Inhibition of omega-conotoxin-sensitive Ca2+ channel currents by internal Mg2+ in cultured rat cerebellar granule neurones.
The effects of changing the intracellular concentrations of either free Mg2+ ions ([Mg2+]i) or Mg(2+)-bound adenosine triphosphate ([Mg.ATP]i) on Ca2+ channel currents were assessed in cultured rat cerebellar granule neurones using the whole-cell patch-clamp technique. Raising [Mg2+]i from 0.06 mM to 1.0 mM inhibited Ca2+ channel currents by approximately 50%. The action of omega-conotoxin GVIA (omega-CgTX), a selective inhibitor of "N"-type Ca2+ channels was also investigated. With increasing [Mg2+]i, the proportion of current irreversibly blocked by omega-CgTX was reduced, and was negligible (approximately 5 pA of current) in the presence of [Mg2+]i values of 0.5 mM or greater. Block of the omega-CgTX-sensitive current accounted for the reduction in total current by concentrations of [Mg2+]i to 0.5 mM. Raising [Mg2+]i had no effect on the rate of decay of Ca2+ currents, but did produce a negative shift in current activation, possibly due to a non-specific interaction with negative surface charge. Altering [Mg.ATP]i from 0.3 to 5.0 mM caused a twofold increase in the size of currents without affecting the proportion of current sensitive to omega-CgTX. [Mg2+]i was also effective in inhibiting the Ca2+ channel current following potentiation by increasing [Mg.ATP]i. These data suggest that omega-CgTX-sensitive current in these cells is selectively inhibited by internal Mg2+ whereas both omega-CgTX-sensitive and -resistant components of current are potentiated by internal Mg.ATP. The mechanism by which Mg2+ inhibits "N"-type channels is unclear, but may involve an open channel block.
Pearson
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Department of Pharmacology, Royal Free Hospital School of Medicine, London, UK.
Dolphin
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D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects* / Q000502:physiology; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D004553:Electric Conductivity; D006098:Granulocytes / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism / Q000502:physiology*; D009474:Neurons / Q000378:metabolism*; D009994:Osmolar Concentration; D010455:Peptides / Q000494:pharmacology*; D051381:Rats; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010455:Peptides; D000255:Adenosine Triphosphate; D020866:omega-Conotoxin GVIA; D008274:Magnesium