title: Inhibition of the renal betaine transporter by calcium ions.
abstract: Chronic upregulation of the renal betaine/GABA transporter (BGT1) by hypertonic stress has been well documented, but it is not known whether BGT1 can be regulated acutely after insertion in the basolateral plasma membrane. Related transporters, such as the rat brain GABA transporter, can be rapidly removed from the plasma membrane through activation of G protein-coupled receptors. The goal of the present study was to determine whether acute changes in extracellular and/or intracellular Ca(2+) will regulate BGT1 transport activity at the plasma membrane level in Madin-Darby canine kidney cells subjected to 24-h hypertonic stress. After brief pretreatment with a Ca(2+)-free solution, the addition of extracellular Ca(2+) in the transport assay produced dose-dependent inhibition of Na(+)-GABA cotransport. Maximum inhibition was 49% at 2 mM Ca(2+) (P < 0.05). Fura 2 imaging confirmed that addition of 2 mM Ca(2+) produced a transient increase in intracellular Ca(2+) that preceded transport inhibition. Acute inhibition of Na(+)-GABA cotransport was reproduced by addition of thapsigargin (5 microM) and ionomycin (10 microM). Amino acid transport system A, assayed as a control, was not inhibited. Brief treatment with phorbol esters reproduced the specific inhibition of Na(+)-GABA cotransport, and the inhibition was blocked by staurosporine. Surface biotinylation confirmed that the response to phorbol esters was accompanied by loss of BGT1 protein from the plasma membrane, and immunohistochemistry showed a shift to an intracellular distribution. We conclude that BGT1 can be inhibited acutely by extracellular Ca(2+) through a mechanism involving BGT1 protein internalization, and protein kinase C may play a role.
authors.affiliation: Department of Cellular and Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana, USA. skempson@iupui.edu
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Kempson
authors.forename: Jason M
authors.initials: JM
authors.lastname: Edwards
authors.forename: Michael
authors.initials: M
authors.lastname: Sturek
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000737:chemistry / Q000187:drug effects / Q000502:physiology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D050485:GABA Plasma Membrane Transport Proteins; D006982:Hypertonic Solutions; D015759:Ionomycin / Q000494:pharmacology; D007668:Kidney / Q000737:chemistry* / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D010703:Phorbol Esters / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology; D015398:Signal Transduction / Q000502:physiology; D019311:Staurosporine / Q000494:pharmacology; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D050485:GABA Plasma Membrane Transport Proteins; D006982:Hypertonic Solutions; D010703:Phorbol Esters; C072207:betaine plasma membrane transport proteins; D015759:Ionomycin; D019284:Thapsigargin; D011493:Protein Kinase C; D019311:Staurosporine; D002118:Calcium