PHOSPHORYLATION AND ACTIVITY OF NA+ H+ EXCHANGER ISOFORM-1 OF IMMORTALIZED LYMPHOBLASTS IN DIABETIC NEPHROPATHY/

In both essential hypertension and diabetic nephropathy (DN), the ubiq uitous cellular Na+/H+ exchanger (NHE) exhibits altered kinetics with increased transport activity, The mechanism for this phenotype and its dependence on the presence of serum are unknown, but increased lympho blast NHE activity in DN has been attributed to a defect in post-trans lational processing of NHE-1 rather than an increased cellular exchang er number, Phosphorylation of NHE-1 has been proposed to play a role i n its activation in a variety of cell models, We have examined, theref ore, the role of NHE-1 phosphorylation and the effect of serum in dete rmining the increased NHE-1 activity in lymphoblasts from patients wit h DN. Cells from these patients exhibited increased NHE activity in th e presence and absence of fetal calf serum (range 42-59%, P < 0.005, a nalysis of variance) and an increased proliferation rate (P < 0.01) wh en compared with cells from both normoalbuminuric diabetic patients an d nondiabetic control subjects, However, NHE-1 abundance was very simi lar among all groups in the presence and absence of serum, indicating that increased NHE activity in cells of nephropathy patients was due t o an increased turnover number, This nephropathy phenotype was not acc ompanied by an increased net phosphorylation of NHE-1 in the presence or absence of serum, Our findings suggest that increased NHE-1 activit y in cells of DN patients is independent of the presence of serum and is not attributable to altered NHE-1 phosphorylation, Additional post- translational mechanisms for activation of NHE-1, therefore, may be in volved,