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 pH(i) = 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 thal 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 [H-3]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 intritisic abnormalities in cell function, independent of the metabolic disturbances of diabetes, are characteristic of IDDM patients who develop nephropathy.