Timolol may inhibit aqueous humor secretion by cAMP-independent action on ciliary epithelial cells

The beta -adrenergic antagonist timolol reduces ciliary epithelial secretion in glaucomatous patients. Whether inhibition is mediated by reducing cAMP is unknown. Elemental composition of rabbit ciliary epithelium was studied by electron probe X-ray microanalysis. Volume of cultured bovine pigmented ciliary epithelial (PE) cells was measured by electronic cell sizing; Ca2+ activity and pH were monitored with fura 2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. Timolol (10 muM) produced similar K and Cl losses from ciliary epithelia in HCO3-/CO2 solution but had no effect in HCO3-/ CO2-free solution or in HCO3-/CO2 solution containing the carbonic anhydrase inhibitor acetazolamide. Inhibition of Na+/H+ exchange by dimethylamiloride in HCO3-/CO2 solution reduced Cl and K comparably to timolol. cAMP did not reverse timolol's effects. Timolol (100 nM, 10 muM) and levobunolol (10 muM) produced cAMP-independent inhibition of the regulatory volume increase (RVI) in PE cells and increased intracellular Ca2+ and pH. Increasing Ca2+ with ionomycin also blocked the RVI. The results document a previously unrecognized cAMP-independent transport effect of timolol. Inhibition of Cl-/HCO3- exchange may mediate timolol's inhibition of aqueous humor formation.