Methylmercury (MeHg) causes a multiphasic disruption of intraneuronal cation regulation. Release of Ca 2+ from internal stores and entry of extracellular Ca 2+ (Ca 2+ e ) contribute to the temporally distinct early (first Ca 2+ phase) and late (second Ca 2+ phase) components of increased intracellular Ca 2+ concentration ((Ca 2+ ) i ). The present study was designed to explore the mechanisms mediating the second Ca 2+ phase. Fluorescence intensity was monitored from single NG108-15 cells loaded with fura-2 before and during acute application of 2 μM MeHg. Nifedipine (1 or 10 μM but not 0.1 μM) significantly delayed the time-to-onset of the second Ca 2+ phase. Nifedipine (1 μM but not 0.1 μM) also caused a concentration-dependent delay in the onset of both the first Ca 2+ phase which is independent of Ca 2+ e and the elevation of non-Ca 2+ cation (non-Ca 2+ phase). The L-type dihydropyridine (DHP) Ca 2+ channel agonist Bay K-8644 (10 nM) had no effect on the time-to-onset of the second Ca 2+ phase. Neither the N-type Ca 2+ channel blocker ω-conotoxin GVIA (up to 1 μM) nor the nonselective Ca 2+ channel blocker Ni 2+ (1 mM) altered the time-to-onset of the second Ca 2+ phase. Removal of Na + e or addition of the voltage-dependent Na + channel antagonist tetrodotoxin (TTX, 1 μM) significantly delayed the onset of the second Ca 2+ phase. In a manner similar to that for 1 μM nifedipine, TTX also delayed the onset of the other phases. Thus, we hypothesize that MeHg depolarizes the plasma membrane leading to an increase in the activation of voltage-dependent Na + and Ca 2+ channels which promotes, directly or indirectly, the influx of Ca 2+ during the second Ca 2+ phase.
Toxicology and Applied Pharmacology – Elsevier
Published: Dec 1, 1995
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