Mechanisms of integumental amino acid transport in marine bivalves

The marine bivalve integument accumulates amino acids from seawater at rates that can significantly supplement other nutritional strategies. Reviewed here are studies that used both intact bivalve gill tissue and membranes isolated from gills to examine the mechanism of integumental transport. The evidence supports a model for secondary active transport in which an inward flux of Na⁺ down its electrochemical gradient provides the energy to support uptake of amino acids. Indeed, the Na⁺-cotransport paradigm appears to be a ubiquitous strategy by which animal cells energize uphill transport of organic solutes. What appears to distinguish integumental processes from others are the conditions under which they must routinely work: exposed to ambient concentrations of substrate of < 1 μM and to cytoplasmic concentrations that exceed 0.1 M, transport of amino acid occurs against gradients that can exceed 10⁶ to 1! Somewhat paradoxically, these transporters, which are characterized by an extremely high affinity for amino acid (K(t) values of ≃ 5 μM), work so well because they have a low affinity for Na⁺; low cytoplasmic Na⁺ concentrations prevent the system from acting as an avenue for efflux, whereas the very high concentrations of Na⁺ in seawater are adequate to fully activate the influx mode of operation. The combination of these kinetic characteristics results in a class of transporters that makes efficient use of the scarce nutritional resource represented by dissolved organic material in near-shore waters.