Calcium Transients and Transmitter Secretion in Different Parts of Frog Nerve Endings in Different Conditions of Calcium
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Calcium Transients and Transmitter Secretion in Different Parts of Frog Nerve Endings in Different Conditions of Calcium Ion Influx E. F. Khaziev,1,2,3 D. V. Balashova,3 A. N. Tsentsevitsky,1,2 E. A. Bukharaeva,1 and D. V. Samigullin1,2,3
Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 105, No. 10, pp. 1262–1270, October, 2019. Original article submitted June 25, 2019. Revised version received July 7, 2019. Accepted July 7, 2019. Experiments on frog neuromuscular preparations were performed to study the characteristics of the calcium response and the quantum secretion of acetylcholine in different pats of extended nerve terminals in different conditions of calcium influx. A calcium-sensitive fluorescent dye was used to analyze Ca2+ influx (Ca2+ transients) into the proximal and distal parts of nerve endings in conditions of increased K+ ion content, in response to blockers of N- and L-type calcium channels, and on blockade of calcium-activated potassium channels. These studies showed that at a uniform distribution density of voltage-gated calcium channels along nerve endings, the proximal-to-distal decrement in calcium transients and quantum secretion intensity persisted in conditions of additional opening of voltage-gated calcium channels by potassium depolarization, on “thinning” of these channels using specific blockers, but changed on blockade of calcium-activated potassium channels. Keywords: neuromuscular junction, calcium transient, proximal-to-distal decrement, quantum composition, calcium channels, potassium channels.
current [2, 3]. Ca2+ influx in response to the first stimulus is presently evaluated by recording calcium transients – fluorescent signals reflecting changes in the emission intensity of a calcium-sensitive dye in response to binding Ca2+ [4]. Frog nerve endings are quite characteristic in terms of size and the number of terminal branches [5, 6]. Action potential shape changes along extended terminals, from a predominantly negative direction to a completely positive signal [7–10]. Changes in presynaptic action potentials are accompanied by changes in the probability of neurotransmitter release in different parts of the terminal [11–14]. The causes of differences in measures of acetylcholine quantum release along the terminal may be the morphological features of different areas, particularly the cross-sectional diameter of the terminal, which determines the length of the synaptic contact, the number of active secretion zones in each part [15, 16], differences in the density and conductivity of voltage-gated ion (potassium, sodium, and calcium)
Neuromuscular synapses in vertebrates are classical and widely used system for studying the mechanisms underlying the realization and modulation of synaptic transmission of excitation in fast chemical synapses. It is well known that Ca2+ influx after presynaptic action potentials triggers the secretion and determines the intensity of release of neurotransmitter quanta [1]. However, it is quite difficult to obtain accura
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