Hypotonicity-induced exocytosis of the skate anion exchanger skAE1: role of lipid raft regions.

Upon hypotonic volume expansion, skate erythrocytes lose solutes via a pathway that requires participation of anion exchangers (AEs). Three skate AE isoforms (skAEs) are expressed, and at least skAE1 has been shown to mediate this effect when expressed in oocytes. Under isoosmotic conditions, only a small fraction of skAE1 is expressed on the external plasma membrane. Under these conditions, a portion of skAE1 may be found in non-ionic detergent-insoluble regions. However, the detergent-insoluble material is found intracellularly. Cellular volume expansion by hypoosmotic volume expansion but not volume expansion by isoosmotic medium by permeant solutes (ethylene glycol, diethyl urea, or ammonium chloride) stimulates the appearance of skAE1 in the external plasma membrane, and a significant portion of this is found in detergent-insoluble regions. Upon hypoosmotic volume expansion nearly half of the skAE1 is found as oligomers. SkAE1 in these detergent-insoluble fractions is highly tyrosine phosphorylated. These data suggest that volume expansion by hypoosmotic medium stimulates movement of skAE1 from an intracellular pool contained in detergent-insoluble lipid rafts to the plasma membrane. This skAE1 associates to form oligomers that could be involved in the solute efflux that occurs upon volume expansion.