Effects of Intracellular Mg 2+ on the Properties of Large-Conductance,Ca 2+ -Dependent K + Channels in Rat Cerebrovascular Smooth Muscle Cells

The effects of intracellular magnesium ions, Mg²⁺_i on large-conductance, Ca²⁺-dependent K⁺ channels (BK channels) of adult rat cerebrovascular smooth muscle cells (CVSMCs) were studied using patch clamp techniques and cells enzymatically dispersed from basilar, middle, and posterior cerebral arteries. Recordings used inside-out membrane patches and took place at 20–24°C. One millimeter [Mg²⁺]_i produced a fast block of BK channel currents, as well described by the Woodhull model of channel occlusion by a charged species. However, the affinity and voltage-sensitivity of Mg²⁺_i block were dependent on the concentration of free intracellular calcium ions, [Ca²⁺]_i. Calcium ions may stabilize a channel conformation in which Mg²⁺_i binding sites are relocated closer to the inner membrane surface. In the presence of 1 μM [Ca²⁺]_i, 0.5 mM [Mg²⁺]_i shifted the Boltzmann curve relating BK channel open probability, P_o, to membrane voltage leftward on the voltage axis, without any change in its slope. The enhancing effect of Mg²⁺_i on P_o was, therefore, insensitive to membrane potential. Quantitative considerations suggest that physiological levels of Mg²⁺_i tonically facilitate BK channel activation. Alterations of [Mg²⁺]_i during hyper- or hypomagnesemia may contribute to the dilation or contraction of cerebral vessels seen under these two conditions.