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C S Long

Publications and source records attributed to C S Long.

At least 37 records · Page 2Linked to original sources
PubMed1996

Alpha1-adrenergic receptor subtype mRNAs are differentially regulated by alpha1-adrenergic and other hypertrophic stimuli in cardiac myocytes in culture and in vivo. Repression of alpha1B and alpha1D but induction of alpha1C.

The three cloned alpha1-adrenergic receptor (AR) subtypes, alpha1B, alpha1C, and alpha1D, can all couple to the same effector, phospholipase C, and the reason(s) for conservation of multiple subtypes remain uncertain. All three alpha1-ARs are expressed natively in cultured neonatal rat cardiac myocytes, where chronic exposure to the agonist catecholamine norepinephrine (NE) induces hypertrophic growth and gene transcription. We show here, using RNase protection, that the alpha1-AR subtype mRNAs respond in distinctly different ways during prolonged NE exposure (12 72 h). Alpha1B and alpha1D mRNA levels were repressed by NE, whereas alpha1C mRNA was induced. Changes in mRNA levels were mediated by an alpha1-AR, were not explained by altered mRNA stability, and were reflected in receptor proteins by [3H]prazosin binding. alpha1-AR-stimulated phosphoinositide hydrolysis and myocyte growth were not desensitized. Three other hypertrophic agonists in culture, endothelin-1, PGF2alpha, and phorbol 12-myristate 13-acetate, also induced alpha1C mRNA and repressed alpha1B mRNA. In myocytes from hearts with pressure overload hypertrophy, alpha1 mRNA changes were identical to those produced by NE in culture. These results provide the first example of a difference in regulation among alpha1-AR subtypes expressed natively in the same cell. Transcriptional induction of the alpha1C-AR could be a mechanism for sustained growth signaling through this receptor and is a common feature of a hypertrophic phenotype in cardiac myocytes.

Adrenergic alpha-Agonists↗
PubMed1996

Alterations in skeletal muscle gene expression in the rat with chronic congestive heart failure.

Congestive heart failure is often associated with skeletal muscle abnormalities that contribute to early fatigue and acidosis. Up to the present time, however, the mechanisms responsible for these changes are unclear. Myocardial infarctions were produced by coronary ligation in adult Sprague-Dawley rats. At 20 weeks, 10 control rats, and 15 animals with heart failure [defined by elevated LVEDP (26.1 +/- 3.1 v 2.5 +/- 0.5 mmHg) and RV hypertrophy (300 +/- 21 g v 158 +/- 9 mg)] underwent in vivo measurements of total body, and soleus total protein and myosin heavy chain (MHC) synthesis by [3H]leucine constant infusion. Soleus muscle was also analysed for protein content, and MHC isoenzyme content by SDS-PAGE. Northern blotting also was used to determine levels of the mRNA's encoding type I, IIa, IIb, and IIx MHC, alpha-skeletal actin, COX III, SDH and GAPDH. Soleus muscles in heart failure rats were smaller than controls (112 +/- 6 v 126 +/- 5 mg) and the degree of atrophy was significant when corrected for body mass (0.38 +/- 0.02 v 0.46 +/- 0.02 mg/g. P = 0.007). Although there was no significant difference in plasma leucine flux (an index of whole-body protein synthesis), soleus muscle total and MHC synthesis was reduced in heart failure animals. Whereas the Type I MHC isoenzyme (beta MHC) was the only MHC detected in the soleus of control animals, type II MHC isoenzyme comprised 11.8 +/- 3.1% of the MHC in the heart failure group. Furthermore, steady-state mRNA levels encoding beta MHC were significantly depressed in the heart failure rats, where those encoding Types IIb and IIx MHC were increased. Steady-state mRNA levels of alpha-skeletal actin, cytochrome C oxidase (COX III) and succinate dehydrogenase (SDH) were also significantly depressed. This animal model of chronic heart failure is associated with quantitative and qualitative alterations in skeletal muscle gene expression that are similar to those reported in skeletal muscle of patients with chronic heart failure. The altered phenotype and impaired metabolic capacity may contribute to exercise intolerance in CHF.

Animals↗
PubMed1996

Heart failure in rats causes changes in skeletal muscle morphology and gene expression that are not explained by reduced activity.

In patients with congestive heart failure, skeletal muscle is characterized by a smaller proportion of slow-twitch oxidative fibers and reduced oxidative enzyme activity. However, whether these changes result from disuse or occur as a direct consequence of heart failure is unresolved. To address this issue, 18 rats with heart failure 8 weeks after left coronary artery ligation and 13 sham-operated control rats underwent quantification of locomotor activity by a photocell activation technique, measurements of hemodynamics and infarct size, histochemical and morphological analyses of the soleus and plantaris muscles, and Northern analyses of muscle contractile protein and oxidative enzyme mRNA expression. Although the rats with heart failure had elevated left ventricular end-diastolic pressures (24.1 +/- 2.6 mm Hg) and a mean infarct size of 35.1 +/- 4.1%, activity levels were similar to those found in the sham-operated rats (3849 +/- 304 versus 3526 +/- 130 counts per hour). With heart failure, there was a significant reduction of type I fibers in the soleus muscle and type IIa fibers in the plantaris muscle, with corresponding increases in intermediate staining of type IIab fibers in both muscles. This was associated with a 17% decrease in citrate synthase activity in both the soleus and plantaris muscles (26.2 +/- 1.6 versus 30.7 +/- 3.4 and 29.1 +/- 2.4 versus 35.7 +/- 3.4 mumol/L per minute per gram, respectively [P < .05]). In the soleus muscle, mRNA for both beta-myosin heavy chains and cytochrome C oxidase III (normalized to 18S RNA) was reduced (0.27 +/- 0.02 versus 0.65 +/- 0.02 and 0.23 +/- 0.04 versus 0.64 +/- 0.02 U), whereas the messages for IIx and IIb myosin heavy chains were increased. A similar decrease in messages for cytochrome oxidase and the primary myosin isoform was observed in the plantaris muscle. Both soleus beta-myosin heavy chain and cytochrome C oxidase expression show significant inverse relationships to left ventricular end-diastolic pressure and infarct size. In contrast, there was no relationship between either beta-myosin heavy chain or cytochrome C oxidase expression and locomotor activity. These results indicate that in rats heart failure produces changes in skeletal muscle gene expression at the pretranslational level that cannot be explained by inactivity.

Animals↗
PubMed1995

Chronic hypoxia differentially regulates alpha 1-adrenergic receptor subtype mRNAs and inhibits alpha 1-adrenergic receptor-stimulated cardiac hypertrophy and signaling.

BACKGROUND: After myocardial ischemia and/or infarction, surviving cardiac myocytes in and around the injured zone develop hypertrophy to compensate for the loss of contractile units due to myocyte injury and death. One of the factors that may be involved in the development of hypertrophy after ischemic injury is norepinephrine (NE), an agent that induces hypertrophy of cardiac myocytes through the alpha 1-adrenergic receptor (AR). It is not known, however, whether hypoxia, a major component of ischemia, has any direct effect on NE-stimulated hypertrophy. Therefore, we sought to determine whether chronic hypoxia could alter NE-stimulated hypertrophy and if so, whether this alteration was related to alpha 1-AR-mediated signaling and alpha 1-AR changes at both the protein and mRNA levels. METHODS AND RESULTS: We developed a model of chronic hypoxia in cultured neonatal rat cardiac myocytes in which myocytes were exposed to 1% oxygen for 72 hours. Initially, we observed that chronic hypoxia inhibited NE-stimulated hypertrophy, as reflected by decreases in both new protein synthesis and total protein content during chronic hypoxia. Then we found that chronic hypoxia also inhibited alpha 1-AR-transduced phosphatidylinositol hydrolysis, as indicated by a reduction in alpha 1-AR-stimulated inositol phosphate production in hypoxic cells. These observations suggested that the inhibition of NE-stimulated hypertrophy seen during chronic hypoxia was due to impairment of alpha 1-AR-mediated signaling and could result from changes in alpha 1-AR numbers and/or subtype distribution. To address this issue, we determined alpha 1-AR density and subtype distribution by radioligand binding and alpha 1-AR subtype mRNAs, including alpha 1A/D-, alpha 1B-, and alpha 1C-ARs, by RNase protection assays. We found that chronic hypoxia differentially regulated both the pharmacologically defined alpha 1-AR subtypes and the mRNAs for the alpha 1-AR subtypes. Thus, hypoxia for 72 hours coordinately downregulated both the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level. During normoxia, NE increased the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level, but hypoxia for 72 hours prevented these NE-mediated changes. CONCLUSIONS: Chronic hypoxia (1) inhibits alpha 1-AR-mediated hypertrophy of cardiac myocytes and alpha 1-AR-transduced phosphatidylinositol hydrolysis and (2) downregulates both the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level.

Animals↗
PubMed1995

Interleukin-1 beta induces cardiac myocyte growth but inhibits cardiac fibroblast proliferation in culture.

Interleukin-1 (IL-1), initially called "endogenous pyrogen," is primarily known as a mediator of inflammation. However, it also plays many other diverse physiologic roles including the stimulation and inhibition of both primary cells in culture and the interstitial and parenchymal cells of a number of organs including the heart. In the heart, IL-1 expression has traditionally been reported in situations where there is immunologic myocardial injury such as occurs during transplant rejection and congestive heart failure. For this reason, all of the effects of IL-1 have been presumed to be deleterious. Using a cell culture model which allows both the muscle cells (myocytes) and nonmuscle cells (fibroblasts) to be evaluated separately, we have found that IL-1 induces both cardiac myocyte hypertrophy and reinitiates myocyte DNA synthesis. In stark contrast, IL-1 exerts a potent anti-proliferative effect on cardiac fibroblasts. To our knowledge this is the first report concerning the differential effects of IL-1 on myocardial cell growth in culture and, given the inducible expression of IL-1 by myocardial cells during stress, underscores the importance of investigating the complex nature of the intracardiac cell-cell interactions that occur in the heart.

Animals↗
PubMed1994

Distribution of alpha 1C-adrenergic receptor mRNA in adult rat tissues by RNase protection assay and comparison with alpha 1B and alpha 1D.

Two alpha 1-adrenergic receptor (AR) subtypes have been defined by pharmacological studies in rat tissues, the alpha 1A and the alpha 1B, whereas three alpha 1-ARs have been cloned, alpha 1B, alpha 1C, and alpha 1D. It has been reported that alpha 1C mRNA is absent in all rat tissues, making uncertain the correspondence of this cloned subtype, if any, to the native alpha 1-ARs defined by pharmacological criteria. In the present study, a partial alpha 1C-AR cDNA was obtained from rat cardiac myocytes using RT-PCR with degenerate primers. A sensitive RNase protection assay was used to map the distribution of alpha 1C mRNA in adult rat tissues, in comparison with alpha 1B and alpha 1D. alpha 1C mRNA was abundant in heart, brain, aorta, vena cava, vas deferens, submaxillary gland, lung, and kidney; was detected at lower levels in prostate, parotid gland, and skeletal muscle; and was undetectable in liver and spleen. alpha 1B and alpha 1D mRNAs were present in most of the same tissues. In contrast to alpha 1C, however, alpha 1B and alpha 1D were both present in spleen; alpha 1B was the sole alpha 1-AR mRNA in liver; and alpha 1D mRNA was not detected in submaxillary gland, a tissue known to be enriched in the pharmacological alpha 1A. We conclude that the distribution of alpha 1C-AR mRNA in rat tissues is compatible with the idea that the alpha 1C corresponds to the classical native alpha 1A-AR. Although many tissues contain all three alpha 1-AR mRNAs, distinct tissue-specific expression is evident.

Amino Acid Sequence↗
PubMed1994

Neutral endopeptidase in the heart. Neutral endopeptidase inhibition prevents isoproterenol-induced myocardial hypoperfusion in rats by reducing bradykinin degradation.

Peptide mediators may play a role in the control of myocardial perfusion. We found immunohistochemical evidence of the peptide-degrading enzyme neutral endopeptidase (NEP) in cultured rat myocytes. Therefore, we examined the effect of an NEP inhibitor, phosphoramidon, on myocardial perfusion in rats after (1) stimulating sensory nerves with capsaicin and (2) inducing myocardial hypoperfusion with isoproterenol, with or without pretreatment with selective antagonists of the substance P (NK1) and bradykinin (B2) receptors. Three to five sequential determinations of myocardial blood flow were made in anesthetized rats by injecting 100,000 radionuclide-labeled microspheres suspended in 70% dextrose into the left ventricle. Phosphoramidon doubled coronary blood flow in response to a dose of capsaicin that was ineffective in the absence of the inhibitor. Isoproterenol (50 mg/kg IP) caused an immediate fall in blood pressure and coronary blood flow; after 20 minutes, flow had returned to normal but pressure was still subnormal. Administration of phosphoramidon reduced the recovery of blood pressure but greatly increased coronary blood flow. These changes were not altered by a substance P NK1 receptor blocker but were completely abolished by a selective bradykinin B2 receptor blocker. Our data indicate that (1) NEP is present in the rat myocardium, (2) sensory nerve-induced coronary vasodilation is markedly potentiated by NEP inhibition, (3) isoproterenol-induced myocardial hypoperfusion is prevented by NEP inhibition, and (4) this effect of NEP inhibition is due to reduced degradation of bradykinin.

Animals↗
PubMed1994

Cloning of the rat alpha 1C-adrenergic receptor from cardiac myocytes. alpha 1C, alpha 1B, and alpha 1D mRNAs are present in cardiac myocytes but not in cardiac fibroblasts.

alpha 1-Adrenergic receptor (AR) activation in cardiac muscle has several different physiological effects that might be mediated through different alpha 1-AR subtypes. Two alpha 1-AR subtypes have been cloned from the rat, the alpha 1B and the alpha 1D; both are present in adult rat heart. A third subtype, the alpha 1C, cloned from the cow and human, was reported to be absent in the rat. However, we recently found alpha 1C mRNA in adult rat heart by using a partial alpha 1C cDNA. Thus, all three cloned alpha 1-AR subtypes are present in the heart, but it is unknown whether each is expressed in cardiac myocytes or in cardiac fibroblasts. In the present study, the full-length rat alpha 1C-AR was cloned from cultured neonatal cardiac myocytes. alpha 1C mRNA transcripts of 3, 9.5, and 11 kb were present in adult rat heart by Northern blot analysis. alpha 1B-, alpha 1C-, and alpha 1D-subtype mRNAs were each present in isolated adult and neonatal cardiac myocytes by RNase protection assay. In addition, cultured neonatal cardiac myocytes expressed the three alpha 1-AR subtype mRNAs. In contrast, none of the alpha 1-AR mRNAs was detected in cultured neonatal cardiac fibroblasts. In addition, alpha 1-ARs were absent in fibroblasts by [3H]prazosin binding and norepinephrine-stimulated [3H]inositol phosphate production. The absence of alpha 1-ARs in cardiac fibroblasts differs from beta-adrenergic and angiotensin II receptors, which are present in both cardiac fibroblasts and cardiac myocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1993

Beta-adrenergic stimulation of cardiac non-myocytes augments the growth-promoting activity of non-myocyte conditioned medium.

Although the stimulatory action of catecholamines on the heart has been presumed to result exclusively from their direct effects on the cardiac myocytes, little work has been done addressing the effects of catecholamines on the supporting non-myocytes of the heart. We have recently identified medium conditioned by neonatal rat cardiac non-myocytes (NMC-CM) as the source of a growth-promoting factor which leads to cardiac myocyte hypertrophy in culture, suggesting that these non-myocytes may play an active role in myocardial growth. Since cardiac non-myocytes also contain adrenergic receptors (both alpha and beta) on their cell surface, we asked whether adrenergic stimulation of these non-myocytes could supplement the growth-promoting effect of NMC-CM. While isoproterenol (ISO, 0.2 microM) caused no increase in the per cell content of total protein in the non-myocytes, inclusion of ISO in the medium used in the production of NMC-CM augmented the growth promoting effects of this "ISOCM" over control CM. This increase was not seen with the alpha 1 adrenergic agonist phenylephrine suggesting that the stimulatory effect was specific to the beta-adrenergic receptor. Because TGF beta 3 contains an upstream cAMP Response Element, we wonder whether its expression could respond to the increase in cAMP induced by ISO. Non-myocytes treated over 72 h with ISO expressed increased steady state mRNA levels for TGF beta 3 but not that for the closely related TGF beta 1 over this time period. We believe that this is the first report indicating that a potential mechanism for the observed effects of beta-adrenergic stimulation on myocardial cells in culture and possibly relevant in vivo is the contribution of beta-stimulated factor(s) produced by non-myocytes which act in a paracrine fashion on myocardial cells.

1-Methyl-3-isobutylxanthine↗
PubMed1992

Sympathetic modulation of the cardiac myocyte phenotype: studies with a cell-culture model of myocardial hypertrophy.

Myocardial hypertrophy is the common endpoint of many cardiovascular stimuli such as hypertension, myocardial infarction, valvular disease, and congestive failure. Catecholamines have long been implicated in the pathogenesis of myocardial hypertrophy, however, it is very difficult to sort out catecholamine mechanisms in vivo. We have developed a cell-culture model which excludes hemodynamic effects and allows the assignment of receptor specificity to catecholamine effects. Utilizing this system, we have shown that stimulation of the alpha 1 adrenergic receptor leads to the development of myocardial hypertrophy and results in the selective up-regulation of the fetal/neonatal mRNAs encoding skeletal alpha-actin and beta-MHC, a pattern similar to that seen with hypertrophy in-vivo. Utilizing a co-transfection assay, we have also obtained data that suggest that the beta-PKC isozyme is in a pathway regulating transcription of the beta-MHC isogene. Beta adrenergic stimulation of the cultured cardiac myocytes also results in a modest degree of hypertrophy, however, this effect may be dependent upon myocyte contractile activity and may involve, at least in part, the non-muscle cells present in the culture system.

Animals↗
PubMed1991

A growth factor for cardiac myocytes is produced by cardiac nonmyocytes.

Cardiac nonmyocytes, primarily fibroblasts, surround cardiac myocytes in vivo. We examined whether nonmyocytes could modulate myocyte growth by production of one or more growth factors. Cardiac myocyte hypertrophic growth was stimulated in cultures with increasing numbers of cardiac nonmyocytes. This effect of nonmyocytes on myocyte size was reproduced by serum-free medium conditioned by the cardiac nonmyocytes. The majority of the nonmyocyte-derived myocyte growth-promoting activity bound to heparin-Sepharose and was eluted with 0.75 M NaCl. Several known polypeptide growth factors found recently in cardiac tissue, namely acidic fibroblast growth factor (aFGF), basic FGF (bFGF), platelet-derived growth factor (PDGF), tumor necrosis factor alpha (TNF alpha), and transforming growth factor beta 1 (TGF beta 1), also caused hypertrophy of cardiac myocytes in a dose-dependent manner. However, the nonmyocyte-derived growth factor (tentatively named NMDGF) could be distinguished from these other growth factors by different heparin-Sepharose binding profiles (TNF alpha, aFGF, bFGF, and TGF beta 1) by neutralizing growth factor-specific antisera (PDGF, TNF alpha, aFGF, bFGF, and TGF beta 1), by the failure of NMDGF to stimulate phosphatidylinositol hydrolysis (PDGF and TGF beta 1), and, finally, by the apparent molecular weight of NMDGF (45-50 kDa). This nonmyocyte-derived heparin-binding growth factor may represent a novel paracrine growth mechanism in myocardium.

Animals↗
PubMed1991

Sympathetic activity: modulator of myocardial hypertrophy.

The mechanisms regulating myocardial hypertrophy are largely unknown. Furthermore, the hypertrophic phenotype can be associated with either normal or abnormal function. To study the molecular mechanisms involved in myocardial hypertrophy, we have established a cell culture system in which stimulation of the alpha 1-adrenergic receptor leads to the development of myocardial cell hypertrophy. In addition to producing a generalized twofold increase in both cell size, total protein, and total RNA, activation of the alpha 1-receptor produces specific alterations in gene expression that are reflected by changes at both the mRNA and protein levels. In particular, alpha 1 stimulation leads to an increase in the expression of the c-myc oncogene as well as a selective increase in skeletal alpha-actin and beta-myosin heavy-chain isogene expression, isoforms normally found only in fetal/neonatal hearts. Similar changes in gene expression are seen in pressure-load hypertrophy in vivo. Skeletal alpha-actin gene expression is induced preferentially to that of the cardiac actin isogene resulting from a specific preferential increase in gene transcription. Work with subtype-specific inhibitors indicates that it is a particular alpha 1-receptor subtype that is responsible for the development of hypertrophy in culture. The finding that alpha 1 stimulation leads to an increase in protein kinase C activity is suggestive of a potential second messenger involving the phosphorylation of a transcriptional factor or factors.

Animals↗
PubMed1990

Trophic factors for cardiac myocytes.

During early neonatal myocardial growth, cardiac myocytes undergo a terminal differentiation stage after which cellular proliferation no longer occurs. Subsequent growth occurs by means of enlargement/hypertrophy of the existing cells. In an effort to dissect the molecular mechanisms underlying myocardial hypertrophy, a cell culture model of cardiac hypertrophy was developed in our laboratory. In this model, neonatal rat heart muscle cells respond to alpha 1-adrenergic receptor stimulation with an increase in cell size, total protein and the cellular contents of several messenger (m)RNA encoding fetal/neonatal contractile protein isoforms, and also that encoding the proto-oncogene c-myc. Similar changes in gene expression are seen in pressure-load hypertrophy in vivo. Recent observations on the production of peptide growth factors by the myocardium suggest an additional role of cell-cell interaction in cardiac growth and development. Some of the growth factors which have been found in normal myocardium include platelet-derived growth factor B chain, fibroblast growth factor (both acidic and basic), transforming growth factor beta, insulin-like growth factor-1 and insulin-like growth factor-2, and nerve growth factor. We recently identified a heparin-binding growth factor produced by cardiac non-myocytes in culture, which acts in a dose-dependent fashion to produce hypertrophy of cardiac muscle cells in culture. Preliminary work suggests that this is dissimilar from previously identified growth factors. Furthermore, the quantitative response of the cardiac myocytes to our growth factor exceeds that seen for other factors. Further work is necessary to ascertain how these factors and the alpha 1-adrenergic system interact to produce the different clinical forms of myocardial hypertrophy.

Animals↗
PubMed1989

Transcription of early developmental isogenes in cardiac myocyte hypertrophy.

We have developed a cell culture system to study molecular mechanisms important in myocardial hypertrophy. alpha 1-Adrenergic receptor stimulation produces hypertrophy of neonatal rat cardiac myocytes. Myocyte hyperplasia is not induced by alpha 1 stimulation, although alpha 1-adrenergic receptor-mediated DNA synthesis and cell division have been observed in other types of cells. The myocyte hypertrophic response does not require contractile activity. Activation of the alpha 1 receptor also produces highly specific alterations in gene expression, as measured at the mRNA and protein levels. In particular, there is selective up-regulation of two contractile protein isogenes that are expressed in vivo during early development and in pressure-load hypertrophy, skeletal alpha-actin and beta-myosin heavy chain. Studies with an in vitro transcription assay indicate that stimulation of the alpha 1-adrenergic receptor leads to a distinctive temporal sequence of transcriptional activation. Transcription of the skeletal alpha-actin isogene is induced preferentially to that of cardiac alpha-actin. Thus, early developmental isogene induction in alpha 1-stimulated hypertrophy reflects a fundamental change in the transcriptional program of the cardiac myocyte nucleus. The goal now is to define an intracellular pathway connecting the alpha 1-adrenergic receptor in the plasma membrane to activation of RNA polymerase II on the skeletal alpha-actin gene in the cardiac myocyte nucleus. There is evidence that protein kinase C may be one component of this pathway. A model for alpha 1-mediated transcription is presented.

Animals↗
PubMed1989

Alpha 1-adrenergic receptor stimulation of sarcomeric actin isogene transcription in hypertrophy of cultured rat heart muscle cells.

During pressure-load hypertrophy of the adult heart in vivo, there is up-regulation of the mRNA encoding skeletal alpha-actin, the sarcomeric actin iso-mRNA characteristic of mature skeletal muscle and the fetal/neonatal heart. We have shown previously that during alpha 1-adrenergic receptor-stimulated hypertrophy of cultured rat heart myocytes, the induction of skeletal alpha-actin mRNA is greater than that of the mRNA encoding cardiac alpha-actin, the sarcomeric actin iso-mRNA characteristic of the adult heart. To determine if this actin iso-mRNA switch during cardiac hypertrophy reflects changes in the transcriptional status of the myocyte nucleus, we quantified the rate of transcription of actin mRNAs and total RNA, using an in vitro run-on transcription assay with nuclei isolated from the cultured myocytes after stimulation with norepinephrine (NE). Transcription of skeletal alpha-actin was increased at 3 h after NE, reached a maximum 6.1-fold increase at 12 h, and returned to the control level at 24 h. The EC50 for NE was 200 nM, and pharmacologic studies indicated alpha 1-receptor specificity. Transcription of cardiac alpha-actin was also increased rapidly by NE (maximum 4.6-fold vs. control at 3 h). However, cardiac alpha-actin transcription had returned to the control level at 6 h, when NE-stimulated skeletal alpha-actin transcription was still increasing. Transcription of the cytoskeletal (beta) actin gene was not changed significantly by NE treatment. Total RNA transcription was not increased until 6 h after NE (1.5-fold vs. control) and remained elevated through 24 h. Inhibition of protein synthesis did not attenuate NE-stimulated actin gene transcription. Thus the alpha 1-adrenoceptor mediates a rapid, transient, and selective increase in transcription of the sarcomeric actin isogenes during cardiac myocyte hypertrophy. Skeletal alpha-actin, the fetal/neonatal isogene, is induced preferentially to cardiac alpha-actin, the adult isogene. The different kinetics of actin isogene and total RNA transcription and the independence of transcription from protein synthesis suggest that transcriptional induction via the alpha 1 receptor is complex and may involve preexisting regulatory factors. These results are the first to demonstrate that the alpha 1-adrenergic receptor is a molecular mediator of transcriptional changes underlying an isogene switch that is known to be associated with cardiac myocyte hypertrophy.

Actins↗
PubMed1988

Transcriptional repression of an embryo-specific muscle gene.

Skeletal muscle involves both the induction and repression of gene expression. Although activation and up-regulation of several contractile protein genes has been shown to occur via transcriptional mechanisms, the mechanisms by which contractile protein genes are repressed during muscle development remain unknown. However, a post-transcriptional mechanism has been implicated in the repression of thymidine kinase expression during muscle development. The chicken cardiac troponin T (cTNT) gene is expressed in early embryonic skeletal muscle but is abruptly repressed in late embryonic/fetal development. Using run-on transcription assays we demonstrate here that cTNT gene repression occurs at the level of transcription. Thus, transcriptional as well as post-transcriptional mechanisms operate both to activate and repress gene expression during skeletal muscle development.

Actins↗