Ethnic origin and hypertension-associated alterations in sodium-lithium countertransport kinetics

In Caucasian subjects, elevated erythrocyte sodium-lithium countertransport (SLC) activity, displaying an increased affinity for external Na+ (decreased KNa), has a strong association with hypertension and has also been proposed as a potential marker for vascular disease. We have compared SLC activity and the kinetic components, KNa and maximal rate of turnover (Vmax), of the countertransporter in groups of Caucasian, Asian and Black hypertensive subjects matched for ethnicity, age and sex with healthy normotensive controls. SLC activity was the same in all ethnic groups irrespective of the presence of hypertension. Similarly, hypertension had no impact on Vmax values within each ethnic group (normotensive vs. hypertensives: Caucasian, 0.360 +/- 0.186 vs. 0.335 +/- 0.137; Asian, 0.324 +/- 0.078 vs. 0.273 +/- 0.105; black people, 0.192 +/- 0.123 vs. 0.178 +/- 0.082 mmol Li/l erythrocytes h). However, in black people compared with the other two ethnic groups, Vmax was lower for both controls and hypertensives (P < 0.05; ANOVA). Median KNa values in hypertensive subjects were consistently lower than their normotensive counterparts in all ethnic groups (P < 0.01; Kruskal-Wallis); Caucasians, (89.1 vs. 41.2 mmol Na; P = 0.01), Asians (121.1 vs. 33.1; P = 0.04) and black people (74.4 vs. 27.2 mmol Na; P = 0.02; Wilcoxon). The results show that Vmax is altered in black people independently of the presence of hypertension. This contrasts with KNa which, for each ethnic group studied, is reduced in the hypertensive compared with the normotensive state.