Time course of AQP-2 and ENaC regulation in the kidney in response to PPAR agonists associated with marked edema in rats

Abstract

Peroxisome-proliferator-activated receptor (PPAR-γ) agonists improve insulin sensitivity, but are associated with edema. Increased distal tubule sodium and water reabsorption through the epithelial sodium channel (ENaC) and aquaporin-2 (AQP-2) have been suggested to play mechanistic roles. To determine the molecular regulation of these proteins, we treated male, Sprague–Dawley rats daily by gavage with either vehicle, rosiglitazone (RGZ, 50 mg/kg bw), or PD168 (a test compound causing marked edema, 10 mg/kg bw), for 1, 3, or 5 days ( n = 6/treatment/time). On day 1, urine sodium excretion was significantly reduced by RGZ with a strong trend for PD168 ( p-values 0.047 and 0.053, respectively) indicating early sodium retention. Blood pressure was lowered by RGZ- or PD168 treatment by 12 h. Immunoblotting of whole kidney homogenates (WKHs) and a membrane-enriched fraction (MF) revealed increased band densities for AQP-2 in WKH (29 kDa and glycosylated bands) by both drugs at 1 day. However, at 5 days, the 29-kDa band was significantly decreased (∼30% of vehicle). α-ENaC was increased by RGZ at 3 days; however both agents decreased α-ENaC by 5 days. In contrast, β- and γ-ENaC (85 kDa) were unchanged or decreased at all times by both agents. However, the 70-kDa band of γ-ENaC (active band) in MF was increased in density (120–600%) by both agents on days 3–5. Overall, both agents resulted in early alterations in banding patterns for AQP-2 and ENaC subunits, many of which are described as activating changes. However, later reduction in AQP-2 and α-ENaC may represent an attempt to re-establish sodium and water balance.