title: Differential activation of system A and betaine/GABA transport in MDCK cell membranes by hypertonic stress.
abstract: Accumulation of osmolytes by renal cells is due in part to increased uptake via specific transporters. These include amino acid transport system A and the betaine/GABA transporter (BGT1). Transport changes have been characterized using intact cells which makes the intracellular mechanisms difficult to determine. In this study the hypertonic upregulation of system A and BGT1 was studied directly at the membrane level in Madin-Darby canine kidney (MDCK) cells. Both system A and BGT1 transport systems were detected in an isolated membrane fraction containing plasma membranes. System A transport was increased in membranes prepared from cells after 6 h hypertonic stress (449 mosmol/kg) but BGT1 activity was minimal and not different from isotonic controls. The increase in system A was blocked by inhibitors of RNA and protein synthesis. BGT1 transport was induced in membranes prepared after 24 h hypertonicity. At this time system A activity in the membrane fraction remained increased, unlike the downregulation observed in intact MDCK cells. We conclude that differential upregulation of system A and BGT1 by hypertonic stress is due to intrinsic changes in these transporters at the membrane level. In contrast, the downregulation of system A in intact cells when hypertonicity is prolonged for 24 h is likely due to the action of an intracellular repressor that is not present in the isolated membranes.
authors.affiliation: Department of Physiology and Biophysics, Indiana University School of Medicine, Indianapolis, IN 46202-5120, USA. skempson@iupui.edu
authors.forename: S A
authors.initials: SA
authors.lastname: Kempson
mesh_terms: D000222:Adaptation, Physiological / Q000187:drug effects; D000818:Animals; D001622:Betaine / Q000378:metabolism*; D001693:Biological Transport, Active / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D004285:Dogs; D050485:GABA Plasma Membrane Transport Proteins; D007424:Intracellular Fluid / Q000502:physiology; D007668:Kidney; D012462:Saline Solution, Hypertonic; D013312:Stress, Physiological / Q000378:metabolism*; D046249:Transfer RNA Aminoacylation / Q000187:drug effects; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D050485:GABA Plasma Membrane Transport Proteins; D012462:Saline Solution, Hypertonic; C072207:betaine plasma membrane transport proteins; D001622:Betaine; D005680:gamma-Aminobutyric Acid