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Articles by A. K Stewart
Total Records ( 15 ) for A. K Stewart
  A. K Stewart , A Yamamoto , M Nakakuki , T Kondo , S. L Alper and H. Ishiguro
 

Pancreatic ductal epithelium produces a HCO3-rich fluid. HCO3 transport across ductal apical membranes has been proposed to be mediated by both SLC26-mediated Cl/HCO3 exchange and CFTR-mediated HCO3 conductance, with proportional contributions determined in part by axial changes in gene expression and luminal anion composition. In this study we investigated the characteristics of apical Cl/HCO3 exchange and its functional interaction with Cftr activity in isolated interlobular ducts of guinea pig pancreas. BCECF-loaded epithelial cells of luminally microperfused ducts were alkalinized by acetate prepulse or by luminal Cl removal in the presence of HCO3-CO2. Intracellular pH recovery upon luminal Cl restoration (nominal Cl/HCO3 exchange) in cAMP-stimulated ducts was largely inhibited by luminal dihydro-DIDS (H2DIDS), accelerated by luminal CFTR inhibitor inh-172 (CFTRinh-172), and was insensitive to elevated bath K+ concentration. Luminal introduction of CFTRinh-172 into sealed duct lumens containing BCECF-dextran in HCO3-free, Cl-rich solution enhanced cAMP-stimulated HCO3 secretion, as calculated from changes in luminal pH and volume. Luminal Cl removal produced, after a transient small depolarization, sustained cell hyperpolarization of ~15 mV consistent with electrogenic Cl/HCO3 exchange. The hyperpolarization was inhibited by H2DIDS and potentiated by CFTRinh-172. Interlobular ducts expressed mRNAs encoding CFTR, Slc26a6, and Slc26a3, as detected by RT-PCR. Thus Cl-dependent apical HCO3 secretion in pancreatic duct is mediated predominantly by an Slc26a6-like Cl/HCO3 exchanger and is accelerated by inhibition of CFTR. This study demonstrates functional coupling between Cftr and Slc26a6-like Cl/HCO3 exchange activity in apical membrane of guinea pig pancreatic interlobular duct.

  P Dai , A. K Stewart , F Chebib , A Hsu , J Rozenfeld , D Huang , D Kang , V Lip , H Fang , H Shao , X Liu , F Yu , H Yuan , M Kenna , D. T Miller , Y Shen , W Yang , I Zelikovic , O. S Platt , D Han , S. L Alper and B. L. Wu
 

Mutations of the human SLC26A4/PDS gene constitute the most common cause of syndromic and nonsyndromic hearing loss. Definition of the SLC26A4 mutation spectrum among different populations with sensorineural hearing loss is important for development of optimal genetic screening services for congenital hearing impairment. We screened for SLC26A4 mutations among Chinese and U.S. subjects with hearing loss, using denaturing HPLC (DHPLC) and direct DNA sequencing. Fifty-two of 55 Chinese subjects with deafness accompanied by enlargement of the vestibular aqueduct (EVA) exhibited at least one mutant SLC26A4 allele, whereas SLC26A4 mutations were found in only 2 of 116 deaf Chinese patients without EVA. The spectrum of SLC26A4 mutations differed among Chinese and U.S. subjects and included 10 previously unreported SLC26A4 variants: 4 in the Chinese population (p.E303Q, p.X329, p.X467, p.X573) and 6 in the U.S. population (p.V250A, p.D266N, p.F354S, p.D697A, p.K715N, p.E737D). Among the seven novel in-frame missense mutations, five encoded SLC26A4 proteins with substantially reduced Cl/anion exchange activity as expressed and measured in Xenopus oocytes, but four of these were sufficiently active to allow study of anion selectivity. The only mutant polypeptide exhibiting complete loss of anion exchange function, p.E303Q, was expressed at or near the oocyte surface at near-wild-type levels. Two variants, p.F354S and p.E737D, displayed selective reduction in relative rate of Cl/HCO3 exchange compared with similarly measured rates of Cl/Cl and Cl/I exchange. Our data show that mutation analysis of the SLC26A4 gene is of high diagnostic yield among subjects with deafness and bilateral EVA in both China and the U.S. However, the pathogenicity of monoallelic SLC26A4 gene variants in patients with hearing loss remains unclear in many instances.

 
 
 
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