The thiol-oxidizing agent diamide increases transmitter release by decreasing calcium requirements for neuromuscular transmission in the frog

Abstract

Diamide, which in concentrations of10 −5 M and higher oxidizes glutathione intracellularly, produces a dose-related increase in the frequency of miniaure end-plate potentials (MEPPs). With high enough doses, quantal release is blocked, apparently through exhaustion. The early phase of MEPP frequency increase is accompanied by an increase in EPP amplitude that may reach more than 10-fold and is therefore not produced by depolarization of axon terminals. Subsequently, EPP amplitude is reduced and falls to zero, associated with failure of invasion of the nerve action into the terminals while the MEPP frequency remains elevated. Both facilitation and PTP follow the time course of change in EPP amplitude. The increase in MEPP frequency with diamide does not require external Ca 2+ but raising external Ca 2+ increases the MEPP rate in the presence of diamide. External Ca 2+ is necessary for EPP appearance and also potentiates the diamide effects. Conversely diamide reduces the requirements for Ca 2+ in releasing ACh. Diamide substitutes for external Ca 2+ in K + evoked MEPP release and in the absence of external Ca 2+, diamide-evoked MEPP release is increased by raising external Mg 2+ levels. The action of diamide may be dependent on the actual release of Ca 2+ from intracellular stores or it may work through mimicking some of the actions of Ca 2+. The action of diamide bears close resemblance to the effects of prolonged stimulation of the motor axon at 10 Hz.