Abnormal regulation of Mg2 transport via Na/Mg exchanger in sickle erythrocytes

Erythrocyte magnesium (Mg2) deficiency has been demonstrated in sickle cell dis-ease to contribute to erythrocyte dehydra-tion, K loss, and thus sickling. No studies have assessed the functional properties of the Na/Mg exchanger in sickle cell disease. Using Mg2-loaded erythrocytes, we measured Mg2 efflux induced by ex-tracellular Na. We estimated that the Na/Mg exchanger had higher maximal ve-locity, higher affinity for Na, and lower cooperativity for Mg2 in sickle than in normal erythrocytes. The activity of the exchanger was markedly decreased by hypotonic and hypertonic conditions in normal erythrocytes but not in sickle erythrocytes. Studies of density-sepa-rated erythrocytes showed that the activ-ity of the exchanger decreased as the mean cellular hemoglobin concentration increased in normal but not in sickle erythrocytes. Inhibition of protein kinase C (PKC) activity by calphostin C and chelerythrine increased the activity of the exchanger in normal but not in sickle erythrocytes. Inhibition of serine/threo-nine phosphatases did not affect the activ-ity of the exchanger in either normal or sickle erythrocytes. Altogether, these data indicate that the Na/Mg exchanger is ab-normally regulated in sickle erythrocytes. Therefore, Mg2 depletion in sickle eryth-rocytes might be mediated by an up-regulated Na/Mg exchanger, possibly by dephosphorylation of the transporter or a closely associated regulator. (Blood. 2005