid composition of axoplasm was measured and compared with those of coelomic fluid, muscle and axon sheath. The axoplasmic composition is unique. Axoplasm/coelomic fluid concentration ratios are all much larger than 1. The axoplasmic amino acid concentrations are (mmol/kg plasm): cysteic acid (104), aspartic acid (75), glutamic acid (10), taurine (64), serine (5), glycine (191) and alanine (5). Other amino acids or primary amines, if present, must have concentrations of less than 1 mm.3. The size of the sheath amino acid pool is 12% or less of the axoplasmic pool.4. The amino acid pool of axons soaked in sea water for up to 24 h is stable. Removal of Na from sea water causes a large increase of net efflux and net production of amino acids.5. Net amino acid production can not be detected in sheath. Metabolic production occurs in axoplasm with little accumulation. Time scales for production and net efflux are therefore similar.6. The Myxicola axon has a vigorous amino acid metabolism and transport systems capable of relatively large fluxes. Homeostasis is strongly linked to Na and may involve Na-coupled co-transport. Conservation of transmembrane amino acid gradients could be promoted in part by trans inhibition of efflux by external Na.7. Taurine is a useful model substrate because it is not catabolized in Myxicola and its net efflux is sensitive to Na. [(3)H]taurine efflux was measured from injected axons. Fluxes and internally recorded action potentials are stable for up to 6 h.8. Axon sheaths take up [(3)H]taurine from 10 mm-taurine sea water with an apparent half-time of 5 h. [(3)H]taurine washout from the apparent extracellular space has a half-time of 5 min. Washout from sheath cells has a half-time of 2-3 h. Sheath is not an important parallel compartment for taurine fluxes in the axon.9. Taurine efflux has a Q(10) of 1.8.10. Taurine efflux is insensitive to external taurine concentrations up to 10 mm.11. Taurine efflux is sensitive to external Na, but only if internal Na is high.12. Taurine is transported by a low-affinity Na-dependent system in Myxicola axon. Results could be explained by a carrier which is more mobile in the empty state than in the substrate-loaded state. Trans inhibition of taurine efflux by external Na is an important property of the system, and contributes to conservation of axoplasmic taurine.