REVERSE NA+ CA2+ EXCHANGE CONTRIBUTES TO GLUTAMATE-INDUCED INTRACELLULAR CA2+ CONCENTRATION INCREASES IN CULTURED RAT FOREBRAIN NEURONS/

Activation of ionotropic glutamate receptors causes increases in intra cellular Ca2+ concentration ([Ca2+](i)) and intracellular Na+ concentr ation in neurons. It has been suggested that reversal of the plasma me mbrane Na+/Ca2+ exchanger (NCE) may account in part for the rise in [C a2+](i). Recently, KB-R7943 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]iso thiourea methanesulfonate) was reported to selectively inhibit the rev erse mode of the NCE in non-neuronal cells. We investigated the effect s of KB-R7943 on glutamate-stimulated increases in [Ca2+](i). In cultu red rat forebrain neurons loaded with indo-1 acetoxymethyl ester, KB-R 7943 inhibited the reverse mode of NCE (IC50 = 0.7 mu M). When tested against kainate- (100 mu M), N-methyl-D-aspartate- (30 mu M), glutamat e- (3 mu M), or KCl- (50 mM) induced [Ca2+](i) transients (15 sec, in the presence of Na+ and Ca2+)(i) KB-R7943 inhibited these transients w ith IC50 values of 6.6, 8.2, 5.2, and 2.9 mu M, respectively. [Ca2+](i ) increases caused by a higher concentration of glutamate (100 mu M) a lso were inhibited by KB-R7943 (10 mu M). However, KB-R7943 had no eff ect on peak [Ca2+](i) changes caused by prolonged application of gluta mate and did not inhibit glutamate-induced neuronal injury. KB-R7943 d id not inhibit N-methyl-D-aspartate- or kainate-induced whole-cell cur rents, nor did it substantially inhibit voltage-sensitive Ca2+ current s, excluding a direct inhibition of these ion channels. These results suggest that reverse NCE contributes to the immediate rise in [Ca2+](i ) resulting from glutamate receptor activation. However, reverse NCE b ecomes less important as the stimulus time is increased, and Ca2+ entr y by this route is not critical for the expression of excitotoxic inju ry.