Pathways mediating Ca2+ entry in rat cerebellar granule cells following in vitro exposure to methyl mercury.
Cell imaging and the Ca(2+)-sensitive fluorophore fura-2 were used to examine methyl mercury's effect on Ca2+ homeostasis in rat cerebellar granule cells, a cell type preferentially targeted by methyl mercury. In vitro methyl mercury exposure (0.2-5.0 microM) induced a biphasic rise in fura-2 fluorescence ratio, consisting of a small first phase due to Ca2+ release from intracellular store(s) and a much larger second phase which required Ca2+ influx. The time-to-onset of these fura-2 fluorescence changes was inversely correlated with methyl mercury concentration. When examining various Ca2+ entry pathways as possible targets contributing to Ca2+ influx, we found that excitatory amino acid pathways were not directly involved. In contrast, the voltage-dependent Ca2+ channel blockers nifedipine and omega-conotoxin-MVIIC significantly delayed the time-to-onset of both phases, a response inconsistent with mere inhibition of Ca2+ entry. The nonselective voltage-dependent Ca2+ channel blocker Ni2+ had no effect on the methyl mercury response. Because methyl mercury alters cell membrane potentials, we hypothesized that voltage-dependent Na+ channels were activated initially; however, tetrodotoxin did not alter the methyl mercury-induced increases in fura-2 fluorescence ratio. Thus, methyl mercury alters Ca2+ homeostasis in cerebellar granule cells through nifedipine- and omega-conotoxin-MVIIC-sensitive pathways, suggesting that L-, N-, and/or Q-type Ca2+ channels may play a role in methyl mercury's mode of action or entry.
Marty
M S
MS
Department of Pharmacology and Toxicology, Michigan State University, East Lansing 48824, USA.
Atchison
W D
WD
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D002531:Cerebellum / Q000187:drug effects* / Q000378:metabolism*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D008767:Methylmercury Compounds / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013779:Tetrodotoxin / Q000494:pharmacology
D002121:Calcium Channel Blockers; D018691:Excitatory Amino Acid Antagonists; D008767:Methylmercury Compounds; D013779:Tetrodotoxin; C004925:methylmercuric chloride; D002118:Calcium