Early days in the research to localize skeletal muscle acetylcholinesterases.

Very early in the study of the mechanism of neuromuscular transmission in skeletal muscles, it was clear that the hydrolysis of acetylcholine by muscle cholinesterases within the time of the refractory period required a very high concentration of the enzyme near the motor terminals. David Nachmansohn and George B. Koelle and their collaborators obtained the first biochemical and histochemical data consistent with this prediction.

Relationship between muscle myosin isoforms and contractile features in rabbit fast-twitch denervated muscle.

The effects of 8-day-old rabbit fast-twitch gastrocnemius denervation on the type of myosin isoforms and on contractile features (maximum velocity Vmax and contraction time (CT) of the muscle were followed between 15 and 60 days postnatal. The myosin isoforms and the Vmax and CT values of the denervated gastrocnemius displayed large changes during this period. These changes, which led at 2 months postnatal to a muscle displaying the properties of a slow-twitch muscle did not occur in synchrony: complete conversion to slow-type myosin isoforms occurred only at 60 days postnatal, whereas complete conversion to slow-twitch Vmax and CT values occurred as soon as 35 days postnatal.

Response to denervation of rabbit soleus and gastrocnemius muscles. Time-course study of postnatal changes in myosin isoforms, fiber types, and contractile properties.

In contrast to general belief, the response of rabbit muscles to denervation is maturation to slow-like type muscles [7]. We report now an investigation by biochemical, morphological, and mechanical studies of the time course effects of muscle denervation on the slow-type soleus and fast-type gastrocnemius to help elucidate the mechanism of maturation of rabbit denervated muscles to slow-like muscles. In both muscles, denervation induced selective progressive atrophy of most fast fibers and hypertrophy of many slow fibers which displayed wide Z-lines; this was accompanied by the appearance of hybrid LC1F- and LC1E-associated slow myosins.

The effect of denervation on myosin isoform synthesis in rabbit slow-type and fast-type muscles during terminal differentiation. Denervation induces differentiation into slow-type muscles.

The soleus and gastrocnemius medialis of eight-day-old rabbits were denervated and the effects were examined after fifty-two days by biochemical, cytochemical and mechanical methods. The contralateral soleus exhibited the properties of slow-type muscle, namely a predominance of slow-type myosin isoforms and slow-type oxidative fibers, slow twitch and low maximal velocity for shortening. The contralateral gastrocnemius exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening.

Opposite regulations by androgenic and thyroid hormones of V1 myosin expression in the two types of rabbit striated muscle: skeletal and cardiac.

The finding that V1 cardiac myosin is expressed in masticatory skeletal muscles of the rabbit provided a unique opportunity for comparing the hormonal regulation of V1 in skeletal and cardiac muscles. Thyroid hormones had no significant effect on the postnatal expression of V1 in masticatory muscles, but increased this expression in cardiac ventricles. In contrast, androgenic hormones reduced V1 expression in masticatory muscles, but did not affect it significantly in cardiac ventricles.

Reinnervation of denervated extensor digitorum longus of the rat by the nerve of the soleus does not induce the type I myosin synthesis directly but through a sequential transition of type II myosin isoforms.

The fast-contracting extensor digitorum longus (EDL) muscle of 1-month-old rats was denervated and reinnervated by the nerve innervating the slow-contracting soleus muscle. After variable periods of time, the myosin isoform content of the EDL was analyzed by sensitive electrophoretic techniques, which allowed to discriminate between the slow-type I and the three, IIA, (IID or IIX) and IIB, fast-type II myosin isoforms. Compared to the control EDL, which contains predominantly the IIB isoform, the operated muscles contained variable proportions of all the isoforms.

Effect of testosterone and thyroid hormone on the expression of myosin in the sexually dimorphic levator ani muscle of rat.

During postnatal development, the myosin transition from embryonic and neonatal isoforms to adult isoforms has been shown to occur with half-transition times of about 20 and 32 days in the male and female levator ani muscles, respectively. We show that this difference could not be attributed to the testosterone male hormone, since treatment of newborn females by testosterone did not modify the half-transition time. However, treatment of females by thyroid hormone accelerated the myosin transition of the female muscle, which then occurred at almost the same time as the transition of the male muscle.

Species- and muscle type-dependence of perinatal isomyosin transitions.

The progressive transition from developmental to adult myosin isoforms during perinatal development was quantified in four muscles (diaphragm, gastrocnemius medialis, masseter and tongue) of four mammals (guinea-pig, hamster, rabbit and rat). It was observed that the timing of transition varied for each muscle, and differed according to the mammal as well. This suggests that the synthesis of adult myosin isoforms may be partly related to the specialized contractile function of a given muscle in a given species.

The same myosin isoforms are found in the female and male sexually dimorphic levator ani muscle of the rat, but their postnatal transitions are not synchronous.

The levator ani of the female adult rat is greatly atrophied in comparison to the same muscle in males. In the present study, the female levator ani was, nevertheless, found to contain type IIb myosin isoforms similar to those contained in the male muscle. These adult type isoforms were, however, synthesized later in the female than in the male levator ani: the half-transition times of the myosin transition curve were 20 days postnatal in the male and 35 days postnatal in the female.

Muscle-specific response to thyroid hormone of myosin isoform transitions during rat postnatal development.

Transitions from embryonic and neonatal to adult-type-II isomyosins are known to be related to the increase in the thyroid hormone plasma concentration during postnatal development. These transitions have been shown, however, to occur at different times, depending on the muscle, suggesting that each muscle responds differently to the thyroid hormone. We have investigated quantitatively the effects of experimental hypothyroidism and hyperthyroidism on isomyosin transitions from birth until the 45th postnatal day in eight rat muscles: diaphragm, intercostals, gastrocnemius medialis, soleus, plantar muscles of the foot, tongue muscle, levator ani and bulbocavernosus complex, and masseter.

Myosin isoform transitions in regeneration of fast and slow muscles during postnatal development of the rat.

Regeneration of rat fast (gastrocnemius medialis) and slow (soleus) muscles was examined after degeneration of myofibers had been achieved by injection of cardiotoxin into the hindleg during the first week after birth. Myogenesis in the regenerating muscles was compared to postnatal myogenesis in the contralateral and in control muscles. Synthesis of embryonic and neonatal myosin isoforms was initiated 3 days after injury.

Specific programs of myosin expression in the postnatal development of rat muscles.

The expression of myosin during postnatal development was studied in a dozen muscles of the rat. All muscles displayed the usual sequential transitions from embryonic to neonatal and to adult isomyosins. However, we observed that these transitions did not take place uniformly.

Regeneration after cardiotoxin injury of innervated and denervated slow and fast muscles of mammals. Myosin isoform analysis.

The regeneration of adult rat and mouse slow (soleus) and fast (sternomastoid) muscles was examined after the degeneration of myofibers had been achieved by a snake venom cardiotoxin, under experimental conditions devised to spare as far as possible the satellite cells, the nerves, and the blood vessels of the muscles. Three days after the injury, no myosin was detectable in selected portions of the muscles. New myosins of embryonic, neonatal, and adult types started to be synthesized during the following two days.

Regeneration of muscles after cardiotoxin injury. I. Cytological aspects.

Regeneration of several adult rat and mouse skeletal muscles was studied after degeneration of muscle fibers had been obtained by the selective action of the cardiotoxin of Naja mossambica mossambica venom. Experimental conditions were set up to ensure minimal damage to satellite cells and also the nerves and blood vessels of the original muscles. As in the other types of experimental regeneration, the structure of the regenerated muscle appeared in many respects different from that of the normal muscle.

[Myosin isoforms synthesized during regeneration of fast contraction skeletal muscle regeneration, in the presence of motor nerve and after denervation. Study in adult rats and mice].

Adult rat and mouse fast contracting skeletal muscles were injured by a cardiotoxin. New myosins of embryonic, neonatal and adult types appeared 4 and 5 days after the treatment in both innervated and denervated muscles. Although their structure remained altered, innervated--but not denervated--muscles rapidly recovered a normal isomyosin pattern.

[The regulator role of thyroid hormones in myogenesis. Analysis of isoforms of myosin in muscular regeneration].

Fast-twitch muscle regeneration has been studied in experimental hyper- and hypothyroid adult rats. The degeneration of the muscle fibres was achieved through the injection of a snake venom cardiotoxin and the synthesis of new isomyosins was examined 7, 10, 15, and 21 days after the injury. As early as the 7th day after the toxin treatment, that is 3 days after the start of the regeneration, the muscles of hyperthyroid rats do not contain any neonatal myosins and synthesize only adult myosins.

[Dedifferentiation and remodeling of motor endplates following experimental localized lesions of muscle fibers].

Local anaesthetics, cardiotoxin and mechanical injuries may cause necrosis of muscle fibres while leaving the motor nerve fibres and their terminals intact. With local injuries to mouse muscles carried out by freezing or cutting we made a point of preserving both the nerve terminals and the muscle fibre portions on which these terminals were located. It was thus possible to follow the changes induced at endplates by these lesions.

Structure of the subsynaptic sarcoplasm in the interfolds of the frog neuromuscular junction.

After aldehyde fixation of frog muscles, complex organelles, which appear specific to the subsynaptic sarcoplasm, were observed at the neuromuscular junction. These organelles have a cylindrical shape; their diameter ranges generally between 150 and 300 nm, and their length between 500 nm and several micrometres. They are situated between the folds formed by the postsynaptic membrane beneath the nerve terminal branches, and, like these folds and the filament bundles of the interfolds, are orientated perpendicular to the axis of the terminal branches.

[Structural changes in motor endplates in frog muscle treated with vinblastine].

Two kinds of abnormalities have been observed in motor nerve endings of muscles soaked in vinblastine: the occurrence of paracrystalline structures, which could derive from the precipitation of one or several proteins in the axon terminal, and the presence of large vesicles, often of festooned shape. These abnormal vesicles seem to result from the fusion of synaptic vesicles and could explain the appearence of spontaneous giant potentials which are most probably produced by the release of big packets of ACh.

[Differentiation factors of active zones of presynaptic membranes].

The differentiation of the subsynaptic areas depends very probably on a local influence exerted by the axon terminals; but conversely, as suggested by obervations on the development of neuromuscular junctions of "fast" and "slow" muscle fibres in Anura, complementing the results of previous degeneration experiments on frog muscles, the subsynaptic areas might intervene in the differentiation of "active zones" of presynaptic membranes.