Ventricular Myosin Research Articles

Complete sequence and characterization of chick ventricular myosin heavy chain in the developing atria

We isolated five complementary DNA (cDNA) clones, encoding the chick ventricular myosin heavy chain (MyHC) by reverse transcription polymerase chain reaction (RT-PCR). The entire cDNA consists of 5995 nucleotides with the 52 bp 5′-untranslated region and the 129 bp 3′-untranslated region. The complete cDNA encodes 1937 amino acids. Expression of the chick ventricular MyHC gene was also studied by Northern blot analysis. This gene continued to be strongly expressed in the ventricle during cardiac development. On the other hand, its expression was moderate in the early embryonic atria, and was down-regulated during development. In the adult atria, this gene was expressed at very low levels. To determine the localization of the ventricular MyHC protein, an immunohistochemical study was performed. The ventricular MyHC was present in early embryonic atrial myocytes. During development, the expression of this protein in the atrial myocytes was down-regulated, but continued to be present in the atrial conduction system. Our results indicate that the ventricular MyHC appears in the primary atrial myocardium and is then localized in the conduction cells of the atria.

Recovery after Cardioplegia in the Hypertrophic Rat Heart

Background.Enhanced recovery after cardioplegic arrest has been observed in rat hearts with hypertrophy induced by hemodynamic overload. We hypothesize that this is related to altered characteristics of hypertrophied myocardium—reflected by increased V3 isomyosin and glycolytic potential—other than increased left ventricular mass.Materials and methods. Isolated hearts from age-matched nonoperated and sham-operated control rats and from aortic-banded, hyperthyroid, and hypothyroid rats—groups in which hypertrophy and V3 as a percentage of left ventricular myosin vary independently—underwent 2 h of multidose cardioplegic arrest at 8°C followed by reperfusion at 37°C. Left ventricular V3 isomyosin was evaluated after separation by gel electrophoresis.Results. Moderate left ventricular hypertrophy was produced by aortic banding or hyperthyroidism and atrophy by hypothyroidism. V3 isomyosin was increased in banded (28%) and hypothyroid (75%) rats compared to control (12%) and hyperthyroid rats (7%). Myocardial glycogen content closely paralleled %V3. At 30 min of working reperfusion, functional recovery (assessed as percentage prearrest cardiac output) was 66 ± 4 and 68 ± 5% in control and hyperthyroid hearts and 81 ± 2 and 80 ± 5% in hearts from banded and hypothyroid rats (each P < 0.05 vs controls), respectively. At 30 min, hearts from banded and hypothyroid rats were also more efficient (as indexed by cardiac output at constant mean aortic pressure/myocardial oxygen consumption) than control and hyperthyroid hearts.Conclusions. The data suggest that recovery is related not to increased mass but to other changes in overload hypertrophy. Increased percentage V3 isomyosin and glycogen reflect these changes and may themselves contribute to improved functional recovery after cardioplegic arrest, as may increased postischemic efficiency.

Jaw-closing muscles of kangaroos express alpha-cardiac myosin heavy chain.

The masseter muscle of eutherian grazing mammals typically express beta or slow myosin heavy chain (MyHC). Myosins in the masseter of 4 species of kangaroos and a slow limb muscle of one of them were compared with their cardiac myosin by pyrophosphate and sodium dodecyl sulphate (SDS) gel electrophoresis, immunoblotting and immunohistochemistry. It was found that ventricular muscle contains three isoforms homologous to V1 (alpha-MyHC homodimer), V2 (heterodimer) and V3 (beta-MyHC homodimer) of eutherian cardiac muscle, and that the masseter contained V1, with traces of V2 and V3, in great contrast to eutherian ruminants, which express only V3. A polyclonal antibody (anti-KJM) was raised in rabbits against red kangaroo masseter myosin. After cross-absorption against limb muscle myofibrils, anti-KJM specifically reacted in Westerns with MyHCs from masseter but not limb muscles, and immunohistochemically with masseter, but not limb muscle fibers. In pyrophosphate Western blots, anti-KJM reacted with V1 but not with V3. However, a monoclonal antibody specific for eutherian slow myosin stained all kangaroo slow muscle fibers but only weakly stained scattered fibers in the masseter. The SDS-PAGE revealed that light chain composition of masseter and ventricular myosins is identical, but isoforms of both light chains of kangaroo limb slow myosin were observed. These results confirm that kangaroo jaw muscle express alpha-MyHC rather than beta-MyHC. The difference in MyHC gene expression between marsupial and eutherian grazers may be related to the fact that kangaroos are not ruminants, and have only a single chance to comminute food into fine particles, hence the need for the greater speed and power of muscle contraction associated with V1 containing muscle fibers.

ラット心筋の局所的ミオシンアイソザイム構成比に及ぼすランニングトレーニングの影響

We examined the effect of running training on regional cardiac myosin isozyme composition in rats. Male Sprague-Dawley strain rats (4 weeks old) were used, and divided into two groups: sedentary control (C) and trained (T) groups. The T group was trained by treadmill running (40 m/min, 1h/day, 5 days/week, for 12 weeks) . At 16 weeks old, their hearts were excised. The left ventricle was separated into the subendocardium (Endo) and subepicardium (Epi) by dissecting the ventricle at the mid-wall. The ventricular myosin isozymes were examined by electrophoresis on pyrophosphate gel under non-dissociating conditions. The results showed the following: 1) The relative heart weight of the T group was significantly higher than the C group. 2) Left ventricular myosin isozyme composition showed a region-specific distribution in the C rats, and the proportion of V3 myosin or β-myosin heavy chain in the Endo was significantly higher than that of the Epi. However, the training had no effect on the cardiac myosin isozyme in either portion. 3) The activity of citrate synthase did not show transmural gradient in the ventricle of C animals. Training had no effect on the activities of either portion. 4) The activity of lactate dehydrogenase (LDH) showed transmural gradient in the ventricle of C rats. Training-induced changes in the activity of LDH were found in both portions, therefore, training abolished the transmural gradient in the activity of LDH, suggesting a corresponding redistribution of the myocardial work load.These results indicate that running training might induce the redistribution of the myocardial work load, whereas the stimulation apparently has no effect on the regional distribution of cardiac myosin isozyme composition.

Restricted Expression of Cardiac Myosin Genes Reveals Regulated Aspects of Heart Tube Assembly in Zebrafish

The embryonic vertebrate heart is divided into two major chambers, an anterior ventricle and a posterior atrium. Although the fundamental differences between ventricular and atrial tissues are well documented, it is not known when and how cardiac anterior–posterior (A–P) patterning occurs. The expression patterns of two zebrafish cardiac myosin genes, cardiac myosin light chain 2 (cmlc2) and ventricular myosin heavy chain (vmhc), allow us to distinguish two populations of myocardial precursors at an early stage, well before the heart tube forms. These myocardial subpopulations, which may represent the ventricular and atrial precursors, are organized in a medial–lateral pattern within the precardiac mesoderm. Our examinations of cmlc2 and vmhc expression throughout the process of heart tube assembly indicate the important role of an intermediate structure, the cardiac cone, in the conversion of this early medial–lateral pattern into the A–P pattern of the heart tube. To gain insight into the genetic regulation of heart tube assembly and patterning, we examine cmlc2 and vmhc expression in several zebrafish mutants. Analyses of mutations that cause cardia bifida demonstrate that the achievement of a proper cardiac A–P pattern does not depend upon cardiac fusion. On the other hand, cardiac fusion does not ensure the proper A–P orientation of the ventricle and atrium, as demonstrated by the heart and soul mutation, which blocks cardiac cone morphogenesis. Finally, the pandora mutation interferes with the establishment of the early medial–lateral myocardial pattern. Altogether, these data suggest new models for the mechanisms that regulate the formation of a patterned heart tube and provide an important framework for future analyses of zebrafish mutants with defects in this process.

Желудочковый и предсердный кардиальный миозин при дилатационной кардиомиопатии: сравнительное исследование иммунореактивности

Желудочковый и предсердный кардиальный миозин при дилатационной кардиомиопатии: сравнительное исследование иммунореактивности

Tuning the human heart molecular motors by myosin light chains.

Cardiac contraction is triggered by the cyclic interaction of the "molecular motor" protein myosin with the actin filament, consuming ATP as the energy source to produce tension or shortening. The myosin heavy chain (MHC) contains the actin- and ATP-binding sites and represents the molecular motor of muscle contraction. This review describes the various subunits of human heart myosin in health and disease and discusses their functions. Two different MHC genes (alpha and beta) with distinct biochemical features are expressed in the human heart. Alpha-MHC confers a higher ATPase activity and higher shortening velocity to the heart than beta-MHC. Motor function is regulated by myosin light chain (MLC) isoforms. Expression of the atrial MLC-1 isoform in the hypertrophied human ventricle increases cross-bridge cycling and contractility. It is suggested that MLC-1 acts as a MHC/actin tether. Weakening of this tether increases myosin function. MLC-2 slows the rate of tension development of myosin. This relative inhibition is relieved upon phosphorylation of the MLC-2 perhaps caused by "swing-out" of cross-bridges from the myosin filament. Mutations in all ventricular myosin subunits have been found in patients with hypertrophic cardiomyopathy.

Subclass Specificity of Autoantibodies against Myosin in Patients with Idiopathic Dilated Cardiomyopathy: Pro-inflammatory Antibodies in DCM Patients

Detection of antimyosin antibodies in non-inflammatory cardiac disease undermines their disease specificity as a sensitive marker of damage in dilated cardiomyopathy (DCM) patients. Antibody subclass specificity could provide a more sensitive marker of disease and possibly discriminate the humoral autoimmune responses in different cardiac diseases. Frequency and reactivity of autoantibodies against α- and β-isoforms of myosin heavy chain (mhc) were evaluated by ELISA for IgG, IgM, and subclasses IgG1, IgG2, and IgG3 in patients with DCM (NYHA III/IV, n = 82), end stage ischemic heart disease (E-IHD: NYHA III/IV, n = 62), mild ischemic heart disease (NYHA I/II, n = 27), and controls (n = 54). Autoantibodies against atrial and ventricular myosin were raised in heartfailure patients compared to mild-IHD and controls but with different antigen affinities. Reactivity in E-IHD was significantly raised against (ventricular) β-mhc compared with only mild-IHD patients, suggesting a relative increase in ventricular specific antibodies in IHD patients with a higher NYHA class. IgG subclass analysis for IgG1, IgG2, and IgG3 against α- and β-mhc showed statistically raised levels of IgG3 only in DCM patients and a significantly higher reactivity of IgG2 in heartfailure patients versus controls. The results demonstrate immunological heterogeneity of antimyosin antibodies developed in different clinical entities. Pro-inflammatory characteristics of IgG3 antibodies in a select group of patients with DCM may contribute to autoimmune mechanisms of injury in these patients.

A Post-transcriptional Compensatory Pathway in Heterozygous Ventricular Myosin Light Chain 2-Deficient Mice Results in Lack of Gene Dosage Effect during Normal Cardiac Growth or Hypertrophy

Our previous study of homozygous mutants of the ventricular specific isoform of myosin light chain 2 (mlc-2v) demonstrated that mlc-2v plays an essential role in murine heart development (Chen, J., Kubalak, S. W., Minamisawa, S., Price, R. L., Becker, K. D., Hickey, R., Ross, J., Jr., and Chien, K. R. (1998) J. Biol. Chem. 273, 1252-1256). As gene dosage of some myofibrillar proteins can affect muscle function, we have analyzed heterozygous mutants in depth. Ventricles of heterozygous mutants displayed a 50% reduction in mlc-2v mRNA, yet expressed normal levels of protein both under basal conditions and following induction of cardiac hypertrophy by aortic constriction. Heterozygous mutants exhibited cardiac function comparable to that of wild-type littermate controls both prior to and following aortic constriction. There were no significant differences in contractility and responses to calcium between wild-type and heterozygous unloaded cardiomyocytes. We conclude that heterozygous mutants show neither a molecular nor a physiological cardiac phenotype either at base line or following hypertrophic stimuli. These results suggest that post-transcriptional compensatory mechanisms play a major role in maintaining the level of MLC-2v protein in murine hearts. In addition, as our mlc-2v knockout mutants were created by a knock-in of Cre recombinase into the endogenous mlc-2v locus, this study demonstrates that heterozygous mlc-2v cre knock-in mice are appropriate for ventricular specific gene targeting.

Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium.

1. The effects of ventricular myosin heavy chain (MHC) composition on the kinetics of activation and relaxation were examined in both chemically skinned and intact myocardial preparations from adult rats. Thyroid deficiency was induced to alter ventricular MHC isoform expression from approximately 80% alpha-MHC/20% beta-MHC in euthyroid rats to 100% beta-MHC, without altering the expression of thin-filament-associated regulatory proteins. 2. In single skinned myocytes, increased expression of beta-MHC did not significantly affect either maximal Ca2+-activated tension (P0) or the Ca2+ sensitivity of tension (pCa50). However, unloaded shortening velocity (V0) decreased by 80% due to increased beta-MHC expression. 3. The kinetics of activation and relaxation were examined in skinned multicellular preparations using the caged Ca2+ compound DM-nitrophen and caged Ca2+ chelator diazo-2, respectively. Myocardium expressing 100% beta-MHC exhibited apparent rates of submaximal and maximal tension development (kCa) that were 60% lower than in control myocardium, and a 2-fold increase in the half-time for relaxation from steady-state submaximal force. 4. The time courses of cell shortening and intracellular Ca2+ transients were assessed in living, electrically paced myocytes, both with and without beta-adrenergic stimulation (70 nM isoproterenol (isoprenaline)). Thyroid deficiency had no affect on either the extent of myocyte shortening or the resting or peak fura-2 fluorescence ratios. However, induction of beta-MHC expression by thyroid deficiency was associated with increased half-times for myocyte shortening and relengthening and increased half-time for the decay of the fura-2 fluorescence ratio. Qualitatively similar results were obtained in both the absence and the presence of beta-adrenergic stimulation although the beta-agonist accelerated the kinetics of the twitch and the Ca2+ transient. 5. Collectively, these data provide evidence that increased beta-MHC expression contributes significantly to the observed depression of contractile function in thyroid deficient myocardium by slowing the rates of both force development and force relaxation.

Investigating the relaxation, following diazo-2 laser flash photolysis, of a skinned trabecular preparation from SHR hypertrophied left ventricle.

Rat models of cardiac hypertrophy are characterised by a shift in left ventricular myosin isoform from V1 (adult) to V3 (foetal), the latter being associated with a slowing of the acto-myosin ATPase rate. The aim of this study was to examine hypertrophy effects on relaxation by investigating a chemically skinned cardiac preparation from the SHR, where all the cellular membranes are rendered non-functional allowing the myofibrils to be studied in isolation. On comparison, following photolysis of the photolabile caged Ca2+ chelator diazo-2, it can be seen that the SHR fibres relax at a slower rate than their age-matched WKY counterparts. We suggest that, since the thin filament regulatory proteins seem not to be affected by cardiac hypertrophy in the rat, this result can be attributed to the shift in left ventricular myosin isoforms. The reduced relaxation rate in the SHR could be the result of a slowing of the dissociation of actin and myosin during the cross-bridge cycle. These results have previously been published in abstract form [1].

Effect of trimetazidine and verapamil on the cardiomyopathic hamster myosin phenotype.

1. In this study we investigated whether long-term trimetazidine (anti-ischaemic drug) therapy alters the ventricular myosin heavy chain (MHC) isoform composition in a model of cardiomyopathy. 2. MHC isoforms were analysed in the native state by electrophoresis in a pyrophosphate buffer. Myosin isoform patterns were studied in cardiac muscle from cardiomyopathic hamsters (CMH) of the BIO 14:6 strain during the time course of the disease and compared with those of healthy golden hamsters (F1B). The correlation between myosin profile and Ca2+-activated ATPase activity was determined from 220 days. 3. At the stage of insufficiency (350 days), CMH presented the most abnormal phenotype with 53% V1-24% V3 compared to 79% V1-7% V3 (P<0.001), in F1B. Trimetazidine was administered to cardiomyopathic hamsters from the early stage of active disease (30 days) to the congestive stages (220-350 days). Within 65 days, trimetazidine treatment, in CMH and F1B, reduced V1 to a low level (53% and 62%, respectively), which remained constant throughout the treatment. This level was similar to that in 220 and 350 days-old untreated-CMH. In sharp contrast, a standard calcium blocker, verapamil, administered to CMH in the same conditions resulted in a higher V1 (about 70%) and higher global myosin ATPase activity from 220 days. 4. Previous results in terms of hypertrophy and survival, compared to these results, suggest that verapamil and trimetazidine treatments reveal a dissociation between ventricular hypertrophy and isomyosin distribution. In addition, the shift in favour of V3 may not necessarily be an aggravating factor of the disease but an adaptative compensatory event.

In vitro motility assay of atrial and ventricular myosin from pig.

The role of myosin isoforms in determining contractile filament velocity in the atrium and ventricle of the pig heart was studied by measuring the motion of fluorescently labeled actin over myosin (in vitro motility assay). A rapid and relatively simple method for purification of myosin from small tissue samples was used. The relative extent of light chain-2 phosphorylation was about 30% in both atrial and ventricular myosin extracts. Although the extracted myosin was not free from contaminating proteins, mainly actin, the mean velocity at optimal pH and 32 degrees C of both atrial (3.3 microns/s) and ventricular (2.3 microns/s) myosin were similar to those obtained using extensively purified myosin. The filament sliding velocities using isolated myosin and actin are lower than those estimated from previously published experiments on skinned fiber preparations, which might reflect an influence on sliding velocity by the filament organization or regulatory proteins in the muscle fiber. However, the ratio between velocities of atrial and ventricular myosin was similar in the motility assay (1.5) and muscle fiber experiments (1.6), which might suggest that these two methods reflect the same fundamental processes in cardiac contraction and that the difference in filament sliding velocity between the atrium and ventricle of the pig heart is determined my their myosin isoforms.

Myocardial adaptive changes and damage in ischemic heart disease.

Changes in two of the elements of myocardial subcellular organelles relating to cardiac energetics, ventricular myosin isozymes and mitochondrial DNA mutations, were examined using left ventricular tissue samples obtained at autopsy from patients with ischemic heart disease. Myosin isozymes were examined in tissues from nine patients with ischemic heart disease and 12 control patients with cancer but no heart disease. Extracted myosin was separated by pyrophosphate gel electrophoresis. The relative concentration of each component was determined by densitometry. Mitochondrial DNA mutations were evaluated in tissues from ten patients with myocardial infarction and 11 control patients with cancer but no heart disease. DNA was extracted and mitochondrial DNA mutations were detected by the polymerase chain reaction. Two bands were revealed by pyrophosphate gel electrophoresis. These contained VM-A, which exhibited faster electrophoretic mobility and was present in lower concentrations, and VM-B, which had a lower mobility and a higher concentration, respectively. SDS polyacrylamide gel electrophoresis showed that these two components contained the heavy chain and light chains 1 and 2 of myosin. VM-A concentrations tended to be higher in patients with ischemic heart disease than in controls. A 7.4-kb deletion was detected between the D-loop and the ATPase 6 genes of mitochondrial DNA from the myocardium of 6 out of 10 patients with myocardial infarction. The relative amounts of the two myosin isozymes could be altered by ischemic heart disease, although the functional significance of these components is unclear. The changes in the two myosin isozymes might be an adaptive change to disordered energy metabolism, but this change was small. The myocardial mitochondrial DNA deletions in patients with myocardial infarction were thought to result from ischemic damage.

Innervation regulates myosin heavy chain isoform expression in developing skeletal muscle fibers

The influence of innervation on primary and secondary myogenesis and its relation to fiber type diversity were investigated in two specific wing muscles of quail embryo, the posterior (PLD) and anterior latissimus dorsi (ALD). In the adult, these muscles are composed almost exclusively of pure populations of fast and slow fibers, respectively. When slow ALD and fast PLD muscles developed in ovo in an aneurogenic environment induced after neural tube ablation, the cardiac ventricular myosin heavy chain (MHC) isoform was not expressed. The adult slow MHC isoform, SM2, appeared by embryonic day 7 (ED 7) in normal innervated slow ALD but was not expressed in denervated muscle. Analysis of in vitro differentiation of myoblasts from fast PLD and slow ALD muscles isolated from ED 7 control and neuralectomized quail embryos showed no fundamental differences in the pattern of MHC isoform expression. Newly differentiated fibers accumulated cardiac ventricular, embryonic fast, slow SM1 and SM3 MHC isoforms. Nevertheless, the expression of slow SM2 isoform in myotubes formed from slow ALD myoblasts only occurred when myoblasts were cultured in the presence of embryonic spinal cord. Our studies demonstrate that the neural tube influences primary as well as secondary myotube differentiation in avian forelimb and facilitates the expression of different MHC, particularly slow SM2 MHC gene expression in slow myoblasts.

Angiotensin II increases left ventricular mass without affecting myosin isoform mRNAs.

We studied the effect of chronic (7 days) angiotensin II (Ang II) infusion in nonpressor and pressor doses on cardiovascular mass and expression of alpha- and beta-myosin heavy chain genes in the left ventricle in normotensive Wistar rats. An increased left ventricular mass was observed in rats receiving non-pressor and pressor doses of Ang II, but only high doses increased arterial pressure. Normalization of arterial pressure during Ang II infusion by losartan, a specific Ang II receptor antagonist, or hydralazine had different effects on left ventricular mass. Losartan prevented the increased left ventricular mass, and hydralazine did not affect left ventricular mass. Northern blot analysis showed that the switch in left ventricular myosin isoform mRNA from the adult to the fetal pattern occurred only in rats given the pressor Ang II dose. Both losartan and hydralazine, in parallel with the normalization of arterial pressure, prevented this myosin isoform switch. Thus, these data suggest that the Ang II-induced increase in left ventricular mass was not dependent on pressure overload, but the switch in myosin isoform mRNA from the adult to the fetal pattern was dependent on pressure overload.

Induction of avian cardiac myogenesis by anterior endoderm

An experimental system was devised to study the mechanisms by which cells become committed to the cardiac myocyte lineage during avian development. Chick tissues from outside the fate map of the heart (in the posterior primitive streak (PPS) of a Hamburger & Hamilton stage 4 embryo) were combined with potential inducing tissues from quail embryos and cultured in vitro. Species-specific RT-PCR was employed to detect the appearance of the cardiac muscle markers chick Nkx-2.5 (cNkx-2.5), cardiac troponin C and ventricular myosin heavy chain in the chick responder tissues. Using this procedure, we found that stage 4-5 anterior lateral (AL) endoderm and anterior central (AC) mesendoderm, but not AL mesoderm or posterior lateral mesendoderm, induced cells of the PPS to differentiate as cardiac myocytes. Induction of cardiogenesis was accompanied by a marked decrease in the expression of rho-globin, implying that PPS cells were being induced by anterior endoderm to become cardiac myocytes instead of blood-forming tissue. These results suggest that anterior endoderm contains signaling molecules that can induce cardiac myocyte specification of early primitive streak cells. One of the cardiac muscle markers induced by anterior endoderm, cNkx-2.5, is here described for the first time. cNkx-2.5 is a chick homeobox-containing gene that shares extensive sequence similarity with the Drosophila gene tinman, which is required for Drosophila heart formation. The mesodermal component of cNkx-2.5 expression from stage 5 onward, as determined by in situ hybridization, is strikingly in accord with the fate map of the avian heart. By the time the myocardium and endocardium form distinct layers, cNkx-2.5 is found only in the myocardium. cNkx-2.5 thus appears to be the earliest described marker of avian mesoderm fated to give rise to cardiac muscle.

Effect of "Enhancer" Sequences on Ventricular Myosin Light Chain-2 Promoter Activity in Heart Muscle and Nonmuscle Cells

Positive (HF-1 and HF-2) and negative (HF-3) elements responsible for cardiac-specificity of the rat ventricular myosin light chain-2 (MLC-2v) promoter are contained in a 250 base pair region. The effect of the simian virus 40 (SV40) enhancer or 3 copies of the HF-1, HF-2 and HF-3 elements on MLC-2v promoter activity and specificity was assessed in neonatal rat cardiac myocytes and cardiac nonmuscle cells as well as rat heart myoblast H9c2 and glioma (nonmuscle) C6 cell Lines. The SV40 enhancer increased promoter activity by at least 10-fold in both muscle and nonmuscle cell types; however, there was a decrease in cardiac ventricular myocyte-specificity. In contrast, the 3 copies of HF-1, HF-2 and HF-3 elements stimulated MLC-2v promoter activity ∼3-fold in neonatal ventricular cardiac myocytes alone and, effectively, displayed about a 5-fold increase in specificity over the wild type MLC-2v promoter.

The developmental switch in ventricular myosin expression in vivo is not triggered by an increase in cyclic AMP.

Ventricular myosin heavy chain (HC) expression undergoes a rapid change from the beta to the alpha isoform shortly after birth. Thyroid hormone is required for this transition to occur, but the time course of developmental changes in circulating thyroid hormone levels suggests that it cannot be the trigger for this event. In this study, the possibility was examined that cyclic AMP (cAMP), either acting separately or as a mediator of the permissive actions of thyroid hormone, triggers the developmental transition in ventricular myosin HC expression. One-day-old euthyroid or propylthiouracil-hypothyroid rat pups were treated with a membrane-permeable cAMP analogue, 8-bromo-cAMP (8-Br-cAMP) or triiodothyronine (T3). Two and four hours after acute treatment, the relative synthetic rates of alpha and beta myosin HC were measured using an in vivo pulse labelling technique. Myosin HC isoforms were separated electrophoretically and then quantitated by densitometry. Acute treatment in vivo with 8-Br-cAMP did not alter the relative rate of alpha myosin HC synthesis in euthyroid animals at either 2 or 4 h. Hypothyroidism significantly reduced the relative rate of alpha HC synthesis and obturated the transition from beta to alpha HC. The effect on synthesis was rapidly reversed by acute treatment with T3, but not with 8-Br-cAMP. Thus, an increase in cAMP does not appear to trigger the developmental change in myosin isoform expression, nor does it appear to mediate the stimulatory effect of thyroid hormone on alpha myosin HC synthesis. Instead, thyroid hormone is likely to be acting directly on the alpha HC gene by binding to the thyroid response element. The identity of stimuli that mediate the increased responsiveness to thyroid hormone during development remains to be determined, but it is not due to an increase in the levels of cAMP in perinatal cardiocytes in vivo.

Ring fibres express ventricular myosin in stretch overloaded quail muscle.

Acta Physiologica ScandinavicaVolume 152, Issue 4 p. 429-430 Ring fibres express ventricular myosin in stretch overloaded quail muscle J. ANTONIO, J. ANTONIO Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USASearch for more papers by this authorW.J. GONYEA, Corresponding Author W.J. GONYEA Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USADepartment of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USA, 75235-9039.Search for more papers by this author J. ANTONIO, J. ANTONIO Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USASearch for more papers by this authorW.J. GONYEA, Corresponding Author W.J. GONYEA Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USADepartment of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas, Texas, USA, 75235-9039.Search for more papers by this author First published: December 1994 https://doi.org/10.1111/j.1748-1716.1994.tb09825.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. Volume152, Issue4December 1994Pages 429-430 RelatedInformation