Multiple pathways for amino acid transport in Mytilus gill

The epidermal tissues of marine mussels can accumulate amino acids from surrounding sea water. In the present study, gill tissue isolated from the California coastal mussel, Mytilus californianus, was used in conjunction with intact, actively pumping mussels to study epidermal transport processes. There appeared to be at least four pathways for this uptake: i) a β-neutral pathway which transports taurine; ii) an α-acidic pathway specific for substrates such as aspartate; iii) an α-neutral pathway having a general specificity for this class of compound, but which also accepts the basic amino acid, lysine; and iv) a second α-neutral pathway, also of broad specificity, capable of accepting the imino acid, proline, as a substrate. Replacement of Na^α in sea water with choline reduced uptake of leucine, taurine, aspartate, and proline by more than 95%, and reduced lysine uptake by 75%, suggesting that Na^α-independent pathways play no significant role in epidermal transport in the gill. Isolated gill tissue was used to estimate the maximum transport capacities (J_max's) of the pathways, which ranged from approximately 5 to 25 μmol/(g·hr). Apparent Michaelis constants (K_t^*'s) of the epidermal transporters were estimated using a convection-diffusion model introduced previously (Wright and Secomb, Am J Physiol 247:R346-R355, 1984). These K_t^*'s ranged from 1 to 5 μM. The characteristics of the epidermal transporters are such that they can play a significant role in both animal nutrition and in the reacumulation of endogenous amino acids lost from surface cells through passive diffusion.