Extracellular magnesium and contractile responses to noradrenaline in the rat tail artery

The effects of variations in extracellular magnesium concentration (Mg2+)0 on contractile responses to noradrenaline (NA) were studied in isolated rat tail arteries. Mg2+-free or high Mg2+ exposures caused (respectively) enhancement or inhibition of NA dose-response curves. Increases in (Mg2+)0 significantly increased NA-induced phasic contractions due to Ca release from a "membrane-bound" pool. Increases in (Mg2+)0 diminished the magnitude of Ca2+-induced relaxations due to a "membrane-stabilizing" effect. It is hypothesized that high (Mg2+)0 enhances NA-induced mobilization of stored calcium, as well as preventing the membrane-stabilizing effect of calcium ions through competition with Ca2+ at intracellular and membrane sequestrating sites, thus interfering with Ca2+ sequestration.