Using whole-cell patch-clamp recording, we demonstrate that exposure of single rat brain capillary endothelial cells to different extracellular Mg2+ concentrations (0.3, 4.8 and 9.6 mM) affects the conductance of K+ and Ca2+ currents elicited under control conditions (1.2 mM). Extracellular Mg2+ concentrations ([Mg2+]o) of 4.8 and 9.6 mM reversibly depress outward K+ currents by about 30 ± 12% (n =10) and 3% ± 13%(n = 10), at all activating potentials, respectively. Using identical concentrations reversibly depressed the Ca2+ current by about 40 ± 16% (n = 8) and 46 ± 18% (n = 6), respectively. Using a low Mg2+ concentration of 0.3 mM, the K+ current activation was unexpectedly and mildly increased by about 15 ± 5% (n = 5), and the inward Ca2+ current was attenuated. When studying this effect of low [Mg2+]o on ‘pure’ Ca2+ currents, free of outward currents, we found that this inward current was depressed by about 38 ± 16% (n = 8), and its threshold for activation, in the current-voltage relationship, was shifted to more negative potentials. It is concluded that high [Mg2+]o hinders the entry of Ca2+ through low-voltage activated Ca2+ channels and thereby attenuates a Ca2+-regulated K+ conductance. At a low [Mg2+]o (0.3 mM), Mg2+ shifts the steady-state inactivation of the voltage-activated Ca2+ channel to more negative potentials by about 8 mV (n = 6), probably due to a negative screening effect, i.e. a reduction of positive charges on the cell membrane. This may contribute to an apparent increase in K+ conductance by an, as yet, unknown mechanism.
