ENDOGENOUS PROSTAGLANDIN-E(2) MEDIATES INHIBITION OF RAT THICK ASCENDING LIMB CL REABSORPTION IN CHRONIC HYPERCALCEMIA

The hypothesis that endogenous PGE2 mediates defective thick ascending limb (TAL) Cl reabsorption (percent delivered load: FR(Cl)%) in rats with vitamin D-induced chronic hypercalcemia (HC) was tested by measuring FR(Cl)% in loop segments microperfused in vivo in HC and control rats treated acutely with indomethacin (Indo) or its vehicle, and obtaining the corresponding outer medullary [PGE2]. Microperfusion conditions were developed in which FR(Cl)% was exclusively furosemide sensitive. To determine the cellular mechanism, tubules were perfused acutely with forskolin (FSK), cAMP, or the protein kinase C inhibitor staurosporine (SSP). Outer medullary [PGE2] in HC rats was 9 to 10 times greater than control and could be normalized by Indo. FR(Cl)% was 20% lower in HC rats infused with vehicle, and Indo, FSK, and cAMP returned FR(Cl)% to normal despite sustained HC. Indo or FSK had no effect on FR(Cl)% in control rats and Indo did not prevent inhibition of FR(Cl)% by luminal PGE2 (1 muM). Luminal SSP (10(-7), 10(-8) M) in HC did not return FR(Cl)% to control values. We conclude that impaired TAL FR(Cl)% in HC occurs at a pre-cAMP site and is due to endogenous PGE2 and not to HC.