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N Farman

Publications and source records attributed to N Farman.

At least 19 recordsLinked to original sources
PubMed2001

Functional role and immunocytochemical localization of the gamma a and gamma b forms of the Na,K-ATPase gamma subunit.

The gamma subunit of the Na,K-ATPase is a member of the FXYD family of type 2 transmembrane proteins that probably function as regulators of ion transport. Rat gamma is present primarily in the kidney as two main splice variants, gamma(a) and gamma(b), which differ only at their extracellular N termini (TELSANH and MDRWYL, respectively; Kuster, B., Shainskaya, A., Pu, H. X., Goldshleger, R., Blostein, R., Mann, M., and Karlish, S. J. D. (2000) J. Biol. Chem. 275, 18441-18446). Expression in cultured cells indicates that both variants affect catalytic properties, without a detectable difference between gamma(a) and gamma(b). At least two singular effects are seen, irrespective of whether the variants are expressed in HeLa or rat alpha1-transfected HeLa cells, i.e. (i) an increase in apparent affinity for ATP, probably secondary to a left shift in E(1) <--> E(2) conformational equilibrium and (ii) an increase in K(+) antagonism of cytoplasmic Na(+) activation. Antibodies against the C terminus common to both variants (anti-gamma) abrogate the first effect but not the second. In contrast, gamma(a) and gamma(b) show differences in their localization along the kidney tubule. Using anti-gamma (C-terminal) and antibodies to the rat alpha subunit as well as antibodies to identify cell types, double immunofluorescence showed gamma in the basolateral membrane of several tubular segments. Highest expression is in the medullary portion of the thick ascending limb (TAL), which contains both gamma(a) and gamma(b). In fact, TAL is the only positive tubular segment in the medulla. In the cortex, most tubules express gamma but at lower levels. Antibodies specific for gamma(a) and gamma(b) showed differences in their cortical location; gamma(a) is specific for cells in the macula densa and principal cells of the cortical collecting duct but not cortical TAL. In contrast, gamma(b) but not gamma(a) is present in the cortical TAL only. Thus, the importance of gamma(a) and gamma(b) may be related to their partially overlapping but distinct expression patterns and tissue-specific functions of the pump that these serve.

Adenosine Triphosphate↗
PubMed2001

Vasopressin regulates water flow in a rat cortical collecting duct cell line not containing known aquaporins.

Transepithelial water movements and arginine-vasopressin (AVP)-associated ones were studied in a renal cell line established from a rat cortical collecting duct (RCCD(1)). Transepithelial net water fluxes (J(w)) were recorded every minute in RCCD(1) monolayers cultured on permeable supports. Spontaneous net water secretion was observed, which was inhibited by serosal bumetanide (10(-5) m), apical glibenclamide (10(-4) m) and apical BaCl(2) (5 x 10(-3) m). RT-PCR, RNAse protection and/or immunoblotting experiments demonstrated that known renal aquaporins (AQP1, AQP2, AQP3, AQP4, AQP6 and AQP7) were not expressed in RCCD(1) cells. AVP stimulates cAMP production and sodium reabsorption in RCCD(1) cells. We have now observed that AVP significantly reduces the spontaneous water secretory flux. The amiloride-sensitive AVP-induced increase in short-circuit current (I(sc)) was paralleled by a simultaneous modification of the observed J(w): both responses had similar time courses and half-times (about 4 min). On the other hand, AVP did not modify the osmotically driven J(w) induced by serosal hypertonicity. We can conclude that: (i) transepithelial J(w) occurs in RCCD(1) cells in the absence of known renal aquaporins; (ii) the "water secretory component" observed could be linked to Cl- and K = secretion; (iii) the natriferic response to AVP, preserved in RCCD(1) cells, was associated with a change in net water flux, which was even observed in absence of AQP2, AQP3 or AQP4 and (iv) the hydro-osmotic response to AVP was completely lost.

Animals↗
PubMed2001

Multiple aspects of mineralocorticoid selectivity.

Aldosterone regulates renal sodium reabsorption through binding to the mineralocorticoid receptor (MR). Because the glucocorticoid receptor (GR) is expressed together with the MR in aldosterone target cells, glucocorticoid hormones bound to GR may also intervene to modulate physiological functions in these cells. In addition, each steroid can bind both receptors, and the MR has equal affinity for aldosterone and glucocorticoid hormones. Several cellular and molecular mechanisms intervene to allow specific aldosterone regulatory effects, despite the large prevalence of glucocorticoid hormones in the plasma. They include the local metabolism of the glucocorticoid hormones into inactive derivatives by the enzyme 11beta-hydroxysteroid dehydrogenase; the intrinsic properties of the MR that discriminate between ligands through differential contacts; the possibility of forming homo- or heterodimers between MR and GR, leading to differential transactivation properties; and the interactions of MR and GR with other regulatory transcription factors. The relative contribution of each of these successive mechanisms may vary among aldosterone target cells (epithelial vs. nonepithelial) and according to the hormonal context. All these phenomena allow fine tuning of cellular functions depending on the degree of cooperation between corticosteroid hormones and other factors (hormonal or tissue specific). Such interactions may be altered in pathophysiological situations.

11-beta-Hydroxysteroid Dehydrogenase Type 2↗
PubMed2001

Location and function of the epithelial Na channel in the cochlea.

In the cochlea, endolymph is a K-rich and Na-poor fluid. The purpose of the present study was to check the presence and to assess the role of epithelial Na channel (ENaC) in this organ. alpha-, beta-, and gamma-ENaC subunit mRNA, and proteins were detected in rat cochlea by RT-PCR and Western blot. alpha-ENaC subunit mRNA was localized by in situ hybridization in both epithelial (stria vascularis, spiral prominence, spiral limbus) and nonepithelial structures (spiral ligament, spiral ganglion). The alpha-ENaC-positive tissues were also positive for beta-subunit mRNA (except spiral ganglion) or for gamma-subunit mRNA (spiral limbus, spiral ligament, and spiral ganglion), but the signals of beta- and gamma-subunits were weaker than those observed for alpha-subunit. In vivo, the endocochlear potential was recorded in guinea pigs under normoxic and hypoxic conditions after endolymphatic perfusion of ENaC inhibitors (amiloride, benzamil) dissolved either in K-rich or Na-rich solutions. ENaC inhibitors altered the endocochlear potential when Na-rich but not when K-rich solutions were perfused. In conclusion, ENaC subunits are expressed in epithelial and nonepithelial cochlear structures. One of its functions is probably to maintain the low concentration of Na in endolymph.

Action Potentials↗
PubMed2001

Membrane topology and immunolocalization of CHIF in kidney and intestine.

Corticosteroid hormone-induced factor (CHIF) is an aldosterone-induced gene, the function of which is yet unknown. It is specifically expressed in kidney collecting duct (CD) and distal colon and is upregulated by either Na+ deprivation or K+ loading. Hence, it may play a role in epithelial electrolyte transport. Previous studies have characterized regulation and tissue distribution of CHIF mRNA but provided no information on the protein itself. The present paper addresses this issue by using Western blotting, immunochemistry, and in vitro translation. CHIF is an approximately 8-kDa membranal protein, and protease digestion experiments suggest that its COOH tail faces the cell interior. The protein is abundant in distal colon, kidney medulla, and papilla but cannot be detected in a variety of other tissues. Confocal immunocytochemistry demonstrates that CHIF is present in the basolateral membrane of CD principal cells and distal colon surface cells, with occasional intracellular staining. Dexamethasone and low Na+ intake increase the abundance of CHIF. Unlike previous Northern data, induction of CHIF protein by low-Na+ intake was apparent not only in the distal colon but also in the kidney.

Animals↗
PubMed2001

Coordinate control of Na,K-atpase mRNA expression by aldosterone, vasopressin and cell sodium delivery in the cortical collecting duct.

We have examined the respective influence of aldosterone, vasopressin and cell sodium delivery on Na+,K+-ATPase expression. The level of expression of the mRNA encoding for the alpha1- and beta1-subunits of Na+,K+-ATPase was evaluated in cortical collecting duct (CCD) cells from rats under different aldosterone status, in cells from the rat CCD cell line RCCD1 treated or not with vasopressin and in CCD cells from mice inactivated or not for the a-subunit of the epithelial sodium channel. The amount of mRNA was determined by in situ hybridization. Both aldosterone and vasopressin up-regulate transcripts encoding for the alpha1-subunit of Na+,K+-ATPase while beta1 is unaltered. Interestingly, when cell sodium entry was largely reduced (alphaENaC knock-out mice), the amount of transcripts encoding for the alpha1-subunit of Na+,K+-ATPase was significantly decreased in spite of high plasma aldosterone concentrations. No effect was observed on beta1-subunit. Altogether, these results suggest a coordinated hormonal and ionic control of Na+,K+-ATPase expression by different transcriptional pathways (steroid-receptor, cAMP-dependent and Na+dependent) in CCD cells. These regulations affect only alpha1-subunit of Na,K+-ATPase but not beta1.

Adrenalectomy↗
PubMed2000

Introduction

Explore the source record for details and available documents.

Journal Article↗
PubMed2000

Mineralocorticoid selectivity: molecular and cellular aspects.

Aldosterone acts in mineralocorticoid-sensitive cells by binding to the mineralocorticoid receptor (MR). Because the MR displays similar affinity for aldosterone and glucocorticoid hormones and because these latter hormones are 100- to 1000-fold more abundant than aldosterone in the plasma, mechanisms are required to avoid permanent illicit occupancy of MR by glucocorticoid hormones. The main mechanism of mineralocorticoid selectivity is enzymatic: the 11beta hydroxysteroid dehydrogenase (HSD2) metabolizes glucocorticoid hormones into derivatives with a low affinity for MR. The cell biology and regulation of HSD2 are reviewed in this article, as well as its implications in human hypertension. Other factors play a role in mineralocorticoid selectivity: the MR itself, the possibility to form homodimers (MR-MR), or heterodimers (with the glucocorticoid receptor). All of these cellular events participate to successive dynamic equilibriums, which allow fine tuning of transcriptional regulation of target genes, depending on the target tissue and the hormonal status.

11-beta-Hydroxysteroid Dehydrogenases↗
PubMed2000

Identification and role of aldosterone receptors in the cardiovascular system.

The cardiovascular system is now recognized as an important mineralocorticoid target. All -components required for specific and selective aldosterone effects are present in the cardiovascular system. Mineralocorticoid receptors (MR) are expressed in the heart and large blood vessels together with the 11 B-hydroxysteroid dehydrogenase type II, which ensures the enzymatic protection of MR against glucocorticoids. The recent description of local vascular and cardiac aldosterone biosynthesis strongly supports an autocrine/paracrine hormonal action. Establishment of transgenic mice models of targeted overexpression of the mineralocorticoid receptor should facilitate new insights into the molecular and cellular mechanisms of aldo-sterone actions in the cardiovascular system.

Aldosterone↗
PubMed1999

Vasopressin stimulates long-term net chloride secretion in cortical collecting duct cells.

The classical short-term effect (within minutes) of arginine vasopressin (AVP) consists in increasing sodium, chloride and water transport in kidney cells. More recently, long-term actions (several hours) of the hormone have been evidenced on water and sodium fluxes, due to transcriptional enhancement in the expression of their transporters. The present study demonstrates that AVP is also responsible for a long-term increase in net chloride secretion. In the RCCD(1) rat cortical collecting duct cell line, 10(-8) M AVP induced, after several hours, an increase in net (36)Cl(-) secretion. This delayed effect of AVP was inhibited by basal addition of 10(-4) M bumetanide and apical addition of 10(-4) M glibenclamide, suggesting chloride entry at the basal membrane through a Na(+)/K(+)/2Cl(-) and apical secretion through a chloride conductance. An original acute cell permeabilization method was developed to allow for entry of antibodies directed against the regulatory region (R) of the cystic fibrosis transmembrane regulator (CFTR) into the cells. This procedure led to a complete and specific blocking of the long-term net chloride secretion induced by AVP. Finally, it was observed that CFTR transcripts steady-state level was significantly increased by AVP treatment. Besides the well-documented short-term effect of AVP on chloride transport, these results provide evidence that in RCCD(1) cells, AVP induces a delayed increase in transepithelial net chloride secretion that is mediated by a Na(+)/K(+)/2Cl(-) co-transporter and CFTR.

Animals↗
PubMed1999

Molecular and cellular determinants of mineralocorticoid selectivity.

Aldosterone plays a major role in the regulation of renal sodium reabsorption, of extracellular fluid volume and blood pressure. Such specific mineralocorticoid physiological adaptations occur despite the large prevalence of glucocorticoid hormones over aldosterone in the plasma. Indeed both classes of hormones bind with the same affinity to the mineralocorticoid receptor, but several mechanisms allow selective and tissue-specific aldosterone effects. They represent a series of mutually interacting selectivity filters, which have not yet been fully documented. The main determinants of aldosterone selective effects include an enzymatic protection of the mineralocorticoid receptor, the intrinsic properties of the mineralocorticoid receptor towards different ligands, and numerous possibilities of interaction between corticosteroid receptors (forming different homo or heterodimers) and other transcription factors.

Aldosterone↗
PubMed1999

Developmental expression of sodium entry pathways in rat nephron.

During the past several years, sites of expression of ion transport proteins in tubules from adult kidneys have been described and correlated with functional properties. Less information is available concerning sites of expression during tubule morphogenesis, although such expression patterns may be crucial to renal development. In the current studies, patterns of renal axial differentiation were defined by mapping the expression of sodium transport pathways during nephrogenesis in the rat. Combined in situ hybridization and immunohistochemistry were used to localize the Na-Pi cotransporter type 2 (NaPi2), the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2), the thiazide-sensitive Na-Cl cotransporter (NCC), the Na/Ca exchanger (NaCa), the epithelial sodium channel (rENaC), and 11beta-hydroxysteroid dehydrogenase (11HSD). The onset of expression of these proteins began in post-S-shape stages. NKCC2 was initially expressed at the macula densa region and later extended into the nascent ascending limb of the loop of Henle (TAL), whereas differentiation of the proximal tubular part of the loop of Henle showed a comparatively retarded onset when probed for NaPi2. The NCC was initially found at the distal end of the nascent distal convoluted tubule (DCT) and later extended toward the junction with the TAL. After a period of changing proportions, subsegmentation of the DCT into a proximal part expressing NCC alone and a distal part expressing NCC together with NaCa was evident. Strong coexpression of rENaC and 11HSD was observed in early nascent connecting tubule (CNT) and collecting ducts and later also in the distal portion of the DCT. Ontogeny of the expression of NCC, NaCa, 11HSD, and rENaC in the late distal convolutions indicates a heterogenous origin of the CNT. These data present a detailed analysis of the relations between the anatomic differentiation of the developing renal tubule and the expression of tubular transport proteins.

11-beta-Hydroxysteroid Dehydrogenases↗
PubMed1999

Epithelial sodium channel in human epidermal keratinocytes: expression of its subunits and relation to sodium transport and differentiation.

The amiloride-sensitive epithelial sodium channel (ENaC) is a main determinant of sodium absorption in renal and colonic epithelial cells. Surprisingly, it is also expressed in non-transporting epithelia such as the epidermis. To gain insight into the putative role of ENaC in keratinocytes, we have evaluated its expression in human skin and in cultured human keratinocytes. Our results indicate that (1) ENaC is expressed in the epidermis and in cultured keratinocytes, at the mRNA and at the protein levels, (2) the ratio of expression of the different ENaC subunits is drastically modified at the protein level during cell growth and differentiation, with a selective upregulation of the &bgr; subunit, (3) no transepithelial sodium transport function is apparent in cultured keratinocytes, but patch-clamp recordings indicate the existence of functional sodium channels with properties similar to those of the cloned ENaC and (4) ENaC inhibition does not alter keratinocyte proliferation, but it significantly decreases the frequency of dome formation in confluent keratinocyte cultures. These results document for the first time the characteristics of ENaC subunit expression in human keratinocytes, and suggest that ENaC may be important during differentiation.

Amiloride↗
PubMed1998

Cloning and expression of a novel pH-sensitive two pore domain K+ channel from human kidney.

A complementary DNA encoding a novel K+ channel, called TASK-2, was isolated from human kidney and its gene was mapped to chromosome 6p21. TASK-2 has a low sequence similarity to other two pore domain K+ channels, such as TWIK-1, TREK-1, TASK-1, and TRAAK (18-22% of amino acid identity), but a similar topology consisting of four potential membrane-spanning domains. In transfected cells, TASK-2 produces noninactivating, outwardly rectifying K+ currents with activation potential thresholds that closely follow the K+ equilibrium potential. As for the related TASK-1 and TRAAK channels, the outward rectification is lost at high external K+ concentration. The conductance of TASK-2 was estimated to be 14.5 picosiemens in physiological conditions and 59.9 picosiemens in symmetrical conditions with 155 mM K+. TASK-2 currents are blocked by quinine (IC50 = 22 microM) and quinidine (65% of inhibition at 100 microM) but not by the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine, and Cs+. They are only slightly sensitive to Ba2+, with less than 17% of inhibition at 1 mM. As TASK-1, TASK-2 is highly sensitive to external pH in the physiological range. 10% of the maximum current was recorded at pH 6. 5 and 90% at pH 8.8. Unlike all other cloned channels with two pore-forming domains, TASK-2 is essentially absent in the brain. In human and mouse, TASK-2 is mainly expressed in the kidney, where in situ hybridization shows that it is localized in cortical distal tubules and collecting ducts. This localization, as well as its functional properties, suggest that TASK-2 could play an important role in renal K+ transport.

Amino Acid Sequence↗
PubMed1998

Expression of TWIK-1, a novel weakly inward rectifying potassium channel in rat kidney.

Several K+ conductances have been identified in the kidney, with specific properties and localization in distinct cell types and membrane domains. On the other hand, several K+ channels have been characterized at the molecular level. By immunolocalization, we show that a new inward rectifying K+ channel, TWIK-1, is specifically expressed in distinct tubular segments and cell types of the rat kidney. In the proximal tubule, TWIK-1 prevails in the initial portions (convoluted part), where it is restricted to the apical (brush-border) membrane. In the collecting duct, immunofluorescence was intracellular or confined to the apical membrane and restricted to intercalated cells, i.e., in cells lacking aquaporin-2, as shown by double immunofluorescence. TWIK was also expressed in medullary and cortical parts of the thick limb of the loop of Henle, identified with an anti-Tamm-Horsfall protein antibody (double immunofluorescence). The intensity of TWIK-1 immunolabeling was unchanged in rats fed a low-Na+ or a low-K+ diet. Because TWIK-1 shares common properties with the low-conductance apical K+ channel of the collecting duct, we propose that it could play a role in K+ secretion, complementary to ROMK, another recently characterized K+ channel located in principal cells of the cortical collecting duct and in the loop of Henle.

Animals↗
PubMed1997

Transcriptional regulation of sodium transport by vasopressin in renal cells.

We have examined whether arginine vasopressin (AVP) can induce a long-term modulation of transepithelial ion transport in addition to its well known short-term effect. In the RCCD1 rat cortical collecting duct cell line, an increase in both short-circuit current and 22Na transport was observed after several hours of 10(-8) M AVP treatment (a concentration above the in vivo physiological range). This delayed effect was partially prevented by apical addition of 10(-5) M amiloride and was blocked by 10(-6) M actinomycin D and 2 x 10(-6) M cycloheximide. The amounts of mRNA encoding the alpha1 (not beta1) subunit of Na+/K+-ATPase and the beta and gamma (not alpha) subunits of the amiloride-sensitive epithelial Na+ channel were significantly increased by AVP treatment. The increase in mRNA was blocked by actinomycin D, not by amiloride, suggesting a Na+-independent increase in the rate of transcription of these subunits. The translation rates of the alpha1 subunit of Na+/K+-ATPase and the beta and gamma subunits of the rat epithelial sodium channel increased significantly, whereas the translation rates of the other subunits remained unchanged. Finally, the number of Na+ channels present in the apical membrane of the cells increased, as demonstrated by enhanced specific [3H]phenamil binding.

Amiloride↗
PubMed1997

Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct.

Arginine vasopressin (AVP) and corticosteroid hormones are involved in sodium reabsorption regulation in the renal collecting duct. Synergy between AVP and aldosterone has been well documented, although its mechanism remains unclear. Both aldosterone and glucocorticoid hormones bind to the mineralocorticoid receptor (MR), and mineralocorticoid selectivity depends on the MR-protecting enzyme 11 beta hydroxysteroid deshydrogenase (11-HSD), which metabolizes glucocorticoids into derivatives with low affinity for MR. We have investigated whether the activity of 11-HSD could be influenced by AVP and corticosteroid hormones. This study shows that in isolated rat renal collecting ducts, AVP increases 11-HSD catalytic activity. This effect is maximal at 10(-8) M AVP (a concentration clearly above the normal physiological range of AVP concentrations) and involves the V2 receptor pathway, while activation of protein kinase C or changes in intracellular calcium are ineffective. The stimulatory effect of AVP on 11-HSD is largely reduced after adrenalectomy, and is selectively restored by infusion of aldosterone, not glucocorticoids. We conclude that this synergy between AVP and aldosterone in controlling the activity of 11-HSD is likely to play a pivotal role in resetting mineralocorticoid selectivity, and hence sodium reabsorption capacities of the renal collecting duct.

1-Methyl-3-isobutylxanthine↗