DIFFERENTIAL ACTIVATION OF SYSTEM-A AND BETAINE GABA TRANSPORT IN MDCK CELL-MEMBRANES BY HYPERTONIC STRESS/

Accumulation of osmolytes by renal cells is due in part to increased u ptake via specific transporters. These include amino acid transport sy stem A and the betaine/GABA transporter (BGT1). Transport changes have been characterized using intact cells which makes the intracellular m echanisms difficult to determine. In this study the hypertonic upregul ation of system A and BGT1 was studied directly at the membrane level in Madin-Darby canine kidney (MDCK) cells. Both system A and BGT1 tran sport systems were detected in an isolated membrane fraction containin g plasma membranes. System A transport was increased in membranes prep ared from cells after 6 h hypertonic stress (449 mosmol/kg) but BGT1 a ctivity was minimal and not different from isotonic controls. The incr ease in system A was blocked by inhibitors of RNA and protein synthesi s. BGT1 transport was induced in membranes prepared after 24 h hyperto nicity. At this time system A activity in the membrane fraction remain ed increased, unlike the downregulation observed in intact MDCK cells. We conclude that differential upregulation of system A and BGT1 by hy pertonic stress is due to intrinsic changes in these transporters at t he membrane level. In contrast, the downregulation of system A in inta ct cells when hypertonicity is prolonged for 24 h is Likely due to the action of an intracellular repressor that is not present in the isola ted membranes. (C) 1998 Elsevier Science B.V. All rights reserved.