K⁺ transport in red blood cells from human umbilical cord

The current study was designed to characterise K⁺ transport in human fetal red blood cells, containing mainly haemoglobin F (HbF, and termed HbF cells), isolated from umbilical cords following normal parturition. Na⁺/K⁺ pump activity was comparable to that in normal adult human red cells (which contain HbA, and are termed HbA cells). Passive (ouabain-resistant) K⁺ transport was dominated by a bumetanide (10 μM)-resistant component, inhibited by [(dihydroxyindenyl)oxy]alkanoic acid (100 μM), calyculin A (100 nM) and Cl^- removal, and stimulated by N-ethylmaleimide (1 mM) and staurosporine (2 μM) - all consistent with mediation via the K⁺-Cl^- cotransporter (KCC). KCC activity in HbF cells was also O₂-dependent and stimulated by swelling and urea, and showed a biphasic response to changes in external pH. Peak activity of KCC in HbF cells was about 3-fold that in HbA cells. These characteristics are qualitatively similar to those observed in HbA cells, notwithstanding the different conditions experienced by HbF cells in vivo, and the presence of HbF rather than HbA. KCC in HbF cells has a higher total capacity, but when measured at the ambient PO₂ of fetal blood it would be similar in magnitude to that in fully oxygenated HbA cells, and about that required to balance K⁺ accumulation via the Na⁺/K⁺ pump. These findings are relevant to the mechanism by which O₂ regulates membrane transporters in red blood cells, and to the strategy of promoting HbF synthesis as a therapy for patients with sickle cell disease.