The Contribution of L-Type Calcium Channels to Acetylcholine Secretion in Frog and Mouse Neuromuscular Junctions with Active and Inactivated Voltage-Gated Potassium Channels

© 2020, Springer Science+Business Media, LLC, part of Springer Nature. We report here experiments addressing the contribution of L-type Ca2+ channels to evoked acetylcholine secretion from frog and mouse motor nerve endings with active and inactivated voltage-gated K+ channels. These studies evaluated the effects of the specific L-type Ca2+ channel blocker nitrendipine on the quantum composition of endplate currents and the time course of the secretion of acetylcholine quanta in intact preparations and after preliminary blockade of voltage-gated K+ channels with 4-aminopyridine (4-AP) in medium with depressed and physiological Ca2+ levels. A fluorescence method was used to measure calcium transients reflecting the integral influx of Ca2+ into nerve endings; computer modeling was applied to the processes underlying exocytosis in the presence of the two types of Ca2+ channel (N and L) and with different durations of nerve ending action potentials. In frog synapses, L-type Ca2+ channels were found to contribute to evoked acetylcholine secretion in the presence of active K+ channels, but only in the presence of a depressed Ca2+ level in the medium; on inactivation of voltage-gated K+ channels, the contribution of L-type channels to the secretory process became less significant. At a physiological Ca2+ level, the involvement of L-type channels in evoked acetylcholine secretion was apparent, as in mouse synapses, only in conditions of blockade of voltage-gated K+ channels

Calcium Transients and Transmitter Secretion in Different Parts of Frog Nerve Endings in Different Conditions of Calcium Ion Influx

© 2020, Springer Science+Business Media, LLC, part of Springer Nature. 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