Epithelial Amino Acid Transport in Marine Mussels: Role in net Exchange of Taurine Between Gills and Sea water

ABSTRACT The exchange of taurine across epidermal epithelia of Mytilus edulis and M. califonianus was studied using radiotracer and chromatographic (HPLC) methods. Gill levels of taurine in both species ranged from 60 to 70μmol g −1 wet weight. Net uptake of taurine, determined using HPLC, occurred down to ambient concentrations as low as 10 nmol 1 −1. The rate of taurine loss from mussels was about 0·02 μmolg −1 wet body weight h −1, and when exposed to amino-acid-free sea water, external taurine concentration increased until a steady-state of about 15 nmol 1 −1 was achieved. Mussels accumulated inhibitors of taurine transport at rates which were directly related to their relative inhibitory capacities: β-alanine>β-aminobutyric acid ≃ γ-aminobutyric acid (GABA). Addition of large concentrations (50–200 μmoll −1) of GABA resulted in a rapid increase in taurine concentration in test solutions. This increase was consistent with a model in which GABA both competitively inhibits the reaccumulation of endogenous taurine lost from epidermal tissues, and accelerates the exchange diffusion of taurine from surface cells. We suggest that epidermal taurine transport in Mytilus assists in the maintenance of large intracellular taurine concentrations, and can serve to reaccumulate up to 30% of the taurine lost from surface tissues by passive processes.