Membrane-associated carbonic anhydrase activity in the brain of CA II-deficient mice

Membrane-associated carbonic anhydrase (CA) activity is of importance for transepithelial transport of ions and fluid. Histochemical studies have indicated its presence in the brain, but the data are difficult to evaluate because of interference from cytoplasmic CA isozymes, of which CA II is the predominant one. CA II-deficient mice offer a possibility to study the location of membrane-associated CA-activity, without interference from CA II. The location of CA activity in the brain of CA II-deficient and normal mice was studied by the cobalt-phosphate histochemical method, and that of CA I, CA II and CA III by an immunocytochemical method. The brains of both types of mice lacked cytoplasmic isozymes CA I and CA III, and the CA II-deficient mice also lacked CA II. In the normal mice, oligodendrocytes and choroid epithelium stained for CA II in the cytoplasm. In normal and CA (II)D-mice there was an intense membrane associated histochemical CA activity in neuronal processes. Neuronal perikarya were not stained. Endothelial membranes of brain capillaries showed strong histochemical CA-activity. Choroid epithelial cells had histochemical CA activity in the cytoplasm and along apical and baso-lateral cell membranes. The results suggest that membrane-associated CA-activity found along neuronal processes probably modulates pH of the extracellular fluid and thus neuronal activity. CA II and the membrane-associated CA of choroidal epithelium are probably involved in the secretion of cerebrospinal fluid.