Erythrocyte sodium/lithium countertransport and renal lithium clearance in a random sample of untreated middle-aged men

1. It has been proposed that the enhanced erythrocyte Na⁺/Li⁺ countertransport observed in many patients with essential hypertension could be a marker of abnormal renal proximal tubular function. We thus investigated the relationship of blood pressure and Na⁺/Li⁺ countertransport to an index of proximal tubular function such as renal Li⁺ clearance. 2. The study was carried out in a sample of 299 untreated male subjects (aged 21-59 years) randomly selected from a population at work. Na⁺/Li⁺ countertransport was measured in a representative sub-group of 176 men. 3. We did not detect statistically significant correlation of either blood pressure or Na⁺/Li⁺ countertransport (V(max.)) with fractional excretion of Li⁺, while confirming the existence of a significant continuous association of blood pressure and body mass index with Na⁺/Li⁺ countertransport (P <0.01). 4. A sub-sample of 57 participants belonging to the lowest or the highest quintiles of Na⁺/Li⁺ countertransport distribution repeated the Li⁺ clearance study after moderate Na⁺ restriction. 5. Although fractional excretions of Na⁺ and Li⁺ were reduced on the low Na⁺ diet (both P <0.001), they did not differ significantly between groups. 6. Our results are at variance with the findings of a recent case-control study in a young age group and suggest that further studies are necessary before a conclusion can be drawn as to the suitability of Na⁺/Li⁺ countertransport as a marker of Na⁺ reabsorption in the renal proximal tubule.