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+-adenosinetrip hosphatase (Na+-K+-ATPase) activity independently of adenosine 3',5'-c yclic monophosphate generation. The present study used PT suspensions to investigate the signaling pathway responsible for this hormonal act ion. PTH-(1-34) and PTH(3-34) significantly increased the release of a rachidonic acid (AA) compared with control tubules, suggesting activat ion of phospholipase A(2) (PLA(2)). AA, 10(-6) M, mimicked the inhibit ion of the pump by 10(-8) M PTH-(3-34), and together were not additive . Eicosatetraynoic acid, 3 mu M, a general inhibitor of AA metabolism, blocked the PTH action. Indomethacin, 10 mu M, an inhibitor of AA-dep endent cyclooxygenase, did not prevent the PTH action, but 2 mu M 7-et hoxyresorufin, 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) M PTH-(3-34), was not additive with PTH, and was maximally i nhibitory at 10(-7) M. To further investigate the signaling pathway re sponsible for PTH activated PLA(2), we tested the effect of PTH on cyt oplasmic 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 a ction. We conclude that PTH inhibits PT Na+-K+-ATPase activity by acti vating a Ca2+- and PTX-insensitive PLA(2) involved in an AA-dependent cytochrome P-450 pathway responsible for the generation of 20-HETE, a metabolite with natriuretic properties.