MEMBRANE-BOUND CARBONIC-ANHYDRASE IN HUMAN RETINAL-PIGMENT EPITHELIUM

Purpose. Inhibition of carbonic anhydrase (CA) by acetazolamide causes a decrease in the standing potential of the retinal pigment epitheliu m (RPE) and an increase in the rate of subretinal fluid absorption, an d it may improve cystoid macular edema. These effects are thought to b e mediated by the RPE. Given the solubility coefficient of acetazolami de, the drug is most likely to act by direct inhibition of membrane-bo und CA (CA TV). To identify a substrate for acetazolamide in the RPE, the distribution of CA activity and the isoform of CA in the RPE membr ane were investigated. Methods. Carbonic anhydrase activity was determ ined by Hansson's technique in fresh human eyes from donors of both se xes and different ages. The presence of the membrane-bound isoform CA IV was investigated immunohistochemically at the light and electron mi croscopic level, as well as by Western blotting in fresh RPE, and in a dult and fetal RPE cultures. Results. Hansson's histochemical method d emonstrated CA activity on the apical and basolateral cell membrane of the RPE. Using the gamma-globulin fraction of a polyclonal antibody a gainst pure CA IV, immunocytochemistry showed labeling for CA IV on th e apical RPE membrane of morphologically polarized human adult and fet al RPE cultures. Gel electrophoresis and Western blotting demonstrated a major immunoreactive band at 55 kDa in homogenates, which was consi stently reduced to approximately 35 kDa by incorporation of 0.1% Trito n X-100 detergent. Conclusions. These results suggest that the clinica l effects of carbonic anhydrase inhibitors on RPE function may be medi ated via membrane-bound carbonic anhydrase activity in RPE and that CA TV is responsible for activity on the apical surface.