The causal relationship SKI-606 between lipid abnormalities and defective NH4+ production and/or transport has been tested in vitro in cell culture but not in vivo (6). We examined causality in vivo by reducing plasma FFA and the development of renal steatosis in ZDF rats with TZD treatment. The amelioration of lipid abnormalities in TZD-treated ZDF rats was accompanied by changes in urinary acidification parameters including urinary pH, NH4+, and TA to values close to those in lean rats, while TZD treatment in lean rats, in the absence of lipid abnormalities, had no effect on urinary acidification (Fig. 2, A�CC). Of note, urinary citrate was lower in ZDF rats compared with their lean littermates, and this difference was also reversed by TZD treatment (Fig. 2D).

These differences could not be attributed to acid-base disturbances or differential acid or alkali intestinal absorption, since acid-base status and urinary sulfate and potassium excretion were not different between the groups (Table 1). Fig. 2. Improvement of urinary acidification defects with TZD treatment in ZDF rats. Eight-week-old ZDF rats and lean littermates were treated with TZD for 4 wk. Urinary pH (A), urinary NH4+ excretion (B), urine titratable acidity (TA; C), and urine citrate excretion … NHE3 antigen and activity in ZDF rats treated with TZD. The Na+/H+ exchanger NHE3 is critical for NH4+ transport in the proximal tubule (24, 33�C35), and reduced expression and activity of NHE3 at the proximal tubule brush-border membrane may account at least in part for the decreased urinary NH4+ and pH in ZDF rats (6).

We next tested whether the amelioration of lipid abnormalities and urinary acidification defects in ZDF rats treated with TZD is accompanied by a corresponding increase in proximal tubule NHE3 expression and function. Rosiglitazone treatment was previously shown to have no effect on renal cortical NHE3 protein in ZDF rats (40), but this finding does not distinguish between brush-border NHE3, which participates in luminal transport, and intracellular NHE3. In addition, NHE3 activity can change without commensurate changes Anacetrapib in protein abundance (8, 31). To determine the effect of TZD on NHE3 function at the lumen of the proximal tubule, we measured NHE3 antigen and Na+/H+ exchange activity in isolated proximal tubule BBMV. TZD treatment was accompanied by a significant increase in brush-border NHE3 protein abundance and activity in ZDF rats, while having no detectable effect in lean rats (Fig. 3). Fig. 3. Effect of TZD treatment on brush-border membrane Na+/H+ exchanger 3 (NHE3) antigen and Na+/H+ exchange function in ZDF and lean rats. Eight-week-old ZDF rats and lean littermates were treated with TZD for 4 wk. A: representative immunoblot. B: summary …