Developmental changes in carbonic anhydrase II in the rat kidney

We examined the distribution and maturational changes of carbonic anhydrase I (CAI) and carbonic anhydrase II (CAII) in microdissected nephron segments of Sprague-Dawley rats. CAI and CAII proteins were measured by enzyme-linked immunosorbent assay. CAI was not detected in any nephron segment in 7-week-old rats. CAII was present in the collecting ducts, proximal tubules, and thick ascending limbs of loop of Henle in 7-week-old rats. CAII contents were significantly higher in the early proximal tubules (S1) than in second (S2) and late (S3) portions of the proximal tubules, while the contents in S1 were less than in cortical collecting ducts (CCD), outer stripe and inner stripes of the outer medullary collecting ducts (OMCDo and OMCDi). CAII content in each of S1, CCD, and OMCD of 1-week-old rats was only 14% or less of that of adults, but increased steeply during the 2nd and 3rd weeks of life, reaching almost 40% at 3 weeks of age and 97% at 7 weeks. Our results indicate that CAII is present throughout the entire nephron of the rat, and that CAII content in S1, CCD, and OMCD increases exponentially during the first 7 weeks of life. Our data suggest that the immature low levels of CAII may explain, at least in part, the limited capacity of urinary acidification during neonatal life. Further studies are necessary to establish the role of such changes in CAII content in acid-base homeostasis during neonatal life.