Abstract-The proximal nephron reabsorbs 60% to 70% of the fluid and sodium and most of the filtered bicarbonate via Na/H exchanger 3. Enhanced proximal nephron transport is implicated in hypertension. Our findings show that a fructose-enriched diet causes salt sensitivity. We hypothesized that fructose stimulates luminal Na/H exchange activity and sensitizes the proximal tubule to angiotensin II. Na/H exchange was measured in rat proximal tubules as the rate of intracellular pH (pH i ) recovery in fluorescent units/s. Replacing 5 mmol/L glucose with 5 mmol/L fructose increased the rate of pH i recovery (1.8±0.6 fluorescent units/s; P<0.02; n=8). Staurosporine, a protein kinase C inhibitor, blocked this effect. We studied whether this effect was because of the addition of fructose or removal of glucose. The basal rate of pH i recovery was first tested in the presence of a 0.6-mmol/L glucose and 1, 3, or 5 mmol/L fructose added in a second period. The rate of pH i recovery did not change with 1 mmol/L but it increased with 3 and 5 mmol/L of fructose. Adding 5 mmol/L glucose caused no change. Removal of luminal sodium blocked pH i recovery. With 5.5 mmol/L glucose, angiotensin II (1 pmol/L) did not affect the rate of pH i recovery (change, -1.1±0.5 fluorescent units/s; n=9) but it increased the rate of pH i recovery with 0.6 mmol/L glucose/5 mmol/L fructose (change, 4.0±2.2 fluorescent units/s; P<0.02; n=6). We conclude that fructose stimulates Na/H exchange activity and sensitizes the proximal tubule to angiotensin II. This mechanism is likely dependent on protein kinase C. These results may partially explain the mechanism by which a fructose diet induces

Albuminuria and tubular atrophy are among the highest risks for CKD progression to ESRD. A parsimonious mechanism involves leakage of albumin-bound nonesterified fatty acids (NEFAs) across the damaged glomerular filtration barrier and subsequent reabsorption by the downstream proximal tubule, causing lipoapoptosis. We sought to identify the apical proximal tubule transporter that mediates NEFA uptake and cytotoxicity. We observed transporter-mediated uptake of fluorescently labeled NEFA in cultured proximal tubule cells and microperfused rat proximal tubules, with greater uptake from the apical surface than from the basolateral surface. Protein and mRNA expression analyses revealed that kidney proximal tubules express transmembrane fatty acid transporter-2 (FATP2), encoded by , but not the other candidate transporters CD36 and free fatty acid receptor 1. Kidney FATP2 localized exclusively to proximal tubule epithelial cells along the apical but not the basolateral membrane. Treatment of mice with lipidated albumin to induce proteinuria caused a decrease in the proportion of tubular epithelial cells and an increase in the proportion of interstitial space in kidneys from wild-type but not mice. microperfusion and experiments with NEFA-bound albumin at concentrations that mimic apical proximal tubule exposure during glomerular injury revealed significantly reduced NEFA uptake and palmitate-induced apoptosis in microperfused proximal tubules and or FATP2 shRNA-treated proximal tubule cell lines compared with wild-type or scrambled oligonucleotide-treated cells, respectively. We conclude that FATP2 is a major apical proximal tubule NEFA transporter that regulates lipoapoptosis and may be an amenable target for the prevention of CKD progression.

Fructose-enriched diets cause salt-sensitive hypertension. Proximal tubules (PTs) reabsorb 70% of the water and salt filtered through the glomerulus. Angiotensin II (Ang II) regulates this process. Normally, dietary salt reduces Ang II allowing the kidney to excrete more salt, thereby preventing hypertension. We hypothesized that fructose-enriched diets enhance the ability of low concentrations of Ang II to stimulate PT transport. We measured the effects of a low concentration of Ang II (10−12 mol/L) on transport-related oxygen consumption (QO2), and Na/K-ATPase and Na/H-exchange (NHE) activities and expression in PTs from rats consuming tap water (Control) or 20% fructose (FRUC). In FRUC-treated PTs, Ang II increased QO2 by 14.9 ± 1.3 nmol/mg/min (p < 0.01) but had no effect in Controls. FRUC elevated NHE3 expression by 19 ± 3% (p < 0.004) but not Na/K-ATPase expression. Ang II stimulated NHE activity in FRUC PT (Δ + 0.7 ± 0.1 Arbitrary Fluorescent units (AFU)/s, p < 0.01) but not in Controls. Na/K-ATPase activity was not affected. The PKC inhibitor Gö6976 blocked the ability of FRUC to augment the actions of Ang II. FRUC did not alter the inhibitory effect of dopamine on NHE activity. We conclude that dietary fructose increases the ability of low concentrations of Ang II to stimulate PT Na reabsorption via effects on NHE.