The roles of arachidonic acid and calcium in the angiotensin II-induced inhibition of Na+ uptake in renal proximal tubule cells

ABSTRACT

Angiotensin II (ANG II) has a biphasic effect on Na+ transport in proximal tubule low doses of ANG II increase the Na+ transport, whereas high doses of ANG II inhibit it. However, the mechanisms of high dose ANG II-induced inhibition on Na+ uptake are poorly understood. Thus the aim of the present study was to investigate signal transduction pathways involved in the ANG II-induced inhibition of Na+ uptake in the primary cultured rabbit renal proximal tubule cells (PTCs) in hormonally defined serum-free medium. ANG II (10-9 M)-induced inhibition of Na+ uptake was blocked by losartan (10-8 M, AT1 antagonist), but not by PD123319 (10-8 M, AT2 antagonist) (P < 0.05). ANG II-induced inhibition of Na+ uptake was also completely abolished by neomycin (10-4 M, PLC inhibitor), W-7 (10-4 M, calmodulin antagonist), and AACOCF3 (10-6 M, PLA2 inhibitor) (P < 0.05). ANG II significantly increased (3H)arachidonic acid (AA) release compared to control. The ANG II-induced (3H)AA release was blocked by losartan, AACOCF3, neomycin, and W-7, but not by PD123319. ANG II-induced (3H)AA release in the presence of extracellular Ca2+ was greater than in Ca2+-free medium, and it was partially blocked by TMB-8 (10-4 M, intracelluar Ca2+ mobilization blocker). However, in the absence of extracellular Ca2+, it was completely blocked by TMB-8. In addition, econazole (10-6 M, cytochrome P-450 monooxygenase inhibitor) and indomethacin (10-6 M, cyclooxygenase inhibitor) blocked ANG II-induced inhibition of Na+ uptake, but NGDA (10-6 M, lipoxygenase inhibitor) did not affect it. In conclusion, PLA2-mediated AA release is involved in ANG II-induced inhibition of Na+ uptake and is modulated by (Ca2+)i in the PTCs.