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In general, the mammalian whole body mass-specific metabolic rate correlates positively with maximal urine concentration (U-max) irrespective of whether or not the species have adapted to arid or mesic habitat. Accordingly, we hypothesized that the thick ascending limb (TAL) of a rodent with markedly higher whole body mass-specific metabolism than rat exhibits a substantially higher TAL metabolic rate as estimated by Na+-K+-ATPase activity and Na+-K+ -ATPase alpha 1-gene and protein expression. The kangaroo rat inner stripe of the outer medulla exhibits significantly higher mean N+-K+-ATPase activity (similar to 70%) compared with two rat strains (SpragueDawley and Munich-Wistar), extending prior studies showing rat activity exceeds rabbit. Furthermore, higher expression of N+-K+-ATPase alpha 1-protein (similar to 4- to 6-fold) and mRNA (similar to 13-fold) and higher TAL mitochondrial volume density (similar to 20%) occur in the kangaroo rat compared with both rat strains. Rat TAL N+-K+-ATPase al-protein expression is relatively unaffected by body hydration status or, shown previously, by dietary Na+, arguing against confounding effects from two unavoidably dissimilar diets: grain-based diet without water (kangaroo rat) or grain-based diet with water (rat). We conclude that higher TAL N+-K+-ATPase activity contributes to relationships between whole body mass-specific metabolic rate and high U More vigorous TAL N+-K+-ATPase activity in kangaroo rat than rat may contribute to its steeper Na+ and urea axial concentration gradients, adding support to a revised model of the urine concentrating mechanism, which hypothesizes a leading role for vigorous active transport of NaCl, rather than countercurrent multiplication, in generating the outer medullary axial osmotic gradient.