Red cell membrane Na+ transport systems in hereditary spherocytosis: relevance to understanding the increased Na+
1Haematology Laboratory for Anaemia Research, IDIBAPS, Hospital Clinic i Provincial, University of Barcelona, Spain. jlvives@medicina.ub.es
Abstract:
Red blood cells (RBCs) in hereditary spherocytosis (HS) show high sodium (Na+) and potassium (K+) movement across the membrane, resulting in dehydration. In general, these abnormal cation fluxes have been interpreted as "increased leaks" due to passive or electrodiffusional permeability of the RBC membrane. A study to elucidate the contribution of concomitant ouabain-resistant pathways (Na-K-2Cl cotransport and Na-Li countertransport) to abnormal Na+ permeability present in RBCs of subjects with HS has been undertaken. Accordingly, erythrocyte Na+ and K+ content and transmembrane cation movements via the Na-K pump, Na-K-2Cl cotransport, Na-Li countertransport, and Na+ passive diffusion, were measured in 25 non-splenectomized patients with HS and compared with the results obtained from the study of 11 patients with congenital non-spherocytic haemolytic anaemia (CNSHA) due to hereditary elliptocytosis (7 cases) and RBC enzyme defects (4 cases) and of 30 normal controls. Compared to the controls, patients with HS exhibited a highly significant (P<0.001) increase in all the Na+ transmembrane movements via passive diffusion (411+/-243 vs 105+/-40), Na-K pump (2615+/-970 vs 1874+/-359), Na-K-2Cl cotransport (males: 371+/-138 vs 190+/-42; females: 401+/-134 vs 104+/-44) and Na-Li countertransport (207+/-131 vs 98+/-41). This was associated with increased Na+ and decreased K+ content, resulting in a reduction of total cation (Na+ + K+) RBC concentration. Furthermore, significant correlations were also found between the patients' RBC cationic content and the mean corpuscular haemoglobin concentration (MCHC) (r=0.51, P<0.05) and between the Na+ passive leak and the haematocrit value (r=-0.44, P<0.05). In the patients with CNSHA, a less significant (P<0.01) increase of active (Na-K pump) and passive (leak) transmembrane permeability to Na+ was associated with normal transmembrane movements via Na-K-2Cl cotransport and Na-Li countertransport. The present study demonstrates that in HS, RBCs are characterized by a variable, but always significant increase of all the membrane transport systems leading to the extrusion of Na+, and that these abnormalities, regardless of their relation to membrane structural defects, may probably be valuable for the differential diagnosis between HS and other congenital defects of RBCs.
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