Evidence for bumetanide-sensitive, Na(+)-dependent, partial Na-K-Cl co-transport in red blood cells of a primitive fish.

Tracer uptake studies identified the major routes for K+ transport in hagfish red cells, resolving them into ouabain-sensitive, loop diuretic-sensitive, and residual components. The K½ values for ouabain, bumetanide, and furosemide were 10−5, 6 × 10−7, and 5 × 10−6 M, respectively. The properties of the Na–K–Cl co-transporter were investigated further by varying K+, Na+, and Cl− concentrations. The measured K½ values were similar to those for human red cells. Finally, the stoichiometry of Na:K:Cl uptake was determined, giving 1:1 for K+:Cl−; in contrast, no significant Na+ flux could be measured, although Na+ content must be present for measurable bumetanide-dependent K+ or Cl− flux to occur. The Na–K–Cl transport therefore shows Na+-dependent KCl co-transport or partial flux of the system.Key words: Na–K–Cl co-transport, hagfish red blood cells, Eptatretus stouti.