The Roles of Arachidonic Acid and Calcium in the Angiotensin II-induced Inhibition of Na<SUP>&#8290;</SUP> Uptake in Renal Proximal Tubule Cells

The Roles of Arachidonic Acid and Calcium in the Angiotensin II-induced Inhibition of Na<SUP>&#8290;</SUP> Uptake in Renal Proximal Tubule Cells

<P> Angiotensin II (ANG II) has a biphasic effect on Na<SUP>&#8290;</SUP> transport in proximal tubule: low doses of ANG II increase the Na<SUP>&#8290;</SUP> transport, whereas high doses of ANG II inhibit it. However, the mechanisms of high dose ANG II-induced inhibition on Na<SUP>&#8290;</SUP> 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<SUP>&#8290;</SUP> uptake in the primary cultured rabbit renal proximal tubule cells (PTCs) in hormonally defined serum-free medium. ANG II (10<SUP>&#8291;9</SUP> M)-induced inhibition of Na<SUP>&#8290;</SUP> uptake was blocked by losartan (10<SUP>&#8291;8</SUP> M, AT<SUB>1</SUB> antagonist), but not by PD123319 (10<SUP>&#8291;8</SUP> M, AT<SUB>2</SUB> antagonist) (P＜0.05). ANG II-induced inhibition of Na<SUP>&#8290;</SUP> uptake was also completely abolished by neomycin (10<SUP>&#8291;4</SUP> M, PLC inhibitor), W-7 (10<SUP>&#8291;4</SUP> M, calmodulin antagonist), and AACOCF<SUB>3</SUB> (10<SUP>&#8291;6</SUP> M, PLA<SUB>2</SUB> inhibitor) (P＜0.05). ANG II significantly increased [<SUP>3</SUP>H]arachidonic acid (AA) release compared to control. The ANG II-induced [<SUP>3</SUP>H]AA release was blocked by losartan, AACOCF<SUB>3</SUB>, neomycin, and W-7, but not by PD123319. ANG II-induced [<SUP>3</SUP>H]AA release in the presence of extracellular Ca<SUP>2&#8290;</SUP> was greater than in Ca<SUP>2&#8290;</SUP>-free medium, and it was partially blocked by TMB-8 (10<SUP>&#8291;4</SUP> M, intracelluar Ca<SUP>2&#8290;</SUP> mobilization blocker). However, in the absence of extracellular Ca<SUP>2&#8290;</SUP>, it was completely blocked by TMB-8. In addition, econazole (10<SUP>&#8291;6</SUP> M, cytochrome P-450 monooxygenase inhibitor) and indomethacin (10<SUP>&#8291;6</SUP> M, cyclooxygenase inhibitor) blocked ANG II-induced inhibition of Na<SUP>&#8290;</SUP> uptake, but NGDA (10<SUP>&#8291;6</SUP> M, lipoxygenase inhibitor) did not affect it. In conclusion, PLA<SUB>2</SUB>-mediated AA release is involved in ANG II-induced inhibition of Na<SUP>&#8290;</SUP> uptake and is modulated by [Ca<SUP>2&#8290;</SUP>]<SUB>i</SUB> in the PTCs.