Agonist-specific role for Na+-H+ antiport in prostaglandin release from microvessel endothelium

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Abstract
Rabbit coronary microvascular endothelial (RCME) cells synthesize prostaglandin (PG) I2 and PGE2 in response to stimulation with human thrombin, ATP, and the Ca2+ ionophore, A23187. Replacement of extracellular Na+ with choline or N-methylglucamine reduced thrombin-stimulated PG secretion but did not significantly affect either ATP- or A23187-stimulated PG secretion. Pretreatment of RCME cells with Na+ channel or Na+ -Ca2+ exchange blockers did not alter PG release in response to any of these three agonists. Pretreatment of RCME cells with the specific Na+ -H+ antiport blockers 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 microM) and 5-(N,N-hexamethylene)-amiloride (HMA, 0.1 microM) significantly reduced thrombin but not A23187- or ATP-stimulated PG secretion. Studies with the intracellular pH indicator dye 2,7-bis(carboxyethyl)-5(6)-carboxyfluorescein demonstrated thrombin activation of Na+ -H+ antiport, an effect blocked by either HMA or EIPA. Since manipulations known to inhibit Na+ -H+ exchange (EIPA, HMA, replacement of Na+ with choline or N-methylglucamine) reduced thrombin-stimulated RCME cell PG release, we conclude that activation of Na+ -H+ exchange is involved in the coupling of thrombin interaction with RCME cells to subsequent phospholipase activation and PG release.

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