Abstract 553: Prostaglandin E2 EP3 Receptor Downregulates COX2 Expression in the Medullary Thick Ascending Limb (mTAL) Induced by Hypertonic NaCl Intake

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We recently showed that a novel negative feedback mechanism involving PGE2 acting on EP3 receptors regulates cyclooxygenase-2 (COX-2) expression in the thick ascending limb (TAL) induced by the selective COX-2 inhibitors, celecoxib and rofecoxib. In the present study we tested the hypothesis that inhibition of EP3 facilitates COX-2 expression in the TAL induced by ingestion of hypertonic NaCl or exposure of mTAL cells to hypertonic media. COX-2 protein expression in the outer medulla (OM) increased 2.2±0.3 fold in mice given free access to 1% NaCl in the drinking water for 3 days compared with tap water; p<0.05. The increase was associated with a 4-fold elevation in COX-2 mRNA accumulation (tap water: 0.5±0.1 vs 1% NaCl: 1.9±0.4; p<0.05), and higher PGE2 production by freshly isolated mTAL tubules (tap water: 87.6±9.4 vs 1% NaCl: 203.3±19.3 pg/mg protein; p<0.05). EP3 mRNA levels also increased approximately 2-fold in OM of mice ingesting 1% NaCl (tap water: 0.7±0.2 vs 1% NaCl: 1.3±0.2). Administration of a selective EP3 receptor antagonist (L-798106: 20μg/kg/day) for 2 days increased COX-2 mRNA accumulation in mTAL tubules (L-798106: 2.1±0.1 fold change vs control; p<0.05), and the elevation in COX-2 protein expression induced by 1% NaCl was increased an additional 50% in mice given L-798106. COX-2 mRNA accumulation in primary cultures of mTAL cells increased 2-fold in response to media made hypertonic by addition of NaCl (400 mosmol/kg H2O), compared with cells incubated in isotonic media. L-798106 increased COX-2 mRNA 2-fold in isotonic media and 4-fold in cells exposed to 400 mosmol/kg H2O. PGE2 production by mTAL cells increased approximately 4-fold in response to challenge with 400 mosmol/kg H2O for 9 hr, and was inhibited in cells transiently transfected with a lentivirus shRNA construct (EGFP-C2-ex5) to silence COX-2 (286.1±14.8 to 64.3±5.2 pg/mg protein; p<0.05). Collectively, the data suggest that local hypertonicity in the mTAL is associated with an increase in COX-2 expression concomitant with elevated EP3 receptor expression, which attenuates COX-2 activity in this segment of the nephron. Moreover, PGE2 signaling via EP3 receptors in the TAL may be part of a mechanism that regulates mTAL COX-2 expression and function in response to diverse stimuli.

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