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 10 super(7) 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 sub(t) of 6 mu M and J sub(max) of 7 mu mol times g/h. 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. The pattern of structural specificity suggested that betaine transport in the gill is confined to the alanine-proline pathway. Uptake of 0.5 mu M betaine into gills was inhibited by 97% when Na super(+) in seawater was replaced with Li super(+). Activation of betaine transport in the gill was a near-linear function of the external Na super(+) concentration up through 100% artificial seawater (ASW, 425 mM Na super(+)). 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 super(+). Acclimation of the gill for 1 h in 60% AW increased transport from 17 to 53% of control uptake, i.e., near to the level defined by ambient [Na super(+)]. 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.