Abstract 639: Fructose Potently Stimulates NKCC2 Trafficking in rat Thick Ascending Limbs


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Abstract

Fructose consumption has increased dramatically in the last 30 years and is directly linked to the epidemic of obesity, type-2 diabetes and also hypertension. A moderately high fructose intake induces salt-sensitive hypertension and enhances urinary excretion of fructose. Despite the potential implications of fructose in renal salt handling, very little is known about its effect along the nephron. The thick ascending limb (TAL) reabsorbs 25% of filtered NaCl, primarily via the apical Na/K/2Cl cotransporter (NKCC2). Enhanced NKCC2 activity and trafficking is related to salt-sensitive hypertension in genetic animal models. Thus, we hypothesized that fructose acutely stimulate NKCC2 trafficking to the apical surface of TALs. To test this hypothesis we measured membrane and total NKCC2 expression as well as phosphorylation of NKCC2 at Ser126 and Thre96,101 in suspensions of rat TALs. We found that acute treatment (20 minutes) of TALs with fructose potently enhanced apical surface NKCC2 in a concentration-dependent manner (Baseline: 100%; fructose 1 mM= 122 ± 5%, fructose 5 mM= 49 ± 10%, fructose 10 mM = 201 ± 59%; p<0.05). Acute fructose treatment did not increase NKCC2 phosphorylation at Ser126 or Thre96,101 and did not affect total NKCC2 expression. To test if the effect of fructose on apical surface NKCC2 was specific to this hexose, we studied the effect of glucose and fructose 1,6 bisphosphate (1,6BP). We found that 20 minute-incubation with glucose (5 mM) and fructose 1,6BP (5 mM) did not affect baseline surface NKCC2 levels (baseline = 100, glucose (5 mM)= 101 ± 7%, fructose 1,6BP (5 mM)= 92 ± 6%, not significant). We concluded that acute fructose treatment, at physiologically relevant concentrations, increases membrane NKCC2 expression without enhancing its phosphorylation. Compared to β-adrenergic receptor stimulation and vasopressin (which enhance apical surface NKCC2 in TALs by 25-40%, fructose (but not glucose) is a very potent agonist of NKCC2 trafficking to the apical membrane of rat TALs. Our data suggest that fructose-stimulated TAL NaCl reabsorption could contribute to the salt-sensitive hypertension caused by a high fructose diet.

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