Blockage of Neuronal Low-Threshold Ca2+ Channels by Extracellular Mg2+
Mg2+ belongs to the category of abundant extracellular and intracellular cations and has biologically relevant chemical properties (for review see Flatman 1984). Mg2+ ions are small compared with other earth alkaline ions and have a large hydration shell. Water substitution in Mg2+ aquocomplexes by ligands is known to occur by 3–4 orders of magnitude slower than in Ca2+ aquocomplexes (Diebler et al. 1969). Specificity of the binding of a comparably small metal ion like Mg2+ demands that the ligand be more tightly bound than the water to be substituted. Thus, Mg2+ ions are expected to pass with difficulty through small, water-filled channels of the excitable membrane. On the other hand, if Mg2+ ions are to compete effectively with other cations for passage, a ligand is required that binds the Mg2+ ion sufficiently tightly. Channels that are specifically permeable to divalent cations appear to be interesting candidates for studying these assumptions.
KeywordsTail Current Dorsal Root Ganglion Cell Negative Membrane Potential Water Substitution Helix Neuron
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