Myosin isoenzymes of fish hearts

1. Myosin extracted from ventricular and atrial muscles of some fish species were analysed by native and SDS gel electrophoresis. 2. Within the single heart, distinct types of native myosin were present in ventricular and atrial tissues. 3. Ventricular and atrial isomyosins contained two classes of light chain subunits. 4. The present results support the suggestion that the presence of multiple molecular forms of myosin in the heart is a common property of all vertebrates.     

Different distributions of microtubules, desmin filaments and isomyosins during the onset of cardiac hypertrophy in the rat

To elucidate the role of the cytoskeleton in the development of adult heart, microtubules and intermediate filaments of desmin were studied in young and adult rat heart myocytes during the onset of growth, after mechanical overloading induced by aortic stenosis. Such overloading is known to cause heart hypertrophy by stimulating overall protein synthesis, and to initiate a shift in myosin isozymes. For this study, we used double immunolabelling of isolated myocytes with specific antibodies raised against tubulin, desmin, and the two main isomyosins V1 and V3. Whereas desmin remained unchanged, tubulin was redistributed in arrays parallel to the long axis of the myocytes, and was densest around the nuclei. Alterations in the microtubule pattern were observed very early after aortic stenosis, during the onset of heart growth; they were transitory, and did not occur simultaneously in all myocytes. Chronological examination of myocytes labelling with both antitubulin and anti V3 myosin clearly suggested that the transitory alteration in the microtubule pattern was an early event preceding the change in the expression of the myosin gene. Results, observed in young rats, in which mitosis is stimulated by overloading, and in adult rats, exhibiting no mitosis, showed that microtubules are involved in the development of cells in which mitosis does not occur. This work provides the first evidence of a correlation in functional adult heart, between the reorganization of cytoplasmic microtubules and the onset of growth.     

Myosin structure and thyroidian control of myosin synthesis in urodelan amphibian skeletal muscle

Electrophoretic analysis in non-dissociating conditions reveals three types of myosin in adult urodelan amphibian skeletal muscles: 3 isoforms of fast myosin (FM), one isoform of intermediate myosin (IM) and one or two isoforms of slow myosin (SM). Each type is characterized by a specific heavy chain HCf (FM), HCi (IM) and HCs (SM), respectively. In all urodelan species, as in mammals, fast isomyosins associate HCf and the three fast light chains LC1f, LC2f, and LC3f. In most urodelan species the intermediate myosin contains LC1f and LC2f and can be considered as an homodimer of the alkali LC1f. However, in Euproctus asper, IM is characterized by the association of both slow and fast LC with HCi. Slow myosin is a hybrid molecule associating HCs with slow and fast LC. During metamorphosis, a myosin isoenzymic transition occurs consisting in the replacement of three larval myosins (LM) characterized by a specific heavy chain (HCI), by the adult isomyosins with lower electrophoretic mobilities. At the same time there is a change in the ATPase myofibrillar pattern, with the larval fiber types being replaced by adult fibers of types I, IIA and IIB. In the neotenic and perennibranchiate species, which do not undergo spontaneous metamorphosis, sexually mature larval animals present a change in the myosin isoenzymic profile, but no complete transition. The coexistence of larval and adult isomyosins and the persistence of transitional fibers of type IIC in the skeletal muscle are demonstrated. Experimental hypo- and hyperthyroidism indicate that thyroid hormone stimulates the regression of the larval isomyosins, possibly through indirect pathways. In contrast, the appearance and the persistence of the adult isomyosins seem to be independent of thyroid hormone. Thus, the control of the isoenzymic transition in the skeletal muscle of urodelan amphibians appears to imply indirect mechanisms, operating differently on each of the two phases of the complete transition.     

Myosin heavy-chain-based isomyosins in developing, adult fast-twitch and slow-twitch muscles.

A modified method of electrophoresis under nondenaturing conditions made it possible to separate rat muscle extracts of defined myosin heavy chain (HC) and light chain (LC) composition into subsets of developmental, fast and slow myosin heavy-chain-based isomyosins. The fastest migrating isomyosins were the neonatal isomyosins (nM1, nM2, nM3), followed by the slightly slower migrating embryonic isomyosins (eM1, eM2, eM3, eM4). Of the nine adult fast isomyosins, the HCIIb-based isomyosins (FM1b, FM2b, FM3b) were the fastest migrating. These were followed by the HCIId-based isomyosins (FM1d, FM2d, FM3d). The HCIIa-based isomyosins (FM1a, FM2a, FM3a) were the slowest. Our results suggest that FM3a is identical with the so-called intermediate isomyosin (IM) described in the literature. The slow myosin heavy-chain-based isomyosins (SM1, SM2, SM3) migrated far behind the fast isomyosins. Whereas the gross electrophoretic mobilities of each of these isomyosin triplets is determined by the specific heavy chain complement, the different mobilities of the bands within each triplet result from different alkali light chain combinations. Thus, the fastest triplet bands of the neonatal (nM1) and adult fast isomyosins (FM1b, FM1d, FM1a) represent the LC3f homodimers, the slowest (nM3, FM3b, FM3d, FM3a) the LC1f homodimers, and the intermediate bands (nM2, FM2b, FM2d, FM2a) the LC1f/LC3f heterodimers. Different proportions of the adult fast isomyosin triplet bands indicate that the affinity for LC3f decreases in the order HCIIb, HCIId, HCIIa. The three slow isomyosins represent LC1sa (SM1) and LC1sb (SM3) homodimers and a LC1sa/LC1sb heterodimer (SM2). Circumstantial evidence suggests an inverse order in rabbit muscle where SM1 and SM3 most likely represent LC1sb and LC1sa homodimers, respectively.     

Myosin types and fiber types in cardiac muscle. I. Ventricular myocardium

Antisera against bovine atrial myosin were raised in rabbits, purified by affinity chromatography, and absorbed with insolubilized ventricular myosin. Specific anti-bovine atrial myosin (anti-bAm) antibodies reacted selectively with atrial myosin heavy chains, as determined by enzyme immunoassay combined with SDS-gel electrophoresis. In direct and indirect immunofluorescence assay, anti-bAm was found to stain all atrial muscle fibers and a minor proportion of ventricular muscle fibers in the right ventricle of the bovine heart. In contrast, almost all muscle fibers in the left ventricle were unreactive. Purkinje fibers showed variable reactivity. In the rabbit heart, all atrial muscle fibers were stained by anti-bAm, whereas ventricular fibers showed a variable response in both the right and left ventricle, with a tendency for reactive fibers to be more numerous in the right ventricle and in subepicardial regions. Diversification of fiber types with respect to anti-bAm reactivity was found to occur during late stages of postnatal development in the rabbit heart and to be influenced by thyroid hormone. All ventricular muscle fibers became strongly reactive after thyroxine treatment, whereas they became unreactive or poorly reactive after propylthiouracil treatment. These findings are consistent with the existence of different ventricular isomyosins whose relative proportions can vary according to the thyroid state. Variations in ventricular isomyosin composition can account for the changes in myosin Ca2+-activated ATPase activity previously observed in cardiac muscle from hyper- and hypothyroid animals and may be responsible for the changes in the velocity of contraction of ventricular myocardium that occur under these conditions. The differential distribution of ventricular isomyosins in the normal heart suggests that fiber types with different contractile properties may coexist in the ventricular myocardium.     

Ventricular myosin from young and adult animals with respect to the thyroid state

Studies were conducted to analyze the effect of the thyroid hormone on ventricular myosin during ontogenesis of mice, rats and rabbits. Hypothyroidism was induced in mice and rats by administering propylthiouracyl in drinking water. Rabbits were made hyperthyroid by chronic administration of thyroxine. The change in the thyroid state of rats and rabbits influenced young and adult animals differently depending on whether V1 or V3 was the major ventricular isomyosin form present. Measurements of Ca2+-ATPase activity of myosins from young and old control animals and from animals with changed thyroid state showed that hypothyroidism in rats is associated with a greater decrease of myosin ATPase in young rats which contain V1 isomyosin only, when compared with old rats which contain a preponderance of V3 isomyosin and less of the V1 form. In rabbits, ATPase activity of ventricular myosin was more elevated after thyroxine administration in adult rabbits, which contain V3 isomyosin only, than in young rabbits in which myosin consists of V1 and V3 isomyosins. Ventricular myosins of young and adult mice did not differ in their ATPase activity and the treatment of mice with propylthiouracyl had only slight effect on myosin ATPase. It can be concluded based on these results that the hypothesis concerning hypothyroidism inducing transformation of V1 into V3 isomyosin does not hold generally.     

Assessment of the mechanical activity of cardiac isomyosins V1 and V3 by the in vitro motility assay with regulated thin filament

The dependences of thin filament sliding velocity on the calcium concentration in solution (pCa 5 to 8) for rabbit cardiac myosin isoforms V1 and V3 were determined in a set of experiments using an in vitro motility assay with a reconstructed thin filament. The constructed pCa-versus-velocity curves had a sigmoid shape. It was demonstrated that the sliding velocity of regulated thin filament at the saturating calcium concentration (pCa 5) did not differ from the actin sliding velocity for each isoform. The determined values of Hill’s cooperativity coefficient for isomyosins V1 and V3 were 1.04 and 0.75, respectively. It was demonstrated that isomyosin V3 was more sensitive to calcium as compared with isomyosin V1. Using the same assay, the dependence of thin filament sliding velocity on the concentration of the actin-binding protein α-actinin (analog of a force-velocity dependence) was determined at the saturating calcium concentration for each myosin isoform (V1 and V3). The results suggest that the calcium regulation of V1 and V3 contractile activity follows different mechanisms.     

Study of the interaction between rabbit cardiac contractile and regulatory proteins. An <Emphasis Type="Italic">in vitro</Emphasis> motility assay

A series of experiments was performed in an in vitro motility assay with reconstructed thin filaments to obtain pCa-force relationships for cardiac isomyosins V1 and V3. Two concentrations of each isomyosin (200 and 300 microg/ml) on the surface of a flow cell were tested. Isometric force was estimated as the amount of actin-binding protein, alpha-actinin, stopping thin filament movement. It was found that the amount of alpha-actinin stopping the movement at saturating calcium concentration for V3 was twice higher than for V1 at both concentrations of isoforms. Hill coefficients of cooperativity (h) were determined for pCa-force relationships. The value of h did not differ significantly for isoforms at 300 microg/ml of protein (h was 1.56 for V1 and 1.54 for V3). However, the Hill coefficient was higher for V3 isoform at 200 microg/ml (h = 2.00 and 1.76 for V3 and V1, respectively). Importantly, the Hill coefficient increased for both isoenzymes when their concentrations were decreased. The connection between Hill coefficient and cooperative interactions between cardiac contractile and regulatory proteins is analyzed in detail.     

Myosin light chain patterns in histochemically typed single fibers of the rat skeletal muscle.

1. This study was conducted to investigate the relationship between histochemical fiber types and myosin light chain patterns in rat single muscle fibers. 2. The hybrid of fast and slow light chains was observed in type I and II fibers of the soleus and type II fibers of the red portion of lateral head of the gastrocnemius muscles. 3. We also observed 7 types of light chain composition. Of the 7 types, 5 types were explainable by assuming the coexistence of isomyosins with either fast or slow light chains. However, the other 2 types could not be accounted for without hypothesizing the presence of isomyosins with promiscuous light chain distribution.     

Physiological adaptation of the heart to pathological overloading

Chronic overloading of the rat heart induces a cascade of adaptational events that compensate for the increase in work. At the myocardial level there are two types of adaptational mechanisms: qualitative, represented by the isomyosin changes leading to an improved efficiency; and quantitative, the hypertrophy. We present new approaches exploring possible adaptational changes at other levels within the myocardial cell. Studies of heart overload were performed either in young rats with experimental aortic stenosis or in humans with chronic compensatory hypertrophy. By means of double immunofluorescence labeling of isolated myocytes with anti-V1 and anti-V3 myosin immunoglobulins, we showed that the shift from high- to low-ATPase isomyosins occurs rapidly after aortic stenosis (2-3 days). Cardiac myocytes were shown to be poor in tubulin but a microtubule pattern was clearly visualized by an immunofluorescence approach. Their role in the onset of adaptational processes after aortic stenosis in not yet clear. On the other hand, we showed that in humans, contrary to small rodents, the adaptational process at the isomyosin level is very small or nonexistent.     

Influence of myocardial isomyosins on cardiac performance and oxygen consumption

There was 21% isomyosin V1 in the 12 month SHR (Spontaneous Hypertensive Rat) and 70% isomyosin V1 in the 6 month WKY (Wystar-Kyoto), nevertheless there was no difference in maximum developed pressure nor maximum dP/dt in the isovolumically beating hearts of the two sets of animals. [Hearts were perfused with hypercalcemic perfusate in the presence of isoproterenol (10(-7)M)]. There was, however, a a 32% reduction in oxygen consumption per gram of dry weight per beat in the 12 month SHR as compared to the 6 mo WKY. Associated with a shift towards isomyosins V2 and V3 in the 6 and 12 month WKY and SHR there was no corresponding change in maximum dP/dt nor developed pressure, but there was a conservation in oxygen consumption.     

Diversity of native myosin and myosin heavy chain in fish skeletal muscles

• 1.1. Polymorphism of native myosin and myosin heavy chain (MHC) of fish skeletal muscles was analysed by pyrophosphate and SDS-gel electrophoreses.
• 2.2. Depending on the species, three or four myosin isoforms were detected in the white muscle, one or two isoforms in the pure red muscle, and four isomyosins were found in the red muscle composed of red and pink (intermediate) fibres.
• 3.3. It is suggested that all main types of fish muscle fibre (red, intermediate and white) differ in myosin isoform content.
• 4.4. Myosin heavy chain of the red muscle is a distinct protein from that of the white muscle. However, structural differences between these proteins vary among species.

     

Subunit composition of rodent isomyosins and their distribution in hindlimb skeletal muscles

Three adult skeletal muscle sarcomeric myosin heavy chain (MHC) genes have been identified in the rat, suggesting that the expressed native myosin isoforms can be differentiated, in part, on the basis of their MHC composition. This study was undertaken to ascertain whether the five major native isomyosins [3 fast (Fm1, Fm2, Fm3), 1 slow (Sm), and 1 intermediate (Im)], typically expressed in the spectrum of adult rat skeletal muscles comprising the hindlimb, could be further differentiated on the basis of their MHC profiles in addition to their light chain composition. Results show that in muscles comprised exclusively of fast-twitch glycolytic (FG) fibers and consisting of Fm1, Fm2, and Fm3, such as the tensor fasciae latae, only one MHC, designated as fast type IIb, could be resolved. In soleus muscle, comprised of both slow-twitch oxidative and fast-twitch oxidative-glycolytic fibers and expressing Sm and Im, two MHC bands were resolved and designated as slow/cardiac beta-MHC and fast type IIa MHC. In muscles expressing a mixture of all three fiber types and a full complement of isomyosins, as seen in the plantaris, the MHC could be resolved into three bands. Light chain profiles were characterized for each muscle type, as well as for the purified isomyosins. These data suggest that Im (IIa) consists of a mixture of fast and slow light chains, whereas Fm (IIb) and Sm (beta) isoforms consist solely of fast- and slow-type light chains, respectively. Polypeptide mapping of denatured myosin extracted from muscles expressing contrasting isoform phenotypes suggests differences in the MHC primary structure between slow, intermediate, and fast myosin isotypes. These findings demonstrate that 1) Fm, Im, and Sm isoforms are differentiated on the bases of both their heavy and light chain components and 2) each isomyosin is distributed in a characteristic fashion among rat hindlimb skeletal muscles. Furthermore, these data suggest that the ratio of isomyosins in a given muscle or muscle region is of physiological importance to the function of that muscle during muscular activity.     

Effects of taurine and verapamil on heart calcium in hamsters and rats

Heart calcium was measured in hamsters and rats following 30 days of drinking (i) tap water, (ii) taurine solution (T) or (iii) a taurine uptake inhibitor (GES). The regimes were duplicated in animals receiving verapamil (V). Heart calcium was reduced in both species by T + V; a comparable effect was obtained with GES alone; T alone had no effect. In the hamster, the GES effect was reversed by GES + V; V alone had no effect. In rats, the separate and combined effects of GES and V on heart calcium were identical. Possible mechanisms and underlying species differences are discussed.     

Distribution of isomyosin in cultured cardiac myocytes as determined by monoclonal antibodies and adenosine triphosphatase activity

The distribution of isomyosin in cardiac muscle cells in culture has been investigated with monoclonal antibodies and Ca2+-activated myosin ATPase cytochemical staining. With immunofluorescent studies using monoclonal antibodies to isomyosins V1 and V3, the cardiac myocytes grown in a serum-free and thyroxine (T4)-free medium for 7 days contained a predominant population of cells which were strongly reactive to anti-V3 antibody. A small population of myocytes in this culture exhibited weak or no reaction to anti-V3 antibody. When cultures were exposed to anti-V1 antibody, the predominant cardiac myocyte population showed little or no reactivity to this antibody, whereas a small population of the myocytes were strongly reactive. The myosin ATPase staining reaction of the positive myocyte population was significantly less pronounced than that of the V3-negative population which showed a strong reaction. The staining pattern changed dramatically after exposure of cultured myocytes to thyroid hormone for 7 days. Most of the cells were found to react strongly with anti-V1 antibody, while some cells showed little reactivity and some were not stained at all. A small number of cardiac myocytes in this culture showed little or no reactivity to anti-V1 antibody but were strongly reactive to anti-V3 antibody. The predominant anti-V1-positive myocyte population exhibited strong myosin ATPase staining as compared to a smaller V3-positive myocyte population which showed very weak staining. The cytochemical results of ATPase staining in cardiac myocytes agreed well with ATPase activity as determined on pyrophosphate gels containing isomyosin derived from cultured cardiac myocytes with or without T4. This study has demonstrated that cultured myocytes contain a small population of muscle cells which is not responsive to thyroid hormone or to the lack of it.     

Regulation of myosin isoenzyme composition in fetal and neonatal rat ventricle by endogenous thyroid hormones

The effect of thyroid hormone on the expression of ventricular isomyosins V1, V2, and V3 was studied in fetal and neonatal rats. Between 15 and 21 days gestation, V3 accounts for 80-90% of fetal ventricular myosin. After birth, there is a rapid transition from the fetal V3 isotype to an equal mixture of V1 and V3 at 3 days, and to 100% V1 at 3 weeks of age. The endogenous serum levels of thyroxine (T4) and triiodothyronine (T3) increase from trace amounts in the fetus to adult levels at 2-3 weeks of age; this increase correlates with the maximal expression of V1 during the same period. Expression of the V1 isomyosin can be eliminated in the neonatal rat if endogenous thyroid hormone synthesis is suppressed by propylthiouracil (PTU) treatment. In the PTU-treated rats, V3 is the only isomyosin synthesized between 1 and 30 days of age. In fetal ventricle, the amount of V1 is also decreased but not completely eliminated by PTU treatment. Conversely, the relative amount of V1 can be increased in the fetal ventricle by increasing the fetal serum concentrations of T4 and T3 to adult physiological levels. In these fetal ventricles, V1 represents greater than 85% of the total myosin. Likewise, the expression and accumulation of V1 could be stimulated in ventricles of PTU-treated, 12-day-old rats by administration of pharmacological or physiological doses of T3. Within 4 to 8 h after an initial dose of T3, V1 accumulates to 5-10% of the ventricular myosin, and by 72 h comprises 60-80% of the myosin. These results indicate that endogenous thyroid hormone induces the synthesis of ventricular heavy chain alpha, which as a dimer forms the V1 isomyosin, or plays a permissive role for the continued synthesis of heavy chain alpha in ventricles of fetal and neonatal rats.     

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.     

The expression of myosin heavy chain isoforms in normal and hypertrophied chicken slow muscle

Hypertrophy was produced in the anterior latissimus dorsi (ALD) muscle of 5-wk-old chickens by application of a load to the humerus. After 4 wk, hypertrophied ALD muscles were greater than 2.5 times heavier than contralateral control ALD muscles. Two isomyosins are distinguishable in normal ALD muscles by their different electrophoretic mobilities. It is shown here that the faster migrating SM-1 isomyosin decreases in abundance with age and that the application of an overload enhances both the rate and extent of this process. Monoclonal antibodies were selected by an immunotransfer technique that were specific for the heavy chains associated with either SM-1 or SM-2, or cross-reacted with both isoforms. The cellular distribution of the SM-1 and SM-2 isomyosins was analyzed by immunofluorescent technique using these antibodies. Anti-SM-1 and anti-SM-2 antibodies reacted with separate populations of cells, whereas the third antibody reacted with all myocytes in the normal ALD muscle. These data suggest that there is an exclusive cellular distribution of myosin heavy chains associated with SM-1 and SM-2 proteins. Immunofluorescent analysis of hypertrophied muscle showed the anti-SM-2-specific antibody reacting with all myocytes, whereas the anti-SM-1-specific antibody reacted with none. This is consistent with the elimination of the SM-1 isoform in hypertrophied muscles.     

Leaf habit influences nitrogen remobilization in Vaccinium species.

The effect of N supply on plant growth and leaf demography of a deciduous and an evergreen Ericaceae was studied in relation to their internal cycling of N. Mature ramets of Vaccinium myrtillus (deciduous) and Vaccinium vitis-idaea (evergreen) were established in sand culture for 1 year with an adequate supply of a balanced nutrient solution. During one growing season, the plants were given two levels of N supply enriched with 15N and eight sequential destructive harvests were taken. Recovery of unlabelled N in the new shoot was used to determine the remobilization of N from storage. Initially, growth was unaffected by N supply. After May, High N enhanced growth for both species but the nature of their growth response differed. For both species, new shoot biomass and leaf number increased but root biomass production was affected for V. myrtillus only. Whole plant biomass production was similar for both species under High N, but was greater for V. vitis-idaea under Low N. The amount of N remobilized to support new shoot growth was similar for the two species and was independent of N current supply. N was remobilized predominantly from previous year leaves for V. vitis-idaea and from previous year stems and roots for V. myrtillus. The contribution of remobilization to new shoot N was similar for the two species, but depended on N supply. Remobilization was faster in V. myrtillus, but lasted longer in V. vitis-idaea. The results are discussed in relation to species growth in N-poor environments, focusing on the extent to which species-differences in the dynamic of N remobilization and growth may explain their adaptation to constant and/or changeable N supply.     

Developmental changes in myosin isoforms from slow and fast latissimus dorsi muscles in the chicken

In the course of muscle differentiation, changes in fibre-type population and in myosin composition occur. In this work, the expression of native myosin isoforms in developing fast-twitch (posterior latissimus dorsi; PLD) and slow-tonic (anterior latissimus dorsi; ALD) muscles of the chick was examined using electrophoresis under nondissociating conditions. The major isomyosin of 11-day-old embryonic PLD comigrated with the adult fast myosin FM3. Two additional components indistinguishable from adult fast FM2 and FM1 isomyosins appeared successively during the embryonic development. The relative proportion of these latter isoforms increased with age, and the adult pattern was established by the end of the 1st month after hatching. Between day 11 and day 16 of embryonic development, PLD muscle fibres also contained small amounts of slow isomyosins SM1 and SM2. This synthesis of slow isoforms may be related to the presence of slow fibres within the muscle. At all embryonic and posthatch stages, ALD was composed essentially of slow isomyosins that comigrated with the two slow components SM1 and SM2 identified in adult. Several studies have reported that the SM1:SM2 ratio decreases progressively throughout embryonic and posthatching development, SM2 being predominant in the adult. In contrast, we observed a transient increase in SM1:SM2 ratio at the end of embryonic life. This could reflect a transitional neonatal stage in myosin expression. In addition, the presence in trace amounts of fast isomyosins in developing ALD muscle could be related to the presence of a population of fast fibres within this muscle.