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      Combined Effects of Carbonic Anhydrase Inhibitor and Adenosine A 1 Receptor Antagonist on Hemodynamic and Tubular Function in the Kidney

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          Abstract

          Background: Carbonic anhydrase inhibitors (CAI) reduce proximal reabsorption, activating tubuloglomerular feedback (TGF) and reducing glomerular filtration rate (GFR). Adenosine A<sub>1</sub> receptors (A<sub>1</sub>R) mediate the TGF response and stimulate proximal reabsorption. Methods: Clearance and micropuncture studies were performed in Wistar rats to determine whether blockade of A<sub>1</sub>R (KW3902 0.3 mg/kg i.v.) would prevent CAI (benzolamide 5 mg/kg i.v.) from lowering GFR, whether CAI and KW3902 exert additive effects on sodium excretion, and to what extent such interactions depend on events in the glomerulus, proximal tubule, or distal nephron. Results: KW3902 raised GFR and prevented CAI from lowering GFR. KW3902 and CAI caused additive diuresis and natriuresis. KW3902 and CAI increased lithium clearance, but their effects were redundant. CAI increased the dependence of proximal reabsorption on active chloride transport. KW3902, alone, did likewise, but to a lesser extent than CAI. Adding KW3902 to CAI lessened the shift toward active chloride transport. Conclusions: The data reveal that A<sub>1</sub>R mediate glomerular vascular resistance whether or not TGF is activated, that additive effects of CAI and KW3902 on salt excretion occur, in part, because KW3902 inhibits reabsorption downstream from the macula densa, and that KW3902 likely inhibits proximal reabsorption by interfering with apical sodium-hydrogen exchange.

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          Most cited references 24

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          Adenosine and kidney function.

          In this review we outline the unique effects of the autacoid adenosine in the kidney. Adenosine is present in the cytosol of renal cells and in the extracellular space of normoxic kidneys. Extracellular adenosine can derive from cellular adenosine release or extracellular breakdown of ATP, AMP, or cAMP. It is generated at enhanced rates when tubular NaCl reabsorption and thus transport work increase or when hypoxia is induced. Extracellular adenosine acts on adenosine receptor subtypes in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate (GFR) by constricting afferent arterioles, especially in superficial nephrons, and acts as a mediator of the tubuloglomerular feedback, i.e., a mechanism that coordinates GFR and tubular transport. In contrast, it leads to vasodilation in deep cortex and medulla. Moreover, adenosine tonically inhibits the renal release of renin and stimulates NaCl transport in the cortical proximal tubule but inhibits it in medullary segments including the medullary thick ascending limb. These differential effects of adenosine are subsequently analyzed in a more integrative way in the context of intrarenal metabolic regulation of kidney function, and potential pathophysiological consequences are outlined.
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            Mediation of tubuloglomerular feedback by adenosine: evidence from mice lacking adenosine 1 receptors.

            Adenosine is a determinant of metabolic control of organ function increasing oxygen supply through the A2 class of adenosine receptors and reducing oxygen demand through A1 adenosine receptors (A1AR). In the kidney, activation of A1AR in afferent glomerular arterioles has been suggested to contribute to tubuloglomerular feedback (TGF), the vasoconstriction elicited by elevations in [NaCl] in the macula densa region of the nephron. To further elucidate the role of A1AR in TGF, we have generated mice in which the entire A1AR coding sequence was deleted by homologous recombination. Homozygous A1AR mutants that do not express A1AR mRNA transcripts and do not respond to A1AR agonists are viable and without gross anatomical abnormalities. Plasma and urinary electrolytes were not different between genotypes. Likewise, arterial blood pressure, heart rates, and glomerular filtration rates were indistinguishable between A1AR(+/+), A1AR(+/-), and A1AR(-/-) mice. TGF responses to an increase in loop of Henle flow rate from 0 to 30 nl/min, whether determined as change of stop flow pressure or early proximal flow rate, were completely abolished in A1AR(-/-) mice (stop flow pressure response, -6.8 +/- 0.55 mmHg and -0.4 +/- 0.2 in A1AR(+/+) and A1AR(-/-) mice; early proximal flow rate response, -3.4 +/- 0.4 nl/min and +0.02 +/- 0.3 nl/min in A1AR(+/+) and A1AR(-/-) mice). Absence of TGF responses in A1AR-deficient mice suggests that adenosine is a required constituent of the juxtaglomerular signaling pathway. A1AR null mutant mice are a promising tool to study the functional role of A1AR in different target tissues.
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              Identification of a chloride-formate exchanger expressed on the brush border membrane of renal proximal tubule cells.

              A key function of the proximal tubule is retrieval of most of the vast quantities of NaCl and water filtered by the kidney. Physiological studies using brush border vesicles and perfused tubules have indicated that a major fraction of Cl(-) reabsorption across the apical membrane of proximal tubule cells occurs via Cl(-)-formate exchange. The molecular identity of the transporter responsible for renal brush border Cl(-)-formate exchange has yet to be elucidated. As a strategy to identify one or more anion exchangers responsible for mediating Cl(-) reabsorption in the proximal tubule, we screened the expressed sequence tag database for homologs of pendrin, a transporter previously shown to mediate Cl(-)-formate exchange. We now report the cDNA cloning of CFEX, a mouse pendrin homolog with expression in the kidney by Northern analysis. Sequence analysis indicated that CFEX very likely represents the mouse ortholog of human SLC26A6. Immunolocalization studies detected expression of CFEX, but not pendrin, on the brush border membrane of proximal tubule cells. Functional expression studies in Xenopus oocytes demonstrated that CFEX mediates Cl(-)-formate exchange. Taken together, these observations identify CFEX as a prime candidate to mediate Cl(-)-formate exchange in the proximal tubule and thereby to contribute importantly to renal NaCl reabsorption. Given its wide tissue distribution, CFEX also may contribute to transcellular Cl(-) transport in additional epithelia such as the pancreas and contribute to transmembrane Cl(-) transport in nonepithelial tissues such as the heart.
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                Author and article information

                Journal
                KBR
                Kidney Blood Press Res
                10.1159/issn.1420-4096
                Kidney and Blood Pressure Research
                S. Karger AG
                1420-4096
                1423-0143
                2007
                November 2007
                20 September 2007
                : 30
                : 6
                : 388-399
                Affiliations
                Division of Nephrology-Hypertension, University of California, and Veterans Affairs San Diego Healthcare System, San Diego, Calif., USA
                Article
                108625 PMC2814144 Kidney Blood Press Res 2007;30:388–399
                10.1159/000108625
                PMC2814144
                17890869
                © 2007 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 7, Tables: 1, References: 51, Pages: 12
                Categories
                Original Paper

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