Intrarenal distribution of the colonic H,K-ATPase mRNA in rabbit.

BACKGROUND: Evidence suggests that the colonic H,K-ATPase isoform is expressed in the kidney and that a mRNA species highly homologous to the rat and guinea pig HKalpha2 is expressed in the cortical collecting duct (CCD) of the rabbit. The goals of this study were to determine if this mRNA is the rabbit homologue of HKalpha2 or a novel isoform and to determine intrarenal distribution of the HKalpha2 mRNA in rabbit. METHODS: 5'-RACE and Dye Deoxy Terminator chemistry were used to determine the full-length sequence of the rabbit HKalpha2 mRNA. The intrarenal distribution of HKalpha2 mRNA was determined in microdissected nephron segments, connecting tubule (CNT), and CCD cells isolated by immunodissection, as well as in the three cell types of the CCD. Principal cells and alpha- and beta-intercalated cells were isolated by fluorescence-activated cell sorting. HKalpha2 mRNA levels were determined by quantitative reverse transcription-polymerase chain reaction (RT-PCR) or single-nephron RT-PCR (SN-RTPCR). RESULTS: The full-length sequence of the rabbit kidney HKalpha2 mRNA was determined. This transcript is identical to the one expressed in rabbit distal colon. In microdissected nephron segments, strong HKalpha2 amplicons were present in the CNT, CCD, and outer medullary collecting duct (OMCD), whereas no signal was detected in the proximal tubule, distal convoluted tubule, think ascending limb, and inner medullary collecting duct. Roughly comparable levels of HKalpha2 mRNA were present in all three CCD cell types, and the highest levels were observed in a subpopulation most likely corresponding to CNT cells. CONCLUSIONS: These results suggest that the HKalpha2 mRNA is expressed in rabbit collecting duct is identical in size and sequence to the one expressed in rabbit distal colon. HKalpha2 mRNA in the rabbit kidney is selectively expressed in the CNT, CCD, and OMCD, and all three collecting duct subtypes express its mRNA.

Expression of colonic H-K-ATPase mRNA in cortical collecting duct: regulation by acid/base balance.

In addition to the gastric isoform of H-K-ATPase, the colonic isoform is also expressed in the kidney, but its intrarenal localization and exact function are not known. The goal of this study was to determine whether the colonic H-K-ATPase is expressed in the rabbit cortical collecting duct (CCD) and whether it is regulated by changes in acid/base balance. With quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) with RNA isolated from immunodissected rabbit CCD cells and degenerate oligonucleotide primers, a PCR product of the predicted size (approximately 430 bp) was amplified. The amplified DNA was further characterized by nested PCR and sequencing. Direct sequencing of the 434-bp PCR product revealed 83% identity at the nucleotide level and an 80.4% identity at the deduced amino acid level to the rat colonic H-K-ATPase. With the same primers and cDNA originating from rabbit distal colon, a DNA fragment with a size and nucleotide sequence identical to that originating from CCD cells was amplified. Furthermore, using PCR screening, we isolated and sequenced a 1.5-kb cDNA clone from a rabbit CCD library. The predicted amino acid sequence of the protein encoded by this cDNA is 85 and 82% identical to the corresponding regions of the guinea pig and rat colonic H-K-ATPase, respectively, and 70% identical to the H-K-ATPase recently cloned from Bufo marinus, whereas it shows only 45 and 42% homology to the rat Na-K-ATPase alpha 1-subunit and the rat gastric H-K-ATPase, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression cloning of the aldosterone target cell-specific 11 beta-hydroxysteroid dehydrogenase from rabbit collecting duct cells.

11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) is thought to confer aldosterone specificity to mineralocorticoid target cells by protecting the mineralocorticoid receptor from occupancy by endogenous glucocorticoids. We have recently described a novel isoform of 11-OHSD in the renal aldosterone target cells (11 beta-OHSD/CD) that differs from the previously characterized isoform (11 beta-OHSD-1). Unlike 11-OHSD-1, the collecting duct enzyme catalyzes irreversible dehydrogenation, has a very high affinity for its substrate, and is tissue-specific. We report here the isolation, sequence, and characterization of a complementary DNA (cDNA) encoding the rabbit collecting duct 11 beta-OHSD/CD or 11 beta-OHSD type 2. The cDNA, isolated using expression screening in Xenopus oocytes, is 1.9 kilobases in length and encodes a protein of 406 amino acids with a predicted molecular mass of 44,130 daltons. The cloned enzyme has a Michaelis constant (Km) for corticosterone of 6.6 +/- 3 nM, catalyzes exclusively dehydrogenation, and uses only NAD as cofactor. The cloned enzyme shows 85% and 75% amino acid identity to the recently cloned human type 2 11 beta-OHSD and sheep kidney 11 beta-OHSD, respectively, whereas the overall homology to rat liver 11 beta-OHSD-1 is less than 20% The messenger RNA for this 11 beta-OHSD is expressed at very high levels in the renal collecting duct and at much lower levels in the colon. The intrarenal distribution was determined by reverse-transcription polymerase chain reaction in isolated nephron segments or cell types. The messenger RNA is present only in aldosterone target cells within the kidney, at highest levels in principal cells, at lower levels in intercalated cells, and in inner medullary cells. These data suggest that the 11 beta-OHSD cDNA from rabbit collecting duct cells encodes the enzyme that confers aldosterone selectivity to mineralocorticoid target cells.