Na/Li Countertransport Abnormalities in Type 1 Diabetes With and Without Nephropathy Are Familial

Abstract

OBJECTIVE—To determine whether there is a familial abnormality in erythrocyte Na/Li countertransport (CT) kinetics in the approximate one-third of type 1 diabetic patients that succumb to a familial predisposition to nephropathy.

RESEARCH DESIGN AND METHODS—Erythrocyte Na/Li CT kinetics were measured in nondiabetic first-degree relatives of type 1 diabetic patients with nephropathy (DNrel) (n = 32) or without nephropathy (DCrel) (n = 22) and normal control subjects ( n = 25).

RESULTS—Increases in outside-site Na ion association rate constant and turnover rate of Na/Li countertransport (CT) in DNrels caused increases in V_max/K_m and V_max, respectively. Thiol alkylation with N-ethylmaleimide (NEM) modifies these kinetic parameters abnormally in nephropathy. With Na ions at the outside site of the transporter, thiol alkylation causes a large decrease in V_max; but in their absence, V_max is decreased in normal control subjects, unchanged in DCrels, or increased in DNrels. The relationship between V_max values after thiol alkylation with or without Na ions was different in DNrels (P < 0.001). Kinetic parameters with and without thiol alkylation identified 60% of DNrels and 20% of DCrels as abnormal. The single-flux rate assay of Na/Li CT did not give this discrimination, and its use may cause discrepancy between studies.

CONCLUSIONS—Clinically normal untreated DNrels have the same abnormality in Na/Li CT as the affected patients. DNrels had a metabolic syndrome with increased BMI and plasma triglycerides, but no elevation in blood pressure. Na/Li CT can detect those type 1 diabetic patients at risk of nephropathy who have a familial abnormality in a membrane thiol protein.