20-HETE MEDIATES THE EFFECT OF PARATHYROID-HORMONE AND PROTEIN-KINASE-C ON RENAL PHOSPHATE-TRANSPORT

Parathyroid hormone (PTH) is a major inhibitor of renal proximal tubul e (PT) sodium-dependent phosphate (Na+-Pi) cotransport. PTH is thought to exert its effect on Pi transport in the PT via the protein kinase A (PKA) and C (PKC) intracellular signalling pathways. PKC-dependent p hosphorylation of phospholipase A(2) stimulates arachidonic acid (AA) release, the latter a potent inhibitor of Pi transport. In turn, AA is metabolized to 20-hydroxyeicosatetraenoic acid (20-HETE) in the PT. I n addition, 20-HETE production is stimulated by PTH. We therefore expl ored the possibility that 20-HETE may mediate the PTH/PKC inhibition o f renal Na+-Pi cotransport, To this end, we tested the effect of 20-HE TE on Na+-Pi cotransport in proximal tubule-like cells. Exposure of op ossum kidney (OK) cells for 4 h to 20-HETE (10(-7) M) decreased Na+-de pendent uptake of (32)Pi (from 0.26 +/- 0.02 to 0.19 +/- 0.01 nmol/mg protein.min) by similar to 25% (P< 0.001). The inhibition was due to a reduction in V-max. 20-HETE had no significant effect on either the a pical amiloride-sensitive and insensitive Na-22 uptakes or on basolate ral ouabain-sensitive Rb-86 uptake, and was specific for Pi. These res ults indicate that 20-HETE specifically inhibits Na+-dependent Pi tran sport in OK cells and that it may be a mediator of PTH action in the P T.