Endothelium-Dependent Influence of Small Changes in Extracellular Magnesium Concentration on the Tone of Feline Middle Cerebral Arteries

The aim of this study was to investigate the effect of small alterations in the extracellular magnesium concentration on the tone of feline middle cerebral arteries and to examine the role of the endothelium in these responses. We measured the isometric tension of isolated arterial rings placed between two stainless steel wires in a tissue chamber containing Krebs-Henseleit solution aerated with a gas mixture containing 95 % O2 and 5 % CO2 at 37°C. After precontrac-tion with noradrenaline, a decrease of the extracellular magnesium concentration from 1.2 mM to 1.0 and 0.8 mM resulted in sustained relaxations, whereas elevation of the extracellular magnesium concentration from 0.8 mM to 1.2 mM caused an increase in vascular tone when the endothelium was intact The magnesium deficiency-related dilations were absent in endothe-lium-denuded vessels and were inhibited by 5 x 10" ' M oxyhemoglobin and 1 0 s M methylene blue, suggesting the involvement of an endothelium-derived relaxing factor in this vascular response. However, 5 x l 0 ~ 7 M nifedipine or 3xlO~s M dichlorobenzamil did not affect the magnesium deficiency-related relaxations. Therefore, nifedipine-sensitive calcium channels or the sodium/calcium antiport system are not involved in this vascular action of magnesium. We conclude that small alterations in the extracellular magnesium concentration, possibly withi