A potential mechanism for proximal tubule angiotensin II-mediated sodium flux associated with receptor-mediated endocytosis and arachidonic acid release.

Angiotensin II (Ang II) receptors in the proximal nephron significantly affect renal salt and water reabsorption. The predominant tubular-epithelial cell Ang II receptor, type 1 Ang II receptors (AT1R), is a member of the superfamily of G-protein-coupled receptors. Tubular cell AT1R are unusual as they are polarized to apical (AP) and basolateral (BL) cell membranes. In a proximal tubule cell model that mimics the in vivo polar distribution of AT1R (LLC-PKC14 cells transfected with rabbit AT1R), we have determined that AP and BL AT1R display differential rates of endocytosis and recycling. In addition, AP AT1R recycling was selectively inhibited by quinacrine, a non-specific inhibitor of phospholipase A2 (PLA2) activity and HELSS, a mechanism-based inhibitor of calcium-independent PLA2 activity. PLA2 catalyzes the hydroysis of sn-2 fatty acyl bonds, liberating free fatty acids and lysophosphoslipids, and proximal tubule AT1R are known to couple to PLA2 as a signaling pathway. In this model, AP Ang II treatment selectively stimulated [3H]-arachidonic acid ([3H]-AA) release. Furthermore, inhibiting AP AT1R endocytosis+recycling with either phenylarsine oxide (PAO) or HELSS, decreased AP Ang II-stimulated [3H]-AA release. In other studies, Ang II internalization also has been linked to vectorial sodium (Na+) flux. Preliminary studies in our model also suggested that HELSS inhibited AP Ang II-stimulated Na+ flux (AP --> BL), raising the possibility that AP AT1R targeting in proximal tubular epithelium, the endocytic profile of these receptors, and AP AT1R-mediated arachidonic acid release combine to serve as additional contributors to Ang II-mediated Na+ flux in the nephron.