Characteristics of influex and net flux of amino acids in Mytilus californianus.

1. Intact mussels and an in vitro preparation of isolated gill tissue were employed to study characteristics of the influx and net flux of amino acids in Mytilus californianus. 2. The kinetics of influx of 14C-labelled amino acids were complicated by the presence of a rapidly labelled extracellular compartment. 3. Correction of influx data for the extracellular compartment revealed influx of 14C-cycloleucine and 14C-glycine to be mediated by a transport mechanism adequately described by Michaelis-Menten kinetics. Passive diffusion plays no significant role in influx. 4. Influx and net flux of glycine into intact Mytilus were examined. From high concentrations (greater than 20 micronM) the influx of 14C-glycine was equivalent to the net influx of primary amine as determined by fluorescamine. At low ambient concentrations (greater than 20 micronM), influx of 14C-glycine occurred more rapidly than net influx as determined by fluorescamine. The data suggest that influx of labelled substrate is accompanied by efflux of unknown primary amine(s). In the absence of labelled substrate, efflux continues until a steady-state concentration of 2-5 micronM in the medium is achieved.5. The rate of influx of 14C-cycloleucine into isolated gill tissue, and the concentration gradient which can be developed by gill tissue are reduced when sodium is replaced in the medium. 6. The efflux of primary amines from isolated gill tissue is stimulated by the replacement of sodium in the medium with choline. 7. The data are consistent with a sodium-coupled mechanism for the transport of amino acid into gill tissue. However, energetic considerations cast doubt on the adequacy of such a mechanism to account fully for the observed trans-epidermal transport.