title: Sodium-lithium countertransport is not a marker of proximal tubular sodium clearance.
abstract: 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 males 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.
authors.affiliation: Department of Medicine, Wellington School of Medicine, New Zealand.
authors.forename: T J
authors.initials: TJ
authors.lastname: Maling
authors.forename: R W
authors.initials: RW
authors.lastname: Siebers
mesh_terms: D000328:Adult; D015415:Biomarkers; D001794:Blood Pressure / Q000502:physiology; D004912:Erythrocytes / Q000378:metabolism; D006801:Humans; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008094:Lithium / Q000097:blood / Q000378:metabolism* / Q000652:urine; D008297:Male; D012964:Sodium / Q000097:blood / Q000378:metabolism*
chemical_list: D015415:Biomarkers; D008094:Lithium; D012964:Sodium