CARBONIC-ANHYDRASE-II AND CARBONIC-ANHYDRASEIV MESSENGER-RNA IN RABBIT NEPHRON SEGMENTS - STIMULATION DURING METABOLIC-ACIDOSIS

Carbonic anhydrase (CA) facilitates renal bicarbonate reabsorption and acid excretion. Cytosolic CA II Catalyzes the buffering of intracellu lar hydroxyl ions by CO2, whereas membrane-bound CA IV catalyzes the d ehydration of carbonic acid generated from the secretion of protons. A lthough CA II and IV are expressed in rabbit kidney, it is not entirel y clear which segments express which isoforms. It was the purpose of t his study to characterize the expression of CA II and CA IV mRNAs by s pecific segments of the nephron using semiquantitative reverse transcr iption-polymerase chain reaction (RT-PCR) and to determine the effect of chronic metabolic acidosis on CA expression by those segments. Indi vidual nephron segments (usually 1-2 mm) were isolated by microdissect ion and subjected to RT-PCR. Amplification was performed simultaneousl y for CA IV, CA II. and malate dehydrogenase (MDH), a housekeeping gen e. The intensities of the PCR products were quantitated by densitometr y. CA IV mRNA was expressed by S1 and S2 proximal tubules and by outer medullary collecting duct from inner stripe (OMCDi) and outer stripe and initial inner medullary collecting duct (IMCDi). CA II mRNA was ex pressed by S1, S2, and S3 proximal tubules, thin descending limb, conn ecting segment (CNT), and all collecting duct segments. Acid loading i nduced CA IV mRNA expression in S1 and S2 proximal tubules and in OMCD i and IMCDi. CA II mRNA was induced by acidosis in all three proximal segments and nearly all distal segments beginning with CNT No upregula tion of MDH mRNA expression occurred. These adaptive increases in CA I I and IV mRNAs are potentially important in the kidney's adaptation to chronic metabolic acidosis.