Maturation of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV facilitates renal acidification by catalyzing th e dehydration of luminal H2CO3. CA IV is expressed in proximal tubules, med ullary collecting ducts, and A-intercalated cells of the mature rabbit kidn ey (Schwartz GJ, Kittelberger AM, Barnhart DA, and Vijayakumar S. Am J Phys iol 278: F894-F904, 2000). In view of the maturation of HCO3- transport in the proximal tubule and collecting duct, the ontogeny of CA IV expression w as examined. During the first 2 wk, CA IV mRNA was expressed in maturing co rtex and medulla at similar to 20% of adult levels. The maturational increa se was gradual in cortex over 3-5 wk of age but surged in the medulla, so t hat mRNA levels appeared higher than those in the adult medulla. In situ hy bridization showed very little CA IV mRNA at 5 days, with increases in deep cortex and medullary collecting ducts by 21 days. Expression of CA IV prot ein in the cortex and medulla was minimal at 3 days of age but then apparen t in the juxtamedullary region, A-intercalated cells and medullary collecti ng ducts by 18 days; there was little labeling of the proximal straight tub ules of the medullary rays. Thus CA IV expression may be regulated to accom modate the maturational increase in HCO3- absorption in the proximal tubule . In the medullary collecting duct, there is a more robust maturation of CA IV mRNA and protein, commensurate with the high rate of HCO3- absorption i n the neonatal segment.