Na⁺/H⁺ Antiport Activity and Cell Growth in Cultured Skin Fibroblasts of IDDM Patients With Nephropathy

IDDM patients with incipient and overt nephropathy have been found to exhibit an overactivity of RBC sodium-lithium countertransport. To explore the physiological relevance of this finding, we measured the activity of Na⁺/H⁺ antiport in serially passaged cultured skin fibroblasts from IDDM patients with and without nephropathy and from normal, nondiabetic control subjects. Na⁺/H⁺ antiport activity (measured as the rate of amiloride-sensitive Na⁺ influx at pH₁ = 6.4, extracellular pH = 8.0, and [Na⁺] = 1 mM) was elevated significantly in IDDM patients with nephropathy compared with IDDM patients without nephropathy and nondiabetic control subjects (13.35 ± 3.8 vs. 8.54 ± 2.0 vs. 7.33 ± 2.3 nmol Na⁺ · mg protein⁻¹ · min⁻¹; P < 0.006 and P < 0.001, respectively). A kinetic analysis of Na⁺/H⁺ antiport activity showed that the raised activity in IDDM patients with nephropathy was caused by an increased V_max for extracellular Na⁺. K^m values were similar in the three groups. pH-stimulated amiloride-sensitive Na⁺ influx also was higher under baseline conditions and after serum stimulation in cells from IDDM patients with nephropathy. pH^I values were significantly higher, both during active proliferation and after 10-min exposure to serum, in cells from IDDM patients with nephropathy, compared with IDDM patients without nephropathy and nondiabetic control subjects. Serum-stimulated incorporation of [³H]thymidine into DNA was greater in IDDM patients with nephropathy than in the other two groups (35.7 ± 18.9- vs. 17.4 ± 7.5- vs. 11.9 ± 8.7-fold stimulation above baseline; P < 0.01 for both). These data from cultured skin fibroblasts suggest that intrinsic abnormalities in cell function, independent of the metabolic disturbances of diabetes, are characteristic of IDDM patients who develop nephropathy.