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K⁺-Cl^- cotransport: 'to be or not to be' oxygen sensitive

Peter K. Lauf

Under 20 years ago, K⁺-Cl^- cotransport (Cot) was discovered in red blood cells after hyposmotic cell swelling (Dunham & Ellory, 1981) and cellular thiol modification (Lauf & Theg, 1980), and later shown to be electroneutral (Kaji, 1993; Lauf & Adragna, 1996). Early hypotheses of membrane mechanical stress versus chemical processes (Lauf, 1985) addressed the activation mechanism. Chemically stimulated K⁺-Cl^- Cot is ATP dependent (Lauf, 1985), and volume-stimulated K⁺-Cl^- Cot, exhibiting a lag phase, is inhibited by serine/threonine (ST) protein phosphatase (PP) inhibitors okadaic acid and calyculin. A two state equilibrium model (Jennings & Al-Rohil, 1990) proposed a phosphorylated (A) resting- and a dephosphorylated (B) active-transporter controlled by ST kinases (STPK) and STPP. Low cellular Mg²⁺ ([Mg²⁺]_i) activates K⁺-Cl^- Cot (Lauf 1985), abolishes a characteristic bell-shaped 'acid' flux activation (Brugnara et al. 1989) and reveals an internal pH sensor (Lauf & Adragna, 1998). Two isoforms of K⁺-Cl^- Cot, KCC1 and KCC2, have been expression cloned (Gillen et al. 1996).