Allosteric effects of Mg2+ on the gating of Ca2+-activated K+ channels from mammalian skeletal muscle.
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.
D000494:Allosteric Regulation; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008954:Models, Biological; D009132:Muscles / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D051381:Rats
D007473:Ion Channels; D008274:Magnesium; D011188:Potassium; D002118:Calcium