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The aim of this study was to analyze Na,Li countertransport in erythrocytes from adolescents with insulin dependent diabetes mellitus (IDDM) and to see if those with elevated values present distinct clinical features, in particular as regards arterial pressure and urinary albumin excretion (UAE). Twenty-nine adolescents with IDDM (17 males, 12 females, mean age 15 +/- 0.6 years, mean diabetes duration 11.4 +/- 0.7 years) and fifteen healthy age-matched control subjects (8 males, 7 females, age 14.5 +/- 1 years) were investigated. Diabetic adolescents had a RBC Na,Li countertransport activity higher than age matched normal controls; geometric mean 283 (95% limits 259-340) vs. 193 (169-252) mumol/l RBC/h; p less than 0.01. Seven out of 29 subjects had values higher than the 95th percentile of normal subjects (Counter +). Both systolic and diastolic arterial pressures were significantly higher in Counter + than in Counter - patients. No significant differences were found as regards age, body mass index, diabetes duration, HbA1c, fructosamine, serum potassium, triglycerides, creatinine clearance and UAE. The logarithm of systolic pressure was independently positively correlated with ln Na,Li countertransport (r = 0.38; p less than 0.05), ln [Nai] (r = 0.38; p less than 0.05), and ln body mass index (r = 0.5; p less than 0.01) in diabetic patients. The main finding of this study is that diabetic adolescents with a high erythrocyte Na,Li countertransport rate have an arterial pressure significantly higher than patients with normal Na,Li countertransport fluxes.