Modulatory Role of Magnesium on the Contractile Response of Rat Aorta to Several Agonists in Normal and Calcium-free Medium

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Abstract
Acute withdrawal of external Mg2+ increased basal tone of rat isolated aorta incubated in the presence of Ca2+. Above normal levels of Mg2+ (1–4 Mm) inhibited basal tone while much higher levels of the divalent cation (64–256 Mm) evoked contractile responses regardless of the presence of Ca2+. Contractile responses to noradrenaline (1μm) and KCl (80 Mm) were inhibited by addition of cumulative concentrations of Mg2+. Acetylcholine-induced contractions in the presence of physiological concentrations of Mg2+ (1 Mm) decreased gradually to the basal tone, but a sustained contraction was observed in the absence of this ion. In Ca2+-free medium, acetylcholine-induced phasic responses indicate the existence of an acetylcholine-sensitive Ca2+ store. KCl induced contraction only in Krebs solution, although a small residual contraction could be observed in Ca2+-free medium in some experiments. Mg2+-depletion in the extracellular medium increased contractile responses induced by acetylcholine and KCl in Ca2+-free medium. These results suggest that extracellular Mg2+ modulates basal tone, Ca2+ channels and responsiveness to various agents in the absence of Ca2+.

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