Distribution of sodium-lithium countertransport and blood pressure in caucasians five to eighty-nine years of age

Case-control studies suggest that increased erythrocyte sodium-lithium countertransport may predict increased susceptibility to the development of essential hypertension. To characterize interindividual variation in sodium-lithium countertransport and its relation to blood pressure levels in the general population, we studied 1,475 Caucasians between 5 and 89 years of age (711 males and 764 females) ascertained through 266 households with children in the schools of Rochester, Minnesota. Individuals who were taking antihypertensive agents or combinations of estrogen and progesterone were not included in the sample. A third-order polynomial regression on age accounted for only a small fraction of variability in sodium-lithium countertransport (2.8% in males, p<0.001; 2.1% in females, p<0.01), whereas a fourth-order regression on age accounted for a large proportion of variability in systolic blood pressure (45.7% in males, p<0.001; 52.5% in females, p<0.001) and diastolic blood pressure (39.8% in males, p<0.001; 33.0% in females, p<0.001). Mean sodium-lithium countertransport was higher in males than females at all ages; but the rank order of male and female means for systolic and diastolic blood pressure was age dependent. Positively skewed distributions for age-, height-, and weight-adjusted sodium-lithium countertransport in male and female cohorts between 5–19.9, 20–49.9, and 50–89.9 years of age were explained significantly better by postulating a mixture of two partially overlapping sodium-lithium countertransport distributions rather than a single normal distribution (p<0.01). Among men in the 20–49.9-year-old cohort, adjusted sodium-lithium countertransport values in the upper distribution were associated with higher systolic and diastolic blood pressure (mean±SD) than values in the lower distribution (for systolic blood pressure: 115±11 vs. 111±11 mm Hg, p<0.07; for diastolic blood pressure: 71.2±8.0 vs. 68.4±8.6 mm Hg, p<0.08). Among females in the 50–89.9-year-old cohort, adjusted sodium-lithium countertransport values in the upper distribution were associated with significantly greater diastolic blood pressure than values in the lower distribution (77±10 vs. 70±9 mm Hg, p<0.03). We conclude that 1) changes in systolic and diastolic blood pressure levels with age and gender are not accompanied by similar changes in sodium-lithium countertransport; 2) two different sodium-lithium countertransport distributions are evident at all ages and in both sexes, as expected if a factor responsible for increased sodium-lithium countertransport were a presymptomatic predictor of essential hypertension; and 3) among individuals not receiving antihypertensive drugs, increased sodium-lithium countertransport makes only a small contribution to the prediction of higher blood pressure in specific age and sex subgroups from the general population.