Mechanisms of sodium azide-induced changes in intracellular calcium concentration in rat primary cortical neurons

An intracellular calcium ([Ca(2+)](i)) increase is involved in sodium azide (NaN(3))-induced neurotoxicity, an in vitro model of brain ischemia. In this study the questions of possible additional sources of calcium influx, besides glutamate receptor activation, and of the time-course of NaN(3) effects have been addressed by measuring [Ca(2+)](i) in rat primary cortical cultures with the FURA-2 method. Basal [Ca(2+)](i) of neuronal populations was concentration-dependently increased 30 min, but not 24h, after a 10-min NaN(3) (3-30 mM) treatment; conversely, the net increase induced by electrical stimulation (10Hz, 10s) was consistently reduced. All the above effects depended on glutamate release and consequent NMDA receptor activation, since the NMDA antagonist MK-801 (1 microM) prevented them, and the spontaneous efflux of [(3)H]-d-aspartate from superfused neurons was concentration-dependently increased by NaN(3). In single neuronal cells, NaN(3) application progressively and concentration-dependently increased [Ca(2+)](i) (to 177+/-5% and 249+/-7% of the controls, 4 and 12 min after a 10mM-treatment, respectively). EGTA (5mM) pretreatment reduced the effect of 10mM NaN(3) (to 118+/-5% at 4 min, and to 148+/-10% at 12 min, respectively), while 1 microM cyclosporin A did not. Both MK-801 and CNQX (a non-NMDA glutamate antagonist, 10 microM) prevented NaN(3) effect at 4 min (to 147+/-8% and 153+/-5%, respectively), but not at 12 min after NaN(3) treatment. Conversely, 10 microM verapamil and 0.1 microM omega-conotoxin (L- and N-type calcium channel blockers, respectively) significantly attenuated NaN(3) effects at 12 min (to 198+/-8% and 164+/-5%, respectively), but not at 4 min; the P/Q-type calcium channel blocker, agatoxin, 0.3 microM, was ineffective. These findings show that the predominant source of calcium increase induced by NaN(3) is extracellular, involving glutamate receptor activation in a first step and calcium channel (mainly of the N-type) opening in a second step.