Organic osmolyte channels: transport characteristics and regulation

Erythrocytes of the skate (Raja erinacea) exposed to hypotonic stress swell and then undergo a volume regulatory decrease by releasing taurine and other osmolytes. Previous studies showed that taurine release occurs via a volume-activated, Na(+)-independent, bi-directional transporter that has the properties of a size-limited channel. We now report on the transport characteristics of this channel and its regulation. Kinetic, competition and inhibitor studies indicate that polyols (myo-inositol) and trimethylamines (betaine) are transported by the same channel as taurine. Although the identity of the channel is still unknown a variety of evidence suggests that band 3 is involved in either channel formation or regulation. Hypotonicity causes phosphorylation and structural changes in band 3. Under isotonic conditions band 3 is predominantly in the dimeric form. Hypotonicity causes a shift to tetrameric band 3. We hypothesize that the band 3 tetramer either forms or regulates an osmolyte channel. The finding that expression of band 3 protein increases osmolyte channel activity in Xenopus oocytes supports this hypothesis.