Modulatory effect of extracellular Mg2+ ions on K+ and Ca2+ currents of capillary endothelial cells from rat brain.
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 34 +/- 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.
Delpiano
M A
MA
Max-Planck-Institut für molekulare Physiologie, Dortmund, Germany.
Altura
B M
BM
D000818:Animals; D001921:Brain / Q000098:blood supply*; D015220:Calcium Channels / Q000187:drug effects*; D002196:Capillaries / Q000166:cytology / Q000378:metabolism; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D004730:Endothelium, Vascular / Q000166:cytology / Q000378:metabolism*; D015640:Ion Channel Gating; D008274:Magnesium / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects*; D051381:Rats
D015220:Calcium Channels; D015221:Potassium Channels; D008274:Magnesium