MODULATORY ROLE OF MAGNESIUM ON THE CONTRACTILE RESPONSE OF RAT AORTATO SEVERAL AGONISTS IN NORMAL AND CALCIUM-FREE MEDIUM

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 pre sence of Ca2+. Contractile responses to noradrenaline (1 muM) 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. I n Ca2+-free medium, acetylcholine-induced phasic responses indicate th e existence of an acetylcholine-sensitive Ca2+ store. KCl induced cont raction only in Krebs solution, although a small residual contraction could be observed in Ca2+-free medium in some experiments. Mg2+-deplet ion in the extracellular medium increased contractile responses induce d by acetylcholine and KCl in Ca2+-free medium. These results suggest that extracellular Mg2+ modulates basal tone, Ca2+ Channels and respon siveness to various agents in the absence of Ca2+.