Kinetic characteristics of the erythrocyte sodium-lithium countertransporter in black normotensive subjects compared with three other ethnic groups

Abstract

Sodium-lithium countertransport activity has been proposed as a marker for hypertension and is lower in black compared to Caucasian subjects both with and without vascular disease. The question arises of what is the primary kinetic locus of the altered behaviour of the countertransporter in black subjects and whether this is also seen in normotensive subjects from other non-Caucasian ethnic groups. We studied the sodium-lithium countertransporter in four ethnic groups (black [n = 45], Caucasian [n = 45], Chinese [n = 40], and South Asian [n = 39]) of age-matched normotensive males (age range 18–35 y) with no first-degree family history of hypertension or diabetes. The clinical and laboratory characteristics of the subjects were similar in all four ethnic groups. Minor differences noted were: significantly higher mean height, weight and serum creatinine concentration (P < 0.001) and significantly lower plasma triglyceride concentration (P = 0.02) in the black compared to the other study groups. Sodium-lithium countertransport activity (mmol Li/l RBC h) was significantly lower in the black subjects (0.113 [0.013–0.265]) compared with the other groups (Caucasian, 0.247 [0.037–0.614]; Chinese, 0.210 [0.100–0.707]; South Asian, 0.211 [0.037–0.617]; P < 0.001). no differences were noted between the four study groups in respect of k_Na. Mean (s.d.) V_max values (mmol Li/l RBC h) were also reduced in the black subjects (0.152 [0.088]) compared to the other ethnic groups (Caucasian, 0.376 [0.159]; Chinese, 0.364 [0.182]; South Asian, 0.329 [0.155]; P < 0.001) and there was a strong relationship between countertransport activity and v_max (r > 0.52; P < 0.001; for each of the study groups). the differences in mean v_max noted between the Caucasian, South Asian and Chinese subjects were not significant.These results show that, when compared with three other selected ethnic groups, black subjects demonstrate an altered behaviour with respect to V_max of the sodium-lithium countertransporter, which occurs in the absence of demonstrable vascular pathology.