Ethnic differences in red blood cell sodium/lithium countertransport and metabolic correlates of hypertension: An international collaborative study

Abstract

Arterial hypertension is frequently associated with metabolic abnormalities. An abnormal activity of the erythrocyte sodium/lithium countertransport (Na/Li CT), an ion transport system under strong genetic control, is also found in people with hypertension and concomitant metabolic abnormalities. However, little information exists with regard to these clinical associations in different racial groups. The aim of this international collaborative study was to investigate Na/Li CT and the metabolic correlates of hypertension in two comparable samples of normotensive and hypertensive populations in the cities of Naples, Italy, and Shanghai, China, using identical, carefully standardized techniques. Blood pressure, anthropometric and metabolic variables, Na/Li CT, and 24-h urinary Na and K excretion were measured in untreated essential hypertensive (HPT) and normotensive (NT) individuals selected by age (35–60 years), body mass index (BMI; < 30 kg/m 2), and blood pressure (BP; HPT, DBP ≥ 95 mm Hg; NT, DBP < 90 mm Hg). The analysis of variance with adjustment for age was used to compare the groups. In the Neapolitan population, hypertensive individuals had higher serum triglyceride ( P < .05) and uric acid levels ( P < .001) than the normotensive group and also had a reduced glucose tolerance ( P < .01) and an enhanced insulin response to the oral glucose tolerance test (OGTT) ( P < .05). No such differences were seen between normotensive and hypertensive Chinese participants. The Neapolitan population (both NT and HPT) had a higher BMI ( P < .01) than their Chinese peers. In the comparison of hypertensive patients in Shanghai and in Naples, the Neapolitans were heavier ( P < .001), had a lower HDL/total cholesterol ratio ( P < .01), an elevated fasting blood glucose ( P < .05), and also a higher glucose ( P < .001) and insulin response ( P < .001) to OGTT. By contrast, they showed a significantly lower urinary Na/K ratio ( P < .001). Na/Li CT was significantly increased in HPT both in Naples (286 ± 24 v 224 ± 13 μmol/L RBC × h; P < .05, M ± SE) and in Shanghai (388 ± 45 v 265 ± 30 μmol/L RBC × h; P < .05). Furthermore, Na/Li CT was significantly and inversely associated with HDL cholesterol both in the Neapolitan ( P < .01) and in the Chinese ( P < .05) population, whereas it was directly correlated with serum triglyceride ( P < .001) and serum uric acid ( P = .001) only in the Neapolitan population. These results indicate that essential hypertension is associated with a higher prevalence of obesity, impaired glucose tolerance, and hyperinsulinemia in Naples than in Shanghai; and Na/Li CT is linked to both high blood pressure and metabolic abnormalities in the Italian sample, whereas it is an isolated marker of hypertension in the Chinese sample.