Allosteric regulation of BK channel gating by Ca(2+) and Mg(2+) through a nonselective, low affinity divalent cation site.
The ability of membrane voltage to activate high conductance, calcium-activated (BK-type) K(+) channels is enhanced by cytosolic calcium (Ca(2+)). Activation is sensitive to a range of [Ca(2+)] that spans over four orders of magnitude. Here, we examine the activation of BK channels resulting from expression of cloned mouse Slo1 alpha subunits at [Ca(2+)] and [Mg(2+)] up to 100 mM. The half-activation voltage (V(0.5)) is steeply dependent on [Ca(2+)] in the micromolar range, but shows a tendency towards saturation over the range of 60-300 microM Ca(2+). As [Ca(2+)] is increased to millimolar levels, the V(0.5) is strongly shifted again to more negative potentials. When channels are activated by 300 microM Ca(2+), further addition of either mM Ca(2+) or mM Mg(2+) produces similar negative shifts in steady-state activation. Millimolar Mg(2+) also produces shifts of similar magnitude in the complete absence of Ca(2+). The ability of millimolar concentrations of divalent cations to shift activation is primarily correlated with a slowing of BK current deactivation. At voltages where millimolar elevations in [Ca(2+)] increase activation rates, addition of 10 mM Mg(2+) to 0 Ca(2+) produces little effect on activation time course, while markedly slowing deactivation. This suggests that Mg(2+) does not participate in Ca(2+)-dependent steps that influence current activation rate. We conclude that millimolar Mg(2+) and Ca(2+) concentrations interact with low affinity, relatively nonselective divalent cation binding sites that are distinct from higher affinity, Ca(2+)-selective binding sites that increase current activation rates. A symmetrical model with four independent higher affinity Ca(2+) binding steps, four voltage sensors, and four independent lower affinity Ca(2+)/Mg(2+) binding steps describes well the behavior of G-V curves over a range of Ca(2+) and Mg(2+). The ability of a broad range of [Ca(2+)] to produce shifts in activation of Slo1 conductance can, therefore, be accounted for by multiple types of divalent cation binding sites.
Zhang
X
X
Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Solaro
C R
CR
Lingle
C J
CJ
D000200:Action Potentials; D000818:Animals; D001665:Binding Sites / Q000502:physiology*; D002118:Calcium / Q000493:pharmacokinetics / Q000494:pharmacology*; D002412:Cations / Q000737:chemistry; D003001:Cloning, Molecular; D015640:Ion Channel Gating / Q000502:physiology*; D007700:Kinetics; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008274:Magnesium / Q000493:pharmacokinetics / Q000494:pharmacology*; D051379:Mice; D008956:Models, Chemical; D009865:Oocytes; D015221:Potassium Channels / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated; D014982:Xenopus laevis
D002412:Cations; D051036:Large-Conductance Calcium-Activated Potassium Channels; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D008274:Magnesium; D002118:Calcium