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

Abstract

The integumental epithelium of Mytilus californianus gill accumulates amino acids directly from seawater against chemical gradients exceeding 10(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 Kt of 6 microM and Jmax of 7 mumol.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 microM 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+.(ABSTRACT TRUNCATED AT 250 WORDS)