Abstract
Adrenoceptor-mediated Cl- transport in cultured rabbit corneal endothelium was examined using a Cl--sensitive fluorescent dye. The intracellular Cl- concentration ([Cl-]i) in the endothelial cells was estimated to be about 30 mM. Noradrenaline (0.001-0.1 mM) transiently decreased the [Cl-]i in a dose-dependent manner. Such a decrease in [Cl--]i was completely antagonized by pretreatment with the α-adrenoceptor antagonist phentolamine (0.1 mM). The selective α2-adrenoceptor agonist UK 14304-18 (5-bromo-6-[(4H, 5H-imidazol-2-yl)amino]quinoxaline, 0.1 mM) persistently decreased the [Cl-]i, but neither the α1-adrenoceptor agonist phenylephrine (0.1 mM) nor the β-adrenoceptor agonist isoproterenol (0.1 mM) had any effect. The α2-adrenoceptor agonist/antagonist yohimbine (0.1 mM) persistently and more strongly decreased the [Cl-]i than UK 14304-18 did. The yohimbine-induced decrease in the [Cl-]i was not further altered by UK 14304-18 or phenylephrine, but partly reversed by noradrenaline, isoproterenol and an adenylate cyclase activator, forskolin (0.1 mM). The yohimbine-induced decrease in [Cl-]i was inhibited by the carbonic anhydrase inhibitor acetazolamide (1 mM), and Cl--/HC03- exchange inhibitors, 4-acetamido-4'' -isothiocyanostilbene-2, 2'' disulfonic acid and 4, 4'' -diisothiocyanostilbene-2, 2''-disulfonic acid, but not by the H+-ATPase inhibitor N, N'' dicyclohexylcarbodiimide. The forskolin-induced recovery in [Cl-]i was inhibited by the Na+/K+/Cl- cotransport inhibitor bumetanide (0.1 mM), but not by the Cl-channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid. These findings suggest that corneal endothelial cells extrude Cl- upon α2-adrenoceptor stimulation and accumulate Cl- upon β-adrenoceptor stimulation under low [Cl-]i conditions, probably via acceleration of Cl-/HC03- exchange and Na+/K+/ Cl- cotransport, respectively.
