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; [Ca.sup.2+] activity and pH were monitored with fura 2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, respectively. Timolol (10 [micro]M) produced similar K and Cl losses from ciliary epithelia in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] solution but had no effect in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-free solution or in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] solution containing the carbonic anhydrase inhibitor acetazolamide. Inhibition of [Na.sup.+]/[H.sup.+] exchange by dimethylamiloride in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] solution reduced Cl and K comparably to timolol, cAMP did not reverse timolol's effects. Timolol (100 nM, 10 [micro]M) and levobunolol (10 [micro]M) produced cAMP-independent inhibition of the regulatory volume increase (RVI) in PE cells and increased intracellular [Ca.sup.2+] and pH. Increasing [Ca.sup.2+] with ionomycin also blocked the RVI. The results document a previously unrecognized cAMP-independent transport effect of timolol. Inhibition of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] exchange may mediate timolol's inhibition of aqueous humor formation. electron probe X-ray microanalysis; cell volume; cell pH; cell calcium; chloride/bicarbonate exchanger; sodium/hydrogen exchanger