Organic osmolyte channels: Transport characteristics and regulation

Abstract

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.

Volume Expansion Stimulates p72 and p56 in Skate Erythrocytes

“…p72 syk may play an important role in the volume-activated taurine efflux of the skate erythrocyte. Skate band 3, which is a substrate of p72 syk , is thought to act as a channel or channel activator for the volume-activated taurine efflux in fish erythrocytes (1,3). A very recent study using oocyte expression of trout band 3 has further reinforced its role in volume-activated solute fluxes (4).…”

“…Cells were pelleted (400 g for 2 min at room temperature), and the plasma and buffy coat were removed. Erythrocytes were resuspended in 5 volumes of isotonic (940 mosmol/liter) elasmobranch incubation medium (940 EIM) 1 lyn (polyclonal), anti-pp60 src (monoclonal GD11), anti p56lck (polyclonal, carboxyl terminus) were purchased from Upstate Biotechnology (Lake Placid, NY) and prebound to protein A-Sepharose (Amersham Pharmacia Biotech). Cells were lysed in 9 volumes (450 l) of Nonidet P-40 lysis (IP) buffer (25 mM HEPES, pH 7.4, 225 mM NaCl, 1% v/v Nonidet P-40, 5 mM sodium orthovanadate, 1 mM phenylmethylsulfonyl fluoride, 2 mM benzamidine with 10 g/ml each leupeptin, aprotinin, and pepstatin A).…”

“…Among the most commonly utilized solutes to accomplish the volume decrease is the ␤-amino acid taurine, which can accumulate from 10 -100 mM concentration in many cells (1). In erythrocytes of a number of species, the volume-stimulated taurine efflux appears to occur by a transport pathway that involves band 3 (2,3).…”

Regulation of Sorbitol Efflux in Different Renal Medullary Cells: Similarities and Diversities

“…The experiments in which an increase of calcium permeability under isotonic conditions activates sorbitol release provide in addition information about the role of the intracellular ionic strength -another parameter discussed for the regulation of organic osmolyte channels [31] -in the activation of the sorbitol channel. Since ionic strength remains constant under isotonic conditions, such a role of this parameter in regulating the sorbitol channel seems very unlikely.…”

Inhibition of volume-stimulated taurine efflux and tyrosine kinase activity in the skate red blood cell

“…In fish erythrocytes this taurine efflux is thought to occur via a channel operating through the band 3 anion exchange protein [9,10]. When the red blood cell (RBC) of the little skate, Raja erinacea, is exposed to conditions which induce cell swelling, the cytoplasmic domain of the band 3 protein undergoes a rapid increase in protein tyrosine phosphorylation by the tyrosine kinase p72syk [12,13].…”