Long-term culture of contractile mammalian heart cells in a defined serum-free medium that limits non-muscle cell proliferation

Statins as antioxidant therapy for preventing cardiac myocyte hypertrophy

Cardiac hypertrophy is a major cause of morbidity and mortality worldwide. The hypertrophic process is mediated, in part, by small G proteins of the Rho family. We hypothesized that statins, inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase, inhibit cardiac hypertrophy by blocking Rho isoprenylation. We treated neonatal rat cardiac myocytes with angiotensin II (AngII) with and without simvastatin (Sim) and found that Sim decreased AngII-induced protein content, [3H] leucine uptake, and atrial natriuretic factor (ANF) promoter activity. These effects were associated with decreases in cell size, membrane Rho activity, superoxide anion (O2*-) production, and intracellular oxidation, and were reversed with L-mevalonate or geranylgeranylpyrophosphate, but not with farnesylpyrophosphate or cholesterol. Treatments with the Rho inhibitor C3 exotoxin and with cell-permeable superoxide dismutase also decreased AngII-induced O2*- production and myocyte hypertrophy. Overexpression of the dominant-negative Rho mutant N17Rac1 completely inhibited AngII-induced intracellular oxidation and ANF promoter activity, while N19RhoA partially inhibited it, and N17Cdc42 had no effect. Indeed, Sim inhibited cardiac hypertrophy and decreased myocardial Rac1 activity and O2*- production in rats treated with AngII infusion or subjected to transaortic constriction. These findings suggest that statins prevent the development of cardiac hypertrophy through an antioxidant mechanism involving inhibition of Rac1.

Phenotypic stability of chick cardiomyocytes in serum-free media. Preservation of muscarinic receptor expression

Chick cardiomyocytes cultured in fetal bovine serum (FBS)-supplemented media are phenotypically unstable, becoming noncontractile and unresponsive to stimuli after several days. We report a culturing protocol that preserves the differentiated cardiomyocyte phenotype for at least 9 days in culture. Cardiomyocytes isolated from 11-day chicken embryos, and cultured in either Dulbecco's Modified Earle's Medium (DMEM)/Ham's F12 medium with N-2 supplement or Medium 199 (M199) with 10% FBS continued to beat spontaneously for 4-5 days; only cells cultured in N-2-supplemented medium exhibited spontaneous beating beyond 5 days. Immunostaining for alpha-actinin after 9 days in culture revealed that myofibrils persisted in N-2-supplemented cells, while no myofibrils were observed in the FBS-supplemented cells. For cells in FBS-supplemented media, [3H]thymidine incorporation rates were 7.5 and 3 times greater than that of cells in N-2-supplemented media at Days 4 and 9 in culture, respectively. The effect of growth media on the binding parameters of the muscarinic antagonist, [3H]N-methyl-scopolamine (NMS), was also compared. While B(max) decreased 34% between Days 4 and 9 for cells maintained in N-2-supplemented media, a 77% decrease was observed for cells cultured in FBS-supplemented media. The phenotypic stability of this preparation makes it feasible for the first time to use these cells in experiments that require more than 4 days to complete.

Adrenergic stimulation regulates Na(+)/Ca(2+)Exchanger expression in rat cardiac myocytes

The Na/Ca exchanger protein encoded by the NCX1 gene provides the predominant mechanism for calcium efflux during cardiac relaxation. Because beta -adrenergic stimulation increases expression of Ca(2+)channels (Ca(2+)influx) in cardiac myocytes, we tested the hypothesis that isoproterenol would concomitantly augment expression of NCX1. Four hour treatment of neonatal myocytes with isoproterenol significantly increased NCX1 gene and protein expression, and increased the rate of transcript initiation. Alpha-adrenergic stimulation significantly decreases NCX1 mRNA levels. Calcium transient measurements revealed that for cells that had been pretreated with isoproterenol there was a faster relaxation rate of the Ca(2+)transient in the presence of thapsigargin, indicating an enhanced rate of intracellular Ca(2+)removal. We conclude that effectors that increase calcium channel expression in neonatal myocytes also augments NCX1 gene and protein expression over a similar time course, and that this is due to enhanced NCX1 transcription. The regulation of expression of NCX1 by adrenergic pathways may play an important role in regulation of excitation-contraction coupling in cardiac myocytes.

Expression of coxsackievirus and adenovirus receptor in hearts of rats with experimental autoimmune myocarditis

The expression of coxsackievirus and adenovirus receptor (CAR) was dominant in the brains and hearts of mice until the newborn phase. There is no detailed information concerning the relation between the expression of CAR and development of hearts. It is also uncertain whether CAR is able to be induced in adult hearts after cardiac injury. We demonstrated that CAR was abundant in the hearts of newborn rats but was barely detectable in the hearts of adult rats. The expression of CAR in rat hearts with experimental autoimmune myocarditis, which was induced by immunization of purified cardiac myosin, was serially investigated. Active myocarditis was observed from day 15 after immunization. By immunohistochemistry, cardiomyocytes were strongly stained for CAR antibody from days 24 to 42. CAR mRNA was also detected from days 18 to 30 by using reverse transcription-polymerase chain reaction. In the next experiment, the induction of CAR on isolated cardiomyocytes was investigated. CAR was barely detectable in cultured cardiomyocytes by Western blot analysis after isolation. This molecule gradually appeared along with the creation of clusters and beating of cardiomyocytes. Furthermore, the induction of CAR in cultured cardiomyocytes increased after supplement with conditioned medium of rat splenocytes activated by concanavalin A. In conclusion, rat CAR is expressed strongly in the hearts of newborn rats and is suppressed in those of adult rats. The expression of CAR is enhanced during the active phase of experimental autoimmune myocarditis and is induced by inflammatory mediators. CAR may play a role in cell-to-cell contact and adhesion of cardiomyocytes.

Atrial chamber-specific expression of the slow myosin heavy chain 3 gene in the embryonic heart

The quail slow myosin heavy chain 3 (slow MyHC 3) gene is expressed in the developing heart and in slow muscles of the developing limb. It is first expressed in the pulsatile cardiac tube in the embryo, and as the heart chamberizes its expression becomes restricted to the atria. To identify regulatory elements responsible for atrial-specific expression, the 5' upstream region of slow MyHC 3 gene was investigated. An atrial regulatory domain (ARD1) between -840 and -680 acts as an atrial cell-specific enhancer in primary cardiocyte cultures. ARD1 also specifies atrial-specific expression in vivo when the ARD1/heterologous promoter was introduced into developing chick embryos by a replication-competent retroviral vector. ARD1 is the first atrial cell-specific enhancer to be identified. Fine deletion and mutation analysis within ARD1 defined a 40-base pair vitamin D3 receptor-like element that controls atrial cell-specific expression of the slow MyHC 3 gene by inhibiting its expression in ventricular cardiocytes.

Regulation of calcium channel expression in neonatal myocytes by catecholamines

Expression of the dihydropyridine (DHP) receptor (alpha 1 subunit of L-type calcium channel) in heart is regulated by differentiation and innervation and is altered in congestive heart failure. We examined the transmembrane signaling pathways by which norepinephrine regulates DHP receptor expression in cultured neonatal rat ventricular myocytes. Using a 1.3-kb rat cardiac DHP receptor probe, and Northern analysis quantified by laser densitometry, we found that norepinephrine exposure produced a 2.2-fold increase in DHP receptor mRNA levels at 2 h followed by a decline to 50% of control at 4-48 h (P < 0.02). The alpha-adrenergic agonist phenylephrine and a phorbol ester produced a decline in mRNA levels (8-48 h). The beta-adrenergic agonist isoproterenol and 8-bromo-cAMP produced a transient increase in mRNA levels. After 24 h of exposure to isoproterenol, 3H-(+)PN200-110 binding sites increased from 410 +/- 8 to 539 +/- 39 fmol/mg (P < 0.05). The number of functional calcium channels, estimated by whole-cell voltage clamp experiments, was also increased after 24 h of exposure to isoproterenol. Peak current density (recordings performed in absence of isoproterenol) increased from -10.8 +/- 0.8 (n = 23) to -13.9 +/- 1.0 pA/pF (n = 27) (P < 0.01). Other characteristics of the calcium current (voltage for peak current, activation, and inactivation) were unchanged. Exposure for 48 h to phenylephrine produced a significant decline in peak current density (P < 0.01). We conclude that beta -adrenergic transmembrane signaling increases DHP receptor mRNA and number of functional calcium channels and that alpha - adrenergic transmembrane signaling produces a reciprocal effect. Regulation of cardiac calcium channel expression by adrenergic pathways may have physiological and pathophysiological importance.

A new model for the research into rhythmic contraction activity of cardiomyocytes in vitro

Heart cells continue to contract rhythmically after isolation and in culture in vitro. We describe a model of heart preparation in vitro that permits quantitative research on the frequency of contractions of cardiomyocytes. The chick embryo heart explants placed on a network of elastic glass fibers continued beating for months, recorded and analyzed with the methods of computer-assisted image analysis. The efficacy of this experimental model for the screening of effects of various agents on the frequency of contractions was examined by following the effects of nifedipine, caffeine, ethanol, and benzamide. The reversibility of the effects and the reproducibility of results were demonstrated quantitatively. The significance of a mechanical elastic load provided by glass fibers for the preservation of long-lasting contractile activity of cardiomyocytes is discussed and the common occurrence of oscillatory contraction processes in various eucaryotic cells is noted.

Thyroid hormone regulation of transmembrane signalling in neonatal rat ventricular myocytes by selective alteration of the expression and coupling of G-protein alpha-subunits

Thyroid hormone exerts profound effects on the activity of the hormone-sensitive adenylate cyclase system in the heart. Distinct guanine nucleotide-binding regulatory proteins (G-proteins) mediate stimulatory and inhibitory influences on adenylate cyclase activity. To examine whether the effects of thyroid hormone on adenylate cyclase involve specific changes in G-protein subunit expression, the influence of tri-iodothyronine (T3) on the biosynthesis and activity of G-proteins in neonatal rat ventricular myocytes was determined. In myocytes challenged with T3 for 5 days, Gs alpha levels increased by 4 +/- 0.5-fold, whereas Gi2 alpha levels declined by more than 80%. T3 down-regulated Gi2 alpha mRNA by 60% within 3 days, but had no effect on Gs alpha mRNA. The basis for the decline in Gi2 alpha mRNA was the T3-mediated suppression of Gi2 alpha gene transcription by 80 +/- 9% within 4 h. The decline in Gi2 alpha mRNA in response to T3 produced a 2-fold decrease in relative rate of synthesis of Gi2 alpha but not in its half-life (46 +/- 7 h). Gs alpha synthesis was not altered by T3, but the half-life of Gs alpha increased from 50 +/- 6 h in control cells to 72 +/- 8 h in T3-treated cells. In addition, T3 provoked the translocation of Gs alpha from the cytoplasmic to the membranous compartment. Membranous Gs alpha increased from 30 +/- 6% to 61 +/- 7% of total cellular Gs alpha, whereas cytoplasmic Gs alpha declined from 68 +/- 6% to 33 +/- 8% within 1 day of exposure to T3. T3-mediated up-regulation of Gs alpha enhanced the activation of myocardial adenylate cyclase by the stimulatory pathway whereas the down-regulation of Gi2 alpha attenuated the deactivation of myocardial adenylate cyclase by the inhibitory pathway.

Cocultures of fetal and adult cardiomyocytes yield rhythmically beating rod shaped heart cells from adult rats

Different models of isolated cardiomyocytes are generally used for biochemical, biophysical, and pharmacological studies. Fetal cardiomyocytes can be easily cultured for several weeks regaining their ability for rhythmical and synchronous contractions. For investigations, differentiated myocytes derived from adult hearts are closer to the in situ situation. Unfortunately, these cells at best exhibit irregular and asynchronous contractions at very low frequencies. Already 1 d after seeding calcium-tolerant rod-shaped adult cardiomyocytes on a suitable substrate, the differentiated cells begin to dedifferentiate forming a confluent monolayer. After 7-10 d their beating activities are like those of fetal cells. Therefore, we tried to combine the advantages of both cell types to achieve fully differentiated cardiomyocytes, rod-shaped and rhythmically beating, isolated from adult hearts. Using contractile fetal cells as a substrate for the adult cardiomyocytes, freshly seeded differentiated adult myocytes are paced by the contraction frequency of the fetal monolayer. As a consequence, the rod-shaped adult cardiomyocytes reach frequencies of more than 140 cycles/min without external electrical stimulation. This model enables us to study cardiomyocytes in a state very similar to the in situ situation with respect to morphology, integrity, and contractile behavior.

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.

On establishing primary cultures of neonatal rat ventricular myocytes for analysis over long periods

INTRODUCTION

Primary cultures of neonatal rat ventricular myocytes include a population of rapidly dividing nonmyocardial cells that can alter the properties of myocytes and complicate experimental interpretations. Without any intervention, nonmyocyte proliferation restricts the utility of primary cultures in biochemical and electrophysiologic studies to 4-5 days. However, with the recent interest in regulation of cardiac gene expression and the effects of growth factors on cardiac function, long-term studies with stable heart cultures are warranted.

METHODS AND RESULTS

In the present study an immunohistochemical staining strategy was developed that allowed for reliable quantitation of myocytes and nonmyocytes in cultures maintained for extended periods under different culture conditions. Density gradient purification of myocytes was found valuable in limiting nonmyocyte levels to < 20% at early times. Further treatment of cultures with a mitotic inhibitor, 0.1 mM bromodeoxyuridine, or 3500 rads of gamma-irradiation effectively blocked the proliferation of nonmyocardial cells, while it had no effect on cardiocyte levels. However, bromodeoxyuridine displayed side effects on the myocytes; the spontaneous beating rate and intracellular glycogen content were markedly depressed. In contrast, a systematic investigation of the properties of the irradiated myocytes, including spontaneous beating rates, dihydropyridine receptors, glycogen content, sarcoplasmic reticulum function, and phosphoinositide signaling, revealed that irradiation did not alter cardiac cell function. Although ionizing radiation can stimulate gene expression in some cell types, gamma-irradiation did not evoke c-fos expression or cause sarcomere formation, responses seen in cardiac cells to several trophic factors.

CONCLUSION

This study establishes a system of stable, functional, primary cultured cardiac cells that can be used in long-term molecular and electrophysiologic studies of at least 2 weeks.

Epidermal growth factor stimulates cAMP accumulation in cultured rat cardiac myocytes

We have previously shown that epidermal growth factor (EGF) augments cAMP accumulation in the heart and stimulates cardiac adenylyl cyclase via a G protein mediated mechanism (Nair et al., 1989). More recently, employing an antibody against the carboxy-terminus decapeptide of Gs alpha, we have demonstrated that Gs alpha mediates the effects of EGF on cardiac adenylyl cyclase (Nair et al., 1990). Since the heart comprises of a variety of cell types, the purpose of the studies presented here was to determine whether or not the effects of EGF on adenylyl cyclase were mediated in cardiac myocytes or noncardiomyocytes. Therefore, cultures of ventricular cardiomyocytes and noncardiomyocytes from neonatal rat hearts were established and characterized. Apart from the differences in cellular morphology, cardiomyocytes but not the noncardiomyocytes employed in our studies expressed the alpha- and beta-myosin heavy chain (MHC) mRNA and the beta-MHC protein. Additionally, as described previously, treatment of cardiomyocytes with thyroid hormone increased alpha-MHC mRNA and decreased the expression of beta-MHC mRNA, indicating that the cardiomyocytes employed in our studies were responding in a physiologically relevant manner. EGF in a time-dependent manner increased cAMP accumulation in the cardiomyocytes but not in noncardiomyocytes. Maximum and half-maximum effects were observed at 100 nM and 2 nM concentrations of EGF, respectively. As determined by the presence of immunoreactive EGF receptors and tyrosine phosphorylation of the 170 kDa protein in membranes of cardiomyocytes and noncardiomyocytes, both the cell populations contained functional EGF receptors. Therefore, the differential effects of EGF on cAMP accumulation in the two cell populations appear to be due to differential coupling of the EGF receptors to the adenylyl cyclase system rather than the absence of EGF receptors in noncardiomyocytes. Consistent with our previous findings in isolated membranes and perfused rat hearts, EGF-elicited increase in cAMP accumulation in cardiomyocytes did not involve activation of beta-adrenoreceptors and was abolished by prior treatment of cells with cholera toxin. Overall, our findings demonstrate that EGF-elicited increase in cAMP accumulation in the heart is the reflection of changes in cAMP content of cardiomyocytes and not noncardiomyocytes.

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.

Proliferating cell nuclear antigen in developing and adult rat cardiac muscle cells

During early development, rat cardiac muscle cells actively proliferate. Shortly after birth, division of cardiac muscle cells ceases, whereas DNA synthesis continues for approximately 2 weeks at a progressively diminishing rate. Little DNA synthesis or cell division occurs in adult cardiocytes. Thus, developing cardiac muscle cells are an ideal system in which to examine the expression of cell cycle-regulated genes during development. We chose to examine proliferating cell nuclear antigen (PCNA), a gene expressed at the G1/S phase boundary of the cell cycle. Northern blots of RNA from cardiac muscle cells from 18-day-old rat fetuses and from day 0, 5, and 14 neonatal as well as adult rat hearts revealed that the PCNA mRNA was found in cardiac muscle cells from all ages. However, because it was possible that this was a result of fibroblast PCNA gene expression, we used reverse transcription followed by polymerase chain reaction to see if it was possible to detect the message for PCNA in cardiac muscle cells from all ages. Because of the great sensitivity of this technique, RNA was recovered from 25 isolated adult cardiac muscle cells. Polymerase chain reaction amplification products for PCNA produced from the RNA isolated from these cells conclusively demonstrated that mRNA for this gene, which normally is associated with proliferating cells, is expressed in adult cardiac muscle cells that no longer divide. Furthermore, Western blot analysis demonstrated that the PCNA protein was found only in embryonic and neonatal cells and not in adult rat cardiac muscle cells. Therefore, it might be inferred from these data that PCNA might be regulated at the posttranscriptional level in adult cardiac muscle cells.

A new method for assessment of cultured cardiac myocyte contractility detects immune factor-mediated inhibition of beta-adrenergic responses

BACKGROUND

Potentially reversible congestive heart failure accompanies disease states associated with an immune cell myocardial infiltrate such as cardiac allograft rejection and inflammatory myocarditis. We therefore examined the hypothesis that immune cells can produce noncytotoxic alterations in cardiac function.

METHODS AND RESULTS

A novel system to evaluate cultured cardiac myocyte contractility was developed using neonatal rat cardiocytes grown on human amniotic membrane segments. Spontaneous synchronous cell beating produced macroscopic distortion of these membranes. Movement of free-floating membranes anchored within a perfusion chamber was visualized under low-power microscopy and measured from recordings of the rhythmic displacement of membrane-adherent markers. Additions of graded concentrations of isoproterenol to the perfusate produced up to threefold increases in the initial contractile phase velocity (contractile index), with an EC50 of 10(-7) M. When the extracellular Ca2+ concentration was increased from 0.9 to 3.6 mM, 2.43-fold increases in this index occurred. Myocytes incubated for 72 hours in the presence of dilutions of medium conditioned by activated rat splenic macrophages and lymphocytes exhibited an isoproterenol contractile index inhibited by 62% compared with control cells. In contrast, responses of supernatant-exposed and control cells to increased extracellular Ca2+ concentrations were not significantly different. Parallel studies of increases in myocyte intracellular adenosine 3':5'-cyclic monophosphate concentrations in response to isoproterenol stimulation demonstrated correlative inhibition that was specific for exposure to medium conditioned by immune cells.

CONCLUSION

Thus, a new method of in vitro cardiac contractility assessment that has significant advantages over existing systems has been developed and characterized. This new method has enabled description of an inhibitor of cardiac contractile function produced by activated immune cells.

In vivo collagen turnover during development of thyroxine-induced left ventricular hypertrophy

Cardiac fibroblasts synthesize large amounts of procollagens, yet only a small fraction of mature collagens accumulate in the extracellular matrix. To determine the roles of intracellular degradation of newly synthesized procollagens and extracellular degradation of mature collagens during normal growth and during thyroxine-induced left ventricular hypertrophy, in vivo left ventricular procollagen synthetic rates were assessed in control rats and rats treated with L-thyroxine for 1, 2, 4, and 8 wk (1 mg.kg-1.day-1). A modification of the flooding infusion method was developed using measurements of cardiac prolyl-tRNA, and tissue-free and protein-bound hydroxyproline specific radioactivities 60 min after intravenous administration of a massive dose of [3H]proline. Degradative rates of newly synthesized procollagens and mature collagens were then derived as the difference between rates of procollagen synthesis and collagen accumulation. Left ventricular procollagen synthetic rates were markedly increased after 1 wk of hormone administration (256 +/- 16 and 166 +/- 13 micrograms/day per left ventricle for thyroxine-treated and control animals, respectively; P less than 0.01). An even greater increase in procollagen synthetic rates was observed after 8 wk (438 +/- 46 and 202 +/- 18 micrograms/day for thyroxine-treated and control animals, respectively; P less than 0.01). Despite increased procollagen synthesis, disproportionate accumulation of fibrillar collagens (assessed as the relative concentration of protein-bound hydroxyproline in left ventricular tissue) did not occur. Derived left ventricular degradative rates for newly synthesized procollagens as well as for mature collagens were increased in thyroxine-treated animals. Increased procollagen synthesis, enhanced flux of newly synthesized procollagens through intracellular degradative pathways, and extensive extracellular matrix remodeling without disproportionate collagen accumulation are characteristics of this form of "physiological" left ventricular hypertrophy.

Immunocytochemical studies of cardiac myocytes and other non-neuronal cells of the fetal human heart in culture

Non-neuronal cell types present in cultures dissociated from the atria and ventricles of human fetal hearts, from 9 to 21 weeks' gestation, were studied using phase-contrast optics and immunocytochemistry. All atrial myocytes and many, if not all, ventricular myocytes observed in culture contained atrial natriuretic peptide-like immunoreactivity. Generally, the atrial natriuretic peptide-like immunoreaction in atrial myocytes was more intense and widespread than in ventricular myocytes. Atrial and ventricular fibroblasts expressed extracellular fibronectin-like immunoreactivity. A population of cells with the appearance and growth properties of endothelial cells was observed in both atrial and ventricular cultures, and was classified as endothelioid since their precise origin was not known. Only a subpopulation of these endothelioid cells contained factor VIII-related antigen immunoreactivity, and some cells that did not display the other growth characteristics of endothelial cells were also factor VIII-related antigen immunoreactive in culture. Glial cells were S-100-like immunoreactive; they were usually more numerous in atrial than ventricular preparations. There was no close association between glial cells and neurones in the atrial cultures.

Beta-adrenoceptor-mediated cAMP accumulation in cardiac cells: effects of nebivolol

The effects of nebivolol, the racemic mixture of the SRRR and RSSS enantiomers, on beta-adrenoceptor-mediated cAMP accumulation in living cardiac cells were compared to those of beta-adrenoceptor antagonists. Serum-free cultivation of cardiac cells from ventricles of 2 to 3-day-old Wistar rats resulted in a population of contractile cardiac cells almost free of mesenchymal non-myocardial cells. Isoproterenol stimulated beta 1- as well as beta 2-adrenoceptor sites. Selective beta 1- and beta 2-receptor site occlusion, in the presence of an appropriate concentration of the selective beta 2-adrenoceptor antagonist, ICI 118-551, or the selective beta 1-adrenoceptor antagonist, CGP 20712-A, showed that the receptor population consisted of mostly the beta 1-adrenergic subtype. The latter could be specifically stimulated by noradrenaline. Nebivolol and d-nebivolol (SRRR) inhibited noradrenaline-induced cAMP accumulation with IC50 values of 22 and 15 nM, respectively. CGP 20712-A was 10 times more active and atenolol was 7 times less active than nebivolol. Both assays, beta-adrenoceptor binding and cAMP accumulation, evidenced beta-adrenoceptor antagonistic properties only for the d-enantiomer of nebivolol (SRRR). 1-Nebivolol (RSSS) showed no beta-adrenergic activity.

Serum-free, chemically defined medium to evaluate the direct effects of growth factors and inhibitors on proliferation and function of neonatal rat cardiac muscle cells in culture

Neonatal rat cardiac myocytes were isolated and cultured to evaluate the effects of growth factors and inhibitors on proliferation, survival, and functions in a serum-free medium. Insulin and transferrin in MCDB 107 nutrient medium elicited DNA and protein synthesis in cells on a fibronectin-coated culture surface in serum-free medium. Insulin was most effective on both DNA and protein synthesis in serum-free culture conditions. The serum-free, hormone-supplemented medium eliminated the contamination of noncardiac myocytes and supported the long-term survival (over 18 d) of cardiac myocytes. Dexamethasone was required to induce optimal contractility with or without insulin and transferrin. Serum contained both negative and positive effectors of DNA and protein synthesis of the cardiac myocytes. Concentrations of serum (above 5%) inhibited DNA and protein synthesis. Low density lipoprotein (LDL) accounted in part for the inhibitory activity. The serum-free culture system provides a useful model to elucidate the role of hormones, growth factors, and drugs in heart cell regeneration and function.

Contraction modulates the capacity for protein synthesis during growth of neonatal heart cells in culture

Neonatal ventricular myocytes that were incubated in a well-defined serum-free medium containing 50 mM KCl did not contract and maintained stable cell size, as assessed by the protein/DNA ratio. The present study utilized KCl-arrested cells to examine the effect of constant rates of synchronous contraction in normal [K+]o (4 mM) as a physiological stimulus for myocyte growth. Cell growth increased following the onset of contraction when measured over 3 days. The rate of protein synthesis was accelerated in parallel by contraction, but the rate of protein degradation remained similar to rates in noncontracting cells. The capacity for protein synthesis was estimated by total RNA content and was increased in contracting as compared with KCl-arrested cells. This increase was accompanied by faster rates of RNA synthesis as determined from the incorporation of [3H]uridine into RNA and the specific activity of the cellular UTP pool. The rate of RNA degradation was accelerated during contraction but the difference between the rates of RNA synthesis and degradation resulted in net RNA accumulation of 49% after 3 days. These data demonstrated that 1) contractile activity stimulated myocyte growth through an increased capacity for protein synthesis and 2) the increased capacity for protein synthesis involved acceleration of the rate of RNA synthesis. Since enhancement of protein synthetic capacity is a common feature of myocyte hypertrophy in vivo and in vitro, this model can be used to examine the regulation of ribosome synthesis during hypertrophic growth.

Amphiphile-induced heart muscle-cell (myocyte) injury: effects of intracellular fatty acid overload

Lipid amphiphile toxicity may be an important contributor to myocardial injury, especially during ischemia/reperfusion. In order to investigate directly the potential biochemical and metabolic effects of amphiphile overload on the functioning heart muscle cell (myocyte), a novel model of nonesterified fatty acid (NEFA)-induced myocyte damage has been defined. The model uses intact, beating neonatal rat myocytes in primary monolayer culture as a study object and 5-(tetradecyloxy)-2-furoic acid (TOFA) as a nonmetabolizable fatty acid. Myocytes incubated with TOFA accumulated it as NEFA, and the consequent NEFA amphiphile overload elicited a variety of cellular defects (including decreased beating rate, depletion of high-energy stores and glycogen pools, and breakdown of myocyte membrane phospholipid) and culminated in cell death. The amphiphile-induced cellular pathology could be reversed by removing TOFA from the culture medium, which resulted in intracellular TOFA "wash-out." Although the development and severity of amphiphile-induced myocyte injury could be correlated with both the intracellular TOFA/NEFA content (i.e., the level of TOFA to which the cells were exposed) and the duration of this exposure, removal of amphiphile overload did not inevitably lead to myocyte recovery. TOFA had adverse effects on myocyte mitochondrial function in situ (decoupling of oxidative phosphorylation, impairing respiratory control) and on myocyte oxidative catabolism (transiently increasing fatty acid beta oxidation, citric acid cycle flux, and glucose oxidation). The amphiphile-induced bioenergetic abnormalities appeared to constitute a state of "metabolic anoxia" underlying the progression of myocyte injury to cell death. This anoxic state could be ameliorated to some extent, but not prevented, by carbohydrate catabolism.

Alterations in cation homeostasis in cultured chick ventricular cells during and after recovery from adenosine triphosphate depletion

Alterations in cation homeostasis during and after recovery from myocardial ischemia may account for some of the reversible and irreversible components of myocardial cell injury. To investigate possible mechanisms involved, we exposed cultured layers of spontaneously contracting chick embryo ventricular cells to media containing 1 mM cyanide (CN) and 20 mM 2-deoxyglucose (2-DG), and zero glucose for up to 6 h, and then allowed cultured cells to recover in serum-free culture medium for 24 h. Changes in Na, K, and Ca contents, 42K uptake and efflux, ATP content, cell water content, and lactate dehydrogenase (LDH) release were measured, and compared with changes produced by exposure to 10(-3) M ouabain and severe hypoxia. Exposure to CN and 2-DG caused marked increase in cell Na (sevenfold) and Ca (fivefold) contents, and a decrease in K content (one-fifth normal), coincident with ATP depletion to one-tenth normal levels. This produced only slight cell injury, evidenced by increased LDH release. Recovery for 24 h resulted in return to near normal values (expressed in nanomoles per milligram of protein) of Na, Ca, and ATP contents. However, there was failure of cell K content to return to normal, associated with a persistent reduced net uptake of 42K, and an increase in the rate of 42K efflux. These abnormalities in K homeostasis were associated with a decrease in cell volume and water content per milligram of protein. More marked ATP depletion (to 1/100 normal values) was produced by hypoxia plus 2-DG and zero glucose, and was associated with much more severe cell injury manifested by LDH loss. Ouabain exposure resulted in a much greater Ca gain (20-30-fold), relative to increase in Na content, than did either CN and 2-DG or hypoxia; and ouabain effects were not reversible (after a 15-fold or greater increase in Ca content was produced) and were associated with significant LDH release. We conclude that these cells are resistant to cell injury caused by moderately severe Ca overload and ATP depletion produced by exposure to CN and 2-DG. However, metabolic inhibition of ATP production produces persistent abnormalities in K homeostasis, associated with functional abnormalities.

Effect of thyroid hormone on slow calcium channel function in cultured chick ventricular cells

The hyperthyroid state is associated with increased myocardial contractility. To clarify responsible mechanisms, we examined the effects of thyroid hormone on slow Ca channels, beta-adrenergic receptors, transsarcolemmal 45Ca flux and cytosolic free calcium in cultured chick ventricular cells. Compared with cells grown without triiodothyronine (T3), cells grown in 10 nM T3 possessed 67% (P less than 0.05) more dihydropyridine 3H-PN200-110 binding sites, 24% (P less than 0.05) more beta-adrenergic antagonist 3H-CGP12177 binding sites, a 57% (P less than 0.05) greater nifedipine-sensitive initial 45Ca uptake rate, and a 31% (P less than 0.05) greater nifedipine-sensitive 45Ca uptake rate in response to BAY k 8644. Time-averaged mean intracellular free Ca concentration ([Ca]i) measured with fura-2, total protein content, and dissociation constant values for 3H-PN200-110 or 3H-CGP12177 binding was not significantly different in the two groups of cells. BAY k 8644 (1 microM) increased mean [Ca]i 2.85- or 2.16-fold in cells grown with or without 10 nM T3, respectively. l-Isoproterenol (1 microM) increased [Ca]i 1.53- or 1.28-fold in cells grown with or without 10 nM T3, respectively. We conclude that thyroid hormone augments transsarcolemmal Ca influx, at least in part via slow Ca channels associated with increased numbers of these channels. T3-treated cells appear to be more responsive to the effects of BAY k 8644 or isoproterenol on [Ca]i.

Effects of gonadectomy and hormonal replacement on rat hearts

To evaluate the effects of sex hormones on heart function and biochemistry, gonadectomy (GX) was performed in postpubertal male (M) and female (F) rats and compared with sham-operated controls (SH). The groups were MSH; MGX; MGX replaced with testosterone 3 mg/day s.c. (MGX + T), FSH, and FGX replaced with estrogen 2 mg/day (FGX + E), progesterone 0.4 mg/day (FGX + P), estrogen and progesterone (FGX + EP), or testosterone 2 mg/day (FGX + T). Body weight was decreased in MGX and was decreased further in MGX + T. Heart weight was decreased in both MGX and MGX + T. Body weights were increased in FGX and FTX + P and were increased further in FGX + T but were normal in FGX + E and FGX + EP. Heart weights were unchanged in F groups except in FGX + T, where it was increased. Cardiac performance in perfused hearts, as measured by stroke work, ejection fraction, fractional shortening and mean velocity of circumferential fiber shortening, was decreased in MGX but was slightly increased in MGX + T. Papillary muscle studies showed increases in time to peak tension and one-half relaxation in MGX, but these were decreased in MGX + T. Isotonic shortening studies showed decreased velocity of shortening in MGX and increased velocity in MGX + T. Heart function was significantly decreased in FGX and FGX + P compared with FSH but was similar to FSH in FGX + E and FGX + EP. FGX + T had greater stroke work and ejection fraction than FSH and FGX.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of closure of cardiac sodium channels in rabbit ventricular myocytes: single-channel analysis

We have examined the kinetics of closure of sodium channels using single-channel recordings in cell-attached and excised membrane patches of rabbit ventricular myocytes. Sodium-channel closure was dependent on membrane potential. The closing rate initially decreased with depolarization. The rate then passed through a minimum and increased at strongly depolarized potentials. We attempted to determine the separate voltage dependence of the deactivation and inactivation rate constants using the method of Aldrich, Corey, and Stevens. In a majority of experiments, the method did not give internally consistent results. As an alternative approach, batrachotoxin was used to remove inactivation and determine the voltage dependence of deactivation rate. The deactivation rate decreased with depolarization. To account for the increase in the closing rate at strongly depolarized test potentials, one must postulate voltage dependence of inactivation. The ensemble average current relaxed with a time course that was usually best described by the sum of two exponentials. The larger of the two rate constants that described the relaxation was strongly voltage-dependent, increasing with depolarization. The larger rate constant may reflect voltage-dependent inactivation. We found evidence of two possible mechanisms for the slow component of relaxation: 1) cardiac sodium channels may open repetitively during a given depolarizing epoch, and 2) channels may return from the inactivated state with low probability and burst for as much as 200 msec with open times that are longer than those during usual gating. The slow component appears to be more prominent in cardiac muscle than in nerve and may play an important role in the control of the action potential duration and the inotropic state of the heart.

A neuraminidase from Trypanosoma cruzi removes sialic acid from the surface of mammalian myocardial and endothelial cells

Trypanosoma cruzi causes Chagasic heart disease, a major public health problem in Latin America. The mechanism of interaction of this protozooan parasite with host cells is poorly understood. We recently found that the infective trypomastigote form a T. cruzi exhibits neuraminidase activity and can desialylate mammalian erythrocytes. However, it is not known if T. cruzi can also modify the surfaces of cardiovascular cells that are directly involved in the most important clinical manifestations of this disease. Accordingly, this study determined whether T. cruzi can remove sialic acid from cultured rat myocardial or human vascular endothelial cells. Sialic acid was labeled metabolically with the precursor 3H-N-acetyl-D-mannosamine. Soluble neuraminidase, isolated from intact T. cruzi trypomastigotes, caused significant release of labeled material from myocardial cells (e.g., 2,174 +/- 27 dpm/h vs. spontaneous release of 306 +/- 30 dpm/h, n = 4, P less than 0.001). Chromatographic analysis showed that the bulk of the radioactivity released by T. cruzi neuraminidase was sialic acid. Intact T. cruzi trypomastigotes also released sialic acid from metabolically labeled myocardial cells in a concentration-dependent manner. In contrast, a noninfective form of T. cruzi, the amastigote, did not desialylate these cells. Galactose oxidase labeling demonstrated newly desialylated glycoproteins on the surface of myocardial cells treated with T. cruzi neuraminidase. Desialylation of myocardial cells was confirmed histochemically by the appearance of binding sites for peanut agglutinin, a lectin that binds to complex oligosaccharide moieties after removal of the terminal sialyl residue. T. cruzi neuraminidase also removed sialic acid from adult human saphenous vein endothelial cells, as determined by both histochemical and metabolic labeling studies. Thus, infective forms of T. cruzi can chemically modify the surfaces of myocardial and vascular endothelial cells by desialylation. This alteration may play a role in the initial interaction of this parasite with these important target cells of the host cardiovascular system.

Regulation of the growth of nonmuscle heart cells in culture

Primary cultures of neonatal rat hearts contain both striated muscle (myocytes) as well as nonmuscle heart cells (NMHC). Although myocytes do not divide in culture, NMHC do increase in number. The growth of NMHC is dependent on the concentration of serum in the media over a range of 1 to 10%. When compared to growth in 10%, cells in 1% serum have a prolonged doubling time and reach a maximum density that is 70% less. Thus, 1% serum which supports normal myocyte development is a useful culture media to also maintain muscle heart cell homogeneity by its failure to support optimum NMHC division.

Molecular forms of immunoactive atrial natriuretic peptide released from cultured rat atrial myocytes

Primary cultures of atrial myocytes were prepared from newborn rats and maintained for 8 days in complete serum-free medium. The culture content of immunoactive atrial natriuretic peptide (ANP) increased from 10 to 25 ng/culture during this time. The cells released immunoactive ANP at a rate of 2 to 3% of culture content per hour in a linear fashion for at least 6 hours. When analyzed by gel filtration the major immunoactive material released by and contained within the cells displayed a molecular weight of approximately 15,000 daltons. The medium and cellular ANP-related peptides were further shown to be indistinguishable by reversed-phase HPLC. When the 15,000 dalton material was incubated with rat serum it was converted to ANP-related material possessing a molecular weight of approximately 3,000 daltons. These results suggest that under basal conditions, atrial myocytes release a large molecular weight form of ANP that is converted in the circulation to a low molecular weight form of ANP, which has been previously identified in plasma.

Factors controlling embryonic heart cell proliferation in serum-free synthetic media

Embryonic chick cardiac cell cultures, plated on collagen-coated dishes, containing serum-free synthetic media proliferate actively. The basic medium contained Ham's F12 nutrient mixture, fetuin, ascorbic acid, and bovine serum albumin. This medium was supplemented with various combinations of factors: endothelial cell growth supplement (ECGS), epidermal growth factor (EGF), insulin (I), transferrin (T), selenium (S), hydrocortisone, and thyroxine or supplemented alone. Basic medium supplemented with ECGS alone contributes to the highest final cell density among all other factors used in various combinations or alone. The final cell density of the control culture with 2% fetal bovine serum was higher than those of all experimental cultures and an additional control culture grown in the basic medium. Combinations of factors without ECGS do not promote significant cell proliferation. Thyroxine is required to induce optimal differentiation and contractility of cardiac myocytes in vitro. Fibronectin and laminin did not show any more influence than collagen did on the growth and maintenance of cardiac myocytes in serum-free media. The proportion of cardiac muscle cells in ECGS-containing media was higher than those in other experimental media and control media with the exception of ECGS and ITS-containing medium that showed lower proportion of cardiac myocytes than that of serum-containing medium on Days 3 and 5. The profiles of incorporation of [3H]thymidine into DNA of heart cells in experimental and control cultures showed a peak in incorporation values within the first week of culture and subsequently declined. Autoradiography studies revealed that cardiac myocytes in culture supplemented with ECGS alone attained a peak in labeling index on Day 1 with approximately 62% labeled cells. Subsequently, the labeling indices declined. Cardiac myocytes grown in media without ECGS showed significantly lower labeling indices than those in ECGS-containing media. This study has demonstrated the influence of ECGS, EGF and ITS in promoting the growth of cardiac myocytes and also in contributing to the maintenance of contractile cardiac myocytes in serum-free, long-term culture. The influence of ECGS on heart cell proliferation is considered to be superior to that of EGF and ITS.

Mechanisms of beta-adrenergic receptor regulation in cultured chick heart cells. Role of cytoskeleton function and protein synthesis

To examine mechanisms by which cardiac tissue regulates the beta-adrenergic receptor and physiological response to beta-adrenergic agonists, we studied the effects of cytoskeletal disrupting agents and inhibition of protein synthesis on receptor properties and contractile response to isoproterenol in intact cultured ventricular cells from embryonic chick heart. Thirty minutes of exposure of intact cells to 1 microM isoproterenol produced loss of the high-affinity state (KD = 4.5 +/- 1.5 nM) of the receptor found in cell membranes with no loss of total receptor number, whereas there was concomitant decline in the contractile response to 1 microM isoproterenol to 41 +/- 16% (SD) of control. Contractile response recovered within 60 minutes of agonist removal to 78 +/- 11% of initial response. There was concomitant recovery of the high-affinity state of the receptor, so that 1 hour after agonist removal there was 72% of the initial proportion of high-affinity receptors. This desensitization of the contractile response, as well as recovery after agonist removal, was markedly blunted by preincubation with cytochalasin B so that contractile responsiveness to isoproterenol was maintained at 77 +/- 13% of the initial response. Colchicine (10 microM) was without effect on the first 30 minutes of agonist-induced desensitization. More prolonged agonist exposure (1 microM isoproterenol for 24 hours) produced colchicine-sensitive loss of receptors from intact cells to 40% of control levels. Full recovery of receptor number occurred over 72 hours; this was completely blocked by cycloheximide (P less than 0.01). Thus, rapid desensitization and resensitization of the beta-receptor-mediated contractile response is associated with alterations in high-affinity agonist binding and appears to be modulated by microfilaments. Receptor down-regulation is dependent on functional microtubules, and recovery of these receptors after agonist removal requires protein synthesis.

Effects of thyroid hormone on calcium handling in cultured chick ventricular cells

Mechanisms underlying thyroid hormone-induced changes in myocardial contractile state were investigated by studying the effects of triiodothyronine (T3) on Ca2+ fluxes across the sarcolemmal membrane and Ca2+ handling by the sarcoplasmic reticulum, using spontaneously contracting monolayers of cultured chick embryo ventricular cells. Cells were grown in serum-free medium containing either no T3 or 10(-8) M-T3 for 48 h. At [Ca2+]o levels of 0.6 and 1.2 mM, the velocity of cell contraction was significantly greater in cells grown in 10(-8) M-T3 than in its absence. At higher [Ca2+]o, no differences in the velocity of contraction were noted. 45Ca2+ exchange kinetic studies showed a biexponential pattern with a rapid and a slow component of uptake in cells grown both with and without 10(-8) M-T3. The rate of the rapid phase of uptake and total Ca2+ content were higher in cells grown in T3, with the increment in content ascribable to the rapidly exchangeable Ca2+ pool. Verapamil partially inhibited the T3-induced increase in the rapidly exchangeable pool. 45Ca2+ uptake in response to a step change to Na+-free medium in the presence of 1 microM-verapamil was significantly greater in cells grown in 10(-8) M-T3 than in T3-free medium. Cells grown in T3 showed 20% greater beating rate than cells grown in its absence. A similar increase in beating rate achieved by lowering [K+]o from 4.0 to 3.0 mM or by electrical stimulation failed to affect the rate of 45Ca2+ uptake or the size of the rapidly exchangeable pool; pacing-induced increases in rate resulted in reduction rather than augmentation of contractile state. Ca2+ efflux rate was greater in cells grown in 10(-8) M-T3 than in T3-free medium, whereas cells loaded with various levels of Ca2+ acutely by incubation at selected [Ca2+]o levels had similar efflux rates. Replacement of Na+ by choline in the efflux medium resulted in elevated Ca2+ efflux rates in cells grown both with and without T3; however, it remained greater in cells grown in 10(-8) M-T3 than in its absence. Caffeine (20 mM) in the efflux medium increased Ca2+ efflux to a greater degree in cells grown in T3 than without it. Caffeine also produced a greater tonic contraction in T3-treated cells than in cells grown in absence of T3 in Na+- and Ca2+-free medium.(ABSTRACT TRUNCATED AT 400 WORDS)