A quantitative study of the calcium-magnesium antagonism in isolated strips of guinea-pig ileum

The effect of increasing magnesium concentration (0.61, 1.22, 2.44 and 4.88 mM) on the acetylcholine dose-response curve of isolated strips of the longitudinal muscle of the guinea pig ileum was studied qualitatively and quantitatively in Krebs solution with 2.55 and 5.10 mM calcium. The quantitative study was based on a hypothesis assuming the competition of both ions for a common binding site in the cell "stores" from which the calcium is mobilized following cholinergic receptor activation. The theoretical equations fit the experimental data and allow the calculation of the calcium and magnesium equilibrium constants. As a cross-check two groups of experiments were done to compare the influence on the acetylcholine dose-response curve of a gradual decrease of extracellular calcium either without or with magnesium. The amounts of magnesium used were those theoretically suitable for maintaining a constant amount of calcium in the hypothetical cell "stores". The results agreed with the predictions form the model proposed thus confirming both the hypothesis and the values obtained for the calcium and magnesium equilibrium constants.