Stimulation of sweet taste receptors expressed in the kidney enhance surface NKCC2 levels in thick ascending limbs (TALs)
Ares G, Caceres P, Kassem KM, and Ortiz PA. Stimulation of sweet taste receptors expressed in the kidney enhance surface NKCC2 levels in thick ascending limbs (TALs). FASEB Journal 2017; 31(1 Suppl):856.16.
The thick ascending limb (TAL) reabsorbs 30% of the filtered NaCl via the Na/K/2Cl cotransporter, (NKCC2). NKCC2 activity is regulated by trafficking into and out of the apical membrane. We previously found that the monosaccharide fructose, acutely (20 minutes) increases surface NKCC2 levels and NKCC2 activity in isolated TALs. In the tongue, fructose and other monosaccharides are sensed by the Gprotein coupled sweet taste receptors (T1R2/T1R3). However, the expression and function of T1R2/T1R3 in the kidney or the TAL specifically is unclear. We hypothesize that T1R2/T1R3 are expressed in TALs and their stimulation increases surface NKCC2 levels. We used surface biotinylation and Western blot to measure surface NKCC2 levels in rat or mouse TAL suspensions. By Western blot we observed a single band for T1R2 and T1R3 expression in TALs. We then used gurmarin (GUR), a selective inhibitor of T1R2/T1R3 in rodents, to inhibit fructosestimulated surface NKCC2 expression. We found that treating TALs with GUR completely blocked fructose-stimulated surface NKCC2 expression (Baseline= 100%; fructose= 132 ± 3%; GUR= 89 ± 9%; GUR + fructose= 93 ± 6% n.s.). To directly stimulate T1R2/T1R3 we used the non-caloric sweetener acesulfame-K (AceK). In TALs, 20 min treatment with AceK increased surface NKCC2 expression (Baseline= 100%; AceK 0.5 mM= 134 ± 7%, AceK 1mM= 157 ± 15%, p<0.05). In GUR-treated TALs, AceK did not stimulate surface NKCC2 levels (GUR= 100%; GUR + AceK= 94 ± 5%, n.s.). To understand the role of T1R2/T1R3 in renal function we obtained knockout mice (KO) with dual deletion of the T1R2/T1R3 genes. Under baseline conditions surface NKCC2 levels was decreased in T1R2/R3 KO TALs (WT: 100, Tas1R2/3 KO: 59 ± 15%, p<0.05) whereas total NKCC2 was not changed (n=4). We concluded that stimulation of sweet taste receptors T1R2/T1R3 increases surface NKCC2 expression in TALs. Our data show for the first time a renal function for stimulation of T1R2/T1R3 receptors, and suggest that some of the deleterious effect of fructose could be due to activation of renal sweet taste receptors.