Model of ionic transport for bovine ciliary epithelium: effects of acetazolamide and HCO3-

The possible existence of transepithelial bicarbonate transport across the isolated bovine ciliary body was investigated by employing a chamber that a llows for the measurement of unidirectional, radiolabeled fluxes of CO2 + H CO3-. No net flux of HCO3- was detected. However, acetazolamide (0.1 mM) re duced the simultaneously measured short-circuit current (Isc). In other exp eriments in which Cl-36(-) was used, a net Cl- flux of 1.12 mu eq.h(-1).cm( -2) (30 muA/cm(2)) in the blood-to-aqueous direction was detected. Acetazol amide, as well as removal of HCO3- from the aqueous bathing solution, inhib ited the net Cl- flux and I-sc. Because such removal should increase HCO3- diffusion toward the aqueous compartment and increase the I-sc, this parado xical effect could result from cell acidification and partial closure of Cl - channels. The acetazolamide effect on Cl- fluxes can be explained by a re duction of cellular H+ and HCO3- (generated from metabolic CO2 production), which exchange with Na+ and Cl- via Na+/H+ and Cl-/HCO3- exchangers, contr ibuting to the net Cl- transport. The fact that the net Cl- flux is about t hree times larger than the I-sc is explained with a vectorial model in whic h there is a secretion of Na+ and K+ into the aqueous humor that partially subtracts from the net Cl- flux. These transport characteristics of the bov ine ciliary epithelium suggest how acetazolamide reduces intraocular pressu re in the absence of HCO3- transport as a driving force for fluid secretion .