The effects of noradrenaline on the amplitude-time characteristics of multiquantum endplate currents and the kinetics of induced secretion of transmitter quanta.

Experiments on frog neuromuscular junctions using a two-electrode membrane potential clamping method were used to study the effects of noradrenaline on the amplitude-time characteristics of multiquantum endplate current (EPC) parameters and the time course of secretion of transmitter quanta during the process of EPC generation. Noradrenaline (10 microM) induced significant increases in EPC amplitude (by 16%), with a decrease in the ratio of the duration of the leading front of the EPC to the duration of the leading front of the miniature endplate current (mEPC). Analysis of the time course of induced secretion, based on sequential subtraction of signals with displacement on the time scale, showed that noradrenaline induced synchronization of the process of secretion of quanta involved in generating multiquantum EPC, resulting in a 25% decrease in parameter P90, which characterizes the extent of synchronization of quantum release. The quantum composition of EPC, measured by dividing the area of induced and spontaneous signals and by analysis of the time course of the secretion of quanta, showed no changes in response to noradrenaline. Thus, in conditions in which responses to single stimuli applied to the motor nerve results in the release of several tens of quanta, noradrenaline can lead to increases in the amplitude of multiquantum EPC by increasing the level of synchronization of secretion of the transmitter quanta forming this signal.

[Effect of noradrenaline on the amplitude and temporal parameters of multiquantal currents and on the kinetics of evoked quantal secretion of a transmitter]. / Vliianie noradrenalina na amplitudno-vremennye kharakteristiki mnogokvantovykh tokov kontsevoi plastinki i kinetiku vyzvannoi sekretsii kvantov mediatora.

Noradrenaline caused a significant increase of the multiquantal endplate currents (EPC) amplitude, the EPC rising phase/the rising phase of miniature EPC ratio diminishing, at that. Noradrenaline seems to synchronise the process of quanta secretion in the EPC generation. Thus when a single stimulation of motor nerve results in release of several tenths of a transmitter, noradrenaline may increase the multiquantal EPC amplitude through synchronising of the transmitter release involved in the generation.

[The time course of the evoked secretion of the mediator quanta in various regions of the frog motor nerve ending]. / Osobennosti vremennogo khoda vyzvannoi sekretsii kvantov mediatora v raznykh otdelakh dvigatel'nogo nervnogo okonchaniia liagushki.

Apart from the fact that the gradient of the velocity of the AP propagation along the nerve terminal and the intensity of secretion do exist, the kinetics of a quanta transmitter release may also be revealed in different parts of the terminal. The velocity of the propagation and the minimum sympatric delay tend to diminish along with moving away from the myelinated part of axon, whereas the synchronicity of the quanta release rises. The distinctions in the time course of secretion in different parts of the terminal were amplified when the calcium ion concentration in the medium was enhanced. The observed peculiarities of the secretion kinetics in different regions of nerve ending seem to compensate for diminishing of the amplitude of multiquantal endplate current.