Interactions Between Calcium and Barium on the Spontaneous Release of Transmitter From Mammalian Motor Nerve Terminals

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Abstract
Electrophysiological monitoring of transmitter release from nerve terminals in rat diaphragm muscles showed that substitution of Ba for Ca produced an initial transient increase in miniature end-plate potential (MEPP) frequency, followed by a decline in frequency to a rate expected in Ca-free solutions. When Ba is added to solutions with up to 2.5 mM Ca, Ba increases MEPP frequency. Part of this acceleratory action of Ba is probably due to its membrane depolarizing action. At higher concentrations of Ca, Ba reduces MEPP frequency. A direct action of Ba in accelerating MEPP frequency is observed when membrane potential is stabilized at higher Ba concentrations in the presence of Ca. It is concluded that Ba is a poor substitute for Ca in accelerating MEPP frequency in Ca-free solutions, but Ba ccelerates release if Ca is also present.