The role of resting potential changes in the contractile failure of frog sartorius muscles during calcium deprivation

174

105

SUMMARY1. Previous work has shown that in calcium-free solutions nerve impulses invade the motor nerve terminals at the neuromuscular junction, but fail to release transmitter. In these conditions, strontium ions applied iontophoretically to a minute part of a junction, or to the whole muscle by bath application, restore to the nerve impulse its ability to release transmitter.2. As with calcium, the transmitter released in the presence of strontium is in the form of packages (quanta) whose release can be predicted from Poisson's Theorem.3. The mean number of quanta released by a nerve impulse increases with the concentration of strontium. Strontium is much less effective than calcium in equimolar concentrations.4. Transmitter quanta released in the presence of strontium evoke larger unit potentials than quanta released in the presence of calcium. The larger size of the Sr-unit potentials is caused by a prolongation of transmitter action, presumably due to a post-synaptic effect of strontium.5. Neuromuscular transmission was blocked in some fibres when the concentration of strontium was raised beyond 10 mm. This junctional block was presumably due to a failure in the propagation of nerve impulses.6. The post-stimulation increase in the frequency of miniature end-plate potentials, which is normally seen in calcium solutions, is also observed when calcium is substituted by strontium. The post-stimulation effect increases with the concentration of strontium.

Section: Discussionmentioning

confidence: 99%

Strontium and quantal release of transmitter at the neuromuscular junction

Dodge¹,

Miledi²,

Rahamimoff³

1969

174

105

“…When the ionic composition of this solution was varied, the tonicity was held constant by adjusting appropriately the concentration of the NaCl. Since solutions deficient in both Ca and Mg are deleterious to nerve and muscle (Biilbring, Hollman & Liillman, 1956;Curtis, 1963;Jenden & Reger, 1963;Frankenhauser, 1957), we added Mg to all the low-Ca solutions. In our initial experiments 4 mM-Mg was used since this concentration of Mg has roughly the same effect on nerve excitability as does the Ringer concentration of Ca (Frankenhauser & Meves, 1958).…”

Section: Methodsmentioning

confidence: 99%

Effects of calcium and magnesium on the frequency of miniature end‐plate potentials during prolonged tetanization

Hurlbut¹,

Longenecker²,

Mauro³

1971

1. End‐plate potentials (e.p.p.s) and miniature end‐plate potentials (min.e.p.p.s) were recorded intracellularly from the cutaneous pectoris nerve‐muscle preparation of the frog during prolonged stimulation at low frequencies (5/sec—50/sec). 2. When Ca was present in the bathing solution, the quantum content of the e.p.p. and the frequency of occurrence of the min.e.p.p.s gradually increased during the period of stimulation. During the first few minutes of stimulation, the min.e.p.p. frequency increased linearly with time, and the rate of increase was dependent on the Ca concentration of the bathing solution. However, Mg had no effect on this Ca‐dependent increase in min.e.p.p. frequency. 3. A large maintained increase in min.e.p.p. frequency also occurred during prolonged stimulation in solutions that contained no added Ca and 1‐2 m M‐EGTA. Under these conditions the increase in min.e.p.p. frequency was dependent on the Mg concentration of the bathing solution and was exponential in time. 4. It is suggested that the rise in min.e.p.p. frequency is caused by an accumulation of Ca or Mg ions in the nerve terminal, and it is suggested that these ions enter the terminal at relatively non‐specific sites distinct from the Ca‐specific sites that trigger the ‘phasic’ release of transmitter.

“…2E) indeed, the rate of fall of tension was slightly faster when these ions were present. In this respect the locust muscle differs from vertebrate 'twitch' muscle fibres (Jenden & Reger, 1963); this point will be discussed in the following paper (Aidley, 1965). The effect of treatment with chelating agents In order to investigate a little further the role of calcium ions in contracture, use was made of the chelating agent ethylenediamine tetra-acetic acid (EDTA).…”

Section: Resutltsmentioning

confidence: 99%

“…Potassium contractures in frog toe muscles, which are composed mainly of twitch fibres, are abolished by treatment of the muscle with calcium-free solutions (Frank, 1960), but Curtis (1963) showed that, under these conditions, contractions in response to depolarizing pulses applied through an intracellular electrode could be restored by passing hyperpolarizing current through the membrane. Jenden & Reger (1963) suggested that the contractile failure of frog sartorius muscles in calcium-free solutions was due to two processes: (i) the fall in resting potential which occurred, and (ii) a rise in the potential at which the coupling process could be 'primed'. Consequently, in vertebrate twitch muscles, the fact that calcium deprivation results in abolition of the contractile response to depolarization does not in itself imply that calcium ions are directly involved in the excitation-contraction coupling process.…”

Section: Calcium Ions and Insect Musclementioning

confidence: 99%

The effect of calcium ions on potassium contracture in a locust leg muscle

Aidley¹

1965

It seems very probable that movement of calcium ions is involved in the excitation-contraction coupling process in vertebrate muscle. The presence of calcium ions in the extracellular fluid appears to be necessary for the production of potassium contractures in vertebrate skeletal (Frank, 1960; Liittgau, 1963), heart (Niedergerke, 1956) and smooth muscle (Edman & Schild, 1962). In contrast to these observations, Hoyle (1961) found that potassium contractures of a locust spiracular muscle were maintained for as long as 17 hr in isotonic solutions of potassium chloride or sulphate in the absence of calcium. These results raised the possibility that excitationcontraction coupling in insect muscles does not involve calcium ions.This paper reports the results of experiments on the effect of calcium ions on isometric potassium contractures in a locust leg muscle. A preliminary report of some of this work has appeared elsewhere (Aidley, 1963). METHODSThe experiments were performed on the extensor tibialis of the mesothoracic leg of the locust Schistocerca gregaria.Tension recording. After light carbon dioxide anaesthesia, the head, abdomen, wings and gut of the insect were removed. The thorax was placed in a T-shaped Perspex chamber with the right mesothoracic leg projecting into the 'stem' of the T (Fig. 1). The leg was thus contained in an isolated cell through which physiological saline could be circulated. The femur was attached to the floor of the cell with Eastman 910 adhesive, and the coxa and base of the femur were firmly fixed with dental cement in the slot in the wall dividing the two sections of the chamber. A silk thread was tied round the tibia (the knot being cemented with Eastman 910 adhesive) and its other end was tied to a 5-link length of fine chain. A movable brass hook engaged the distal link of this chain and served to hold the tibia in the extended position during the dissection. The ventral cuticle and flexor tibialis muscle were removed from the femur and the femoro-tibial joint was then dissected so that contraction of the muscle caused the tibia to move horizontally along its axis. The ventral cuticle of the thorax was removed so as to expose the motor nerves; these could be stimulated by square pulses applied through chlorided silver wire electrodes.Tension was recorded by means of an RCA type 5734 transducer valve. The anode peg was extended by a short length of hypodermic tubing and a stainless-steel hook; the * Present address: Department of Zoology, University of Oxford.