Parathyroid hormone inhibits Na⁺-K⁺-ATPase through a cytochrome P-450 pathway

We have previously shown that parathyroid hormone (PTH)-(1-34) or its analogue PTH-(3-34) inhibits proximal tubule (PT) Na⁺-K⁺- adenosinetriphosphatase (Na⁺-K⁺-ATPase) activity independently of adenosine 3',5'-cyclic monophosphate generation. The present study used PT suspensions to investigate the signaling pathway responsible for this hormonal action. PTH-(1-34) and PTH-(3-34) significantly increased the release of arachidonic acid (AA) compared with control tubules, suggesting activation of phospholipase A₂ (PLA₂). AA, 10^-6 M, mimicked the inhibition of the pump by 10^-5 M PTH-(3-34), and together were not additive. Eicosatetraynoic acid, 3 μM, a general inhibitor of AA metabolism, blocked the PTH action. Indomethacin, 10 μM, in inhibitor of AA-dependent cyclooxygenase, did not prevent the PTH action, but 2 μM 7-ethoxyresorufin, a cytochrome P-450 inhibitor, prevented the PTH effect. 20-Hydroxyeicosatetraenoic acid (20- HETE), the main product of P-450 metabolism in PT, inhibited Na⁺-K⁺-ATPase activity to the same extent as 10^-8 PTH-(3-34), was not additive with PTH, and was maximally inhibitory at 10^-7 M. To further investigate the signaling pathway responsible for PTH-activated PLA₂, we tested the effect of PTH on cytoplasmic free Ca²⁺ ([Ca²⁺](i)). PTH-(1-34), 10^-7 M, did not affect [Ca²⁺](i), although 10^-8 M angiotensin II promoted a Ca²⁺ transient. Treatment of PT with pertussis toxin (PTX) did not prevent the PTH action. We conclude that PTH inhibits PT Na⁺-K⁺-ATPase activity by activating a Ca²⁺- and PTX-insensitive PLA₂ involved in an AA-dependent cytochrome P-450 pathway responsible for the generation of 20-HETE, a metabolite with natriuretic properties.