Ca2+-activated K+ channels from rat muscle transverse tubule membranes were inserted into planar phospholipid bilayers, and the activation of these channels by Ca2+ was studied. On the cytoplasmic side of the channel, calcium ions (in the range 10–100 mumol l-1) increase the opening probability of the channel in a graded way. This ‘activation curve’ is sigmoid, with an average Hill coefficient of about 2. Magnesium ions, in the range 1–10 mmol l-1, increase the apparent affinity of the channel for Ca2+ and greatly enhance the sigmoidicity of the Ca2+ activation curve. In the presence of 10 mmol l-1 Mg2+, the Hill coefficient for Ca2+ activation is about 4.5. This effect depends upon Mg2+ concentration but not upon applied voltage. Mg2+ is effective only when added to the cytoplasmic side of the channel. The results argue that this high-conductance, Ca2+-activated K+ channel contains at least six Ca2+-binding sites involved in the activation process.
Allosteric effects of Mg2+ on the gating of Ca2+-activated K+ channels from mammalian skeletal muscle
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J. Golowasch, A. Kirkwood, C. Miller; Allosteric effects of Mg2+ on the gating of Ca2+-activated K+ channels from mammalian skeletal muscle. J Exp Biol 1 September 1986; 124 (1): 5–13. doi: https://doi.org/10.1242/jeb.124.1.5
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