AJP - Regulatory, Integrative and Comparative Physiology, Vol 263, Issue 2 226-R232, Copyright © 1992 by American Physiological Society
Betaine transport in the gill of a marine mussel, Mytilus californianus
S. H. Wright, T. M. Wunz and A. L. Silva
Department of Physiology, College of Medicine, University of Arizona, Tucson 85724.
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)
