The Lack of Urea Transporters, UT‐A1 and UT‐A3, Increases Nitric Oxide Accumulation to Dampen Medullary Sodium Reabsorption Through ENaC

Abstract

While the role of urea in urine concentration is known, the effect of urea handling by the urea transporters, UT‐A1 and UT‐A3, on sodium balance remains elusive. Serum and urinary sodium is similar between wild‐type mice (WT) and UT‐A3 null (UT‐A3 KO) mice; however, mice lacking both UT‐A1 and UT‐A3 (UT‐A1/A3 KO) have significantly lower serum sodium and higher urinary sodium. Protein expression of renal sodium transporters (NHE3, NCC, NaATPase, NKCC2, ENaC) is unchanged among all three genotypes. WT, UT‐A3 KO and UT‐A1/A3 KO acutely respond to hydrochlorothiazide and furosemide; however, UT‐A1/A3 KO fail to show a diuretic or natriuretic response following amiloride administration indicating that ENaC activity is impaired. Interestingly, IMCD biotinylation showed that UT‐A1/A3 KO have more ENaC at the apical membrane than WT, and single channel analysis of ENaC in isolated, split‐open IMCD revealed that ENaC channel density and open probability is higher in UT‐A1/A3 KO than WT. UT‐A1/A3 KO excrete larger amounts of urinary nitric oxide (NO), a paracrine inhibitor of ENaC, and express 40‐fold more inner medullary nitric oxide synthase 1 (NOS1) mRNA than WT. Because endogenous NO is unstable, ENaC activity was reassessed in split‐open IMCD with the NO donor, PAPA NONOate, and now found to be almost completely abolished in UT‐A1/A3 KO. In summary, loss of both UT‐A1 and UT‐A3 (but not UT‐A3 alone) cause a salt‐wasting phenotype and an elevation in medullary NO production. NO inhibition of ENaC, despite elevated apical protein accumulation in UT‐A1/A3 KO IMCD, appears to be the main contributor to natriuresis in UT‐A1/A3 KO mice.