Vol. 296, Issue 3, 966-971, March 2001

Inhibition of Na⁺/K⁺-ATPase by Endothelin-1 in Human Nonpigmented Ciliary Epithelial Cells

Ganesh Prasanna, Adnan Dibas, Christina Hulet and Thomas Yorio

Department of Pharmacology, University of North Texas Health Science Center, Fort Worth, Texas (G.P., C.H., T.Y.); and Department of Immunology, St. Paul Medical Center, Mary Kay Ash Institute for Cancer Research, Dallas, Texas (A.D.)

Endothelin-1 (ET-1), a potent vasoconstrictor, lowers intraocular pressure in mammals, either by enhancing the outflow of aqueous humor (AH) via the trabecular meshwork and Schlemm's canal or by reducing AH formation at the ciliary epithelium. Aqueous humor production occurs by passive diffusion of water coupled with active transport of ions, mainly involving Na⁺:K⁺:2Cl cotransporter and Na⁺/K⁺-ATPase pump from serosal to aqueous side. Presently, we have evaluated the effects of ET-1 on Na⁺:K⁺:2Cl cotransport and Na⁺/K⁺-ATPase activity in HNPE cells using ⁸⁶Rb⁺ uptake. ET-1 (100 pM-100 nM) decreased mean ⁸⁶Rb⁺ uptake by 15% during a 15-min uptake period. ET-1's effect was not prevented by BQ610, an ET_A receptor antagonist, but was blocked by BQ788, an ET_B receptor antagonist. ET-1's effect was mimicked by sarafotoxin, an ET_B agonist. ET-1-induced reduction in ⁸⁶Rb⁺ uptake was additive with bumetanide, a selective inhibitor of Na⁺:K⁺:2Cl cotransporter but not with ouabain, a selective inhibitor of the Na⁺/K⁺-ATPase. ET-1 did not affect iberiotoxin-sensitive maxi K⁺ channels. This suggests that ET-1-induced reduction in ⁸⁶Rb⁺ uptake is mediated through the inhibition of the Na⁺/K⁺-ATPase via an ET_B-like receptor. These findings are consistent with an ET-1 effect on active ion transport activity in HNPE cells that could explain the reduction in aqueous humor production and the lowering of intraocular pressure.