Intercalated Cells of the Kidney Collecting Duct in Kidney Physiology.

The epithelium of the kidney collecting duct (CD) is composed mainly of two different types of cells with distinct and complementary functions. CD principal cells traditionally have been considered to have a major role in Na+ and water regulation, while intercalated cells (ICs) were thought to largely modulate acid-base homeostasis. In recent years, our understanding of IC function has improved significantly owing to new research findings. Thus, we now have a new model for CD transport that integrates mechanisms of salt and water reabsorption, K+ homeostasis, and acid-base status between principal cells and ICs. There are three main types of ICs (type A, type B, and non-A, non-B), which first appear in the late distal convoluted tubule or in the connecting segment in a species-dependent manner. ICs can be detected in CD from cortex to the initial part of the inner medulla, although some transport proteins that are key components of ICs also are present in medullary CD, cells considered inner medullary. Of the three types of ICs, each has a distinct morphology and expresses different complements of membrane transport proteins that translate into very different functions in homeostasis and contributions to CD luminal pro-urine composition. This review includes recent discoveries in IC intracellular and paracrine signaling that contributes to acid-base regulation as well as Na+, Cl-, K+, and Ca2+ homeostasis. Thus, these new findings highlight the potential role of ICs as targets for potential hypertension treatments.

Quantitative evaluation of Oryeongsan and its action on water regulation in renal inner medullary collecting duct cells.

ETHNOPHARMACOLOGICAL RELEVANCE: Oryeongsan (ORS, Wulingsan) has been reported to possess renal protective effects from renal diseases such as diabetes-induced renal damage, and nephrocalcinosis. AIM OF THE STUDY: This study was conducted to evaluate the quantitative analysis and the inhibitory effect of ORS on hypertonic stress-induced water channel and apoptosis in murine inner medullary collecting duct cell line (mIMCD-3). MATERIALS AND METHODS: Chromatographic and NMR spectroscopic analysis were performed and water balance regulation was determined by Western blot, RT-PCR, and immunofluorescnece. RESULTS: Seven active principles (5-hydroxymethylfurfural, alismoxide, methyl(-)trans-cinnamate, adenine, guanosine, adenosine, and ferulic acid) in ORS were isolated and the structures were identified mainly by NMR spectroscopic analysis. In addition, contents of these metabolites in ORS were evaluated by HPLC analysis. Pretreatment with ORS significantly attenuated the hypertonic stress (175mM NaCl)-induced increase in protein levels of AQP2 and apical membrane insertion. ORS also attenuated osmolyte sodium-myo-inositol transporter (SMIT) expression and tonicity-responsive enhancer binding protein (TonEBP) mRNA under hypertonic stress. Those actions of ORS presented the similar effect of PKA inhibitor which AQP2 expression throughout the inhibition of vasopressin-mediated cAMP/PKA signal pathway. On the other hand, pretreatment with ORS attenuated hypertonic stress-induced cell death. Hypertonic stress-induced Bax or caspase-3 expression was decreased by ORS, resulting in anti-apoptotic effect. CONCLUSIONS: The present data suggest that the beneficial effect of ORS in water balance and apoptosis against hypertonic stress of renal collecting ducts.

Effects of water restriction on gene expression in mouse renal medulla: identification of 3betaHSD4 as a collecting duct protein.

To identify novel gene targets of vasopressin regulation in the renal medulla, we performed a cDNA microarray study on the inner medullary tissue of mice following a 48-h water restriction protocol. In this study, 4,625 genes of the possible approximately 12,000 genes on the array were included in the analysis, and of these 157 transcripts were increased and 63 transcripts were decreased by 1.5-fold or more. Quantitative, real-time PCR measurements confirmed the increases seen for 12 selected transcripts, and the decreases were confirmed for 7 transcripts. In addition, we measured transcript abundance for many renal collecting duct proteins that were not represented on the array; aquaporin-2 (AQP2), AQP3, Pax-8, and alpha- and beta-Na-K-ATPase subunits were all significantly increased in abundance; the beta- and gamma-subunits of ENaC and the vasopressin type 1A receptor were significantly decreased. To correlate changes in mRNA expression with changes in protein expression, we carried out quantitative immunoblotting. For most of the genes examined, changes in mRNA abundances were not associated with concomitant protein abundance changes; however, AQP2 transcript abundance and protein abundance did correlate. Surprisingly, aldolase B transcript abundance was increased but protein abundance was decreased following 48 h of water restriction. Several transcripts identified by microarray were novel with respect to their expression in mouse renal medullary tissues. The steroid hormone enzyme 3beta-hydroxysteroid dehydrogenase 4 (3betaHSD4) was identified as a novel target of vasopressin regulation, and via dual labeling immunofluorescence we colocalized the expression of this protein to AQP2-expressing collecting ducts of the kidney. These studies have identified several transcripts whose abundances are regulated in mouse inner medulla in response to an increase in endogenous vasopressin levels and could play roles in the regulation of salt and water excretion.

Structure, promoter analysis, and chromosomal localization of the murine H(+)/K(+)-ATPase alpha 2 subunit gene.

The H(+)/K(+)-ATPase alpha2 subunit (HK alpha 2) of distal colon and renal collecting ducts plays a critical role in potassium and acid-base homeostasis. The isolation and complete sequence of the murine HK alpha 2 gene are reported. The HK alpha 2 gene contains 23 exons and spans 23.5 kb of genomic DNA. The exon/intron organization is comparable to that of the human ATP1AL1 gene. Primer extension and 5'-rapid amplification of cDNA ends of distal colon RNA were used to map the transcription initiation site. Fluorescence in situ hybridization analysis localized the HK alpha 2 gene to murine chromosome 14C3. Sequence analysis of 7.2 kb of the 5'-flanking region revealed numerous consensus sites for transcription factors, including two potential glucocorticoid response elements. Transient transfection of promoter-luciferase constructs demonstrated strong basal HK alpha 2 promoter activity in renal collecting duct cells but not in fibroblasts or in a medullary thick ascending limb of Henle's loop cell line. Deletion analysis revealed that the proximal 0.2 kb of the promoter was sufficient to confer activity in collecting duct cells. These data should prove important in elucidation of the mechanisms controlling the differential, tissue-specific expression of the HK alpha 2 gene.

Molecular identification of the renal H+,K+-ATPases.

The pharmacological properties of H+,K+-ATPase activity described in the kidney were not necessarily consistent with the properties of the well-characterized gastric H+,K+-ATPase. Recent molecular biology experiments suggest that renal H+,K+-ATPase activity may be the product of several closely related P-type ATPases. At least 3 different pumps containing the HKalpha1, HKalpha2a, and HKalpha2c subunits have been detected in rabbit kidney. The current view is that these HKalpha subunits arose through gene duplication early in evolution and the proteins evolved their differing activities over time. The HKbeta protein associates with HKalpha1 in gastric tissues and is the likely mate for the HKalpha1 subunit in renal tissues. Three distinct beta subunits have been implicated as possible partners for the HKalpha2 subunits, but it remains to be determined which beta subunit predominantly associates with the HKalpha2 subunits in vivo. Sequence analysis suggests the beta subunit was constrained by size and shape of the protein rather than specific amino acid content during the course of evolution. Multiple H+,K+-ATPases in the kidney may be an important adaptation providing redundancy for the essential physiological function of maintaining ionic balance.

Something fishy in the rat brain: molecular genetics of the hypothalamo-neurohypophysial system.

The brain peptides vasopressin and oxytocin play crucial roles in the regulation of salt and water balance. The genes encoding these neurohormones are regulated by cell-specific and physiological cues, but the molecular mechanisms remain obscure. New strategies, involving the introduction of rat transgenes into rats, are being used to address these issues, but the complexity of the rat genome has hampered progress. By contrast, the pufferfish, Fugu rubripes, has a "junk-free" genome. The oxytocin homologue from Fugu, isotocin, has been introduced into rats and is expressed in oxytocin neurons, where it is upregulated by physiological perturbations that upregulate the oxytocin gene. The Fugu and rat lineages separated 400 million years ago, yet the mechanisms that regulate the isotocin and oxytocin genes have been conserved. Fugu genome analysis and transgenesis in the physiologically tractable rat host are a powerful combination that will enable the identification of fundamental components of the neural systems that control homeostasis.

Hyperosmolality stimulates Na-K-ATPase gene expression in inner medullary collecting duct cells.

Primary cultures of inner medullary collecting duct (IMCD) cells of rats were incubated in hyperosmotic media to determine the effects on Na-K-ATPase alpha 1- and beta 1-subunit mRNA expression. Osmolality of the incubation media was raised from 300 up to 500 mosmol/kgH2O by adding NaCl, mannitol, raffinose, or urea. Hyperosmotic media supplemented with NaCl, mannitol, or raffinose caused two- to fourfold increases in the alpha 1-subunit mRNA accumulation and five- to eightfold increases in the beta 1-subunit mRNA accumulation, with peak elevations of both subunits at 12 h after addition. In sharp contrast, hyperosmolar urea medium had no effect at any time. When NaCl or mannitol was added to the media in amounts ranging from 300 to 600 mosmol/kgH2O, the maximal effects on both alpha 1- and beta 1-subunit mRNA accumulation occurred at 500 mosmol/kgH2O. In urea-supplemented medium, however, there was no significant change at any level of osmolality. The upregulation of alpha 1- and beta 1-subunit mRNA induced by hyperosmotic mannitol- or raffinose-supplemented media was markedly inhibited by removal of Na from the culture medium. Furthermore, pretreatment with a protein synthesis inhibitor cycloheximide partially inhibited the upregulation of alpha 1- and beta 1-subunit mRNA in IMCD cells exposed to hyperosmotic media treated with NaCl or mannitol. When IMCD cells were incubated with hyperosmotic media (500 mosmol/kgH2O) supplemented with NaCl or mannitol for 24 h, Na-K-ATPase activity increased by 78.6 and 82.8%, respectively. In contrast, hyperosmolar urea medium had no significant effect on Na-K-ATPase activity. These results demonstrate that 1) hyperosmolality induced by the poorly permeating solutes (NaCl, mannitol, and raffinose) but not the rapidly permeating solute (urea) stimulates both alpha 1- and beta 1-subunit mRNA accumulations in IMCD cells in a time- and an osmolality-dependent manner, 2) the hyperosmolality-induced upregulation of alpha 1- and beta 1-subunit mRNA leads to an increase in Na- K -ATPase activity; and 3) the above upregulation of alpha1- and beta 1-subunit mRNA in response to hyperosmotic media requires, at least in part, the presence of Na in the extracellular medium and the de novo synthesis of intermediate proteins.

Cloning of a pH-sensitive K+ channel possessing two transmembrane segments.

The mammalian renal collecting ducts are responsible for secreting potassium ions into the urine and are a major regulatory site for potassium homeostasis, in which a voltage-independent pH-sensitive K+ channel in the apical membrane plays a central role. Here we describe a complementary DNA encoding a novel K+ channel from rabbit renal cortical collecting tubule cells (RACTK1). RACTK1 has the functional characteristics of the apical K(+)-permeable channel and consists of 284 amino acids, putatively with two transmembrane segments. The sequence of RACTK1, however, shows no homology to known voltage-dependent or -independent K+ channels, and has a different K(+)-driving path and regulatory sites. The study of this protein should provide insight into K+ homeostasis and diseases of K+ metabolism.

Interrelationship between cell pH and cell calcium in rat inner medullary collecting duct cells.

In this study we investigated the interrelationship between cell pH (pHi) and cell calcium (Cai) in cultured inner medullary collecting duct cells of the rat. Confluent monolayers were made quiescent by incubation for 24 h in Dulbecco's modified Eagle's medium supplemented with 0.1% serum before study. Changes in pHi and Cai were measured with the fluorescent probes 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein and fura 2. In nominally bicarbonate-free media containing 110 mM Na+ and 1 mM Cai, cell acidification to pH 6.70 increased Cai from 122 +/- 24 to 243 +/- 33 nM. In the absence of bath calcium, acidification increased Cai from 90 +/- 7 to 144 +/- 13 nM. An increase of pHi to 7.6 reduced Cai almost to baseline. Cell acidification increased inositol trisphosphate (IP3) production, and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester, an IP3 antagonist, partially inhibited the rise in Cai. Elevation of Cai resulted in a sustained cell alkalinization from 7.32 +/- 0.02 to 7.58 +/- 0.04. When Cai was reduced, pHi fell to 7.25 +/- 0.01. We conclude that Cai and pHi participate in a feedback loop that modulates changes in each respective parameter.

Role of mineralocorticoids in adaptation of rabbit cortical collecting duct after loss of renal mass.

The mechanism of compensatory adaptation and hypertrophy of the cortical collecting duct (CCD) was studied by in vitro microperfusion technique after surgical loss of functioning nephrons in the rabbit. Sodium transport was increased at 1 wk (lumen-to-bath sodium transport of 127 +2- 9 vs. 61 +/- 11 pmol.mm-1.min-1 in sham-operated animals, P less than 0.01) and 3 wk (111 +/- 19 vs. 54 +/- 7 pmol.mm-1.min-1, P less than 0.05) but not 16 h (81 +/- 13 vs. 78 +/- 8 pmol.mm-1.min-1) after loss of renal mass. The functional adaptation was accompanied by an increase in the size of the CCD. A rise in plasma aldosterone levels preceded and accompanied the increased sodium transport rate. Adrenalectomy at the time of reduction of renal mass totally prevented the development of both hypertrophy and sodium transport adaptation (sodium transport 15 +/- 8 pmol.mm-1.min-1). When adrenalectomy was combined with clamping the plasma aldosterone level in the nonstressed physiological range, compensatory hypertrophy and adaptation also failed to develop (sodium transport, 66 +/- 10 pmol.mm-1.min-1), but with high-dose aldosterone replacement both the hypertrophy and adaptation of sodium transport (130 +/- 23 pmol.mm-1.min-1) were restored. The results document the importance of increased mineralocorticoid activity in the development of compensatory hypertrophy and adaptation of sodium transport in rabbit CCD after loss of renal mass.