The role of the proximal nephron in salt-sensitive hypertension

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The dramatic rise in U.S. fructose consumption mirrors the increase in the incidence of hypertension. More than 17 million Americans consume >20% of their calories as fructose and crossover studies directly show that fructose increases blood pressure (BP). We reported that fructose causes salt-sensitive hypertension while glucose does not. We also showed that dietary fructose enhances the stimulatory effect of angiotensin II (Ang II) on proximal nephron Na reabsorption, the only nephron segment able to reabsorb and metabolize fructose. We show here that dietary fructose enhances the ability of Ang II to stimulate protein kinase C (PKC) activity and O2- production by proximal tubules. However, the roles of PKC activation and oxidative stress in the proximal nephron in fructose-induced hypertension are poorly understood. In addition to reabsorbing ≈70% of the filtered Na and water, one of the primary functions of the proximal nephron is to secrete proteases that degrade filtered proteins so they can be reabsorbed. Protease-induced activation of the epithelial Na channel (ENaC) in the collecting duct causes Na retention and hypertension in several pathological conditions. Heretofore proximal tubules have not been thought of as a potential source for urinary proteases. Oxidative stress stimulates protease release from non-renal cells. Here we show that dietary fructose enhances proximal nephron expression of two such proteases, trypsin and meprin β, and augments the urinary excretion of these and urokinase. We also show that inhibition of ENaC reverses fructose-induced hypertension. Phospholipase D and increases in intracellular Ca (Cai) are involved in the secretion of many proteins, and both are stimulated by Ang II. Thus, we hypothesize that dietary fructose causes salt-sensitive hypertension by enhancing Ang II-induced increases in PKC activity and oxidative stress in the proximal nephron. This increased oxidative stress stimulates trypsin, urokinase and meprin β expression and release from this segment. These proteases cleave and activate ENaC in collecting ducts, stimulating Na reabsorption. Aim I will test whether dietary fructose enhances the ability of Ang II to stimulate PKC activity and O2- production by proximal tubules thereby contributing to salt retention and elevations in BP. Aim II will test whether dietary fructose enhances the expression and release of trypsin, urokinase and meprin β by proximal tubules via O2-, phospholipase D and Cai-dependent mechanisms. Aim III will test whether fructose-induced trypsin, urokinase and meprin β release from proximal tubules augments Na reabsorption by collecting ducts, thereby contributing to salt retention and elevations in BP. We will use state of the art techniques in imaging, physiology, molecular biology and gene transfer. This project will yield new insights into how dietary fructose causes salt-sensitive hypertension, and which drugs currently used to treat hypertension such as Ang II receptor blockers and diuretics targeting collecting ducts will likely be most efficacious. Since these drugs are currently approved for such use, the recommendations could be adopted immediately.