Antagonism between Na⁺ and Ca²⁺ at the Neuromuscular Junction

Abstract

THE quantity of transmitter released from presynaptic nerve terminals is in part determined by the amplitude of the action potential^1,2 invading the nerve terminals, and by the amount of a specific calcium-compound available at some strategic site³. Large reductions in external Na⁺ concentration, however, which would be expected to reduce the size of the propagated action potential⁴, do not greatly alter the output of acetylcholine at cholinergic nerve endings^5,6. This might be explained if it were supposed that the reduction in Na⁺ concentration also increased the amount of available calcium-compound at the strategic site; Na⁺ deficient solutions might then maintain the release of transmitter in spite of the reduction of the amplitude of the action potential. On this hypothesis a Na⁺ deficient solution might be expected to increase the amount of transmitter released when the calcium, concentration is low. Such an increase in transmitter release has been found in the experiments reported below.