Activation of transmitter release by strontium and calcium ions at the neuromuscular junction

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Abstract
1. The interaction between Ca and Sr ions on quantal transmitter release at the frog neuromuscular junction was studied, using conventional electrophysiological techniques.2. While Ca ions always activate transmitter release, the activating action of Sr ions depends on the Ca ion concentration in the medium; at low [Ca], strontium ions enhance the release, but at higher [Ca] they inhibit it. It is postulated that there is a [Ca] at which Sr ions do not affect transmitter liberation.3. When Sr activates release, its effect and the effect of Ca add in a more than linear fashion.4. Magnesium ions inhibit the release induced by Sr.5. The results can be explained by assuming that Ca and Sr act on the same site, at some stage of the process of quantal transmitter release. The affinity of both ions towards the sites is approximately the same, but the effectiveness of Sr is much smaller.