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Figure 6. Examination of the effects of Mg2+ over a voltage range. (A) Single hemichannel currents obtained with 8-s voltage ramps from −70 to +70 mV applied to excised patches. Shown are examples of three current traces in response to voltage ramps applied in succession to a patch in which the Mg2+ concentration was 1 mM on both sides (left), elevated to 10 mM on the cytoplasmic (middle) and elevated to 10 mM on the extracellular (right). The currents were leak subtracted. (B) Current–voltage relationships of Cx46 hemichannels constructed from ensemble averaged currents from multiple patches in response to ±70 mV, 8-s voltage ramps applied to excised patches as shown in A. The I-V relationship with 10 mM Mg2+on the cytoplasmic side was essentially similar to that obtained in symmetric 1 mM Mg2+, except for a small reduction in unitary current and a small negative shift in Erev. In contrast, the same concentration of Mg2+ applied to the extracellular side showed strong voltage dependence with hemichannels closing at inside negative voltages and opening at positive voltages. The strong voltage dependence of closure observed only when Mg2+ is elevated extracellularly but not cytoplasmically argues against a pore-blocking mechanism and is consistent with an extracellular modulatory site, which when bound with divalent cations causes a positive shift in activation to more positive voltages. Image published in: Verselis VK and Srinivas M (2008) © 2008 Verselis and Srinivas. Creative Commons Attribution-NonCommercial-ShareAlike license Larger Image Printer Friendly View

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