Carbonic anhydrase isoenzymes in the rat kidney. Effects of chronic acetazolamide treatment.

Biochemical, immunocytochemical and histochemical methods were used to study the effect of chronic acetazolamide treatment on carbonic anhydrase (CA) isoenzymes in the rat kidney. Male inbred rats (Lew/Mol) were treated with 15 mg kg-1 day-1 acetazolamide s.c. by Alzet minipump during 2-9 weeks; some animals had a drug-free period of 1-4 weeks before being killed. The renal content of CA II was higher in the acetazolamide-treated rats than in the controls, 178 +/- 10 vs 144 +/- 4.8 micrograms enzyme protein g-1 tissue (mean +/- SE). The distribution of CA isoenzymes did not change during or after chronic acetazolamide treatment. Thus, only CA II was detected in the kidney tubules by immunofluorescence using specific antisera against CA I, CA II and CA III. All animals showed a similar staining pattern, with intense cytoplasmic CA II staining in intercalated cells of collecting ducts, moderate staining in descending thin limbs of Henle, and weak cytoplasmic staining in proximal tubules and chief cells of collecting ducts. All animals also showed histochemical staining of cell membranes in proximal and distal tubules and thick limbs of Henle, suggesting the presence of a membrane-bound isoenzyme (CA IV). The only difference noted by histochemistry and immunocytochemistry was that the intercalated cells appeared bulkier and protruded more markedly into the tubular lumen in treated than in untreated animals. The functional importance of this finding is unclear. The observed changes in CA cannot alone explain why the effect of acetazolamide, in causing loss of bicarbonate and sodium, is self-limited on continued administration.