Increased binding of alpha-bungarotoxin in dystrophic mouse muscle

Single channel properties of synaptic acetylcholine receptors in dystrophic fibers

Neuromuscular transmission in dystrophic mice has been extensively studied through analysis of nerve-evoked endplate potentials. In the present study, single channel ACh-induced activity recorded from endplates of fast twitch muscle fibers from mdx and dy dystrophic mice, was compared with activity recorded from wild-type fiber endplates to ascertain whether expression of these phenotypes leads to changes in ACh receptor properties and synaptic transmission. An 89 pS class of ACh-induced events predominated in recordings from wild-type and both strains of dystrophic mice. The mean open times for this class of events was well described by two exponential components, one with a voltage-independent time constant of approximately 0.3 ms and the other with a time constant of 2-5 ms which increased e-fold with approximately 120 mV of hyperpolarization. The expression of the mdx or the dy phenotype was not associated with significant differences in the conductance, distribution of open durations, or the voltage-dependence of the mean open time for this class of ACh-induced events.

The combined action of exercise and methylprednisolone sodium succinate on the rat gastrocnemius muscle

1. Methylprednisolone sodium succinate (SM) administration reduces the amount of contractile proteins and increases the collagen in the rat gastrocnemius muscle; it also increases the muscle's isotonic contraction capacity. 2. Exercise in combination with SM treatment reduces this capacity and causes also further reductions of the contractile proteins and increases of collagen. 3. There are no changes of either the relative muscle weight or the number of acetylcholine receptors in any experimental group as compared to controls.

The origin of (+)-tubocurarine resistance in dystrophic mice

Intracellular recording, twitch responses and radio-ligand binding techniques were used to study the causes of resistance to (+)-tubocurarine (curare) of extensor digitorum longus (EDL) muscles from dystrophic mice (129 ReJ/strain). The indirectly evoked twitch response of muscles from dystrophic mice was more resistant to block by curare than the twitch response of muscles from normal littermates. The IC50 (concentration producing 50% inhibition of stimulus-evoked contractions) values for the curare block of muscle twitch were 0.78 +/- 0.03 microM and 1.32 +/- 0.05 microM (mean +/- 95% confidence limits) for muscles from normal and dystrophic mice, respectively. There was no difference between muscles from normal and dystrophic mice in the number of alpha-bungarotoxin binding sites per endplate. The amplitudes of both spontaneous miniature endplate potentials (m.e.p.ps) in unblocked preparations and of evoked endplate potentials (e.p.ps) in 1.91 microM curare were greater in muscles from dystrophic mice than in muscles from normal mice. The ratio dystrophic/normal was greater for the e.p.p. amplitudes than for the m.e.p.p. amplitudes. The quantum content of e.p.ps in magnesium-blocked and in cut-fibre preparations was greater in muscles from dystrophic mice than in muscles from normal littermates. Calculation of the binomial parameters n and p in the cut-fibre preparations indicated that this increased quantum content was caused by an increase in the value of p. It is concluded that at least part of the increased resistance to curare of the indirectly evoked twitch response of muscles from dystrophic mice is due to an increase in the quantum content of e.p.ps in these muscles.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Denervation supersensitivity in skeletal muscle: analysis with a cloned cDNA probe

Motor neurons regulate the acetylcholine sensitivity of the muscles they innervate: denervated muscle fiber become "supersensitive" to acetylcholine, due to insertion of newly synthesized acetylcholine receptors (AChRs) in the plasma membrane. We used hybridization analysis with a cloned cDNA specific for AChR alpha-subunit to compare the abundance of AChR mRNA in innervated and denervated adult mouse muscles. Within 3 d of denervation, levels of AChR mRNA increased 100-fold; levels of actin mRNA changed little. The increase in AChR mRNA level was sufficiently large and rapid to account for denervation supersensitivity.

Increased muscle action potentials by 5 Hz prolonged nerve stimulation in neurological and neuromuscular disorders--clinical usefulness for detecting underlying pathophysiology

The time courses of changes in amplitudes of muscle action potentials (MAPs) obtained from gastrocnemius and soleus muscles by 5 Hz prolonged tibial nerve stimulation were studied. Subjects included muscular dystrophy (MD), spinal muscular atrophy, Issacs syndrome, idiopathic muscle spasms, psychiatric disorders such as autism and schizophrenia, and normal controls. In normal subjects, MAPs obtained at 5 minutes from gastrocnemius muscles was 87-102% of those at initiation of the stimulation. In soleus muscles, MAPs at 5 minutes was 95-105% of those at the beginning. In gastrocnemius muscles, MAPs increased in disorders such as Duchenne MD, Fukuyama type congenital MD, facioscapulohumeral MD, myotonic dystrophy, dermatomyositis, Kugelberg-Welander syndrome, viral myelitis, malignant hyperpyrexia, autism and schizophrenia. In soleus muscles, the increase of MAPs was demonstrated in Duchenne MD, Fukuyama type congenital MD, myotonic dystrophy and autism. MAPs remained within normal range in infants with Werdnig-Hoffman disease, Issacs syndrome and idiopathic muscle spasms. In two cases with Duchenne MD, MAPs obtained from gastrocnemius muscles reduced in amplitudes by the administration of dantrolen sodium. While the pathogenesis of the increased MAPs is not clear, several possible factors are discussed. It is considered that this 5 Hz examination may provide an important information for detecting the effect of dantrolen sodium on Duchenne MD, and it is also suggested that the examination will be a useful test for finding latent malignant hyperpyrexia.

Muscular dystrophy in the mouse: neuromuscular transmission and the concept of functional denervation

The results of recent investigations by ourselves and others indicate that no form of denervation exists to any remarkable degree in dystrophic mouse skeletal muscles. This conclusion is based on the following information: Dystrophic nerve terminals liberate normal amounts of transmitter both spontaneously and during impulse-mediated activity. The characteristics of the release process, the size of the available store of transmitter, and the probability of release of transmitter in response to the invasion of an action potential appear to be normal. The sensitivity of the postsynaptic membrane to the transmitter is normal. Action potential generation in response to both direct and indirect excitation is normal. There is no unequivocal pharmacologic evidence of denervation in dystrophic skeletal muscle, even though dystrophic muscle fibers respond to surgical denervation in a normal fashion. Nerve terminal sprouting is extensive, but there is no evidence of collateral reinnervation.

Ouabain binding sites in skeletal muscle from normal and dystrophic mice

The specific binding of tritiated ouabain was used to estimate the density of Na+-K+-ATPase sites ("Na+-pump" sites) in segments of skeletal muscle from normal and dystrophic mice. Ouabain binding was approximately 4 times greater in red (soleus) muscle than in white (superficial gastrocnemius) muscle from normal animals. In dystrophic soleus muscles, ouabain binding was decreased by nearly one-half. Because Na+-K+-ATPase activity is associated with plasma membranes, these observations constitute further evidence for a sarcolemmal abnormality in dystrophic mice.