No abstract

Thiazolidinediones (TZDs) improve insulin resistance by activating a nuclear hormone receptor, peroxisome proliferator-activated receptor γ (PPARγ). However, the use of TZDs is associated with plasma volume expansion through a mechanism that remains to be clarified. Here we showed that TZDs rapidly stimulate sodium-coupled bicarbonate absorption from the renal proximal tubule in vitro and in vivo. TZD-induced transport stimulation is dependent on PPARγ-Src-EGFR-ERK and observed in rat, rabbit and human, but not in mouse proximal tubules where Src-EGFR is constitutively activated. The existence of PPARγ-Src-dependent nongenomic signaling, which requires the ligand-binding ability, but not the transcriptional activity of PPARγ, is confirmed in mouse embryonic fibroblast cells. The enhancement of the association between PPARγ and Src by TZDs supports an indispensable role of Src in this signaling. These results suggest that the PPARγ-dependent nongenomic stimulation of renal proximal transport is also involved in TZD-induced volume expansion.

These results indicate that DA inhibits the Na+:HCO3- cotransporter in renal proximal tubules and also suggest that dysregulation of the cotransporter, possibly through the defect in DA1 receptor signaling, could play an important role in development of hypertension in SHRs.

Regulation of renal proximal transport by angiotensin II (Ang II) is biphasic: low concentrations (picomolar to nanomolar) stimulate reabsorption, but higher concentrations (nanomolar to micromolar) inhibit reabsorption. Traditionally, the stimulatory effect has been attributed to activation of protein kinase C and/or a decrease in intracellular cAMP, whereas the inhibitory action has been attributed to the activation of phospholipase A 2 (PLA 2 ) and the subsequent release of arachidonic acid. The Ang II receptor subtype responsible for these effects and the intracellular signaling pathways involved are not completely understood. We isolated proximal tubules from wild-type, Ang II type 1A receptor (AT 1A )-deficient, and group IVA cytosolic phospholipase A 2 (cPLA 2 ␣)-deficient mice, and compared their responses to Ang II. In wild-type mice, we found that the stimulatory and inhibitory effects of Ang II on Na ϩ -HCO 3 Ϫ cotransporter activity are both AT 1 -mediated but that ERK activation only plays a role in the former. The stimulatory effect of Ang II was also observed in AT 1A -deficient mice, suggesting that this occurs through AT 1B . In contrast, the inhibitory effects of Ang II appeared to be mediated by cPLA 2 ␣ activation because high-concentration Ang II stimulated Na ϩ -HCO 3 Ϫ cotransporter activity when cPLA 2 ␣ activity was abrogated by pharmacological means or genetic knockout. Consistent with this observation, we found that activation of the cPLA 2 ␣/P450 pathway suppressed ERK activation. We conclude that Ang II activates ERK and cPLA 2 ␣ in a concentration-dependent manner via AT 1 , and that the balance between ERK and cPLA 2 ␣ activities determines the ultimate response to Ang II in intact proximal tubules.

Abstract-Although angiotensin (Ang) II is known to regulate renal proximal transport in a biphasic way, the receptor subtype(s) mediating these Ang II effects remained to be established. To clarify this issue, we compared the effects of Ang II in wild-type mice (WT) and Ang II type 1A receptor-deficient mice (AT 1A KO). The Na ϩ -HCO 3 Ϫ cotransporter (NBC) activity, analyzed in isolated nonperfused tubules with a fluorescent probe, was stimulated by 10