SENSITIVITY OF THE SYNAPTIC MEMBRANE NA+ CA2+ EXCHANGER AND THE EXPRESSED NCX1 ISOFORM TO REACTIVE OXYGEN SPECIES/

Two plasma membrane proteins, the Na+/Ca2+ exchanger (NCX) and the Ca2 +-ATPase, are major regulators of free intraneuronal Ca2+ levels as th ey are responsible for extrusion of Ca2+ from the intracellular to the extracellular medium. Because disruption of cellular Ca2+ regulation plays a role in damage occurring under conditions of oxidative stress, studies were conducted to assess the sensitivity of the NCX to reacti ve oxygen species (ROS), Exchanger activity in brain synaptic plasma m embranes and in transfected CHO-K1 cells was inhibited following brief exposure to the peroxyl radical generating azo initiator 2,2'-azobis( 2-amidinopropane)dihydrochloride (AAPH) and to peroxynitrite. Incubati on with hydrogen peroxide did not alter NCX activity, even at 800 mu M concentration. In CHO-K1 cells transiently transfected with the NCX1 isoform of the exchanger, AAPH treatment decreased the maximal transpo rt capacity (V-max), whereas the K-act remained unchanged. Peroxynitri te led to an increase in K-act with no change in V-max. Loss of activi ty following exposure to either AAPH or peroxynitrite was associated w ith the formation of high molecular weight aggregates of NCX, and AAPP H also caused fragmentation of the exchanger protein. These findings s uggest that the NCX is sensitive to biologically relevant ROS and coul d be involved in the loss of Ca2+ homeostasis observed under oxidative stress. 0005-2736/98/$ - see front matter (C) 1998 Elsevier Science B .V. All rights reserved.