Functional demonstration of surface carbonic anhydrase IV activity on rat astrocytes

Buffering of the brain extracellular fluid is catalyzed by carbonic anhydrase (CA) activity. Whereas the extracellular isoform CA XIV has been localized exclusively to neurons in the brain, and to glial cells in the retina, there has been uncertainty regarding the form or forms of CA on the surface of brain astrocytes. We addressed this issue using physiological methods on cultured and acutely dissociated rat astrocytes. Prior work showed that the intracellular lactate-induced acidification (LIA) of astrocytes is diminished by benzolamide, a poorly permeant, nonspecific CA inhibitor. We demonstrate that pretreatment of astrocytes with phosphatidylinositol-specific phospholipase C (PI-PLC) results in a similar inhibition of the mean LIA (by 66 +/- 3%), suggesting that the glycosylphosphatidylinositol-anchored CA IV was responsible. Pretreatment of astrocytes with CA IV inhibitory antisera also markedly reduced the mean LIA in both cultured cortical (by 46 +/- 4%) and acutely dissociated hippocampal astrocytes (by 54 +/- 8%). Pre-immune sera had no effect. The inhibition produced by PIPLC or CA IV antisera was not significantly less than that by benzolamide, suggesting that the majority of detectable surface CA activity was attributable to CA IV. Thus, our data collectively document the presence of CAIV on the surface of brain astrocytes, and suggest that this is the predominant CA isoform on these cells.