KINETIC CHARACTERISTICS OF THE ERYTHROCYTE SODIUM-LITHIUM COUNTERTRANSPORTER IN BLACK NORMOTENSIVE SUBJECTS COMPARED WITH 3 OTHER ETHNIC-GROUPS

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 i s the primary kinetic locus of the altered behaviour of the countertra nsporter in black subjects and whether this is also seen in normotensi ve subjects from other non-Caucasian ethnic groups. We studied the sod ium-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 f amily history of hypertension or diabetes. The clinical and laboratory characteristics of the subjects were similar in all four ethnic group s. Minor differences noted were: significantly higher mean height, wei ght and serum creatinine concentration (P < 0.001) and significantly l ower plasma triglyceride concentration (P = 0.02) in the black compare d 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 [0.03 7-0.614]; Chinese, 0.210 [0.100-0.707]; South Asian, 0.211 [0.037-0.61 7]; P < 0.001). No differences were noted between the four study group s 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 ot her ethnic groups (Caucasian, 0.376 [0.159]; Chinese, 0.364 [0.182]; S outh Asian, 0.329 [0.155]; P < 0.001) and there was a strong relations hip 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 bet ween the Caucasian, South Asian and Chinese subjects were not signific ant. These results show that, when compared with three other selected ethnic groups, black subjects demonstrate an altered behaviour with re spect to V-max of the sodium-lithium countertransporter, which occurs in the absence of demonstrable vascular pathology.