The nature of the antagonism between calcium and magnesium ions at the neuromuscular junction

Calcium and magnesium ions are known to have specific and opposite effects at the prejunctional nerve terminals of several cholinergic synapses. A nerve impulse releases more acetylcholine (ACh) if the concentration of Ca is raised, within limits, or if that of Mg is lowered (del Castillo & Stark, 1952; del Castillo & Engbaek, 1954; Hutter & Kostial, 1954). del Castillo & Katz (1954 a, b) have discussed this antagonism and have put forward tentatively the hypothesis that Ca and Mg compete for some site or carrier molecule, X, in the nerve endings: on the arrival of an impulse, the calcium compound alone breaks down to give Ca and an active form of X, X', which can release or allow the passage of ACh. Thus the output of ACh should depend on the proportion of the total concentration of X which is combined with calcium. In the present experiments, end-plate potentials (e.p.p.'s) in curarized frog muscle have been measured over a range of Ca and Mg concentrations, and the results have been compared with the predictions of this hypothesis.