Role of stored Ca in contractions of rat uterus induced by acetylcholine: effects of preincubation with Mg-free solution.

We have investigated the action of acetylcholine on the release of Ca ions from intracellular stores. The results obtained show that in Ca-free solution acetylcholine induces two different contractions. One of them is a transient contraction that was observed after loading for 5 min in Ca(-) or Ca,Mg(-) solution and its magnitude was not modified by the absence of Mg. Another contraction remains constant after long exposure to Ca(-) medium but decreases in Ca,Mg(-) solution thus indicating that Mg ions are related to it. These results suggest that Mg plays an interesting role in stimulus-contraction coupling mediated by acetylcholine in Ca-free medium. This action can be explained by a release of stored Ca caused by Mg or by a direct effect of the ion on contractile proteins.