Myosin heavy chain messenger RNA and protein isoform transitions during cardiac hypertrophy. Interaction between hemodynamic and thyroid hormone-induced signals

Thyroid hormone therapy in cardiovascular disease

The relationship between thyroid disease states and cardiovascular hemodynamics is well recognized. Although the long-term effects of thyroid hormone are thought to result from changes in myocardial gene expression, attention has recently focused on acute, non-nuclear-mediated actions of L-triidothyronine (T3), the biologically active form of the hormone. Various lines of evidence have documented that T3 can act as a vasodilator and inotrope. With this recognition have come novel treatment strategies targeted at specific clinical conditions including heart failure and cardiac surgery that are associated with impaired cardiovascular performance and low serum T3 levels. An understanding of the mechanisms of action of thyroid hormone on the heart and peripheral vasculature is essential for the rational implementation of thyroid hormone as a therapeutic agent. As outlined in this review, initial clinical experience suggests that the ability of thyroid hormone to increase cardiac output and to lower systemic vascular resistance may provide a novel treatment option for physicians caring for patients with cardiovascular illness.

Identification of a contractile-responsive element in the cardiac alpha-myosin heavy chain gene

The mechanisms by which the cardiac-specific alpha-myosin heavy chain (alpha-MHC) gene responds to contractile activity was studied in cultured cardiomyocytes and in vivo. Deletion analysis of the alpha-MHC promoter transiently transfected into neonatal rat cardiomyocytes localized the contractile-responsive element within -80 to -40 base pairs of the transcriptional start site. Mutational analysis of an E-box motif at position -47 showed that it was necessary for the contractile response both in cultured cardiomyocytes and in the intact heart. Competition gel mobility shift experiments indicated that the protein-DNA complex formed within the -39 to -59 base pair region could be competed by the E-box element at -309 of the alpha-MHC gene and that base substitutions within an E-box motif at -47 eliminated the protein-DNA complex. To identify the contractile-responsive nuclear protein, antibodies specific for E12/E47, an E-box binding basic-helix-loop-helix (bHLH) protein, and antibodies recognizing upstream stimulatory factor (USF), a widely expressed bHLH-leucine zipper transcription factor, were studied for their ability to inhibit cardiomyocyte nuclear protein binding to the E-box motif at -47. Anti-USF antibody abolished formation of the protein-DNA complex, thus identifying the protein as antigenically related to USF and demonstrating that bHLH-leucine zipper proteins are involved in the contractile-induced expression of the cardiac alpha-MHC gene.

Regional myosin heavy chain expression in volume and pressure overload induced cardiac hypertrophy

Although the overall shift towards the V3 myosin heavy chain (MHC) has been shown to be associated with cardiac hypertrophy, quantitative evidence describing regional expression is sparse. The aim of this study was to compare and contrast the regional ventricular myosin isoform expression in two distinct haemodynamic states: pressure and volume overload. Volume overload was achieved using an aortocaval fistula (ACF) model and pressure overload by two-kidney-one-clip (2K1C) hypertension. A separate group (UC-2K1C) had the clip removed 1 week prior to investigation. Sham operated rats (SHAM) served as controls. All groups were studied 4 weeks after surgery. Ventricular tissue samples (approximately 50 mg) were taken from the walls of the right ventricle (RV), septum and left ventricular (LV) free wall. Tissue samples (excluding RV) were divided into endocardium and epicardium, and myosin expression was determined using polyacrylamide gel electrophoresis. Cardiac hypertrophy was substantial in both LV (1.7-fold) and RV (1.9-fold) in ACF rats. The 2K1C rats had similar LV enlargement (1.6-fold) whereas RV hypertrophy was not as great (1.2-fold). Blood pressure (BP) was increased 65% in 2K1C rats, whereas there was no change in ACF rats with respect to SHAM animals. After unclipping (UC-2K1C), LV hypertrophy and BP had returned towards control levels. In general, V3 MHC expression was associated with increasing LV hypertrophy in both 2K1C and ACF models. However, there was a marked endo-epi differential (1.5:1) in the LV free wall and septum of 2K1C rats. In contrast, in ACF rats there was no differential V3 MHC expression in the LV or septal tissue, i.e. expression was similar in both endo- and epi-samples. Elevated expression of V3 MHC persisted despite normotension and regression of cardiac hypertrophy in UC-2K1C rats. Taken together with published results demonstrating that relative transmural myocyte hypertrophy in ACF rats (endo > epi) is in contrast to that seen in 2K1C rats (epi > endo), the present findings reveal that regional V3 myosin expression represents a distinct adaptational component of the overall cardiac hypertrophic response in both volume and pressure overload.

Immunolocalization of rat cardiac beta-MHC protein expression in systemic hypertension and caloric restriction

Previous studies have shown that both systemic hypertension induced by abdominal aortic constriction (Abcon) and 50% caloric restriction (CR) increase left ventricular (LV) beta-myosin heavy chain (MHC) protein expression in the rat. However, these two physiological states have different effects on hemodynamic load, and information regarding beta-MHC localization across the LV wall in these two models may provide insight into the process of adaptation to chronic stress among myocardial cells. Thus the goal of this study was to determine the pattern of beta-MHC protein expression across the LV wall in Abcon and CR models using a beta-MHC-specific antibody. Adult female Sprague-Dawley rats (approximately 225-250 g) were randomly assigned to one of three groups: 1) normal control (NC), 2) Abcon, and 3) CR. After a treatment period of 5 wk, Abcon LVs hypertrophied 52% relative to NC, accompanying the 42% increase in mean blood pressure. CR rats, however, had a normal LV weight-to-body weight ratio. The relative content of LV beta-MHC protein expression, as assessed by native gel electrophoresis, increased from 3% in NC to 25 and 41% in Abcon and CR rats, respectively. Immunohistochemical analysis of beta-MHC expression demonstrated that the increase in beta-MHC protein in the Abcon group occurred primarily on the endocardial side of the LV. In contrast, the increase in beta-MHC protein in the CR LV occurred equally across the entire LV wall. This suggests that CR has a global effect on MHC isoform expression in LV myocardial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of beta-myosin heavy chain expression in systemic hypertension and caloric restriction in the rat heart

In the rat left ventricle, both pressure overload induced by abdominal aortic constriction (Abcon) and caloric restriction (CR) induce an increase in the steady-state level of the beta-myosin heavy chain (MHC) protein and mRNA. Both models also induce a concomitant decrease in the alpha-MHC protein and mRNA. The goals of this study were to 1) determine if the changes in MHC expression in the models are due to altered transcription and 2) identify the relative levels of some key factors interacting with the regulatory regions of these genes. Female Sprague-Dawley rats were randomly assigned to the following groups: 1) normal control (NC), 2) Abcon, and 3) CR. After 5 wk of experimental manipulations, myocardial nuclei were isolated. These nuclei were used for 1) nuclear run-on assays or 2) nuclear extract, which was prepared and used for gel mobility shift assays (GMSAs). Nuclear run-on assays demonstrated that the increase in beta-MHC mRNA and protein expression in both Abcon and CR can be at least partially attributed to increased transcription. The concomitant decrease in alpha-MHC content can similarly be attributed to a decrease in transcription of this gene. Furthermore, GMSAs demonstrate that nuclear extract from each group interact differently with certain elements known to be important for expression in vitro. CR nuclear extracts have a 25.6 +/- 7.2% decrease (P < 0.05 vs. NC) in interaction with a thyroid-responsive element, a potential repressor of beta-MHC transcription.(ABSTRACT TRUNCATED AT 250 WORDS)

Congestive heart failure due to reversible cardiomyopathy in patients with hyperthyroidism

The authors describe the clinical characteristics and response to therapy of seven patients with hyperthyroidism, dilated cardiomyopathy, and low-output cardiac failure. All patients (4 women and 3 men, age 47 +/- 4 years, mean +/- standard error of the mean) were admitted with the primary diagnosis of congestive heart failure. The cause of hyperthyroidism was Graves' disease in six patients, and toxic multinodular goiter in one. On admission, the mean serum T4 was 21 +/- 1 microgram/dL and mean serum T3:411 +/- 77 ng/mL, and serum thyroid-stimulating hormone was suppressed ( < 0.03 microU/mL) in all patients. Two-dimensional echocardiogram showed biventricular or four chamber dilatation and impaired left ventricular performance. Therapy of heart failure and hyperthyroidism resulted in rapid clinical improvement. During follow-up (5 months to 9 years), left ventricular ejection fraction improved from a mean of 28% to a mean ejection fraction of 55% (P < 0.01). Resolution of dilated cardiomyopathy with normalization of systolic function was achieved in five patients, and improvement from severe to mild left ventricular dysfunction was observed in two patients. We conclude that some patients with hyperthyroidism may have a reversible form of dilated cardiomyopathy and "low-output failure." Assessment of thyroid hormone status in patients with heart failure might permit the identification of patients with dilated cardiomyopathy and thyrotoxicosis who are likely to have reversible cardia dysfunction.

Cell death in the failing heart: role of an unnatural growth response to overload

Hypertrophy of the overloaded heart, characterized by an increased number of sarcomeres, provides an adaptive, short-term response. However, when cardiac overload is long-standing, the hypertrophic response appears to cause shortened myocyte survival. The mechanisms responsible for the deleterious effects of chronic myocardial hypertrophy may include a maladaptive growth response of the mature heart. Because terminally differentiated adult cardiac myocytes have little or no capacity to divide, stimuli that promote growth in the overloaded adult heart cannot lead to normal cell division. Instead, overload initiates an unnatural growth response that appears to shorten cardiac myocyte survival, possibly because the same growth factors that mediate the hypertrophic response of the adult heart can also induce programmed cell death (apoptosis). The converting enzyme inhibitors and nitrates, which have growth-inhibitory as well as vasodilator effects, may improve prognosis in heart failure by inhibiting the production of transcription factors. These transcription factors stimulate both the unnatural growth response to overload and stimuli that lead to apoptosis. Since both beta-adrenergic agonists and cytokines, such as tumor necrosis factor-alpha, can stimulate production of similar transcription factors, evidence suggests that beta blockers and vesnarinone improve the prognosis in patients with heart failure possibly because of their ability to inhibit maladaptive growth.

Force-velocity relations of rat cardiac myosin isozymes sliding on algal cell actin cables in vitro

The difference in kinetic properties between two myosin isozymes (V1 and V3) in rat ventricular myocardium was studied by determining the steady-state force-velocity (P-V) relations in the ATP-dependent movement of V1 and V3-coated polystyrene beads on actin cables of giant algal cells mounted on a centrifuge microscope. The maximum unloaded velocity of bead movement was larger for V1 than for V3. The velocity of bead movement decreased with increasing external load applied by the centrifuge microscope, and eventually reached zero when the load was equal to the maximum isometric force (P0) generated by the myosin heads. The maximum isometric force P0 was less than 10 pN, and did not differ significantly between V1 and V3. The P-V curves consisted of a hyperbolic part in the low force range and a non-hyperbolic part in the high force range. The critical force above which the curve deviated from the hyperbola was much smaller for V1 than for V3. An analysis using a model with an extremely small number of myosin heads involved in the bead movement suggested a marked difference in kinetic properties between V1 and V3.

Insulin action on heart and skeletal muscle glucose uptake in essential hypertension

Essential hypertension is characterized by skeletal muscle insulin resistance but it is unknown whether insulin resistance also affects heart glucose uptake. We quantitated whole body (euglycemic insulin clamp) and heart and skeletal muscle (positron emission tomography and 18F-fluoro-2-deoxy-D-glucose) glucose uptake rates in 10 mild essential hypertensive (age 33 +/- 1 yr, body mass index 23.7 +/- 0.8 kg/m2, blood pressure 146 +/- 3/97 +/- 3 mmHg, VO2max 37 +/- 3 ml/kg per min) and 14 normal subjects (29 +/- 2 yr, 22.5 +/- 0.5 kg/m2, 118 +/- 4/69 +/- 3 mmHg, 43 +/- 2 ml/kg per min). Left ventricular mass was similar in the hypertensive (155 +/- 15 g) and the normotensive (164 +/- 13 g) subjects. In the hypertensives, both whole body (28 +/- 3 vs 44 +/- 3 mumol/kg per min, P < 0.01) and femoral (64 +/- 11 vs 94 +/- 8 mumol/kg muscle per min, P < 0.05) glucose uptake rates were decreased compared to the controls. In contrast, heart glucose uptake was 33% increased in the hypertensives (939 +/- 51 vs 707 +/- 46 mumol/kg muscle per min, P < 0.005), and correlated with systolic blood pressure (r = 0.66, P < 0.001) and the minute work index (r = 0.48, P < 0.05). We conclude that insulin-stimulated glucose uptake is decreased in skeletal muscle but increased in proportion to cardiac work in essential hypertension. The increase in heart glucose uptake in mild essential hypertensives with a normal left ventricular mass may reflect increased oxygen consumption and represent an early signal which precedes the development of left ventricular hypertrophy.

Favorable left ventricular remodeling following large myocardial infarction by exercise training. Effect on ventricular morphology and gene expression

Continued adverse remodeling of myocardium after infarction may lead to progressive ventricular dilation and heart failure. We tested the hypothesis that exercise training in a healed myocardial infarction-dysfunction rat model can favorably modify the adverse effects of ventricular remodeling including attenuation of abnormal myosin gene expression. Sprague-Dawley rats were subjected to either proximal LAD ligation or sham operation. At 5 wk after the operation, animals were randomly assigned to sedentary conditions or 6 wk of graduated swim training, creating four experimental groups: infarct sedentary (IS), infarct exercise (IE), sham sedentary (SS), and sham exercise (SE). At 11 wk all rats were sacrificed and analyzed. Compared to sedentary infarct controls, exercise training attenuated left ventricular (LV) dilation and allowed more hypertrophy of the non infarct wall. The exercise-trained hearts also showed a reduction in the estimated peak wall tension. Northern blot analysis showed an increase in beta-myosin heavy chain expression in the hearts of the sedentary infarction group soon after infarction when compared to sham controls. However, with exercise training, there was a significant attenuation of the beta-myosin heavy chain expression in the myocardium. Exercise training in a model of left ventricular dysfunction after healed myocardial infarction can improve the adverse remodeling process by attenuating ventricular dilation and reducing wall tension. The abnormal beta-myosin expression was also attenuated in the exercise trained group. This is evidence that abnormal gene expression following severe myocardial infarction dysfunction can be favorably modified by an intervention.

Cardiac levels of fibronectin, laminin, isomyosins, and cytochrome c oxidase of weanling rats are more vulnerable to copper deficiency than those of postweanling rats

The relative quantities of cardiac laminin, fibronectin, cytochrome c oxidase (CCO), and isomyosin types were studied by gel electrophoresis in male rats fed copper-deficient diets beginning either from the time of weaning for 5 weeks or from 5 weeks postweaning for 6 weeks with one group of copper-repleted rats. Increased levels of fibronectin and V(3) isomyosin but decreased levels of CCO subunit IV and laminin were found in weanling copper-depleted rats. In contrast, postweanling copper-depleted rats exhibited only increased levels of fibronectin and decreased levels of cardiac CCO subunit IV. Repletion of copper-deficient rats for 6 weeks was not sufficient to restore CCO subunit IV to the same level as controls. These results confirm that biochemical lesions in the basal laminae are a result of copper restriction. The decreased nuclear encoded subunits of CCO may help explain some of the mitochondrial pathology observed in dietary copper restriction. Increased V(3) isomyosin levels with low ATPase activity may help to conserve to a limited extent the ATP levels in copper-deficient cardiac tissue. These protein changes are consistent with the known morphological alterations of hearts from copper-restricted rats.

Angiotensin II induces cardiac phenotypic modulation and remodeling in vivo in rats

Cardiac phenotypic modulation and remodeling appear to be involved in the pathophysiology of cardiac hypertrophy and heart failure. We undertook this study to examine whether angiotensin II (Ang II) in vivo, independent of blood pressure, contributes to cardiac phenotypic modulation and remodeling. A low dose (200 ng/kg per minute) of Ang II was continuously infused into rats by osmotic minipump for 24 hours or 3 or 7 days to examine the effects on the expression of cardiac phenotype-related or fibrosis-related genes. This Ang II dose caused a small and gradual increase in blood pressure over 7 days. Left ventricular mRNAs for skeletal alpha-actin, beta-myosin heavy chain, atrial natriuretic polypeptide, and fibronectin were already increased by 6.9-, 1.8-, 4.8-, and 1.5-fold, respectively, after 24 hours of Ang II infusion and by 6.9-, 3.3-, 7.5-, and 2.5-fold, respectively, after 3 days, whereas ventricular alpha-myosin heavy chain and smooth muscle alpha-actin mRNAs were not significantly altered by Ang II infusion. Ventricular transforming growth factor-beta 1 and types I and III collagen mRNA levels did not increase at 24 hours and began to increase by 1.4-, 2.8-, and 2.1-fold, respectively, at 3 days. An increase in left ventricular weight occurred 3 days after Ang II infusion. Treatment with TCV-116 (3 mg/kg per day), a nonpeptide selective angiotensin type 1 receptor antagonist, completely inhibited the above-mentioned Ang II-induced increases in ventricular gene expressions and weight. Hydralazine (10 mg/kg per day), which completely normalized blood pressure, did not block cardiac hypertrophy or increased cardiac gene expressions by Ang II.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular adaptations of the myocardium to chronic exercise

The heart responds positively to programs of chronic dynamic exercise. Hallmark adaptations of the heart include a training bradycardia, increases in end-diastolic dimension and maximal stroke volume, and a general improvement in ventricular performance and contractile function. Of considerable clinical significance are the general observations that chronic exercise renders the myocardium less susceptible to the deleterious effects of acute ischemic episodes and can effectively prevent and/or reverse many of the cardiac functional deficits that are known to occur in settings of chronic hypertension, advanced age, and myocardial infarction. In the text that follows, information gathered over the last 25 to 30 years has been reviewed in an attempt to identify cellular myocardial adaptations, both known and hypothetical, that are responsible for the observed effects of chronic dynamic exercise on the function and morphology of the heart in both normal and selected pathophysiologic settings. Finally, a variety of unresolved issues regarding the ability of chronic exercise to elicit adaptive cardiocyte responses has been identified. In so doing, it is hoped that creative thought and future work in the area will be stimulated.

M-CAT, CArG, and Sp1 elements are required for alpha 1-adrenergic induction of the skeletal alpha-actin promoter during cardiac myocyte hypertrophy. Transcriptional enhancer factor-1 and protein kinase C as conserved transducers of the fetal program in cardiac growth

Induction of the fetal isogenes skeletal alpha-actin (skACT) and beta-myosin heavy chain (beta-MHC) is characteristic of cardiac growth in many models, suggesting a conserved signaling pathway. However, divergent regulation has also been observed. beta-Protein kinase C (PKC) and transcriptional enhancer factor-1 (TEF-1) are involved in induction of beta-MHC in alpha 1-adrenergic-stimulated hypertrophy of cultured cardiac myocytes (Kariya, K., Farrance, I.K. G., and Simpson, P.C. (1993) J. Biol. Chem. 268, 26658-26662; Kariya, K., Karns, L. R., and Simpson, P.C. (1994) J. Biol. Chem. 269, 3775-3782). In the present study, we asked whether the skACT promoter used the same mechanism. A mouse skACT promoter fragment (-113/-46) was induced by both alpha 1-adrenergic stimulation and co-transfection of activated beta-PKC, and contained three required DNA sequence elements: M-CAT, CArG, and Sp1. The skACT M-CAT element bound TEF-1 in cardiac myocytes. Thus the skACT and beta-MHC promoters both require a TEF-1 binding site for activation by alpha 1-adrenergic stimulation, but differ in that skACT also requires a CArG box. These results provide a potential molecular basis for divergent regulation of the fetal program, and also imply that PKC and TEF-1 are conserved transducers for this program during cardiac growth.

Mechanoperception and mechanotransduction in cardiac adaptation: mechanical and molecular aspects

Cardiomyocytes grow in hypertrophy due to a net increase in the synthesis of proteins, especially contractile proteins, in the cell. There is abundant information about the molecular and biochemical changes involved in this process, but it is not completely understood how cells sense mechanical stimuli and how these stimuli are transferred into a biochemical signal inducing the growth response. This mechanotransduction most likely takes place at the cellular membrane. The resulting signal is transferred to the nucleus, where it can initiate alterations in gene expression.

Direct in vivo gene transfer into porcine myocardium using replication-deficient adenoviral vectors

BACKGROUND

Efficient methods of introducing genes into myocardial cells must be developed before local somatic cell gene therapy can be implemented against myocardial disease. Although adenoviral (Ad5) vectors have been used to target rodent hearts and plasmid DNA has been directly injected into the myocardium of rats and dogs, the amounts of recombinant protein produced by these procedures have not been reported, and adenoviral vectors have not been used in large mammalian hearts.

METHODS AND RESULTS

Replication-deficient recombinant adenoviral vectors carrying either the luciferase or lacZ reporter genes were injected directly into the ventricular myocardium of adult domestic swine for evaluation of reporter gene expression. This procedure did not affect regional myocardial function as assessed by systolic wall thickening using ultrasonic crystals. Luciferase activity was detected 3 days after injection, increased markedly at 7 days, and then declined progressively at 14 and 21 days. Luciferase production was comparable in the right and left ventricular walls and increased with increasing amounts of virus, reaching 61 +/- 21 ng at the highest dose examined (3.6 x 10(9) plaque-forming units). The injection of 200 micrograms of plasmid DNA (pRSVL) produced levels of luciferase comparable to 1.8 x 10(8) plaque-forming units of recombinant Ad5; however, when normalized to the number of genes injected, the adenovirus was 140,000 times more efficient than plasmid DNA. Histochemical analysis of beta-galactosidase activity produced by a second Ad5 vector demonstrated that nearly all (> 95%) of the stained cells were cardiomyocytes and that the percentage of cardiomyocytes infected by the virus could be quite high in microscopic regions adjacent to the needle track (up to 75% in fields of 60 to 70 cells); however, Ad5-infected cells were rarely observed farther than 5 mm from the injection site. Furthermore, the Ad5 vector induced pronounced leukocytic infiltration that was far in excess of that seen after injection of vehicle alone.

CONCLUSIONS

This study demonstrates for the first time that direct intramyocardial injection of replication-deficient adenovirus can program recombinant gene expression in the cardiomyocytes of a large animal species with relevance to human physiology. The efficiency of adenovirus-mediated gene transfer is far superior to that of plasmid DNA injection, and this method appears to be capable of producing more recombinant protein. However, the cell-mediated immune response to the Ad5 vector and the limited distribution of reporter gene expression suggest that less immunogenic recombinant vectors and more homogeneous administration methods will be required before Ad5 vectors can be successfully used for phenotypic modulation.

An M-CAT binding factor and an RSRF-related A-rich binding factor positively regulate expression of the alpha-cardiac myosin heavy-chain gene in vivo

Cardiac muscle-restricted expression of the alpha-myosin heavy-chain (alpha-MHC) gene is regulated by multiple elements in the proximal enhancer/promoter. Within this region, an M-CAT site and an A-rich site were identified as potential regulatory elements. Site-specific mutations in each site, individually, reduced activity from the wild-type promoter by approximately 85% in the adult rat heart, demonstrating that these sites were positive regulatory elements. alpha-MHC, beta-MHC, and chicken cardiac troponin T (cTnT) M-CAT sites interacted with an M-CAT-binding factor (MCBF) from rat heart nuclear extracts that was immunologically related to transcriptional enhancer factor 1, a factor that binds within the simian virus 40 enhancer. The factor that bound the A-rich region (ARF) was antigenically related to the RSRF family of proteins, ARF was distinct from myocyte-specific enhancer factor 2 (MEF-2) on the basis of DNA-binding specificity and developmental expression. Like MEF-2, ARF DNA-binding activity was present in the heart and brain; however, no ARF activity was detected in extracts from skeletal muscle or C2C12 myotubes. MCBF and ARF DNA-binding activities were developmentally regulated with peak levels in the 1- to 2-day neonatal heart. The activity of both factors increased nearly fivefold in adult rat hearts subjected to a pressure overload. By comparison, the levels of alpha-MHC binding factor 2 did not change during hypertrophy. Binding sites for MCBF and ARF are present in several genes that are upregulated during cardiac hypertrophy. Our results suggest that these factors participate in the alterations in gene expression that occur during cardiac development and hypertrophy.

An M-CAT binding factor and an RSRF-related A-rich binding factor positively regulate expression of the alpha-cardiac myosin heavy-chain gene in vivo

Cardiac muscle-restricted expression of the alpha-myosin heavy-chain (alpha-MHC) gene is regulated by multiple elements in the proximal enhancer/promoter. Within this region, an M-CAT site and an A-rich site were identified as potential regulatory elements. Site-specific mutations in each site, individually, reduced activity from the wild-type promoter by approximately 85% in the adult rat heart, demonstrating that these sites were positive regulatory elements. alpha-MHC, beta-MHC, and chicken cardiac troponin T (cTnT) M-CAT sites interacted with an M-CAT-binding factor (MCBF) from rat heart nuclear extracts that was immunologically related to transcriptional enhancer factor 1, a factor that binds within the simian virus 40 enhancer. The factor that bound the A-rich region (ARF) was antigenically related to the RSRF family of proteins, ARF was distinct from myocyte-specific enhancer factor 2 (MEF-2) on the basis of DNA-binding specificity and developmental expression. Like MEF-2, ARF DNA-binding activity was present in the heart and brain; however, no ARF activity was detected in extracts from skeletal muscle or C2C12 myotubes. MCBF and ARF DNA-binding activities were developmentally regulated with peak levels in the 1- to 2-day neonatal heart. The activity of both factors increased nearly fivefold in adult rat hearts subjected to a pressure overload. By comparison, the levels of alpha-MHC binding factor 2 did not change during hypertrophy. Binding sites for MCBF and ARF are present in several genes that are upregulated during cardiac hypertrophy. Our results suggest that these factors participate in the alterations in gene expression that occur during cardiac development and hypertrophy.

Regulation of rat ventricular myosin heavy chain expression by serum and contractile activity

To quantitatively analyze the effects of serum stimulation and contractile activity and their interaction on cellular growth and cardiac myosin heavy chain (MHC) gene expression, spontaneously contracting neonatal rat ventricular myocytes in primary culture were maintained in serum-free growth medium or growth medium supplemented with fetal bovine serum. Contractile activity in paired cultures was inhibited by addition of the calcium channel blocker verapamil (10 microM) to the culture medium. Both serum stimulation and contractile activity produced myocyte hypertrophy as assessed by increases in total protein, total RNA, protein-to-DNA ratios, and total MHC protein content. MHC isoenzyme analysis indicated that both MHC-alpha and MHC-beta proteins accumulated in response to serum stimulation and/or contractile activity. The increases in MHC-beta protein resulting from serum stimulation and contractile activity occurred in parallel with increases in MHC-beta mRNA. In contrast, MHC-alpha mRNA levels were relatively unaffected by serum stimulation but appeared to decrease in response to contractile activity. The protein kinase inhibitor staurosporine (5 nM) reduced MHC-beta expression in serum-free, contracting cultures and also prevented the serum-induced increase in MHC-beta mRNA observed in both contracting and arrested myocytes. Staurosporine also increased MHC-alpha mRNA levels in serum-free, contracting, and verapamil-arrested myocytes. These data suggest that both humoral and mechanical factors regulate MHC isoenzyme expression and cellular growth in neonatal ventricular myocytes.

Gene expression of atrial natriuretic peptide in rat papillary muscle. Rapid induction by mechanical loading

The effect of mechanical stretch on protein synthesis and the expression of the gene for atrial natriuretic peptide (ANP) was examined in electrically paced, isolated papillary muscles from rat heart. Incorporation of [3H]phenylalanine into protein increased only in stretched but not in unloaded muscles. Five hours of stretching increased ANP mRNA levels more than threefold as compared to freshly excised papillary muscles. A drastic fall in ANP mRNA levels was observed in unloaded muscles over this time. These data indicate that papillary muscles similar to other ventricular tissue are capable of activating ANP gene expression in response to increased load. The effect occurs in vitro and does not depend on circulating or nervous factors. The unexpected rapid induction of ANP gene expression in such a particular structure of the heart raises the possibility of local actions of ventricular ANP.

Angiotensin receptor regulates cardiac hypertrophy and transforming growth factor-beta 1 expression

The role of angiotensin II via the angiotensin type 1 or type 2 receptor in the development of cardiac hypertrophy was determined in adult male Sprague-Dawley rats subjected to coarctation of the abdominal aorta. Five groups of animals were studied: coarctation, coarctation plus DuP 753, coarctation plus PD 123319, sham plus DuP 753, or sham operation. Type 1 receptor blockade was accomplished with DuP 753 given in the drinking water and type 2 blockade with PD 123319 delivered by osmotic minipumps beginning with the day of surgery until 72 hours after aortic coarctation. Mean carotid blood pressures and the carotid-femoral artery blood pressure gradients were not different among coarctation, coarctation plus DuP 753, and coarctation plus PD 123319 animals. However, ratios of heart weight to body weight were higher in coarctation (4.95 +/- 0.8) or coarctation plus PD 123319 (4.52 +/- 0.5) than in sham animals (3.6 +/- 0.4; P < .005 and .05, respectively). In coarctation plus DuP 753-treated animals heart weight-body weight ratios were not different from sham or sham plus DuP 753 animals (3.9 +/- 0.4 versus 3.6 +/- 0.4 or 3.3 +/- 0.08, respectively). Type 1 receptor mRNA levels were significantly increased in the coarctation group, with the highest levels in the coarctation plus DuP 753 and sham plus DuP 753 groups. To determine whether growth factors were involved in the hypertrophic process, we measured transforming growth factor-beta 1 mRNA levels. Northern analysis demonstrated a twofold increase in coarctation animals compared with sham or coarctation plus DuP 753-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile arrest increases sarcoplasmic reticulum calcium uptake and SERCA2 gene expression in cultured neonatal rat heart cells

We developed protocols with intact cultured neonatal rat myocytes to directly evaluate the function of the sarcoplasmic reticulum (SR) Ca-ATPase (or SERCA2), Na-Ca exchange (Na-CaX), and slow Ca transport systems (mitochondria and sarcolemmal Ca-ATPase). Spontaneously beating control cells were compared with cells cultured for 2 days in the presence of verapamil (verapamil-arrested cells, VA). Intracellular calcium (Cai) transients were measured by use of indo-1 during (1) spontaneous twitches, (2) contractures induced by rapid application of caffeine (CafC, with and without Nao), and (3) twitches induced by brief depolarizations with high [K]o solution (K-twitches). We also measured mRNA levels for the SR Ca-ATPase and Na-CaX in the same experimental preparations. The t1/2 for [Ca]i decline when both the SR Ca uptake and Na-CaX were prevented was the same for control and VA cells (approximately 20 seconds), indicating unaltered slow Ca transport systems. Similarly, there was no significant difference in the t1/2 of CafC when Na-CaX was the main mechanism responsible for [Ca]i decline (t1/2 approximately 1.5 seconds), indicating unaltered Na-CaX. Conversely, we found nearly a twofold increase in the rate of [Ca]i decline during K-twitches (control t1/2, 0.84 +/- 0.05 seconds; VA t1/2, 0.48 +/- 0.06 second; P < .001), indicating an increase in SR Ca-pumping activity in VA cells. This was also reflected by a 56% increase in the peak [Ca]i reached during CafC used to assess maximal SR Ca content (427 +/- 49 nmol/L in control versus 665 +/- 75 nmol/L in VA cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Posttranscriptional modification of myosin heavy-chain gene expression in the hypertrophied rat myocardium

Hypertrophy of the myocardium in response to pressure or volume overload elicits a change in myofibrillar protein content as a result of changes in both transcriptional and translational regulation of gene expression. Hemodynamic overload caused by aortic constriction produced changes in the expression of the two isoforms of myosin heavy chain (MHC) with a 319% increase in beta-MHC mRNA and a 54% decrease in alpha-MHC mRNA (P < 0.01). Cardiac unloading as a result of heterotopic transplantation resulted in a decrease in cardiac mass and a similar shift in MHC isoform expression. In this study. We investigated cardiac gene transcription to understand how different hemodynamic stimuli produce similar cardiac phenotypes. We studied the in vivo activity of the alpha-MHC promoter (-2564 to +421 bp of the transcriptional start site) by directly injecting a recombinant expression plasmid (pAM3LUC) into the ventricular tissue of coarctated animals as well as into the unloaded heterotopic transplanted heart. When expressed as a function of the activity of a constitutively active viral promoter (pSVCAT), pAM3LUC activities were 18.4 +/- 2.9, 24.6 +/- 2.6, and 25.0 +/- 4.5 (x10(4)) luciferase/chloramphenicol acetyltransferase units in the hypertrophied ventricles of 2-, 3-, and 7-day coarctated animals, respectively. These values were not statistically different from pAM3LUC activity in control hearts of sham operated animals even though alpha-MHC mRNA content was decreased by 54% in the hypertrophied myocardium. This disparity between transcriptional activity and mRNA content suggests that alpha-MHC expression in the hypertrophic ventricle is in part regulated by a posttranscriptional mechanism. In contrast, alpha-MHC promoter activity in the unloaded transplanted hearts decreased significantly by 37% compared to control working hearts and suggests that a transcriptional mechanism of regulation of the alpha-MHC gene may account for the phenotypic expression observed in the unloaded myocardium.

Insulin-like growth factor I stimulates myofibril development and decreases smooth muscle alpha-actin of adult cardiomyocytes

Adult rat cardiomyocytes in long-term culture express type 1 insulin-like growth factor (IGF) receptors. In contrast to insulin receptors, type 1 IGF receptors are up-regulated during culturing. IGF-I added to the cells at plating increased granular density and pseudopodia number per cell after 7 days. After 16 days, IGF-I-treated cells showed, as compared with controls, a dramatic increase of the number of newly built sarcomeres and were packed with myofibrils. At the same time, IGF-I suppressed the accumulation of smooth muscle alpha-actin (sm-alpha-actin) in a dose-dependent manner. Under the conditions of this in vitro system, growth hormone had no effect on cell morphology or sm-alpha-actin. sm-alpha-Actin, a nonsarcomeric isoform of actin expressed in early fetal cardiac development, reappears both during long-term culture of adult rat cardiomyocytes and during heart hypertrophy. This study shows that type 1 IGF receptors are up-regulated in adult rat cardiomyocytes in long-term culture and that IGF-I enhances myofibril development and concomitantly down-regulates sm-alpha-actin. This protein forms stress-fiber-like structures and may temporarily serve as a scaffold for the formation of new sarcomeres until myofibrils have developed throughout the cell and the scaffold is no longer needed. Our findings thus allow us to propose another hypothesis for the mechanism leading to overload heart hypertrophy.

Effect of chronic energy deprivation on cardiac thyroid hormone receptor and myosin isoform expression

Thyroid hormone (3,5,3'-triiodothyronine; T3) and its receptor (TR) play an important regulatory role for in vivo and in vitro cardiac myosin heavy chain (MHC) isoform gene expression by activating the alpha- and inhibiting the beta-MHC genes. Previous studies have shown that chronic energy deprivation (CED; 50% of normal caloric intake) in the rat impacts cardiac MHC protein expression and hemodynamic parameters in a pattern typically seen with hypothyroidism; yet, unlike hypothyroidism, circulating T3 levels are not depressed. Therefore, the goal of this study was to determine if the altered MHC isoform expression observed in CED is associated with altered TR expression, both at the mRNA and protein levels. Female rats weighing approximately 250 g were allocated into two groups, designated as normal control (NC) and CED. After 5 wk, the relative content of alpha-MHC protein and mRNA levels decreased in CED ventricles by 20% (P < 0.05). In contrast, the relative content of both beta-MHC protein and mRNA levels increased five- to sixfold in CED (P < 0.05). Although there were no changes in TR mRNA levels relative to 18S rRNA in CED, the total number of nuclear TRs decreased 3.5-fold in the CED group (P < 0.05), from a maximum binding capacity of 840 +/- 130 fmol/mg DNA in NC to 241 +/- 118 fmol/mg DNA in CED, with no change in the affinity of the receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of myocardial insulin-like growth factor-I gene expression in left ventricular hypertrophy

BACKGROUND

Left ventricular hypertrophy is a generalized adaptation to increased afterload, but the growth factors mediating this response have not been identified. To explore whether the hypertrophic response was associated with changes in local insulin-like growth factor-I (IGF-I) gene regulation, we examined the induction of the cardiac IGF-I gene in three models of systolic hypertension and resultant hypertrophy.

METHODS AND RESULTS

The model systems were suprarenal aortic constriction, uninephrectomized spontaneously hypertensive rats (SHR), and uninephrectomized, deoxycorticosterone-treated, saline-fed rats (DOCA salt). Systolic blood pressure reached hypertensive levels at 3 to 4 weeks in all three systems. A differential increase in ventricular weight to body weight (hypertrophy) occurred at 3 weeks in the SHR and aortic constriction models and at 4 weeks in the DOCA salt model. Ventricular IGF-I mRNA was detected by solution hybridization/RNase protection assay. IGF-I mRNA levels increased in all three systems coincident with the onset of hypertension and the development of ventricular hypertrophy. Maximum induction was 10-fold over control at 5 weeks in the aortic constriction model, 8-fold at 3 weeks in the SHR, and 6-fold at 6 weeks in the DOCA salt model. IGF-I mRNA levels returned to control values by the end of the experimental period despite continued hypertension and hypertrophy in all three systems. In contrast, ventricular c-myc mRNA content increased twofold to threefold at 1 week and returned to control levels by 2 weeks. Ventricular IGF-I receptor mRNA levels were unchanged over the time course studied. The increased ventricular IGF-I mRNA content was reflected in an increased ventricular IGF-I protein content, as determined both by radioimmunoassay and immunofluorescence histochemistry.

CONCLUSIONS

We conclude that (1) hypertension induces significant increases in cardiac IGF-I mRNA and protein that occur coordinately with its onset and early in the development of hypertrophy, (2) IGF-I mRNA levels normalize as the hypertrophic response is established, (3) in comparison to IGF-I, both c-myc and IGF-I receptor genes are differentially controlled in experimental hypertension. These findings suggest that IGF-I may participate in initiating ventricular hypertrophy in response to altered loading conditions. The consistency of these findings in models of high-, moderate-, and low-renin hypertension suggests that they occur independently of the systemic renin-angiotensin endocrine axis.

Ribosome distribution in normal and infarcted rat hearts

Distribution of ribosomes throughout the myocardium of normal and infarcted rat hearts was studied by immunofluorescence and laser confocal scanning microscopy. In addition, sections were labelled with peroxidase or immunogold particles for electron microscopic examination. Ligation of the proximal free left coronary artery produced severe myocardial ischaemia, and after 6 days of ligation most of the left ventricular wall was necrotic and partially replaced by granulation tissue. Immunofluorescence microscopy revealed the presence of ribosomes throughout the non-necrotic myocardium. Some cardiac muscle cells located in subendocardial areas and in the border areas surrounding the infarct were particularly intensely stained. Cells constituting the granulation tissue frequently exhibited strong ribosomal immunostaining. Within longitudinally sectioned cardiac muscle cells, ribosomes were organized in strands oriented along the long axis of the cell as well as in a cross-striated pattern. By double labelling of muscle cells with antibodies against ribosomes and Z-line-associated proteins (desmin or alpha-actinin), it was shown that the cross-striated bands of anti-ribosomal staining coincided with the I-bands along the myofibrils. Immunoelectron microscopy confirmed a wide distribution of ribosomes throughout the intermyofibrillar and subsarcolemmal sarcoplasm, and some labelling was also observed within the I-band. The present results indicate that ribosomes are distributed in a characteristic pattern throughout the sarcoplasm of cardiac muscle cells in association with the myofibrils. Furthermore, it is suggested that within viable cardiac muscle cells located adjacent to the infarct, protein synthesis is increased; this might be an important factor in regional development of compensatory hypertrophy of the surviving cardiac muscle cells.

Endothelin: potential role in development and disease

Endothelin is a vasoconstrictor substance, initially isolated from porcine endothelial cell supernatant, which has a structure different from any other mammalian peptide. An extensive array of biological activities has been ascribed to endothelin which, besides having unrivaled vasoconstrictive effects, modulates neurotransmission, regulates other hormones and neurotransmitters, and also has potent hyperplastic/hypertrophic effects. These observations have suggested important roles for endothelin in pathophysiological conditions and also in normal development. Inhibition of endothelin activity can decrease vasoconstriction associated with pathophysiological settings. Inhibition of endothelin activity also decreases mitogenesis and therefore cellular proliferation and growth, thereby supporting a role for endothelin in processes which are an integral part of normal development.

Transforming growth factor-beta in cardiac ontogeny and adaptation

The transforming growth factor-beta (TGF-beta) superfamily comprises a set of regulatory peptides with multiple effects on cell growth and differentiation. The elaborate regulation of TGF-beta s during embryonic development of the heart, the upregulation of TGF-beta after hemodynamic stress, and the impact of TGF-beta on cardiac gene expression together imply a prominent functional role for this family of growth factors in cardiac organogenesis and hypertrophy. Basal and TGF-beta-induced expression of skeletal alpha-actin, one of several genes specifically associated with developing or hypertrophied myocardium, each are contingent on transcriptional activation by serum response factor. A truncated form of the type II TGF-beta receptor, created by deletion of the cytoplasmic kinase domain, acts as a dominant suppressor of TGF-beta signal transduction in cultured cardiac muscle cells and may provide a suitable means to establish the functions of TGF-beta in vivo.

Decreased accumulation of beta 1-adrenergic receptor, G alpha s and total myosin heavy chain messenger RNAs in the left ventricle of senescent rat heart

The expression of genes coding for the beta 1-adrenergic receptor (beta 1-AR), the alpha subunit of Gs and total myosin heavy chain (MHC) was compared between left ventricles (LV's) from young (6-7 weeks old) and old (22 months old) rats. The mRNA levels were quantitated by Northern or Slot blots analyses using specific DNA probes. Ageing was found to be associated with a reduction in beta 1-AR (77%), G alpha s (33%) and, total MHC (51%) mRNA levels with no concomitant change in 18S RNA and poly(A+) mRNA levels. These results indicate that transcriptional and/or post-transcriptional mechanisms participate in the control of beta-adrenergic receptor density during ageing. As in the senescent LV, beta 1-AR mRNA level is reduced in the hypertrophied LV, whereas the level of G alpha s mRNA is reduced in the senescent but not in the hypertrophied LV. From our data we conclude (1) that a dual mechanism may operate during ageing, mechanical factors indirectly regulating beta 1-AR mRNA level, while changes in G alpha s mRNA level do not depend on hemodynamic load and (2) that the re-expression of beta-MHC mRNA does not compensate for the decreased accumulation of alpha-MHC mRNA which results in a large decrease in the level of total MHC mRNA in the senescent LV.

Increased production of endothelin-1 in the hypertrophied rat heart due to pressure overload

Endothelin-1 (ET-1) has been demonstrated to induce hypertrophy in cultured cardiac myocytes. We investigated the production of ET-1 in the heart of aorta-banded rats in vivo. Seven days after the banding of the abdominal aorta, rats developed a significant left ventricular hypertrophy. The tissue content of mature ET-1 and the level of expression of prepro ET-1 mRNA were higher in the left ventricle of aorta-banded rats than in those of sham-operated rats. The expression of prepro ET-1 mRNA in the right ventricle was not different between the two groups. These findings indicate that the production of ET-1 increased in the hypertrophied left ventricle, thereby suggesting the possible involvement of endogenous ET-1 in the development of cardiac hypertrophy due to pressure overload.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

The molecular biology of heart failure

In the mammalian heart, the development of cardiac hypertrophy is a common feature that normally precedes all forms of heart failure. This adaptive process involves molecular changes in the myocardium, including the altered expression of several genes encoding proteins for contraction and relaxation. The expression of myosin heavy chain (MHC) and sarcomeric alpha-actin messenger ribonucleic acid (mRNA) changes qualitatively during cardiac hypertrophy; however, their accumulations are not coordinated. Skeletal alpha-actin transcripts accumulate throughout the ventricles and earlier than beta-MHC transcripts, which accumulate primarily around large coronary vessels. Skeletal alpha-actin transcripts also "hyperaccumulate" relative to cardiac alpha-actin mRNA, whose expression does not change. Expression of MHC isomRNA shows an inverse relation; as beta-MHC accumulates, alpha-MHC decreases in abundance. From nuclear run-on assays, we present evidence that the accumulation of these gene products is at least under partial transcriptional control with developmental growth, suggesting that those changes that occur with hypertrophy and heart failure may be primarily transcriptionally regulated. The expression of the mRNA for the calcium-adenosine triphosphate (Ca(2+)-ATPase) of the sarcoplasmic reticulum changes quantitatively with cardiac hypertrophy without the reexpression of a different isoform. The relative mRNA and protein concentrations for this protein diminish with both cardiac hypertrophy and heart failure, a change that may partially explain the delayed relaxation rates seen in hypertrophied and failing hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

T-type Ca2+ current is expressed in hypertrophied adult feline left ventricular myocytes

Macroscopic T-type Ca2+ currents, which are often observed in fetal and neonatal cardiac muscle cells, were not found in normal (0 of 17) adult feline ventricular myocytes. However, they were present in most (15 of 21) myocytes isolated from adult feline left ventricles with long-standing pressure-overload-induced hypertrophy. This is the first study to provide evidence in a large mammal, such as the cat, that T-type Ca2+ channels may be reexpressed in adults in association with hypertrophy resulting from slow progressive pressure overload. Importantly, this expression was stable for the duration of the hypertrophy process and was not associated with abrupt pressure overload. T-type Ca2+ currents were separated from L-type Ca2+ currents by exploiting the differences in their voltage dependence of steady-state inactivation. Depolarizations from -80 mV revealed a rapidly activating inward current that peaked in magnitude at -30 mV (-1.8 +/- 0.9 [mean +/- SD] pA/pF) and fully inactivated within 100 milliseconds in 15 of 21 hypertrophied myocytes studied. Further depolarizations activated progressively less T-type Ca2+ current, so that at +10 mV the L-type Ca2+ current predominated. In the hypertrophied myocytes that demonstrated both T-type and L-type Ca2+ currents, two distinct peaks occurred in their current-voltage relations. T-type Ca2+ currents were not evident in any of the 17 normal adult feline left ventricular myocytes studied. The purpose of T-type Ca2+ currents in hypertrophy is unclear. However, their presence may make hypertrophied myocardium more prone to spontaneous action potentials and increase the likelihood for arrhythmias in partially depolarized hypertrophied myocardium.

Induction of right ventricular hypertrophy with obstructing balloon catheter. Nonsurgical ventricular preparation for the arterial switch operation in simple transposition

BACKGROUND

Recently, a successful result with a rapid two-stage arterial switch operation (ASO) was reported for patients with transposition of the great arteries (TGA) with low left ventricular pressure. In this procedure, the interval between pulmonary arterial banding and ASO was approximately 1 week. This successful result indicates the possibility of a nonsurgical ventricular preparation procedure using an obstructing balloon catheter prior to ASO.

METHODS AND RESULTS

A 5F atrioseptostomy catheter was inserted directly into the main pulmonary artery in six lambs aged 20 to 38 days. After the chest was closed, the balloon was inflated twice a day for a period of 2 to 2.5 hours. This procedure was performed for 4 consecutive days. After the final inflation, the ratio of right ventricular weight to total ventricular weight was compared with that in an age-matched control group. After the final inflation, the peak systolic right ventricular pressure and the percentage of peak systolic right ventricular to peak systolic aortic pressure rose to 85.6 +/- 4.7 mm Hg (mean +/- 1 SD) and 79.6 +/- 8.6%, respectively. The percentages of the right ventricular weight to the total ventricular weight were significantly higher after the balloon inflation than those in the control group in terms of wet heart weight (29.5 +/- 1.2% versus 23.0 +/- 1.0%; P < .0001) and dry heart weight (27.0 +/- 2.0% versus 21.0 +/- 1.1%; P < .0001).

CONCLUSIONS

The myocardial mass in the right ventricle increased after 4 days of intermittently applied pressure overload. Nonsurgical preparation of the ventricle for ASO in TGA is feasible.

Induction of cardiac insulin-like growth factor I gene expression in pressure overload hypertrophy

Molecular mechanisms regulating the cardiac hypertrophic response to increased hemodynamic load are understood poorly. Insulin-like growth factor I (IGF I) is a mitogen that is thought to play a key role in pre- and postnatal growth. To investigate a possible role of IGF I in the cardiac response to pressure overload, rats underwent banding of the ascending aorta immediately above the aortic valve using a hemoclip, or a sham procedure. An analysis of left-ventricular RNA by Northern hybridization using a 32P-labeled IGF I cDNA revealed four messenger ribonucleic acid transcripts of 7.6, 4.6, 1.7, and 0.9 to 1.2 Kb. Insulin-like growth factor I messenger ribonucleic acid was quantitated by ribonuclease protection assays using a rat exon 3 riboprobe. There was a sustained increase in IGF I mRNA levels that correlated temporally with the development of left ventricular hypertrophy. These results indicate that left ventricular pressure overload is associated with an induction of cardiac IGF I gene expression. Insulin-like growth factor I may play a role in the response to increases in wall stress and likely contribute to cardiac hypertrophy.

Coexpression of alpha and beta myosin heavy-chain isoforms in atria of neonates and infants with congenital heart disease

The relative amounts of cardiac myosin heavy-chain isoforms (MyHC) in right atrial tissue (RA) of 16 neonates and children suffering from congenital heart disease have been investigated. Quantification of MyHC was based on one-dimensional gel electrophoresis and on histometrical evaluation of cyro-sections stained with monoclonal antibodies against alpha- and beta-MyHC. The mean right atrial pressures ranged from 2 to 14 Hg. The RA load was normal in eight patients (5.1 +/- 1.3 mm Hg) and overloaded in eight cases (10 +/- 2.5 mm Hg). The arterial oxygen saturation was normal in 12 and ranged between 85% and 89% in four cases. In all patients a large proportion of atrial myocytes coexpressed alpha- and beta-MyHC. However, in the cases with pressure overloaded RA the amount of beta-MyHC was found to be 1.6 times higher than in the cases with normal pressure. This indicates an adaptational response to overload, as was previously described for the adult human heart. In light of this finding, it seems important to conserve as much as possible of the trained right atrial wall during a Fontan type of operation.

Regulation of myosin heavy chain genes in the heart

Advances in our knowledge of the regulation of cardiac myosin isoforms made possible by molecular cloning of the alpha- and beta-MHCs genes are reviewed. Expression of these genes in heart does not seem to require MyoD or related proteins of the skeletal muscle myogenic program. Cardiac MHC genes are under the control of T3, which stimulates transcription of the alpha-MHC gene and inhibits beta-MHC mRNA production both in vivo and in cultured heart cells. The responsiveness of the genes to T3 varies in different mammals, however. The genes are most responsive in rat and rabbit, intermediate in sensitivity in calf and subhuman primate (baboon), and very resistant in the dog. The human alpha-MHC gene is T3-inducible in ventricle, but the degree of response has not been quantified. Introduction of chimeric plasmids containing 5' flanking sequences of cardiac MHC genes fused to the CAT gene into cultured heart cells and transgenic animals has permitted identification of regulatory elements. Although the genes are closely linked in genomic DNA, they are controlled independently. The element within the alpha-MHC promoter responsible for induction by T3 is located approximately 160 base pairs from the transcription initiation site. Additional transcriptional activators located 5' upstream amplify the response to T3, probably by looping out intervening DNA sequences. The proximal region of the beta-MHC gene contains important regulatory elements, including those required for repression by T3, muscle-specific expression, a MyoD-independent positive element, and a hormone-independent repressor.(ABSTRACT TRUNCATED AT 250 WORDS)

Antithetical accumulation of myosin heavy chain but not alpha-actin mRNA isoforms during early stages of pressure-overload-induced rat cardiac hypertrophy

Myocardial response to a hemodynamic overload involves changes in the expression of isogenes encoding myosin heavy chain (MHC) and actin: beta-MHC/alpha-MHC and skeletal/cardiac alpha-actin mRNA isoform ratios are increased. It is not known whether these changes are due to increased accumulations of the two neosynthesized transcripts, beta-MHC and skeletal alpha-actin, or whether the mRNA isoforms normally present, alpha-MHC and cardiac alpha-actin, are concomitantly decreased. To answer these questions, using dot-blot hybridizations, primer extension, and exonuclease VII mapping assays, we have analyzed the content of sarcomeric MHC and actin mRNAs in the poly(A+) RNA in left ventricles of 23-24-day-old rats 18 and 24 hours after a pressure overload induced by stenosis of the thoracic aorta. The results showed a 1.9-fold increase in poly(A+) RNA after the stenosis. Skeletal/cardiac alpha-actin mRNA isoforms were already increased fivefold (from 0.19 to 0.99) at 18 hours, and this was exclusively due to a 5.5-fold increase in skeletal alpha-actin mRNA. At 24 hours, this ratio was increased ninefold (from 0.14 to 1.22), and this was due to a 4.3-fold increase in the level of skeletal alpha-actin mRNAs (p < 0.001) and a 1.9-fold decrease of cardiac alpha-actin mRNA (p < 0.001), restoring the same proportion of sarcomeric actin mRNA in sham-operated and operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene injection into canine myocardium as a useful model for studying gene expression in the heart of large mammals

We have investigated the regulated expression of genes injected into the heart of large mammals in situ. Reporter constructs using the chloramphenicol acetyltransferase gene under the control of muscle-specific beta-myosin heavy chain (beta-MHC) or promiscuous (mouse sarcoma virus) promoters were injected into the canine myocardium. There was a linear dose-response relation between the level of gene expression and the quantity of plasmid DNA injected between 10 and 200 micrograms per injection site. The level of reporter gene expression did not correlate with the amount of injury imposed on the cardiac tissue. There was no regional variation in expression of injected reporter genes throughout the left ventricular wall. By use of both the mouse sarcoma virus and a muscle-specific beta-MHC promoter, reporter gene expression was one to two orders of magnitude greater in the heart than in skeletal muscle. Expression in the left ventricle was threefold higher than in the right ventricle. Chloramphenicol acetyltransferase activity was detected at 3, 7, 14, and 21 days after injection, with maximal expression at 7 days after injection. Statistical analysis of coinjection experiments revealed that coinjection of a second gene construct (Rous sarcoma virus-luciferase) is useful in the control of transfection efficiency in vivo. Furthermore, using reporter constructs containing serial deletions of the 5' flanking region of the beta-MHC gene, we performed a series of experiments that demonstrate the utility of this model in mapping promoter regions and identifying important regulatory gene sequences in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular mechanisms of cardiac gene expression

Although the physiological properties of the myocardium and their dynamic character have been the focus of intense research during the past three decades, the biochemical and molecular correlates underlying cardiac development and performance have, until recently, remained poorly understood. The development of modern cellular and molecular biology has provided the necessary tools to undertake the study of the mechanisms involved in cardiac development and to understand the basis for important clinical and experimental problems in cardiovascular physiology. Most of the gene encoding contractile proteins have been cloned and characterized. The availability of molecular probes and the ability to introduce genes into individual cell types and tissues of living animals, are the most important breakthroughs of molecular and cell biology. This permits not only to analyze basic mechanisms of gene expression but has also significant practical applications for gene therapy. It is now possible to analyze the role of different regulatory gene sequences and identify their corresponding trans-active factors. In addition, direct gene injection makes it possible to study gene expression in a natural context, under conditions that are physiologically relevant and controllable.

Mechanical properties of rat cardiac skinned fibers are altered by chronic growth hormone hypersecretion

Chronic growth hormone (GH) hypersecretion in rats leads to increased isometric force without affecting the unloaded shortening velocity of isolated cardiac papillary muscles, despite a marked isomyosin shift toward V3. To determine if alterations occurred at the level of the contractile proteins in rats bearing a GH-secreting tumor (GH rats), we examined the mechanical properties of skinned fibers to eliminate the early steps of the excitation-contraction coupling mechanism. We found that maximal active tension and stiffness at saturating calcium concentrations (pCa 4.5) were markedly higher in GH rats than in control rats (tension, 52.9 +/- 5.2 versus 38.1 +/- 4.6 mN.mm-2, p < 0.05; stiffness, 1,105 +/- 120 versus 685 +/- 88 mN.mm-2.microns-1, p < 0.01), whereas values at low calcium concentrations (pCa 9) were unchanged. In addition, the calcium sensitivity of the contractile proteins was slightly but significantly higher in GH rats than in control rats (delta pCa 0.04, p < 0.001). The crossbridge cycling rate, reflected by the response to quick length changes, was lower in GH rats than in control rats (62.0 +/- 2.6 versus 77.4 +/- 6.6 sec-1, p < 0.05), in good agreement with a decrease in the proportion of alpha-myosin heavy chains in the corresponding papillary muscles (45.5 +/- 2.0% versus 94.6 +/- 2.4%, p < 0.001). The changes in myosin heavy chain protein phenotype were paralleled by similar changes of the corresponding mRNAs, indicating that the latter occurred mainly at a pretranslational level. These results demonstrate that during chronic GH hypersecretion in rats, alterations at the myofibrillar level contribute to the increase in myocardial contractility observed in intact muscle.

Increased expression and regional differences of atrial myosin light chain 1 in human ventricles with old myocardial infarction. Analyses using two monoclonal antibodies

BACKGROUND

This study was designed to examine the expression of atrial/fetal-type myosin light chain 1 (ALC1) in human ventricles with old myocardial infarction and in control hearts.

METHODS AND RESULTS

The expression of immunoreactive (ir) ALC1 was examined in the subendocardial and subepicardial myocardium of the infarcted and the noninfarcted regions in the left ventricles with old myocardial infarction (n = 12) and of the control left ventricles (n = 8). For the analysis, we prepared two monoclonal antibodies, KA1 and KB1, that were specific for only ALC1 and for both ALC1 and ventricular myosin light chain 1 (VLC1), respectively. The ir-ALC1 expression ratio [ALC1/(ALC1 + VLC1), %] of the subendocardial myocardium, determined densitometrically by Western blotting with KB1, was significantly higher in the infarcted region (11.4 +/- 7.3%) than in the noninfarcted region (4.7 +/- 2.3%, p < 0.001) and the control ventricle (1.0 +/- 1.5%, p < 0.0001). In the infarcted region, the subendocardial myocardium contained a significantly greater percentage of ir-ALC1 than the subepicardial myocardium (5.8 +/- 6.7%, p < 0.005). The ir-ALC1 expression ratio had a significant negative correlation with the value of tissue protein concentration (milligrams protein per gram wet weight). The immunohistochemical study with KA1 revealed that the surviving myocytes included in the infarcted region, especially in the ventricular aneurysm, expressed ir-ALC1 strongly in comparison with those in the noninfarcted or the control ventricles.

CONCLUSIONS

These results demonstrate increased expression of ALC1 and the regional differences in the failing left ventricles with old myocardial infarction. We conclude that the reexpression of ALC1 in infarcted ventricles occurs as one of the regional responses to increased load and may be a useful biochemical marker for the appearance of fetal-type myocytes.