Kidney-specific WNK1 inhibits sodium reabsorption in the cortical thick ascending limb

Kidneyspecific WNK1 (KS-WNK1) is a variant of full-length WNK1. Previous studies have reported that KS-WNK1 is predominantly expressed in the distal convoluted tubule (DCT) where it regulates sodium-chloride cotransporter. The role of KS-WNK1 in other nephron segments is less clear. Here, we measured the expression of KS-WNK1 transcript in microdissected renal tubules and found that KS-WNK1 was most abundant in the DCT, followed by cortical thick ascending limb (cTAL), connecting tubule, and cortical collecting duct. A high K⁺ diet enhanced the expression of KS-WNK1 in the DCT and cTAL, selectively. It has been reported that a high-K diet suppresses Na⁺ reabsorption in TAL. To understand the role of KS-WNK1 in Na⁺ transport in cTAL and the regulation by dietary K⁺, we examined Na⁺ reabsorption using in vitro microperfusion in cTAL isolated from KS-WNK1-knockout mice and wild-type littermates fed either a control-K⁺ or high-K⁺ diet. Furosemide-sensitive Na⁺ reabsorption in Ctal was higher in KS-WNK1-knockout (KO) mice than in wild-type. A high-K⁺ diet inhibited Na⁺ reabsorption in cTAL from wild-type mice, but the inhibition was eliminated in KS-WNK1-KO mice. We further examined the role of KS-WNK1 using transgenic mice that overexpress KS-WNK1. Na⁺ reabsorption in cTAL was lower in transgenic than in wild-type mice. In whole animal clearance studies, a high-K⁺ diet increased daily urine volume and urinary Na⁺ and K⁺ excretion in wild-type mice, which was blunted in KS-WNK1-KO mice. Thus KS-WNK1 inhibits Na⁺ reabsorption in cTAL and mediates the inhibition of Na⁺ reabsorption in the segment by a high-K diet.