Insulin-dependent diabetic sibling pairs are concordant for sodium- hydrogen antiport activity

Background. Recent findings of enhanced Na⁺/H⁺ antiport activity in cultured fibroblasts and immortalized lymphoblasts from type 1 diabetic patients with nephropathy support the view that a phenotypic or genotypic factor(s) underlies nephropathy risk. This study evaluated the kinetic properties of Na⁺/H⁺ antiporter in cultured fibroblasts from families with two siblings affected by type 1 (insulin-dependent) diabetes. Methods. Seventeen diabetic sibling pairs were studied. The age was 38 ± 10 years (mean ± SD) in probands, the first to develop diabetes, and 39 ± 7 in siblings; the duration of diabetes was, by definition, longer in probands (24 ± 12 vs. 17 ± 8 years in siblings). Na⁺/H⁺ antiport activity was determined using a microfluorometric technique with the pH sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in skin fibroblasts cultured for at least six passages. Results. There were no significant differences between probands and siblings for the following parameters: glycated hemoglobin, 8.3 ± 0.8% in probands and 8.6 ± 1.4% in siblings; creatinine clearance, 103 ± 24 ml/min/1.73 m² in probands and 103 ± 25 in siblings; albumin excretion rate, 6.8 (1 to 860) μg/rain (median and range) in probands and 4.9 (2 to 1334) in siblings. Intracellular pH and buffering capacity were superimposable in the sibling pairs. The V(max) for the antiport was 39.2 ± 14.7 mmol/liter cell/min in probands and 40.3 ± 17.6 in siblings. The internal pH for half-maximal activation (Km) and Hill coefficient was also similar in probands and siblings. There were correlations between probands and siblings in values for intracellular pH (r = 0.51, P < 0.04), V(max) (r = 0.84, P < 0.0001), and buffering capacity (r = 0.53, P < 0.03). Glycated hemoglobin values over five years were not significantly correlated in the sibling pairs (r = 0.3, P > 0.1). V(max) was related with the albumin excretion rate (r = +0.49, P = 0.005) and glycated hemoglobin (r = +0.41, P = 0.017) in the total cohort of sibling pairs. However, multiple regression analysis, using V(max) as the dependent variable, found no correlations between any of the subjects' clinical and demographic variables. Conclusions. Familial concordance for Na⁺/H⁺ antiport activity in long-term cultured skin fibroblasts from type 1 diabetic siblings suggests that at least some of the in vitro phenotypical characteristics of these cells are likely to be genetically determined and to be, at least in part, independent of in vivo metabolic control.