Differential contribution of plasmalemmal Na+/Ca2+ exchange isoforms to sodium-dependent calcium influx and NMDA excitotoxicity in depolarized neurons

Inhibition of Na+,K+-ATPase during NMDA applications greatly increased NMDA-induced excitotoxicity in primary cultures of forebrain neurons (FNs), but not in cerebellar granule cells (CGCs). Because Na+,K+-ATPase inhibition promotes reversal of plasmalemmal Na+/Ca2+ exchangers, we compared the activities of reversed K+-independent (NCX) and K+-dependent (NCKX) Na+/Ca2+ exchangers in these cultures. To this end, we measured gramicidin-induced and Na+-dependent elevation in cytosolic [Ca2+] ([Ca2+](c)) that represents Ca2+ influx via reversed NCX and NCKX; NCX activity was dissected out by removing external K+. The [Ca2+](c) elevations mediated by NCX alone, and NCX plus NCKX combined, were 17 and 6 times more rapid in FNs than in CGCs, respectively. Northern blot analysis showed that FNs preferentially express NCX1 whereas CGCs expressed NCX3. Differences in expression of other isoforms (NCX2, NCKX2, NCKX3 and NCKX4) were less pronounced. We tested whether the NCX or NCKX family of exchangers contributes most to the toxic NMDA-induced Ca2+ influx in depolarized neurons. We found that in FNs, inhibition of NCX alone was sufficient to significantly limit NMDA excitotoxicity, whereas in CGCs, inhibition of both NCX and NCKX was required. The data suggest that the high activity of NCX isoforms expressed in FNs, possibly NCX1, sensitizes these neurons to NMDA excitotoxicity.