Betaine transport in the gill of a marine mussel, Mytilus californianus

The integumental epithelium of Mytilus californianus gill accumulates amino acids directly from seawater against chemical gradients exceeding 10⁷ to 1. In the present report, we confirm the presence of betaine in Mytilus tissue and identify a transport process in the gill for this organic osmolyte. Betaine content of gill tissue from animals acclimated to 100% seawater (980 mosM) was 45 mmol/kg wet wt, similar to that of taurine (53 mmol/kg). The kinetics of betaine uptake were adequately described by the Michaelis-Menten equation, with a K(t) of 6 μM and J(max) of 7 μmol · g^{- 1} · h^-1. Betaine transport was inhibited by L-proline and related structural analogues, and by alanine. L-Proline transport, which involves both high- and low-affinity pathways, was partially inhibited by betaine. The low-affinity proline pathway transports lysine. Betaine and L-lysine showed no reciprocal inhibitory interactions. This pattern of structural specificity suggested that betaine transport in the gill is confined to the alanine- proline pathway. Uptake of 0.5 μM betaine into gills was inhibited by 97% when Na⁺ in seawater was replaced with Li⁺. Activation of betaine transport in the gill was a near-linear function of the external Na⁺ concentration up through 100% artificial seawater (ASW, 425 mM Na⁺). Acute exposure of the gill to 60% ASW inhibited betaine uptake by 83%. Maintenance of normal osmotic concentration (980 mosM, by addition of mannitol to 60% ASW) reduced inhibition to 31%, similar to the level predicted from the availability of Na⁺. Acclimation of the gill for 1 h in 60% ASW increased transport from 17 to 53% of control uptake, i.e., near to the level defined by ambient [Na⁺]. We conclude that betaine transport in Mytilus gill probably plays a role in the routine maintenance of the large intracellular concentrations of betaine involved in controlling normal cell volume.