Alanine and taurine transport by the gill epithelium of a marine bivalve: Effect of sodium on influx

Marine mussels can accumulate amino acids from seawater into the epithelial cells of the gill against chemical gradients in excess of 5×10⁶ to 1. Uptake of both alanine and taurine into gill tissue isolated from Mytilus californianus was found to be dependent upon Na⁺ in the external solution. Uptake of these amino acids was described by Michaelis-Menten kinetics, and a reduction in external [Na⁺] (from 425 to 213 mm) increased the apparent Michaelis constants (alanine, from 8 to 17 μm; taurine, from 4 to 39 μm) without a significant influence on the J_max's of these processes. Five mm harmaline, an inhibitor of Na-cotransport processes in many systems, reduced both alanine and taurine uptake by more than 95%; this inhibition appeared to be competitive in nature, with an apparent K_i of 43 μm for the interaction with alanine uptake. Increasing the external [Na⁺] from 0 to 510 mm produced a sigmoid activation of alanine and taurine uptake with K_Na's of approximately 325 mm. The apparent Hill coefficients for this activation were 7.3 and 7.4 for alanine and taurine, respectively. These data are consistent with uptake mechanisms which require comparatively high concentrations of Na⁺ to activate transport, and which couple several Na⁺ ions to the transport of each amino acid. These characteristics, in conjunction with the previously demonstrated low passive permeability of the apical membrane to amino acids, result in systems capable of i) accumulating amino acids from seawater to help meet the nutritional needs of this animal, and ii) maintaining the high intracellular amino-acid concentrations associated with volume regulation in the gill.