Inhibition of Na+/K+-ATPase by endothelin-1 in human nonpigmented ciliary epithelial cells

Endothelin-1 (ET-1), a potent vasoconstrictor, lowers intraocular pressure in mammals, either by enhancing the outflow of aqueous humor (AH) via the t rabecular meshwork and Schlemm's canal or by reducing AH formation at the c iliary 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. Present ly, we have evaluated the effects of ET-1 on Na+:K+:2Cl(-) cotransport and Na+/K+-ATPase activity in HNPE cells using Rb-86(+) uptake. ET-1 (100 pM-10 0 nM) decreased mean Rb-86(+) uptake by 15% during a 15-min uptake period. ET-1's effect was not prevented by BQ610, an ETA receptor antagonist, but w as blocked by BQ788, an ETB receptor antagonist. ET-1's effect was mimicked by sarafotoxin, an ETB agonist. ET-1-induced reduction in Rb-86(+) uptake was additive with bumetanide, a selective inhibitor of Na+:K+:2Cl(-) cotran sporter but not with ouabain, a selective inhibitor of the Na+/K+-ATPase. E T-1 did not affect iberiotoxin-sensitive maxi K+ channels. This suggests th at ET1-induced reduction in Rb-86(+) uptake is mediated through the inhibit ion of the Na+/K+-ATPase via an ETB-like receptor. These findings are consi stent with an ET-1 effect on active ion transport activity in HNPE cells th at could explain the reduction in aqueous humor production and the lowering of intraocular pressure.