Mechanism of regulation of NA⁺ transport by angiotensin II in primary renal cells

Background. Angiotensin II (Ang II) has a dose-dependent, biphasic effect on the activity of the Na⁺/H⁺ antiport system in the renal proximal tubule (RPT). The aim of the present study was to further delineate the signaling pathways involved in Ang II action. Methods. To examine Ang II signaling, ²²Na⁺ uptake studies were conducted with a primary rabbit RPT cell culture system. The activation of phospholipase A₂ (PLA₂) was assessed by measuring the release of [³H]-arachidonic acid (AA), and changes in intracellular calcium levels were determined by means of confocal microscopy. Results. Low dosages of Ang II (<10^-10 mol/L) stimulated Na⁺ uptake, whereas high dosages of Ang II (>10^-10 mol/L) inhibited Na⁺ uptake. Ang II (>10^-10 mol/L) also caused an increase in AA release associated with an increase in intracellular calcium. Not only did exogenous AA inhibit Na⁺ uptake, but two PLA₂ inhibitors (mepacrine and AACOCF₃) blocked the Ang II- mediated inhibition of Na⁺ uptake. However, the cytochrome P450-dependent epoxygenase inhibitor econazole also blocked the Ang II-induced inhibition of Na⁺ uptake. Inhibition of Na⁺ uptake was obtained by the metabolic product of the epoxygenase 5,6-EET. In turn, the inhibitory effect of 5,6-EET was blocked by indomethacin. Conclusions. The results indicate the involvement of a calcium-dependent PLA₂ in mediating the inhibitory effect of Ang II on Na⁺ uptake. The AA, which is released following PLA₂ activation, acts indirectly, through its own metabolism, via a cytochrome P450 epoxygenase pathway and ultimately cyclooxygenase itself.