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Ira Kurtz

Publications and source records attributed to Ira Kurtz.

At least 37 records · Page 2Linked to original sources
PubMed2004

Expression and localization of rat NBC4c in liver and renal uroepithelium.

Previous studies provided functional evidence for electrogenic Na(+)-HCO(3)(-) cotransport in hepatocytes and in intrahepatic bile duct cholangiocytes. The molecular identity of the transporters mediating electrogenic sodium-bicarbonate cotransport in the liver is currently unknown. Of the known electrogenic Na(+)-HCO(3)(-) cotransporters (NBC1 and NBC4), we previously showed that NBC4 mRNA is highly expressed in the liver. In the present study, we performed RT-PCR, immunoblotting, and immunohistochemistry to characterize the expression pattern of NBC4 in rat liver and kidney. For immunodetection, a polyclonal antibody against rat NBC4 was generated and affinity purified. Of the known human NBC4 variants, only the rat NBC4c ortholog was detected by RT-PCR in rat liver, and the molecular mass of the NBC4c protein was approximately 145 kDa. NBC4c protein was expressed in hepatocytes and in the cholangiocytes lining the intrahepatic bile ducts. In hepatocytes, NBC4c was localized to the basolateral plasma membrane, whereas intrahepatic cholangiocytes stained apically. The NBC1 electrogenic sodium cotransporter variants kNBC1 and pNBC1 were not detected by immunoblotting and immunohistochemistry in rat liver. The pattern of localization of NBC4c in the liver suggests that the cotransporter plays a role in mediating Na(+)-HCO(3)(-) cotransport in hepatocytes and intrahepatic cholangiocytes. Unlike the liver, the rat kidney expressed electrogenic sodium-bicarbonate cotransporter proteins kNBC1 and NBC4c. In kidney, NBC4c also had a molecular mass of approximately 145 kDa and was immunolocalized to uroepithelial cells lining the renal pelvis, where the cotransporter may play an important role in protecting the renal parenchyma from alterations in urine pH.

Amino Acid Sequence↗
PubMed2004

Determinants of plasma water sodium concentration as reflected in the Edelman equation: role of osmotic and Gibbs-Donnan equilibrium.

Edelman et al. have empirically shown that plasma water sodium concentration ([Na(+)](pw)) is equal to 1.11(Na(e) + K(e))/TBW - 25.6 (Edelman IS, Leibman J, O'Meara MP, Birkenfeld LW. J Clin Invest 37: 1236-1256, 1958). However, the physiological significance of the slope and y-intercept in this equation has not been previously considered. Our analysis demonstrates that there are several clinically relevant parameters determining the magnitude of the y-intercept that independently alter [Na(+)](pw):1) osmotically inactive exchangeable Na(+) and K(+); 2) plasma water K(+) concentration; and 3) osmotically active non-Na(+) and non-K(+) osmoles. In addition, we demonstrate quantitatively the physiological significance of the slope in the Edelman equation and its role in modulating [Na(+)](pw). The slope of 1.11 in this equation which Edelman et al. determined empirically can be theoretically predicted by considering the combined effect of the osmotic coefficient of Na(+) salts at physiological concentrations and Gibbs-Donnan equilibrium. In addition, our results demonstrate that the slope has an independent quantitative impact on the magnitude of the y-intercept in the Edelman equation. From a physiological standpoint, the components of both the slope and the y-intercept need to be addressed when considering the factors that modulate [Na(+)](pw).

Animals↗
PubMed2004

Role of potassium in hypokalemia-induced hyponatremia: lessons learned from the Edelman equation.

It is well known that changes in the mass balance of K+ can lead to an alteration in the plasma water sodium concentration ([Na+]pw). We have recently shown that based on the Edelman equation, the [Na+]pw is determined by the total exchangeable Na+ (Nae), total exchangeable K+ (Ke), total body water (TBW), osmotically inactive Nae and Ke, plasma water [K+], intracellular and extracellular osmotically active non-Na+ and non-K+ osmoles, and plasma osmotically active non-Na+ and non-K+ osmoles. In light of these findings, a re-analysis of the role of K+ in modulating the [Na+]pw is required in understanding the pathophysiology of hypokalemia-induced hyponatremia. In this article, we characterize the complex role of K+ in the pathogenesis of hypokalemia-induced hyponatremia using a three-compartment model and the known parameters in the Edelman equation. Our analysis indicates that K+ modulates the [Na+]pw by changing Ke in addition to the parameters in the y-intercept of the Edelman equation. Moreover, the magnitude of potassium-induced changes in the [Na+]pw is determined by the pathophysiologic mechanisms by which changes in Ke occur.

Humans↗
PubMed2004

New insights into the pathophysiology of the dysnatremias: a quantitative analysis.

Recent theoretical considerations have played an important role in advancing our understanding of the physiological mechanisms responsible for perturbing the plasma water sodium concentration ([Na(+)](pw)) in health and disease. Central to these considerations is the original empirical relationship between the [Na(+)](pw) and total exchangeable sodium (Na(e)), total exchangeable potassium (K(e)), and total body water (TBW) initially discovered by Edelman and colleagues (Edelman IS, Leibman J, O'Meara MP, and Birkenfeld LW. J Clin Invest 37: 1236-1256, 1958). The non-zero values of the slope and y-intercept in the Edelman equation are a consequence of the effects of the osmotic coefficient of Na(+) salts at physiological concentrations and Gibbs-Donnan and osmotic equilibrium. Moreover, in addition to Na(e), K(e), and TBW, the physiological components of the y-intercept in this equation play a role in modulating the [Na(+)](pw) and in the generation of the dysnatremias. In this review, the pathophysiological mechanisms underlying the generation and treatment of the dysnatremias are analyzed theoretically and quantitatively. Importantly, the non-zero values of both the slope and y-intercept in the Edelman equation result in several theoretical predictions that can be tested experimentally and have been mathematically incorporated into recently derived equations used to analyze both the generation and the optimal treatment of the dysnatremias. In addition, we review current concepts regarding 1) the role of Gibbs-Donnan and osmotic equilibrium in the determination of the [Na(+)](pw); 2) the modulating effect of osmotically inactive exchangeable Na(+) and K(+) on the [Na(+)](pw); 3) the effect of glucose on the [Na(+)](pw) as reflected by changes in Na(e), K(e), and TBW as well as changes in several components of the y-intercept resulting from the hyperglycemia; and 4) the complex role of K(+) in modulating the [Na(+)](pw).

Body Water↗
PubMed2003

Structural characterization, tissue distribution, and functional expression of murine aminoacylase III.

Many xenobiotics are detoxified through the mercapturate metabolic pathway. The final product of the pathway, mercapturic acids (N-acetylcysteine S-conjugates), are secreted predominantly by renal proximal tubules. Mercapturic acids may undergo a transformation mediated by aminoacylases and cysteine S-conjugate beta-lyases that leads to nephrotoxic reactive thiol formation. The deacetylation of cysteine S-conjugates of N-acyl aromatic amino acids is thought to be mediated by an aminoacylase whose molecular identity has not been determined. In the present study, we cloned aminoacylase III, which likely mediates this process in vivo, and characterized its function and structure. The enzyme consists of 318 amino acids and has a molecular mass (determined by SDS-PAGE) of approximately 35 kDa. Under nondenaturing conditions, the molecular mass of the enzyme is approximately 140 kDa as determined by size-exclusion chromatography, which suggests that it is a tetramer. In agreement with this hypothesis, transmission electron microscopy and image analysis of aminoacylase III showed that the monomers of the enzyme are arranged with a fourfold rotational symmetry. Northern analysis demonstrated an approximately 1.4-kb transcript that was expressed predominantly in kidney and showed less expression in liver, heart, small intestine, brain, lung, testis, and stomach. In kidney, aminoacylase III was immunolocalized predominantly to the apical domain of S1 proximal tubules and the cytoplasm of S2 and S3 proximal tubules. The data suggest that in kidney proximal tubules, aminoacylase III plays an important role in deacetylating mercapturic acids. The predominant cytoplasmic localization of aminoacylase III may explain the greater sensitivity of the proximal straight tubule to the nephrotoxicity of mercapturic acids.

Acetylcysteine↗
PubMed2003

Mapping blood pressure loci in (A/J x B6)F2 mice.

Although the genetics of rare, monogenic, forms of human hypertension are fairly well defined, the genetics of the common polygenic form of human essential hypertension is only emerging. With the ability to control environmental variables, animal models have provided valuable tools with which to study blood pressure (BP) homeostasis. We have now studied BP genetics in a model consisting of 1,521 F2 mice from a series of (A/J x B6) intercrosses kept under standardized conditions. Using whole genome quantitative trait loci (QTL) mapping, we have identified four novel significant BP loci. These included Abbp1 on mouse chromosome MMU1 [maximum LOD score (MLS) at approximately 35 cM = 6.8], Abbp2 on MMU4 (MLS at approximately 25 cM = 9.8), Abbp3 on MMU7 (MLS at approximately 25 cM = 5.4), and Abbp4 on MMU11 (MLS at approximately 58 cM = 6.3). Compared with A/J homozygotes, homozygosity for the B6 alleles of Abbp1, Abbp2, or Abbp4 is independently associated with a 7-12 mmHg increase in BP. In contrast Abbp3 interacts epistatically with a locus on MMU17 (near D17Mit180) to modulate BPs in female (A/J x B6)F2 mice. Interestingly, Abbp4 on MMU11 is homologous to a major confirmed BP locus, BP1, on rat chromosome 10 and to a major confirmed BP locus, HYT1, on human chromosome 17. Defining the molecular differences between the A/J and the B6 alleles at these novel loci with major influences on the BP phenotype will contribute to our understanding of the complex genetics of BP control.

Animals↗
PubMed2003

Identification of membrane topography of the electrogenic sodium bicarbonate cotransporter pNBC1 by in vitro transcription/translation.

The transmembrane topography of the human pancreatic electrogenic sodium bicarbonate cotransporter pNBC1 was investigated using in vitro transcription/translation of HK-M0 and HK-M1 fusion vectors designed to test membrane insertion properties of pNBC1 hydrophobic sequences (H). These vectors encode N-terminal 101 (HK-M0) or 139 (HK-M1) amino acids of the H,K-ATPase alpha-subunit, a linker region and the C-terminal 177 amino acids of the H,K-ATPase beta-subunit that contain five N-linked glycosylation consensus sites (Bamberg, K., and Sachs, G. (1994) J. Biol. Chem. 269, 16909-16919). The glycosylation status of the beta-region was used as a reporter to determine whether a given hydrophobic sequence possesses signal anchor and/or stop transfer properties in the HK-M0 and HK-M1 vectors. The linker region of each vector was replaced either with individual hydrophobic sequences or combinations thereof. The transcription/translation products of these fusion vectors in reticulocyte lysate system +/- microsomal membranes were identified by [(35)S]-autoradiography following separation using SDS-PAGE. The results of the in vitro transcription/translation analysis indicated that 10 (H1, H2N, H3, H5, H6, H7, H8, H9, H11, and H12) out of 12 hydrophobic sequences were able to insert into the plasma membrane. Two hydrophobic sequences, H4 and H10, had no membrane insertion activity even when upstream and downstream sequences were present. These data and immunocytochemical studies indicate that pNBC1 contains 10 transmembrane domains with N- and C-termini oriented intracellularly. This is the first characterization of the membrane topography of a sodium bicarbonate cotransporter.

Amino Acid Sequence↗
PubMed2003

A new quantitative approach to the treatment of the dysnatremias.

Rapid correction of the dysnatremias can result in significant patient morbidity and mortality. To avoid overly rapid correction of the dysnatremias, the sodium deficit equation, water deficit equation, and Adrogue-Madias equation are frequently utilized to predict the change in plasma sodium concentration (Delta[Na+]p) following a therapeutic maneuver. However, there are significant limitations inherent in these equations. Specifically, the sodium deficit equation assumes that total body water (TBW) remains unchanged. Similarly, when using the Adrogue-Madias equation, the volume of infusate required to induce a given Delta[Na+]p is determined by dividing the target Delta[Na+]p by the result of this formula. This calculation also assumes that TBW remains constant. In addition, neither of these equations are applicable in the management of symptomatic syndrome of inappropriate antidiuretic hormone secretion (SIADH) because they fail to consider the subsequent increase in sodium excretion following the administration of infusate. Furthermore, in the treatment of hypernatremia, the water deficit equation is only applicable if the hypernatremia is caused by pure water loss. In hypernatremia caused by hypotonic fluid losses, the water deficit equation does not provide any information on the differential effect of infusates of variable [Na+] and [K+] on the [Na+]p. Finally, all these equations fail to consider any ongoing Na+, K+, or H2O losses. Taking all these limitations into consideration, we have derived two new equations which determine the volume of a given infusate required to induce a target Delta[Na+]p. These equations consider the mass balance of Na+, K+, and H2O, as well as therapy-induced changes in TBW. The first equation is applicable to both hypernatremia and hyponatremia. The second equation is applicable to the management of severe symptomatic SIADH requiring intravenous therapy.

Adult↗
PubMed2003

A simple quantitative approach to analyzing the generation of the dysnatremias.

BACKGROUND: Although the dysnatremias are the most common electrolyte disorders in hospitalized patients, the complexity of the parameters normally used to explain their generation mechanistically is often bewildering to medical students and experts alike. A number of methods have been utilized clinically to analyze retrospectively and predict prospectively the pathogenesis of these disorders. These approaches include the measurements of plasma and urine osmolality, free water clearance, electrolyte free water clearance, and tonicity balance. METHODS: All previous analyses are problematic in that they fail to incorporate mathematically in a single equation the known factors that account quantitatively for changes in the plasma water sodium concentration. In this paper, we have derived a simple formula for use at the bedside based on all known factors that can generate the dysnatremias. The formula incorporates (1) the known empirical relationship between the plasma water Na+ concentration, total body water (TBW), and exchangeable Na+ (Na+(e)) and K+ (K+(e)); (2) changes in mass balance of H2O (VMB) and Na+ + K+ (EMB); and (3) the effect of hyperglycemia. RESULTS: This new equation, unlike all previous qualitative and quantitative approaches, can account mathematically for the simultaneous effects of TBW, Na(e), K(e), EMB, VMB, and the plasma glucose on the plasma water sodium concentration. Clinical examples are provided that demonstrate the utility of this new equation in analyzing the pathogenesis of the dysnatremias. CONCLUSION: The conceptual simplification resulting from the use of this formula should significantly improve the current approaches used in analyzing the generation of the dysnatremias.

Adolescent↗
PubMed2003

Molecular pathogenesis of nephrogenic diabetes insipidus.

There have been significant advances recently in the understanding of the molecular causes of nephrogenic diabetes insipidus. The resistance of the collecting duct to the action of vasopressin in this disorder results from abnormalities in several of the intricate steps that mediate the increase in principal cell hydraulic conductivity in response to the hormone. In this article, we review the current understanding of the known genetic causes of nephrogenic diabetes insipidus that affect the binding of vasopressin to the V2 receptor and subsequent intracellular signaling events, as well as the translocation of aquaporin-2 water channels to the apical membrane. In addition, genetic diseases, which decrease collecting-duct water absorption by diminishing the interstitial medullary osmolarity, are discussed.

Animals↗
PubMed2003

Blindness and auditory impairment caused by loss of the sodium bicarbonate cotransporter NBC3.

Normal sensory transduction requires the efficient disposal of acid (H+) generated by neuronal and sensory receptor activity. Multiple highly sensitive transport mechanisms have evolved in prokaryotic and eukaryotic organisms to maintain acidity within strict limits. It is currently assumed that the multiplicity of these processes provides a biological robustness. Here we report that the visual and auditory systems have a specific requirement for H+ disposal mediated by the sodium bicarbonate cotransporter NBC3 (refs. 7,8). Mice lacking NBC3 develop blindness and auditory impairment because of degeneration of sensory receptors in the eye and inner ear as in Usher syndrome. Our results indicate that in certain sensory organs, in which the requirement to transduce specific environmental signals with speed, sensitivity and reliability is paramount, the choice of the H+ disposal mechanism used is limited.

Animals↗
PubMed2002

The COOH termini of NBC3 and the 56-kDa H+-ATPase subunit are PDZ motifs involved in their interaction.

The electroneutral sodium bicarbonate cotransporter 3 (NBC3) coimmunoprecipitates from renal lysates with the vacuolar H(+)-ATPase. In renal type A and B intercalated cells, NBC3 colocalizes with the vacuolar H(+)-ATPase. The involvement of the COOH termini of NBC3 and the 56-kDa subunit of the proton pump in the interaction of these proteins was investigated. The intact and modified COOH termini of NBC3 and the 56-kDa subunit of the proton pump were synthesized, coupled to Sepharose beads, and used to pull down kidney membrane proteins. Both the 56- and the 70-kDa subunits of the proton pump, as well as a PDZ domain containing protein Na(+)/H(+) exchanger regulatory factor 1 (NHERF-1), were bound to the intact 18 amino acid NBC3 COOH terminus. A peptide truncated by five COOH-terminal amino acids did not bind these proteins. Replacement of the COOH-terminal leucine with glycine blocked binding of both the proton pump subunits but did not affect binding of NHERF-1. The 18 amino acid COOH terminus of the 56-kDa subunit of the proton pump bound NHERF-1 and NBC3, but the truncated and modified peptide did not. A complex of NBC3, the 56-kDa subunit of the proton pump, and NHERF-1 was identified in rat kidney. The data indicate that the COOH termini of NBC3 and the 56-kDa subunit of the vacuolar proton pump are PDZ-interacting motifs that are necessary for the interaction of these proteins. NHERF-1 is involved in the interaction of NBC3 and the vacuolar proton pump.

Animals↗
PubMed2002

Regulation of the sodium bicarbonate cotransporter kNBC1 function: role of Asp(986), Asp(988) and kNBC1-carbonic anhydrase II binding.

The HCO(3)(-) : Na(+) cotransport stoichiometry of the electrogenic sodium bicarbonate cotransporter kNBC1 determines the reversal potential (E(rev)) and thus the net direction of transport of these ions through the cotransporter. Previously, we showed that phosphorylation of kNBC1-Ser(982) in the carboxy-terminus of kNBC1 (kNBC1-Ct), by cAMP-protein kinase A (PKA), shifts the stoichiometry from 3 : 1 to 2 : 1 and that binding of bicarbonate to the cotransporter is electrostaticaly modulated. These results raise the possibility that phosphorylated kNBC1-Ser(982), or other nearby negatively charged residues shift the stoichiometry by blocking a bicarbonate-binding site. In the current study, we examined the role of the negative charge on Ser(982)-phosphate and three aspartate residues in a D986NDD custer in altering the stoichiometry of kNBC1. mPCT cells expressing kNBC1 mutants were grown on filters and mounted in an Ussing chamber for electrophysiological studies. Enhanced green fluorescence protein (EGFP)-tagged mutant constructs expressed in the same cells were used to determine the phosphorylation status of kNBC1-Ser(982). The data indicate that both kNBC1-Asp(986) and kNBC1-Asp(988), but not kNBC1-Asp(989), are required for the phosphorylation-induced shift in stoichiometry. A homologous motif (D887ADD) in the carboxy-terminus of the anion exchanger AE1 binds to carbonic anhydrase II (CAII). In isothermal titration calorimetry experiments, CAII was found to bind to kNBC1-Ct with a K(D) of 160 +/- 10 nM. Acetazolamide inhibited the short-circuit current through the cotransporter by 65 % when the latter operated in the 3 : 1 mode, but had no effect on the current in the 2 : 1 mode. Acetazolamide did not affect the cotransport stoichiometry or the ability of 8-Br-cAMP to shift the stoichiometry. Although CAII does not affect the transport stoichiometry, it may play an important role in enhancing the flux through the transporter when kNBC1-Ser(982) is unphosphorylated.

Amino Acid Motifs↗
PubMed2002

The cystic fibrosis transmembrane conductance regulator interacts with and regulates the activity of the HCO3- salvage transporter human Na+-HCO3- cotransport isoform 3.

Cystic fibrosis transmembrane conductance regulator (CFTR) regulates both HCO(3)(-) secretion and HCO(3)(-) salvage in secretory epithelia. At least two luminal transporters mediate HCO(3)(-) salvage, the Na(+)/H(+) exchanger (NHE3) and the Na(+)-HCO(3)(-) cotransport (NBC3). In a previous work, we show that CFTR interacts with NHE3 to regulate its activity (Ahn, W., Kim, K. W., Lee, J. A., Kim, J. Y., Choi, J. Y., Moe, O. M., Milgram, S. L., Muallem, S., and Lee, M. G. (2001) J. Biol. Chem. 276, 17236-17243). In this work, we report that transient or stable expression of human NBC3 (hNBC3) in HEK cells resulted in a Na(+)-dependent, DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid)- and 5-ethylisopropylamiloride-insensitive HCO(3)(-) transport. Stimulation of CFTR with forskolin markedly inhibited NBC3 activity. This inhibition was prevented by the inhibition of protein kinase A. NBC3 and CFTR could be reciprocally coimmunoprecipitated from transfected HEK cells and from the native pancreas and submandibular and parotid glands. Precipitation of NBC3 or CFTR from transfected HEK293 cells and from the pancreas and submandibular gland also coimmunoprecipitated EBP50. Glutathione S-transferase-EBP50 pulled down CFTR and hNBC3 from cell lysates when expressed individually and as a complex when expressed together. Notably, the deletion of the C-terminal PDZ binding motifs of CFTR or hNBC3 prevented coimmunoprecipitation of the proteins and inhibition of hNBC3 activity by CFTR. We conclude that CFTR and NBC3 reside in the same HCO(3)(-)-transporting complex with the aid of PDZ domain-containing scaffolds, and this interaction is essential for regulation of NBC3 activity by CFTR. Furthermore, these findings add additional evidence for the suggestion that CFTR regulates the overall trans-cellular HCO(3)(-) transport by regulating the activity of all luminal HCO(3)(-) secretion and salvage mechanisms of secretory epithelial cells.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗