The entry of labelled calcium into the innervated region of the mouse diaphragm muscle

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.

Section: Methodsmentioning

confidence: 59%

Section: Introductionmentioning

confidence: 99%

Section: Determination Of the Accumulation Of Camentioning

confidence: 99%

See 1 more Smart Citation

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

Burd

Ferry

Smith

1989

“…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.…”

Section: Antioxidants Andjitter After Ecomentioning

confidence: 99%

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

Kelly¹,

Ferry²,

Bamforth³

et al. 1992

1 Adult male albino mice were injected subcutaneously with an organophosphorous anticholinesterase to initiate excessive variability in the latency of indirectly elicited muscle action potentials (jitter) when assessed 5 days later. 2 Pretreatment of the mice with a single dose of pyridostigmine prevented the development of jitter after subsequent dosing with an organophosphate. 3 Treatment with one dose of pralidoxime (2PAM) prevented the development of jitter if given less than 1 h after treatment with ecothiopate, a reactivatable inhibitor of cholinesterase. Similar treatment with 2PAM after a non-reactivatable inhibitor did not prevent the development of jitter. The repeated administration of 2PAM over 12 h did ameliorate jitter. 4 Pretreatment of mice orally with a-tocopherol and N-acetylcysteine, known to prevent ecothiopateinduced myopathy, did not prevent the development of jitter after ecothiopate. 5 It is concluded that the development of jitter was a consequence of the inhibition of acetylcholinesterase, and although jitter did not develop acutely, the potential for the full development of jitter was achieved about 1 h after intoxication with ecothiopate. The development of jitter did not involve the generation of free radicals. Reduction of the early effects of intoxication with anticholinesterases by pyridostigmine or 2PAM prevented the development of jitter.

“…Since this part of the acetylcholine contraction is completely dependent on extracellular calcium, we conclude that calcium ions enter the myoplasm from the extracellular space and directly activate the myofibrils. It is to be expected that some Ca2+ would enter through the acetylcholine controlled ionic channels since acetylcholine increases endplate permeability to Ca2+, as well as to Na+ and K+ (Takeuchi, 1963;Katz & Miledi, 1969;Evans, 1974). Furthermore, it has been shown that sufficient calcium ions can enter by this route to initiate directly a contraction both in denervated mammalian muscle (Jenkinson & Nicholls,196 1) and in frog muscle (Manthey, 1974 (Ebashi & Endo, 1968 (Dorscheidt-Kiffer & Luttgau, 1974) by delaying contractile inactivation (Andersson & Edman, 1974) it is likely that the 'second phase' tension in this experiment was in fact the first phase, greatly prolonged.…”

Section: Acetylcholine Second Phasementioning

confidence: 99%

The Role of Extracellular Calcium in the Contractions Produced by Acetylcholine in Chronically Denervated Muscle

Hall¹,

Maleque²,

Wadsworth³

1977

I Acetylcholine-induced contractions of the isolated chronically denervated soleus muscle of the mouse consist of two phases, but both phases are equivalent to the contracture phase seen in vivo.2 Low [Ca2+J0 (0.5-1.5 mM) augmented peak tension, as well as the rate of relaxation, of the first phase, but inhibited the second phase. Ethyleneglycol-bis4p-aminoethyl ether)-N,NM-tetraacetic acid (EGTA) or La3+ (2 mM) also inhibited the second phase, but not the first. 3 It was concluded that the first phase requires Ca2+ release from the sarcoplasmic reticulum, and is terminated by inactivation of the contractile process. The second phase is caused by the entry of activator Ca2+ from the extracellular space.4 Increasing [Ca2+]o to 5 or 10 mm after the addition of acetylcholine caused a contraction, starting after a delay of about 50 seconds. EGTA or La3+ added during the second phase of the acetylcholine contraction caused relaxation after a much shorter lag time. 5 It is concluded that most of the Ca2+ entering from the extracellular fluid is taken up by the sarcoplasmic reticulum. 6 The acetylcholine second phase was augmented in low (25 mM) [Na+I0. It is concluded that Na+ and Ca2+ compete for the acetylcholine controlled ionic channels. 7 Isolated chronically denervated diaphragm muscles were less sensitive to acetylcholine and the contraction usually consisted of a first phase only. 8 It is concluded that sequestration of Ca2+ entering from the extracellular fluid is more complete in the diaphragm.