Alteration of cytosolic free calcium homeostasis by SIN-1:: high sensitivity of L-type Ca2+ channels to extracellular oxidative/nitrosative stress in cerebellar granule cells

Exposure of cerebellar granule neurones in 25 mM KCl HEPES-containing Locke's buffer (pH 7.4) to 50-100 muM SIN-1 during 2 h decreased the steady-state free cytosolic Ca2+ concentration ([Ca2+]i) from 168 +/- 33 nM to 60 +/- 10 nM, whereas exposure to greater than or equal to 0.3 mM SIN-1 produced biphasic kinetics: (i) decrease of [Ca2+]i during the first 30 min, reaching a limiting value of 75 +/- 10 nM (due to inactivation of L-type Ca2+ channels) and (ii) a delayed increase of [Ca2+]i at longer exposures, which correlated with SIN-1-induced necrotic cell death. Both effects of SIN-1 on [Ca2+]i are blocked by superoxide dismutase plus catalase and by Mn(III)tetrakis(4-benzoic acid)porphyrin chloride. Supplementation of Locke's buffer with catalase before addition of 0.5-1 mM SIN-1 had no effect on the decrease of [Ca2+]i but further delayed and attenuated the increase of [Ca2+]i observed after 60-120 min exposure to SIN-1 and also protected against SIN-1-induced necrotic cell death. alpha-Tocopherol, the potent NMDA receptor antagonist (+)-MK-801 and the N- and P-type Ca2+ channels blocker omega-conotoxin MVIIC had no effect on the alterations of [Ca2+]i upon exposure to SIN-1. However, inhibition of the plasma membrane Ca2+ ATPase can account for the increase of [Ca2+]i observed after 60-120 min exposure to 0.5-1 mM SIN-1. It is concluded that L-type Ca2+ channels are a primary target of SIN-1-induced extracellular nitrosative/oxidative stress, being inactivated by chronic exposure to fluxes of peroxynitrite of 0.5-1 muM/min, while higher concentrations of peroxynitrite and hydrogen peroxide are required for the inhibition of the plasma membrane Ca2+ ATPase and induction of necrotic cell death, respectively.