K-Cl cotransport modulation by intracellular Mg in erythrocytes from mice bred for low and high Mg levels

Mg is an important determinant of erythrocyte cation transport system(s) ac tivity. We investigated cation transport in erythrocytes from mice bred for high (MGH) and low (MGL) Mg levels in erythrocytes and plasma. We found th at K-Cl cotransport activity was higher in MGL than in NIGH erythrocytes, a nd this could explain their higher mean corpuscular hemoglobin concentratio n, median density, and reduced cell K content. Although mouse KCC1 protein abundance was comparable in MGL and MGH erythrocytes, activities of Src fam ily tyrosine kinases were higher in MGH than in MGL erythrocytes. In contra st, protein phosphatase (PP) isoform 1 alpha (PP1 alpha) enzymatic activity , which has been suggested to play a positive regulatory role in K-Cl cotra nsport, was lower in MGH than in MGL erythrocytes. Additionally, we found t hat the Src family kinase c-Fgr tyrosine phosphorylates PP1 alpha in vitro. These findings suggest that in vivo downregulation of K-Cl cotransport act ivity by Mg is mediated by enhanced Src family kinase activity, leading to inhibition of the K-Cl cotransport stimulator PP1.