Non-quantal acetylcholine release in the mammalian neuromuscular junction: dependence on the extracellular concentration of magnesium and calcium ions.

摘要

It is well known that the signal is transmitted in a chemical synapse from the neuron to the effector cell via both spontaneous and synchronous (evoked) release of relatively stable portions (quanta) of a chemical neurotransmitter. A quantum is considered to be the amount of the neurotransmitter contained in one synaptic vesicle; an increase in the intracellular calcium content results in releasing the neurotrans mitter. An increase in the intracellular calcium level takes place due to opening of the voltagedependent calcium channels of the presynaptic membrane and calcium entrance from the extracellular medium [11]. In the late 1970s, data on nonquantal acetylcho line release started to emerge [2, 3]. Although some progress in studying this type of the neurotransmitter secretion has been made [4], a number of aspects have been still poorly studied. Dependence of nonquantal secretion on extracellular calcium and operation of calcium channels is one of them. Calcium removal from the medium washing the neuromuscular prepa ration was found to result in a complete inhibition of nonquantal acetylcholine secretion, although it is much slower than elimination of both spontaneous and evoked quantal secretion [5]. At the same time, inhibition of nonquantal neurotransmitter secretion by magnesium ions, which are able to block calcium ion channels [6], was found; however, as we had shown earlier, its kinetics is rather rapid.