THE EFFECT OF FACILITATORY CONCENTRATIONS OF DECAMETHONIUM ON THE STORAGE AND RELEASE OF TRANSMITTER AT THE NEUROMUSCULAR JUNCTION OF THE CAT

¹ Department of Pharmacology and Therapeutics, Loyola University Medical Center, Maywood, Illinois

The effects of decamethothum (C10) have been observed in the isolated tenuissimus muscle of the cat. Muscle action potentials and muscle contractions were prevented by transversely cutting the muscle fibers. Trains of end-plate potentials (EPP's) were observed after nerve stimulation by means of intracellular microelectrodes. The following parameters were calculated: quantal size, mobilization rate, quantal content of the first EPP, the available store of quanta and the fraction of the available store released by the first EPP. C10 (10^-7 M) caused a significant increase in the quantal content of the first EPP by an increase in mobilization rate which in turn caused an increase in the available store. Tubocurarine (7.2 x 10^-7 M) antagonized this effect and was shown at this concentration to block postsynaptically while having a faciitatory action presynaptically, increasing the quantal content of the first EPP. Reducing the calcium ion concentration to 50% reduced the quantal content by reducing fractional release, and reducing the sodium ion concentration to 50% reduced the mobilization rate which in turn reduced the available store. Concomitantly, the fractional release was increased such that there was no significant change in quantal content of the first EPP. It has been concluded that there is a cholinoceptive site on the unmyelinated nerve terminal, stimulation of which facilitates the influx of sodium ions.

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