CA II (cytosolic) and CA IV (membrane bound) are believed to facilitate renal HCO3 reabsorption and H+ secretion. We have recently cloned cDNAs encoding rabbit CA II and CA IV and have shown that the mRNAs are expressed abundantly in the kidney and are up-regulated during metabolic acidosis. However, the sites of expression and regulation along the nephron have not been elucidated. We performed reverse transcription-polymerase chain reaction (RT-PCR) on microdissected 1-2 mm nephron segments using methods previously developed in our laboratory (JCI 90:1275,1992). After RT, aliquots of each sample were amplified 40 cycles using primers specific to rabbit CA IV, CA II and malate dehydrogenase (MDH), respectively. Tubules from 3 d acidotic rabbits were run in parallel with those from control animals and with 500 ng total cortex RNA. Ethidium bromide stained gels and Southern autoradiograms were scanned, quantitated and complied. Data from 5-12 of each segment were expressed as a percentage of that obtained using 500 ng total RNA(100%) and compared statistically. CA IV mRNA was detected in IMCDi > OMCDi> S2 > S1 > OMCDo; no CA IV mRNA was seen in glomeruli, S3, DTL, ATL, mTAL, cTAL, DCT, CNT, CCD, or IMCDt. CA II mRNA was detected in OMCDo = OMCDi= S1 = S2 = CCD > IMCDi = IMCDt > S3 = DTL = CNT; no CA II mRNA was seen in glomeruli, ATL, mTAL, cTAL or DCT. MDH was expressed equally among all of the segments. Acidosis caused up-regulation of CA IV mRNA (p<0.05) in S1(44% to 79%), S2 (53% to 105%), and in OMCDi (54% to 97%) Acidosis also up-regulated CA II mRNA in S1 (35% to 79%), S2 (35% to 90%), S3 (27% to 43%), CCD (39% to 73%) and OMCDi (33% to 72%). Acidosis did not change MDH expression. These data show that CA II and CA IV are expressed in a limited number of nephron segments, those mainly involved (in the rabbit) in acid-base transport. CA II and IV mRNA are up-regulated primarily in proximal tubules and collecting ducts appropriate to the need to reabsorb HCO3 and secrete H+ during metabolic acidosis. Correlation at the protein level is needed.

Funded by American Heart Association NY State Affiliate Fellowship and NIH DK50603