Article: PDF OnlyMaling Timothy J. B.; Siebers, Robert W. L.Journal of Cardiovascular Pharmacology: 1990 - p S50-S51 Free Abstract Summary The reported inverse relationship between fractional urinary clearance of lithium (FCLi) and erythrocyte sodium-lithium countertransport (Na-Li CT) in normotensive and hypertensive subjects suggests that Na-Li CT may be a marker of proximal tubular sodium reabsorption. We have refuted this hypothesis in a multiracial study of 57 Caucasian and 48 Maori normotensive and hypertensive maies aged 20–40 years. Na-Li CT was measured in vitro by standard Li efflux methodology, and in vivo by the Li cell:plasma (Li C:P) ratio 24 h after a 1 g oral dose of lithium carbonate. The Na-Li CT and Li C:P ratio were not significantly different in the two races and were strongly correlated within race, confirming the validity of the Li C:P ratio as an in vivo index of in vitro Na-Li CT. There was no correlation in either race between in vitro or in vivo erythrocyte membrane sodium transport indices and FCLi. Copyright © 1990 Wolters Kluwer Health, Inc. All rights reserved.