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 K1/2 values for ouabain, bumetanide, and furosemide were 10(-5), 6 x 10(-7), and 5 x 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 K1/2 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.