Myosin switches in skeletal muscle development of an urodelan amphibian, Pleurodeles waltlii. Comparison with a mammalian, Mus musculus

The characterisation of the 5' regulatory region of a temperature-induced myosin-heavy-chain gene associated with myotomal muscle growth in the carp

We have isolated and characterised the 5' region of a member of the carp myosin heavy chain gene family. Expression of this gene has previously been shown to be induced by an increase in environmental temperature and is restricted to the small-diameter white myotomal muscle fibres which are associated with growth. The whole isoform gene, including potential regulatory sequence 5' to the transcription start site and the 3' untranslated region was cloned in a lambda2001 bacteriophage vector. Studies of the structure of the 5'-end of the gene revealed high amino acid sequence similarity with translated exons 3-7 of mammalian myosin heavy chain genes indicating identical exon/intron boundaries. The overall length of the gene was however only about one half of that in mammals and birds due to shorter introns. The region 5' to the transcription unit was sequenced and revealed the presence of putative TATA and CCAAAT boxes. In order to study the regulation of expression, a series of endonuclease-generated fragments from the 5' flanking sequence were spliced to chloramphenicol acetyltransferase reporter vectors and used in cell transfection assays or direct gene injection into carp skeletal muscle. The 5' flanking region, which contains a consensus sequence known as an E-box (CANNTG) and a MEF2 binding site, was shown to improve the expression of the reporter gene in fish acclimated at 18 degrees C or 28 degrees C. Unlike the coding region, there was little similarity between the 5'-upstream sequence (promoter region) when compared with sequences flanking the 5'-end of the other myosin heavy chain genes in mammals or chicken.

Pattern of muscle fiber type formation in the pig

The aim of this study was to analyze the temporal sequence of expression of the myosin isoforms in the populations of muscle fibers in the pig and to bring more information on the origin of the strikingly different pattern of fiber composition and distribution between the deep medial red (oxido-glycolytic) and superficial white (glycolytic) portions of semitendinosus (ST) muscle. Muscle samples were taken from 49-, 55-, 75-, 90-, 103-, and 113- (birth) day-old fetuses, from 6-, 11-, 21-, 35-, 50-, and 80-day-old piglets, and from a 3-year-old pig. Our results confirm the sequential formation of primary and secondary generation fibers. The use of immunohistochemistry and heterologous monoclonal antibodies (mAb) directed against specific myosin heavy chain (MHC) isoforms revealed a different pattern of gene expression between the two portions of the ST muscle for both generations of fibers. By 75 days of gestation (dg), primary myotubes from the deep medial portion stained positively for the anti-slow MHC mAb and negatively for the adult anti-fast MHC, whereas the opposite was observed in the superficial portion. Secondary fibers never expressed slow MHC until late gestation. Instead, they expressed an adult fast MHC isoform as soon as they formed in the deep medial portion and later on in the superficial portion. From late gestation to the first 3 postnatal weeks, slow MHC began to be expressed in a subpopulation of secondary fibers. These fibers were in the direct vicinity of primary myotubes in the deep medial portion, whereas their location could not be established in the superficial portion. The remaining secondary fibers matured to type IIA in the direct vicinity of these type I fibers and to type IIB at the periphery of the islets. In both portions of the muscle, a subpopulation of secondary fibers, the first ones to express slow MHC, also transitorily expressed a MHC that was identical or closely related to the alpha-cardiac MHC during the early postnatal period. A third generation of small diameter fibers was observed shortly after birth and reacted with the anti-fetal MHC mAb; their destiny remains to be established.(ABSTRACT TRUNCATED AT 400 WORDS)

A new approach of urodele amphibian limb regeneration: study of myosin isoforms and their control by thyroid hormone

In P. waltlii, an urodele amphibian species which undergoes spontaneous metamorphosis, study of native myosin in pyrophosphate gels at various stages of normal development demonstrates a complete larval to fast myosin isoforms transition, which occurs more precociously in forelimb muscles than in the dorsal and ventral muscles. In the neotenic species A. mexicanum, forelimb muscles development also presents a complete myosin isoforms transition which is in contrast with the partial myosin isoforms transition observed in the dorsal muscle. In metamorphosed or neotenic animals of both species aged 1 year, forelimb regeneration is characterized by a complete transition from larval to fast myosin isoforms, that occurs earlier and more rapidly than in normal forelimb development. When forelimb regeneration is studied in P. waltlii aged 4 years, the adult fast and slow isomyosins are expressed very early in the regeneration process. In experimental hypothyroidian P. waltlii, the larval to fast isoforms transition in regenerating forelimb muscles is slightly delayed. Experimental hyperthyroidism accelerates the disappearance of larval isomyosins in regenerating forelimb muscles, both in P. waltlii and A. mexicanum aged 1 year. This work demonstrates that changes in myosin isoform pattern during forelimb regeneration in adult urodele amphibians are different from changes occurring in the normal forelimb development. They take place without any thyroid hormone influence, as opposed to normal development, and appear to be age-dependent.

Myosin, parvalbumin and myofibril expression in barbel (Barbus barbus L.) lateral white muscle during development

Histo- and immunohistochemical techniques have recently been used to study the fibre type and myosin expression in fish muscle during development. In the present work, embryonic, larval and adult myosin isozymes (heavy and light chains) and parvalbumin isotypes were analyzed, from fertization to the adult stage, by polyacrylamide gel electrophoresis of barbel (Barbus barbus L.) trunk muscle extracts. The examined myosins display the sequential transitions from embryonic to larval and adult forms characteristic of higher vertebrates. They are characterized by specific heavy chains but their light chains differ only by the LC1/LC3 stoichiometry with LC3 exceeding LC1 after 10 days. Sarcoplasmic parvalbumins show considerable and unforeseen developmental transitions in their isotype distribution: the PA II isotype first appears after hatching and becomes the predominant form until the length reaches about 6 cm. One month after hatching, the amount of PA II then decreases and the synthesis of PA III and IV further increases to reach the typical adult pattern at a size of 18 cm. These observations show that the distribution of parvalbumin isotypes reflects the stage of development. It suggests a specific role for each isotype in relation to muscle activity. Microscopy illustrates the progressive development of somites, muscles cells, and myofibrils, which accelerates at hatching when movements increase.

Response of slow and fast muscle to hypothyroidism: maximal shortening velocity and myosin isoforms

This study examined both the shortening velocity and myosin isoform distribution of slow- (soleus) and fast-twitch (plantaris) skeletal muscles under hypothyroid conditions. Adult female Sprague-Dawley rats were randomly assigned to one of two groups: control (n = 7) or hypothyroid (n = 7). In both muscles, the relative contents of native slow myosin (SM) and type I myosin heavy chain (MHC) increased in response to the hypothyroid treatment. The effects were such that the hypothyroid soleus muscle expressed only the native SM and type I MHC isoforms while repressing native intermediate myosin and type IIA MHC. In the plantaris, the relative content of native SM and type I MHC isoforms increased from 5 to 13% and from 4 to 10% of the total myosin pool, respectively. Maximal shortening velocity of the soleus and plantaris as measured by the slack test decreased by 32 and 19%, respectively, in response to hypothyroidism. In contrast, maximal shortening velocity as estimated by force-velocity data decreased only in the soleus (-19%). No significant change was observed for the plantaris.

White muscle differentiation in the eel (Anguilla anguilla L.): changes in the myosin isoforms pattern and ATPase profile during post-metamorphic development

Myosin isoforms and their light and heavy chains subunits were studied in the white lateral muscle of the eel during the post metamorphic development, in relation with the myosin ATPase profile. At elver stage VI A1 the myosin isoforms pattern was characterized by at least two isoforms, FM3 and FM2. The fast isomyosin type 1 (FM1) appeared during subsequent development. It increased progressively in correlation with the increase in the level of the light chain LC3f. FM1 became predominant at stage VI A4. At the elver stage VI A1, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed at least two heavy chains, namely type II-1 and II-2. The type II-1 heavy chain disappeared in the yellow eel white muscle, and V8-protease peptide map showed the appearance of a minor heavy chain type II-3 as early as stage VI B. Comparison of myosin heavy chains and myosin isoforms patterns showed the comigration of different myosin isoforms during white muscle development. The myosin ATPase profile was characterized by a uniform pattern as far as stage VI A4. A mosaic aspect in white muscle was observed as early as stage VI B, showing the appearance of small acid labile fibers. This observation suggests that the type II-3 heavy chain is specific to the small fibers.

Influence of hyperthyroidism on maximal shortening velocity and myosin isoform distribution in skeletal muscles

The objectives of this study were 1) to examine the effects of hyperthyroidism on the myosin isoform distribution in slow and fast skeletal muscle, 2) to explore how these effects were manifested with respect to the force-velocity relationship and maximal shortening velocity, and 3) to contrast two different techniques of measuring maximal shortening velocity under normal and hyperthyroid conditions. Adult female Sprague-Dawley rats were randomly assigned to one of two groups: control (n = 8) or hyperthyroid (n = 8). Hyperthyroidism was induced by injections of 3,3',5-triiodo-L-thyronine every other day for 20 wk. We found that hyperthyroidism produced a significant shift in the myosin isoform distribution of the soleus but not the plantaris. The relative amount of the slow myosin isoform was reduced from a control value of 93 to 69% in the hyperthyroid condition. In contrast, both the intermediate and fast myosin-3 isoform pools were substantially increased (P less than 0.001) by approximately fourfold. Hyperthyroidism produced an increase in the maximal shortening velocity of the soleus as measured either by the slack test (+57%; P less than 0.001) or by extrapolation of force-velocity data (+33%; P less than 0.001). The hyperthyroid condition did not, however, affect the mechanical properties of the plantaris.

Comparison of myosin isoenzymes present in skeletal and cardiac muscles of the Arctic charr Salvelinus alpinus (L.). Sequential expression of different myosin heavy chains during development of the fast white skeletal muscle

The expression of myosin isoforms and their subunit composition in the white skeletal body musculature of Arctic charr (Salvelinus alpinus) of different ages (from 77-day embryos until about 5 years old) was studied at the protein level by means of electrophoretic techniques. Myosin from the white muscle displayed three types of light chain during all the developmental stages examined: two myosin light chains type 1 (LC1F) differing in both apparent molecular mass and pI, one myosin light chain type 2 (LC2F) and one myosin light chain type 3 (LC3F). The fastest-migrating form of LC1F seemed to be predominant during the embryonic and eleutheroembryonic periods. The slowest-migrating form of LC1F was predominant in the 5-year-old fish. Between 1 year and 4 years, both types of LC1F were present in similar amounts. Cardiac as well as red muscle myosin from 3-year-old fish had two types of light chain. The myosin light chains from atria and ventriculi were indistinguishable by two-dimensional electrophoresis, but were different from the myosin light chains from red muscle. Neither the light chains from cardiac nor red muscle were coexpressed with the myosin light chains of white muscle at any of the developmental stages examined. Two myosin heavy chain bands were resolved by SDS/glycerol/polyacrylamide gel electrophoresis of the extract from embryos. One of the bands was present in minor amounts. The other, and most abundant, band comigrated with the only band found in the extracts of white muscle myosin from older fish. One-dimensional Staphylococcus aureus V8 protease peptide mapping of these bands revealed some differences during development of the white muscle tentatively interpreted as follows. The myosin heavy chain band present in minor amounts in the embryos may represent an early embryonic form that is replaced by a late embryonic or foetal form in the eleutheroembryos. The foetal myosin heavy chain appears to be present until the resorption of the yolk sack and beginning of the free-swimming stage. A new form of myosin heavy chain, termed neonatal and probably expressed around hatching, is present until about 1 year of age.(ABSTRACT TRUNCATED AT 400 WORDS)

Myosin structure in the eel (Anguilla anguilla L.). Demonstration of three heavy chains in adult lateral muscle

Myosin extracts from central white fibers and peripheral red fibers of the lateral muscle of eel (Anguilla anguilla) were analysed by electrophoresis under non-dissociating conditions, which demonstrated a polymorphism of myosin isoforms. The light and heavy subunit content of the isomyosins was established using SDS-PAGE and two-dimensional electrophoresis. In the central white muscle, 3 myosin isoforms FM3, FM2, FM1, were characterized by 3 types of fast light chain and one fast heavy chain HCf; the existence of a fourth isomyosin is discussed. In the peripheral red muscle, two myosin isoforms were found, SM1 and SM2, each characterized by a specific heavy chain, HCs1 or HCs2, and containing the same slow light chain content. This work demonstrates for the first time the existence of 3 heavy chains in the skeletal muscle of a fish.

Myosin isoforms in red and white muscles of some marine teleost fishes

The myosin content from red and white muscles of three marine fish species, saithe (Pollachius virens. L.), haddock (Melanogrammus aeglefinus, L.), both members of the family Gadidae, and capeline (Mallotus villosus, M.) of the family Osmeridae, was analyzed electrophoretically. Analysis of the native myosin by electrophoresis under non-dissociating conditions revealed two isoforms in red muscles, and three or four in white muscles. The white muscles of the two closely related species had a similar pattern of isoforms. Myosin from the slow red muscles had two types of light chain, LC1S and LC2S, and myosin from the fast white muscles three, LC1F, LC2F, and LC3F. The pattern of light chains in both types of muscles was species-dependent. All the light chains from fish myosins were more acidic than those of the rat diaphragm used as standard. One main type of heavy chain was detected in each kind of muscle. In white muscles of saithe there was an extra band, present in minor amounts. The heavy chains from white muscle myosin had lower electrophoretic mobilities than those from red muscle, and the mobilities of all of them were intermediate between those of the heavy chains type IIa and I of rat diaphragm myosin. In our opinion, there are probably more isomyosins in fish muscles than those detected in the present work and their presence is obscured by comigration with the main types.

Intraspecific myosin light chain polymorphism in the white muscle of herring (Clupea harengus harengus, L.)

The myosin contained in white and red muscles of herring (Clupea harengus harengus) was purified, and its subunit composition analyzed by electrophoretic techniques. The only myosin isoform present in red muscles was made up of one type of heavy chain and two types of light chain. The native myosin from white muscles migrated as one wide band. Analysis of the extracts by SDS/glycerol/PAGE from white muscles revealed one main type of heavy chain. Light chains were identified by SDS-PAGE analysis of electrophoretically purified myosin, and two-dimensional electrophoresis of the extracts demonstrated differences in the light chain composition of white and red muscles. Using this methodology, light chain polymorphism was detected in white muscles among members of the same species.

Electrophoretic study of myosin isoforms in white muscles of some teleost fishes

1. White skeletal muscle myosin of four marine teleost fish species (cod, blue whiting, Norway haddock, and spotted wolf-fish) was analyzed by native, SDS-PAGE, and 2-dimensional electrophoresis. 2. Four types of native myosin were present in cod, blue whiting and Norway haddock. The second fastest migrating form was predominant. 3. Myosin from spotted wolf-fish also resolved into four forms. The fastest migrating form was hardly noticeable. The other three were present in apparently similar amounts. 4. In the myosin from each species there were three types of light chains. The pattern of light chains was species specific. 5. Apparently, there was only one type of heavy chain in myosin from cod, Norway haddock and spotted wolf-fish. One preparation of cod showed an extra band of higher electrophoretic mobility than the main band. In blue whiting we found two bands present in approximately equal amounts.

Thyroidal status and myosin isoenzymic pattern in the skeletal dorsal muscle of urodelan amphibians--the perennibranchiate Proteus anguinus

In the perennibranchiate Proteus anguinus, larval myosin isoforms were shown to coexist for life with the adult isomyosins that appeared at the end of the larval stage. Analysis of the myofibrillar ATPase profile also revealed that a high percentage of immature fibers persisted in adults. A long-term treatment with large amounts of T3 had no effect on juvenile individuals. Applied to subadult animals it promoted a regression of larval myosin isoforms and a reduction in the percentage of immature fiber types. The regulative effect of T3 in the myosin isoenzymic transition may be delayed and depends on metabolic conditions, which suggests it is indirect.

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. Thus, half-transition times for the appearance of the adult intermediate and fast myosin extended from seven days for diaphragm, the most precocious muscle of all those examined, to 23 days for male rat masseter. Besides the large differences between their half-transition times, we noticed that the transition curves displayed different slopes, covering different periods. Differences between muscles mainly affected the neonatal-to-adult transition rather than the embryonic-to-neonatal transition, since the embryonic-type myosin disappeared from all muscles examined except for one, at about the same time, by the end of the first week after birth. In addition, the appearance of slow myosin varied for each muscle and did not follow curves parallel to those for intermediate and fast myosins. These results indicate that each muscle of the rat is subjected to a specific program of myosin isoform transitions during postnatal development.

Correlation among the proton charges and molecular masses of myosin subunits

The molecular masses and isoelectric points of myosin light and heavy chains were calculated from their known primary sequences and their respective distribution in a two-dimensional graph is displayed. Implications for the electrophoretic study of myosin subunits are inferred from this analysis.

Myosin isoenzymes and their subunits in urodelan amphibian fast skeletal muscle. Coexistence of larval and adult heavy chains in neotenic individuals

The distributions of native myosin isoforms were examined by electrophoresis under non-dissociating conditions, in the fast twitch dorsal skeletal muscle of young larvae, neotenic adults and metamorphosed adults of urodelan amphibians. Both heavy and light chains of myosin isoenzymes were analysed. In pyrophosphate acrylamide gel electrophoresis three isoenzymes were demonstrated in larval myosin; other isoforms of lower electrophoretic mobility were observed in metamorphosed adults myosin. Larval and adult isoenzymes were shown to coexist in myosin from neotenic adults. Analysis of heavy chains in denaturing conditions and proteolytic digestion revealed the sequential occurrence during development of two types of heavy chains, one larval and one adult, that coexist in the myosin of neotenic adults only. Analysis of light chain patterns under denaturing conditions revealed the existence of three fast light chains which displayed no modification during the course of development. The neotenic urodelan amphibian species model represents actually the only model in which the coexistence of larval (or neonatal) and adult heavy chains is maintained throughout life in adults.

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. Adult myosins thus appeared more precociously than in development, which implies that the synthesis of myosin isoforms during regeneration does not entirely 'recapitulate' the sequence of myosin transitions observed during normal development. Two weeks after the injury, the isomyosin electrophoretic pattern displayed by regenerated muscles was already the same as that of control muscles; the normal adult pattern was therefore expressed more rapidly in regenerating than in developing muscles. Except for the synthesis of the slow isoform which was generally inhibited in denervated muscles, the same types of myosins were expressed during the early stages of regeneration in denervated as in innervated muscles; long-term denervation prevented however the qualitative and quantitative recovery of the normal myosin pattern.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development

Changes in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. I. Neonate mouse

Changes both in the ATPase myofibrillar profile and in the electrophoretic pattern of myosin isoforms were examined in the mouse dorsal skeletal muscle (longissimus) during postnatal development. In the newborn, only type II C and a few type I fibers were present; differentiation into type II A and II B fibers took place during the 3 weeks following birth. During the same period, a transition from three neonatal isomyosins to four adult isoforms was observed. The two phenomena were related to a marked increase in the serum thyroid hormones levels. Hypothyroidism and hyperthyroidism experiments were performed. Hypothyroidism produced by propylthiouracil treatment of pregnant females and thiourea injections of the litters was shown to induce a complete inhibition of postnatal muscular differentiation. Hyperthyroidism produced by triiodothyronine treatment of the neonate mice significantly accelerated the myosin transition and the switch in the myofibrillar pattern. Our results suggest a primordial role for thyroid hormones in directly regulating the appearance of myosin and fiber adult types and in modulating directly or indirectly the disappearance of the neonatal types.

Regulation by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. II. Urodelan amphibians

In the urodelan amphibian Pleurodeles waltlii, spontaneous anatomical metamorphosis was correlated with an increase in the serum level of thyroxine (T4). It was also accompanied by a change in the myofibrillar ATPase profile of the dorsal skeletal muscle; fibers of larval type were gradually replaced by the adult fiber types I, II A, and II B. Likewise, a myosin isoenzymic transition was observed in dorsal muscle, larval isomyosins were replaced by adult isoforms. In a related species, Ambystoma mexicanum, in which no spontaneous external metamorphosis occurs under standard conditions, the serum T4 level was shown to remain low. During further development, the myofibrillar ATPase profile acquired the adult fiber types, but a high percentage of immature fibers of type II C persisted. Myosin isoenzymic transition was also incomplete; larval isoforms were still distinguished in the neotenic adults. In experimental hypothyroidian P. waltlii, no external metamorphosis occurred; the myofibrillar ATPase profile was of the immature type, and the larval isomyosins persisted. Triiodothyronine induced experimental anatomical metamorphosis in A. mexicanum; only limited changes in the myofibrillar ATPase profile resulted from the treatment, but a complete myosin isoenzymic transition was observed. These results tend to indicate that a moderate increase in the level of thyroid hormone is sufficient to induce the differentiation of adult fiber types, together with the production of adult myosin isoforms in the skeletal dorsal muscle of amphibians, while a pronounced increase would be necessary for repressing the initial larval features.

Isoforms of myosin and actin in human, monkey and rat myometrium. Comparison of pregnant and non-pregnant uterus proteins

Using several electrophoretic procedures, we have compared the forms of myosin and actin in pregnant and non-pregnant uterus of woman, monkey (Macaca fascicularis) and rat. On non-dissociating gels, native myosin of the three species migrates as a single band, of identical mobility independently of the physiological state. Remigration of this band in dissociating conditions shows that it is constituted of two heavy chains of respectively 201 kDa and 205 kDa; the relative proportions of these two bands are different for the three animal species but do not vary during pregnancy. Using two-dimensional gel electrophoresis, we found that the 17-kDa light chain of purified uterus myosin exists under two isoelectric forms, the more acidic one becoming progressively predominant at the end of pregnancy in the human as in the monkey uterus, while we observed no changes in the rat. In two-dimensional gel electrophoresis, actin of human, monkey and rat uterus is present under three isoforms, the most basic one (the gamma form) increasing early in pregnancy in the two primate species but being always the most abundant form in the rat. The ATPase activity of human uterus myosin was found to be similar for the protein extracted from both pregnant and non-pregnant uterus. The changes observed in the 17-kDa light chain and in the actin isoforms might nevertheless participate in the modifications of contractility of the uterus during pregnancy of the primates.

Myosin light chain phosphorylation in intact rat uterine smooth muscle. Role of calcium and cyclic AMP

Myometrial strips from oestrogen-primed rat uterus were exposed to various treatments, isometric contraction was measured, and the extent of myosin light chain phosphorylation determined after rapid freezing in liquid nitrogen. Two-dimensional electrophoresis revealed five spots having the same molecular weight as the light chain, with isoelectric points comprised between 5.15 and 4.95. Two of these spots (pI 5.09 and 5.00) were not present in pure uterine myosin, whether prepared from incubated or nonincubated tissue; they do not represent light chain isoforms or electrophoresis artefacts but rather degradation products appearing during the treatment. Two spots (pI 5.15 and 5.06) were identified as the nonphosphorylated and the phosphorylated forms of the light chain. The fifth minor spot (pI 4.95) may represent a diphosphorylated myosin species. Strips incubated in a normal Ca2+-medium 0.8 mM) exhibited basal contractions and an incorporation of 0.2 mol phosphate per mol light chain. Removal of Ca2+ resulted in almost complete dephosphorylation, coincident with a total relaxation of the muscle. Exposure of the myometrium to carbachol caused tetanic contractions with an increase to 0.5 mol phosphate per mol light chain. Isoproterenol, a beta-adrenergic agonist elevated intracellular cyclic AMP and induced uterine relaxation. Addition of isoproternol to a resting myometrium caused a slight but significant decrease in phosphorylation; its addition prior to carbachol markedly prevented the increase in myosin phosphorylation normally induced by the cholinergic effector. Forskolin (1 microM) increased intracellular cyclic AMP, caused relaxation and a concomitant decrease in basal myosin phosphorylation. Prostaglandin E2-induced elevation in intracellular cyclic AMP was however accompanied by an increase in contraction together with an increase in light chain phosphorylation. The data imply that light chain phosphorylation-dephosphorylation, regulated by Ca2+-dependent mechanisms, is essential for both uterine contraction and relaxation but question the role of cyclic AMP in exclusively mediating relaxation and myosin dephosphorylation in intact myometrium.

Comparison of myosins from the masseter muscle of adult rat, mouse and guinea-pig. Persistence of neonatal-type isoforms in the murine muscle

Adult rat, mouse, and guinea-pig masseter muscles display distinct myosin electrophoretic patterns. The rat muscle contains four main forms which by reference to the myosins of the IIB tensor fasciae latae, of the IIA mylohyoid, and of the red and white portions of the sternomastoid muscles, correspond respectively to the intermediate-type and to the three fast-type isoforms. The mouse masseter muscle contains only three main myosins, the intermediate-type and two fast-type isoforms. The guinea-pig muscle also displays only three bands, whose assignment is, however, less straightforward than in the murine species; their electrophoretic mobilities are not strictly the same as those of their homologous forms in rat and mouse. Comparison with the myosins of the tensor fasciae latae and of the sternomastoid muscles of guinea-pig allows their identification as intermediate and fast-type myosins. In addition to these typical adult-type forms, adult murine masseter muscles are observed to contain between zero and 30% of neonatal-type myosins. The comparison of the developmental transitions of myosins in the rat masseter with those in the skeletal muscles of the same animal indicates a delay in the appearance of the adult as well as in the disappearance of the neonatal-type myosins in the masseter muscle. Both the variability in myosin types with the animal species and the atypical presence of neonatal forms in the murine adults suggest that myosin expression in the masseter muscle is subjected to unusual regulations.