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 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.