Magnesium as a modifier of smooth muscle contractility.

In smooth muscle, Ca2+ enters the cell through two different types of Ca2+ channels, activates contractile filaments and induces contraction. These Ca2+ channels are inhibited by Ca2+ channel blockers and/or nitrocompounds. Mg2+ inhibits both of these Ca2+ channels and relaxes the muscle. A portion of the smooth muscle contraction is due also to the release of Ca2+ from the cellular storage site. Caffeine and norepinephrine release Ca2+ from this store to induce a transient contraction. The contraction induced by caffeine is greatly augmented in the absence of Mg2+. The other effect of Mg2+ deficiency is to inhibit the effects of various vasodilators. Vascular endothelium releases a substance which relaxes vascular smooth muscle and this relaxation is also inhibited by Mg2+ deficiency. Thus, Mg2+ has multiple sites and mechanisms of action in smooth muscle which remain to be clarified.