Am J Physiol Cell Physiol AJP: Renal Physiology
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Am J Physiol Cell Physiol 296: C570-C582, 2009. First published January 7, 2009; doi:10.1152/ajpcell.00436.2008
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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Mechanisms of pH-gradient driven transport mediated by organic anion polypeptide transporters

Simone Leuthold,1 Bruno Hagenbuch,1 Nilufar Mohebbi,2,3 Carsten A. Wagner,2,3 Peter J. Meier,1 and Bruno Stieger1,3

1Division of Clinical Pharmacology and Toxicology, Department of Medicine, University Hospital; 2Institute of Physiology, University of Zurich; and 3Center for Integrative Human Physiology, University of Zurich, 8057 Zurich, Switzerland

Submitted 24 August 2008 ; accepted in final form 6 January 2009

Organic anion transporting polypeptides (humans OATPs, rodents Oatps) are expressed in most mammalian tissues and mediate cellular uptake of a wide variety of amphipathic organic compounds such as bile salts, steroid conjugates, oligopeptides, and a large list of drugs, probably by acting as anion exchangers. In the present study we aimed to investigate the role of the extracellular pH on the transport activity of nine human and four rat OATPs/Oatps. Furthermore, we aimed to test the concept that OATP/Oatp transport activity is accompanied by extrusion of bicarbonate. By using amphibian Xenopus laevis oocytes expressing OATPs/Oatps and mammalian cell lines stably transfected with OATPs/Oatps, we could demonstrate that in all OATPs/Oatps investigated, with the exception of OATP1C1, a low extracellular pH stimulated transport activity. This stimulation was accompanied by an increased substrate affinity as evidenced by lower apparent Michaelis-Menten constant values. OATP1C1 is lacking a highly conserved histidine in the third transmembrane domain, which was shown by site-directed mutagenesis to be critically involved in the pH dependency of OATPs/Oatps. Using online intracellular pH measurements in OATP/Oatp-transfected Chinese Hamster Ovary (CHO)-K1 cells, we could demonstrate the presence of a 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid-sensitive chloride/bicarbonate exchanger in CHO-K1 cells and that OATP/Oatp-mediated substrate transport is paralleled by bicarbonate efflux. We conclude that the pH dependency of OATPs/Oatps may lead to a stimulation of substrate transport in an acidic microenvironment and that the OATP/Oatp-mediated substrate transport into cells is generally compensated or accompanied by bicarbonate efflux.

anion/bicarbonate exchange


Address for reprint requests and other correspondence: B. Stieger, Univ. Hospital, Dept. of Medicine, Division of Clinical Pharmacology and Toxicology, 8091 Zurich, Switzerland (E-mail: bstieger{at}kpt.unizh.ch)






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