Accumulation of extracellular calcium at the endplate of mouse diaphragm after ecothiopate in vitro

Abstract

Experiments were carried out to investigate the accumulation from the extracellular medium of 45Ca2+ by the endplate region of skeletal muscle. Mouse diaphragm muscle was incubated in physiological saline labelled with 45Ca at 37°C for periods of up to 1.5 h. The muscle was divided into junctional and non‐junctional portions and the Ca from the extracellular fluid accumulated at the endplate determined from the 45Ca content of the portions. The accumulation of extracellular Ca at the endplate region of muscles incubated in physiological saline alone was nil, but there was accumulation in the presence of the anticholinesterase ecothiopate iodide 0.5 × 10−6 m (ECO). Stimulation of the phrenic nerve at 0.02 Hz caused no further increase in accumulation, but reduced the amount of spontaneous fasciculation. In tetrodotoxin (TTX) 10−6 m, the accumulation was halved, and in 3.5 mM Mg2+ the accumulation was nil. Carbachol 10−4 m resulted in an accumulation of Ca similar to that in ECO. It is concluded that there was an accumulation of extracellular Ca following excitation of the nerve by stimulation at a low frequency and during the spontaneous fasciculations, and about half of the accumulation of extracellular Ca after ECO in the experiments was due to the postsynaptic action of ACh released non‐quantally from the nerve terminals.

“Calcium bombs” as harbingers of synaptic pathology and their mitigation by magnesium at murine neuromuscular junctions

“…Direct observation of intracellular motor endplate Ca 2+ transients following AChE inactivation with OP compounds has not been reported previously to our knowledge, although several previous studies have described localized contractures of NMJs, and attributed these by inference to Ca 2+ loading 10.3389/fnmol.2022.937974 of motor endplates (Leonard and Salpeter, 1979;Duxson and Vrbová, 1985;Burd and Ferry, 1987;Burd et al, 1989;Ferry and Cullen, 1991). Optical measurements of Ca 2+transients at motor endplates of skeletal muscle fibers are technically difficult to make, due to large and unpredictable movements of muscle fibers in the image plane during stimulation.…”

Protection against the effects of anticholinesterases on the latencies of action potentials in mouse skeletal muscles

“…The prolonged action of ACh at the neuromuscular junction promotes the entry of Ca2+ to the muscle cell at the endplate (Evans, 1974;Miledi et al, 1977;Burd et al, 1989). An increased amount of calcium at the endplate has been found to be one of the earliest effects of ECO given in vivo (Townsend, 1988) and could initiate myopathy by a number of processes including generation of free radicals.…”