Mechanisms that regulate homing function of progenitor cells in myocardial infarction

Cell based therapy has become a new and attractive option for the treatment of cardiac disease and heart failure. Although it has been demonstrated in vitro and in vivo that differentiation of non-differentiated cells (progenitor cells) into cardiomyocytes is able even in adult hearts the potential use of such transdifferentiation processes is limited by the small number of cells that home and engraft in the myocardium and complete the transdifferentiation process. Therefore, cell recruitment to the damaged heart is a major challenge to improve any cell based therapy. This process requires homing and engraftment of stem or progenitor cells. Major strategies to improve stem or progenitor cell homing are based on an improvement of stem or progenitor cell mobilization from the bone marrow. Strategies that have been shown to be successful are those that use granulocyte colony-stimulating factor (G-CSF). But although cell mobilization was indeed successful no major impact on hemodynamics was found. Alternatives are therefore needed and experimental studies use parathyroid hormone, statins, erythropoietin, and others in addition to or as an alternative to G-CSF. Although each of these procedures does have an impact on cell mobilization and homing none of these studies has provided a direct evidence that a major improvement on top of standard pharmacological therapy can be expected from such strategies. In conclusion, improvement of stem cell homing is a major challenge in the development of successful cell based therapies but not yet improved to a clinical relevant status. The underlying concepts of different strategies will be discussed here.

Mechanism of cGMP-mediated protection in a cellular model of myocardial reperfusion injury

OBJECTIVE

Reperfusion injury of the myocardium is characterised by development of cardiomyocyte hypercontracture. Previous studies have shown that cGMP-mediated stimuli protect against reperfusion injury, but the cellular mechanism is still unknown.

METHODS

To simulate ischemia/reperfusion, adult rat cardiomyocytes were incubated anoxically (pH(o) 6.4) and then reoxygenated (pH(o) 7.4). Cytosolic calcium [Ca(2+)](i) (fura-2 ratio), pH(i) (BCECF ratio), cell length, and phospholamban phosphorylation were analysed. Under simulated ischemia cardiomyocytes develop [Ca(2+)](i) overload. When reoxygenated they rapidly undergo hypercontracture, triggered by oscillations of [Ca(2+)](i). We investigated whether cGMP-mediated stimuli can modulate [Ca(2+)](i) or pH(i) recovery and whether this contributes to their protective effect. Membrane-permeable cGMP analogues, 8-bromo-cGMP (1 mmol/L) or 8-pCPT-cGMP (10 micrommol/L), or a receptor-mediated activator of particulate guanylyl cyclase, urodilatin (1 micromol/L), were applied.

RESULTS

The investigated stimuli protect against reoxygenation-induced hypercontracture (cell length as percent of end-ischemic length; control: 68+/-1.6; 8-bromo-cGMP: 88+/-1.5*; 8-pCPT-cGMP: 84+/-2.9*; urodilatin: 87+/-1.1*; n=24; *p<0.05). Recovery from [Ca(2+)](i) overload after 2 min reoxygenation [fura-2 ratio (a.u.); control: 1.43+/-0.15; 8-bromo-cGMP: 1.86+/-0.15*; 8-pCPT-cGMP: 1.92+/-0.19*; urodilatin: 1.93+/-0.24*; n=25; *p<0.05] was accelerated, and the frequency of [Ca(2+)](i) oscillations (min(-1)) was significantly reduced (control: 49+/-5.0 min(-1); 8-bromo-cGMP: 18+/-3.5* min(-1); 8-pCPT-cGMP: 18+/-4.5* min(-1); urodilatin: 16+/-4.1* min(-1); n=24; *p<0.05). cGMP-mediated stimuli increased sarcoplasmic Ca(2+) sequestration (caffeine-releasable Ca(2+) pool: 2-3 fold increase vs. control). Inhibition of sarcoplasmic Ca(2+)-ATPase (SERCA) by thapsigargin (150 nmol/L) or of protein kinase G with KT-5823 (1 micromol/L) abolished the effect of these stimuli on [Ca(2+)](i) recovery. The investigated stimuli significantly enhanced phospholamban phosphorylation.

CONCLUSIONS

We conclude that cGMP-dependent signals activate SERCA via a protein kinase G-dependent phosphorylation of phospholamban. The increase in SERCA activity seems to reduce peak [Ca(2+)](i) and [Ca(2+)](i) oscillation during reoxygenation and to attenuate the excessive activation of the contractile machinery that otherwise leads to the development of hypercontracture.

Noradrenaline-induced increase in protein synthesis in adult rat cardiomyocytes: involvement of only alpha1A-adrenoceptors

Adult rat ventricular cardiomyocytes contain alpha1A- and alpha1B-adrenoceptors (ARs, 20%:80%, assessed by [3H]prazosin binding). We studied which alpha1-AR subtype mediates noradrenaline (NA)-induced increase in rate of protein synthesis, and which signalling pathway is involved. NA (10-9-10-4 M) concentration-dependently increased inositol phosphate (IP) formation (pEC50-value=6.1+/-0.1, n=5) and protein synthesis (assessed as [3H]phenylalanine incorporation; pEC50-value=6.6+/-0.1, n=6). NA-induced IP-formation was partly inhibited by the alpha1B-AR antagonist chloroethylclonidine (CEC, 30 microM; 33+/-9% inhibition, n=5); following CEC-treatment the alpha1A-AR-selective 5-methyl-urapidil (5-MU) inhibited NA-induced IP-formation with a pKi-value of 9.2+/-0.2 (n=6); the alpha1D-AR-selective BMY 7378 was only a weak antagonist (pKi-value <7). NA-induced increase in protein synthesis was insensitive to CEC whereas 5-MU inhibited it with a pKi-value of 9.1+/-0.2 (n=6). NA (1 microM)-induced increase in protein synthesis was inhibited by the protein kinase C (PKC) inhibitor bisindolylmaleimide (IC50-value: 206 nM), the PI 3-kinase inhibitors wortmannin (IC50=3.4 nM) and LY 294002 (IC50=10 microM), and p70s6-kinase inhibitor rapamycin (IC50=123 pM) but not by the p38 MAP-kinase inhibitor SB 203580 (10 microM) or the MEK-inhibitor PD 98059 (25 microM). Moreover, 5-MU (30 nM) but not CEC inhibited NA-induced activation of p70s6-kinase. We conclude that, in adult rat cardiomyocytes, alpha1A- and alpha1B-AR mediate NA-induced IP-formation but only alpha1A-ARs mediate increase in protein synthesis. Alpha1A-AR-mediated increase in protein synthesis involves activation of a PKC, PI 3-kinase and p70s6-kinase but not of ERK- or p38 MAP-kinase.

Redox-sensitive intermediates mediate angiotensin II-induced p38 MAP kinase activation, AP-1 binding activity, and TGF-beta expression in adult ventricular cardiomyocytes

Cardiac hypertrophy as an adaptation to increased blood pressure leads to an increase in ventricular expression of transforming growth factor Cardiac hypertrophy as an adaptation to increased blood pressure leads to an increase in ventricular expression of transforming growth factor b (TGF-b), probably via the renin-angiotensin system. We studied in vivo to determine whether angiotensin II affects TGF-b expression independent from mechanical effects caused by the concomitant increase in blood pressure and in vitro intracellular signaling involved in angiotensin II-dependent TGF-b1 induction. In vivo, the AT1 receptor antagonist losartan, but not reduction of blood pressure by hydralazine, inhibited the increase in TGF-b1 expression caused by angiotensin II. In vitro, angiotensin II caused an induction of TGF-b1 expression in adult ventricular cardiomyocytes and induced AP-1 binding activity. Transfection with "decoys" directed against the binding site of AP-1 binding proteins inhibited the angiotensin II-dependent TGF-b induction. Angiotensin II induced TGF-b expression in a p38-MAP kinase-dependent way. p38-MAP kinase activation was diminished in presence of the antioxidants or diphenyleneiodium chloride, or by pretreatment with antisense nucleotides directed against phox22 and nox, components of smooth muscle type NAD(P)H oxidase. Thus, our study identifies a previously unrecognized coupling of cardiac AT receptors to a NAD(P)H oxidase complex similar to that expressed in smooth muscle cells and identifies p38-MAP kinase activation as an important downstream target.

TGF-beta(1) downregulates PTHrP in coronary endothelial cells

Parathyroid hormone-related peptide (PTHrP) is expressed throughout the cardiovascular system including coronary endothelial cells. Factors involved in the regulation of cardiac PTHrP expression have not been examined before. This study investigates the influence of transforming growth factor (TGF)-beta(1)on ventricular PTHrP expression. Coronary endothelial cells were isolated from ventricles of adult rats and PTHrP protein expression in these cultures was analysed by immunoblotting. TGF-beta(1)caused a concentration-dependent reduction in PTHrP protein within 24 h. In transgenic mice over-expressing TGF-beta(1)ventricular PTHrP protein expression and release was reduced compared to non-transgenic littermates. Similar concerns hold for PTHrP mRNA content (RT-PCR). Since ventricular TGF-beta(1)expression increases under pathophysiological conditions like arterial hypertension, ventricular PTHrP expression was further determined in aging spontaneously hypertensive (SHR-SP) and normotensive rats. TGF- beta(1)expression was increased in SHR-SP and ventricular PTHrP mRNA expression was downregulated at the age of 10 months. PTHrP expression did not recover in elder SHR-SP in which TGF-beta(1)expression was normalized again. Finally, we investigated ventricular PTHrP expression in rats after banding of the ascending aorta which generates a pressure induced hypertrophy without an induction of TGF-beta(1)expression. In ventricles from these animals, PTHrP expression was transiently increased and normalized at day 3. In conclusion, PTHrP expression was reduced under all conditions in which coronary endothelial cells were exposed to TGF-beta(1). PTHrP expression does not correlate with cardiac hypertrophy. Since coronary endothelial cells represent the majority of PTHrP producing cells in the ventricle its downregulation by TGF- beta(1)seems to be relevant for the paracrine effects of PTHrP.

Differential effects of carvedilol and metoprolol on isoprenaline-induced changes in beta-adrenoceptor density and systolic function in rat cardiac myocytes

OBJECTIVE

beta-Blockers improve cardiac function and survival in heart failure patients. The underlying mechanisms are not completely elucidated. Differences between agents might be important for the development of more specific therapeutical approaches. This study investigated whether metoprolol or carvedilol alter beta-adrenergic signaling differently.

METHODS

beta-Adrenoceptor density and systolic function were determined in rat adult ventricular cardiac myocytes.

RESULTS

12 h isoprenaline-treatment (Iso, 1 micromol/l) reduced beta-adrenoceptor density by 33% (P<0.01). The effect was abolished by incubation with isoprenaline plus metoprolol (3 micromol/l), but was more pronounced after coincubation with carvedilol (0.003 micromol/l, P<0.05 Carv vs. Iso). Metoprolol alone had no effect on beta-adrenoceptor density, but carvedilol induced a decrease in receptor density even in absence of isoprenaline (P<0.05 Carv vs. ctr.). The isoprenaline (0.0003-10 micromol/l) induced concentration-dependent increase in myocyte shortening was blunted after 12 h preincubation with Iso (1 micromol/l, P<0.001). This reduction was abolished or partly prevented by coincubation with metoprolol or carvedilol, respectively. Carvedilol decreased the number of receptors which had to be occupied by isoprenaline in order to obtain 50% and 90% increase in myocyte cell shortening. Comparison of guanine nucleotide-dependent binding characteristics of isoprenaline, carvedilol and metoprolol revealed beta-receptor agonist like binding characteristics for carvedilol, but antagonist like binding characteristics for metoprolol.

CONCLUSION

Metoprolol but not carvedilol prevents isoprenaline-induced downregulation of myocyte beta-adrenoceptors. The difference might be due to specific binding properties of the beta-blockers. Restoration of isoprenaline responsiveness by carvedilol might be due to improved coupling of beta-receptors to postreceptor effects.

Beta-adrenoceptor stimulation attenuates the hypertrophic effect of alpha-adrenoceptor stimulation in adult rat ventricular cardiomyocytes

OBJECTIVES

The study investigated whether beta-adrenoceptor antagonists augment the hypertrophic response of cardiomyocytes evoked by norepinephrine.

BACKGROUND

In adult ventricular cardiomyocytes, stimulation of alpha- but not beta-adrenoceptors induces myocardial hypertrophy. Natural catecholamines, like norepinephrine, stimulate simultaneously alpha- and beta-adrenoceptors. We investigated whether beta-adrenoceptor stimulation interferes with the hypertrophic response caused by alpha-adrenoceptor stimulation.

METHODS

Adult ventricular cardiomyocytes isolated from rats were used as an experimental model. Hypertrophic parameters under investigation were stimulation of phenylalanine incorporation and protein mass, stimulation of 14C-uridine incorporation and RNA mass, and increases in cell shape.

RESULTS

Norepinephrine (0.01 to 10 micromol/liter) increased concentration-dependent phenylalanine incorporation; pEC50 value was 5.9 +/- 0.1 (n = 8). The alpha1-adrenoceptor antagonist prazosin (0.1 micromol/liter) suppressed norepinephrine-induced increase in rate of protein synthesis. Conversely, propranolol (1 micromol/liter) and the beta1-adrenoceptor selective antagonists CPG 20712A (300 nmol/liter) or atenolol (1 micromol/liter) augmented increases in phenylalanine incorporation caused by norepinephrine. Addition of the beta2-adrenoceptor antagonist ICI 118,551 (55 nmol/liter) did not influence the hypertrophic effect of norepinephrine. Atenolol augmented the norepinephrine-induced increases of all hypertrophic parameters investigated (i.e., protein mass, uridine incorporation, RNA mass, cell volume, and cross-sectional area). In the presence of norepinephrine, inhibition of beta1-adrenoceptors increased the amount of protein kinase C-alpha and -delta isoforms translocated into the particulate fraction. The effect of pharmacological inhibition of beta1-adrenoceptors could be mimicked by Rp-cAMPS (adenosine-3', 5'-cyclic phosphorothiolate-Rp). The inhibitory effect of beta1-adrenoceptor stimulation on the alpha-adrenoceptor-mediated effect persisted in cardiomyocytes isolated from hypertrophic hearts of rats submitted to aortic banding.

CONCLUSIONS

In isolated ventricular cardiomyocytes from rats, beta1-adrenoceptor stimulation attenuates the hypertrophic response evoked by alpha1-adrenoceptor stimulation.

A N-terminal PTHrP peptide fragment void of a PTH/PTHrP-receptor binding domain activates cardiac ET(A) receptors

1. Adult ventricular cardiomyocytes show an unusual structure-function relationship for cyclic AMP-dependent effects of PTHrP. We investigated whether PTHrP(1 - 16), void of biological activity on classical PTHrP target cells, is able to mimic the positive contractile effect of PTHrP(1 - 34), a fully biological agonist on cardiomyocytes. 2. Adult ventricular cardiomyocytes were paced at a constant frequency of 0.5 Hz and cell contraction was monitored using a cell-edge-detection system. Twitch amplitudes, expressed as per cent cell shortening of the diastolic cell length, and rate constants for maximal contraction and relaxation velocity were analysed. 3. PTHrP(1 - 16) (1 micromol l(-1)) mimicked the contractile effects of PTHrP(1 - 34) (1 micromol l(-1)). It increased the twitch amplitude from 5.33+/-0.72 to 8.95+/-1.10 (% dl l(-1)) without changing the kinetic of contraction. 4. PTH(1 - 34) (10 micromol l(-1)) affected the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 5. RpcAMPS (10 micromol l(-1)) inhibited the positive contractile effect of PTHrP(1 - 34), but not that of PTHrP(1 - 16). 6. The positive contractile effect of PTHrP(1 - 16) was antagonized by the ET(A) receptor antagonist BQ123. 7. Sarafotoxin 6b and PTHrP(1 - 16), but not PTHrP(1 - 34), replaced (3)H-BQ123 from cardiac binding sites. 8. We conclude that N-terminal PTHrP peptides void of a PTH/PTHrP-receptor binding domain are able to bind to, and activate cardiac ET(A) receptors.

Hypertrophic effect of selective beta(1)-adrenoceptor stimulation on ventricular cardiomyocytes from adult rat

We investigated whether selective beta(1)-adrenoceptor stimulation causes hypertrophic growth on isolated ventricular cardiomyocytes from adult rat. As parameters for the induction of hypertrophic growth, the increases of [(14)C]phenylalanine incorporation, protein and RNA mass, and cell size were determined. Isoproterenol (Iso, 10 microM) alone had no growth effect. In the presence of the beta(2)-adrenoceptor antagonist ICI-118551 (ICI, 10 microM), Iso caused an increase in [(14)C]phenylalanine incorporation, protein and RNA mass, cell volume, and cross-sectional area. We showed for phenylalanine incorporation that the growth effect of Iso+ICI could be antagonized by beta(1)-adrenoceptor blockade with atenolol (10 microM) or metoprolol (10 microM), indicating that it was caused by selective beta(1)-adrenoceptor stimulation. The growth response to Iso+ICI was accompanied by an increase in ornithine decarboxylase (ODC) activity and expression. Inhibition of ODC by the ODC antagonist difluoromethylornithine (1 mM) attenuated this hypertrophic response, indicating that ODC induction is causally involved. The growth response to Iso+ICI was found to be cAMP independent but was sensitive to genistein (100 microM) or rapamycin (0.1 microM). The reaction was enhanced in the presence of pertussis toxin (10 microM). We conclude that selective beta(1)-adrenoceptor stimulation causes hypertrophic growth of ventricular cardiomyocytes by a mechanism that is independent of cAMP but dependent on a tyrosine kinase and ODC.

Central role for ornithine decarboxylase in beta-adrenoceptor mediated hypertrophy

OBJECTIVE

TGF-beta stimulation of cardiac myocytes induces a hypertrophic responsiveness to beta-adrenoceptor stimulation. This study investigates whether this beta-adrenoceptor mediated effect depends on induction of ornithine decarboxylase (ODC).

METHODS

Isolated adult ventricular cardiomyocytes from rats were used as an experimental model. Cells were either cultured in 20% (v/v) FCS to activate autocrine released TGF-beta or used without pre-treatment. The hypertrophic response was characterized by an increased 14C-phenylalanine incorporation, RNA and protein mass or by an increased expression of atrionatriurectic factor and ODC. The results on cell cultures were compared to those achieved by isoprenaline perfused mice hearts from transgenic mice overexpressing TGF-beta 1.

RESULTS

ODC activity and expression increased within 2 h in TGF-beta 1 pre-treated cells under isoprenaline. In the presence of ODC inhibitors (alpha-methylornithine or difluoromethylornithine) this increase remained absent and the increases in 14C-phenylalanine incorporation, protein and RNA mass under isoprenaline were abolished. In cells not exposed to TGF-beta no induction of ODC was observed. Isoprenaline also induced ODC in isolated perfused ventricles from transgenic mice overexpressing TGF-beta 1, but not in ventricles from their nontransgenic counterparts.

CONCLUSIONS

This study shows first, a pivotal role for ODC induction in the hypertrophic response of cardiomyocytes to beta-adrenoceptor stimulation and second, that ODC induction in vivo and in vitro requires pre-treatment of cardiomyocytes with TGF-beta. It is concluded that TGF-beta induces a hypertrophic responsiveness to beta-adrenoceptor stimulation that is characterized by ODC induction.

Autocrine regulation of TGF beta expression in adult cardiomyocytes

As shown before, TGF beta acts in an autocrine manner on the induction of hypertrophic responsiveness to beta-adrenoceptor stimulation in cultured ventricular cardiomyocytes of adult rat. We now investigated how TGF beta expression and activation is regulated in these cultures and how beta-adrenoceptor stimulation influences TGF beta -mRNA expression. It was found that freshly isolated cardiomyocytes secrete latent TGF beta in the culture medium. Supplementation of the cultures with 20% FCS resulted in activation of the secreted TGF beta to 4.1+/-0.2 ng/ml active TGF beta after 6 days. Presence of the protease inhibitor aprotinin (50 microg/ml) reduced TGF beta activity by 44+/-5% (n=5, P<0.05). In cultures supplemented with 5% FCS, TGF beta was not activated. Active TGF beta downregulated its mRNA-expression: after 6 days TGF beta(1)-mRNA was reduced to 55.1+/-11.0%, TGF beta(2)-mRNA to 30.1+/-16.5%, and TGF beta(3)-mRNA to 0.3+/-0.4% in 20% FCS-cultures as compared to their expression in freshly isolated cells (n=4, P<0.05). TGF beta-mRNA expression did not change in cultures without active TGF beta. Isoprenaline (1 microm) increased TGF beta(1)-mRNA only in cultures which had been pre-exposed to active TGF beta. This effect was also seen when hearts from normal mice were compared with hearts from transgenic mice overexpressing TGF beta(1): only in hearts from transgenic animals perfusion with isoprenaline increased TGF beta(1)-mRNA. In conclusion, isolated cardiomyocytes release latent TGF beta, which is activated by external proteases. Active TGF beta downregulates its own mRNA expression. Preexposure to TGF beta is necessary for a beta-adrenoceptor-mediated increase in TGF beta(1)-mRNA in cardiomyocytes.

Protection of reoxygenated cardiomyocytes against sarcolemmal fragility: the role of glutathione

This study addressed the question of whether the sarcolemmal fragility of cardiomyocytes after anoxia and subsequent reoxygenation can be altered by modulation of the cellular glutathione state. Isolated ventricular cardiomyocytes (from adult rats) were exposed to 120 min anoxia and subsequently to 30 min reoxygenation. Osmotic stress was generated by reduction of medium osmolarity from 270 to 80 mosmol/l and sarcolemmal fragility assessed by the leakage of lactate dehydrogenase (LDH). Under normoxic conditions 6.7+/-1.0 % of total LDH activity was found extracellularly. Hyposmolar reoxygenation, but not hypoosmolar anoxia, increased LDH release (17.9+/-2.7% of total, P<0.05). Increasing cellular glutathione content by pretreatment with N-acetylcysteine (1 mM) reduced LDH release following hyposmolar reoxygenation (12.3+/-1.9% vs. 18.2+/-2.9% of LDH in medium, P<0.05). Depletion of glutathione content by pretreatment with buthionine sulphoximine (BSO, 200 microM), increased LDH release following osmotic stress already in normoxia (10.5+/-1.8% of LDH in medium; P<0.05 vs. no BSO), and even further after reoxygenation (21.8+/-3. 2%, P<0.05 vs. normoxia). We conclude that the increased sarcolemmal fragility in reoxygenated cardiomyocytes is due to reoxygenation in the presence of reduced antioxidant defence.

Regulation of growth in the adult cardiomyocytes

Cardiomyocytes of adult myocardium increase their cellular mass in response to growth stimuli. They undergo hypertrophic growth but they do not proliferate in contrast to immature cardiomyocytes. Growth stimuli of the adult cardiomyocytes include classical growth hormones, various neuroendocrine factors, and the increase in mechanical load. The signal transduction of alpha1-adrenoceptor stimulation has been investigated in greatest detail and may therefore be taken as a reference for other humoral stimuli. It involves the activation of protein kinase C (PKC) and, downstream of PKC activation, of two separate signaling pathways, one including the mitogen-activated protein kinase and another including PI3-kinase and p70(s6k) as key steps. Activation of the first pathway leads to re-expression of fetal genes, activation of the second pathway to a general activation of protein synthesis, and cellular growth. In neonatal cardiomyocytes, mechanical stretch causes growth by an activation of an autocrine mechanism including angiotensin II and endothelin. This mechanism does not operate, however, in adult cardiomyocytes. A mechanism of mechanotransduction has not yet been identified on adult cardiomyocytes but integrins may play a part. In microgravity, the scenario of myocardial growth stimulation is altered. On the systemic level, there are changes in hemodynamic and neuroendocrine regulation that exert indirect effects on the myocardium. Microgravity may also exert a direct cellular effect by the absence of a constant gravitational load component.

Early response kinase and PI 3-kinase activation in adult cardiomyocytes and their role in hypertrophy

The present study investigated the role of early response kinase (ERK) and phosphatidylinositol 3 (PI 3)-kinase in ventricular cardiomyocytes from adult rat for the hypertrophic response to alpha-adrenoceptor stimulation. Parameters of the hypertrophic response were stimulation of protein synthesis and induction of creatine kinase BB. The alpha-adrenoceptor agonist phenylephrine (10 micromol/l) activated ERK2 and PI 3-kinase. The protein kinase C inhibitor bisindolylmaleimide (5 micromol/l) and the mitogen-activated protein kinase kinase inhibitor PD-98059 (10 micromol/l) but not the tyrosine kinase inhibitor genistein (100 micromol/l) blocked ERK2 activation. Inhibition of ERK2 activation abolished induction of creatine kinase BB by phenylephrine but not the increase in protein synthesis. The PI 3-kinase inhibitor wortmannin (100 nmol/l) blocked protein synthesis under alpha-adrenoceptor stimulation but did not interfere with ERK2 activation. Inhibition of the ERK2 pathway with PD-98059 did not affect PI 3-kinase activation. We conclude that ERK2- and PI 3-kinase-dependent pathways represent two mutually exclusive ways of signaling that lead to different aspects of the hypertrophic response to alpha-adrenoceptor stimulation.

Induction of necrosis but not apoptosis after anoxia and reoxygenation in isolated adult cardiomyocytes of rat

OBJECTIVES

Apoptosis is one feature of myocardial damage after ischemia-reperfusion, but the causes for its induction are unclear. The present study was undertaken to investigate whether apoptosis in cardiomyocytes is directly initiated by their sub-lethal injury that results from ischemia-reperfusion.

METHODS

Ischemia was simulated on isolated ventricular cardiomyocytes of adult rats by anoxia in a glucose free medium, pH 6.4. Induction of apoptosis was detected by (1) DNA laddering of genomic DNA, (2) TUNEL positive cells (terminal deoxynucleotidyl transferase-mediated-UTP nick end labelling) and (3) annexinV-fluorescein isothiocyanate (annexinV-FITC) binding to cells under exclusion of propidium iodide. Necrotic cells were identified by (1) staining with both annexinV-FITC and propidium iodide, (2) unspecific DNA degradation and (3) enzyme release.

RESULTS

Simulated ischemia caused a > 75% loss of high-energy phosphates within 2 h, which was reversible upon reoxygenation at pH 7.4. Even after 18 h of simulated ischemia, creatine phosphate contents recovered to 55.2 +/- 7.3% of control within 1 h. Apoptosis could be induced by UV irradiation (80 J/m2), H2O2 and the NO-donor N2-acetyl-S-nitroso-D,L-penicillinaminamide. In contrast to this, simulated ischemia and reoxygenation could not induce apoptosis in the cells, but with prolonged ischemia more cells became necrotic. After 18 hours of simulated ischemia and 4 h of reoxygenation 41.2 +/- 10.2% myocytes were necrotic (vs. 6.3 +/- 4.4% of control) and only 1.7 +/- 0.5% (vs. 8.7 +/- 4.6% of control) were apoptotic. The percentage of necrotic cells correlated with an increase in lactate dehydrogenase release from 9.9 +/- 0.6% (of total activity) of normoxic controls to 37.9 +/- 5.1% after 18 h of simulated ischemia and 12 h of reoxygenation.

CONCLUSIONS

Simulated ischemia-reoxygenation causes necrosis of isolated cardiomyocytes but is not sufficient for induction of apoptosis.

Intracellular signaling leads to the hypertrophic effect of neuropeptide Y

Signal transduction pathways involved in the hypertrophic effect of neuropeptide Y (NPY) were investigated in adult cardiomyocytes. Reduction of transforming growth factor-beta activity in serum-supplemented media abolished the induction of hypertrophic responsiveness to NPY. In responsive cells, NPY (100 nM) increased protein synthesis, determined as incorporation of [14C]phenylalanine, by 35 +/- 15% (P < 0.05, n = 16 cultures). In these cells, NPY activated pertussis toxin (PTx)-sensitive G proteins and phosphatidylinositol (PI) 3-kinase. PTx and inhibition of PI 3-kinase abolished the hypertrophic effect of NPY. NPY also activated protein kinase C (PKC) and mitogen-activated protein (MAP) kinase. Inhibition of these two kinases attenuated the induction of creatine kinase (CK)-BB but not the growth response to NPY. In conclusion, NPY stimulates protein synthesis in adult cardiomyocytes via activation of PTx-sensitive G proteins and PI 3-kinase and it induces the fetal-type CK-BB via activation of PKC and MAP kinase.

Role of phosphatidylinositol 3-kinase activation in the hypertrophic growth of adult ventricular cardiomyocytes

OBJECTIVE

The present study investigated whether activation of phosphatidylinositol 3-kinase (PI3-kinase) is involved in the stimulation of hypertrophic growth of adult ventricular cardiomyocytes under alpha- or beta-adrenoceptor stimulation.

METHODS

Adult ventricular rat cardiomyocytes were used either directly after isolation (day 1 culture) or after cultivation for 6 days in presence of 20% fetal calf serum (day 7 culture). PI3-kinase activity was determined in extracts of cardiomyocytes after immunoprecipitation with an antibody against the p85 subunit of PI3-kinase. The influence of PI3-kinase inhibition on myocardial growth was determined using the specific PI3-kinase inhibitors wortmannin and LY294002.

RESULTS

In day 1 cultures alpha-adrenoceptor stimulation, but not beta-adrenoceptor stimulation caused activation of PI3-kinase. In response to alpha-adrenoceptor stimulation but not beta-adrenoceptor stimulation an acceleration of protein synthesis (incorporation of 14C-phenylalanine) and an increase in the total masses of cellular protein and RNA was observed. In these cultures inhibition of PI3-kinase attenuated the acceleration of protein synthesis and the increase in cellular masses of protein or RNA in response to alpha-adrenoceptor stimulation. In day 7 cultures alpha- and beta-adrenoceptor stimulation caused activation of PI3-kinase and increased protein synthesis. In these cultures inhibition of PI3-kinase attenuated the growth response to alpha- and beta-adrenoceptor stimulation.

CONCLUSIONS

PI3-kinase activation via protein kinase C-dependent or cAMP-dependent pathways is required for hypertrophic growth of adult cardiomyocytes.

Influence of pHi and creatine phosphate on alpha-adrenoceptor-mediated cardiac hypertrophy

Stimulation of alpha-adrenoceptors on ventricular cardiomyocytes isolated from adult rat hearts leads to cellular alkalization, increases of creatine phosphate concentration, RNA mass, and protein synthesis. This study investigated whether the increase of creatine phosphate concentrations is causally linked to the hypertrophic response of cardiomyocytes under alpha-adrenoceptor stimulation. Cellular alkalization achieved with phenylephrine (10 microM), an alpha-adrenoceptor agonist, was abolished in the presence of the sodium-proton-exchange (NHE)-inhibitor HOE 694 (1 microM). HOE 694 inhibited also the alpha-adrenoceptor-mediated increase in cellular creatine phosphate and the increase in cellular RNA mass. The phenylephrine-induced stimulation of protein synthesis (determined by incorporation of 14C-phenylalanine) was reduced by one-third when HOE 694 was present. beta-Guanidinopropionic acid was added to cardiomyocytes to reduce cellular creatine phosphate concentrations. In these cultures, alpha-adrenoceptor stimulation activated NHE, but creatine phosphate concentrations were not increased. Protein synthesis was augmented to the same extent as in control cultures, but total RNA mass did not increase. From these results we conclude that alpha-adrenoceptor stimulation causes the increase in protein synthesis via activation of NHE, but independent of the concomitant increase in creatine phosphate contents. The effect of alpha-adrenoceptor stimulation on total RNA mass (translational capacity) is also caused by NHE activation, but depends on the changes in creatine phosphate contents as well.

Cardiovascular actions of parathyroid hormone and parathyroid hormone-related peptide

Cardiovascular cells (cardiomyocytes and smooth muscle cells) are target cells for parathyroid hormone (PTH) and the structurally related peptide parathyroid hormone-related peptide (PTH-rP). PTH activates protein kinase C (PKC) of cardiomyocytes via a PKC activating domain previously identified on chondrocytes. Activation of PKC leads to hypertrophic growth and re-expression of fetal type proteins in cardiomyocytes. This hypertrophic effect of PTH might contribute to left ventricular hypertrophy in hemodialysis patients with secondary hyperparathyroidism. PTH-rP is expressed in cardiovascular cells (endothelial cells and smooth muscle cells). It does not mimic the above described actions of PTH but exerts effects of its own on cardiomyocytes. These effects involve activation of protein kinase A, via a N-terminal domain distinct from that identified on PTH, and activation of PKC, via a C-terminally located domain distinct from that found on PTH. On smooth muscle cells PTH and PTH-rP reduce the influence of extracellular calcium, through cAMP-dependent mechanisms. These inhibitory effects on voltage-dependent L-type calcium channels of smooth muscle cells cause vasorelaxation. Present studies concerning cardiovascular actions of either PTH and PTH-rP suggest that increased plasma levels of PTH and PTH-rP influence cardiomyocyte and smooth muscle cell physiology. It can be assumed that PTH-rP acts as a paracrine or autocrine modulator in heart and vessels.

Effects of PTH-rP(107-111) and PTH-rP(7-34) on adult cardiomyocytes

We investigated whether parathyroid hormone-related peptide (PTH-rP), recently found expressed in the heart, exerts growth and contractile effects on adult cardiomyocytes from rat hearts. Synthetic PTH-rP peptides were used covering either a protein kinase C (PKC)-activating domain [PTH-rP(107-111)], or an adenylate cyclase activating domain [PTH-rP(1-34) and PTH-rP(7-34)]. PTH-rP(107-111) (1 micro M) increased creatine kinase BB activity (CK-BB), a CK isoform re-expressed during cardiac hypertrophy, within 24 h by 62+/-12%. This induction was abolished in the presence of the mitogen-activated-protein (MAP)-kinase-kinase inhibitor PD 98059. PTH-rP(107-111) activated p42-MAP-kinase within 15 min, increased protein synthesis (19+/- 4%), total protein mass (19+/-5%), cell volume (45+/-7%), and cross-sectional area (38+/-9%) of cardiomyocytes. Activation of p42-MAP-kinase and increase in protein synthesis were abolished in presence of bisindolylmaleimide, a PKC inhibitor. PTH- rP(107-111) did not directly influence contractile activity but reduced the contractile response to isoprenaline. In contrast, PTH-rP(1-34) and PTH-rP(7-34) induced spontaneous contractile activity in 3-day-old cultures. This induction was abolished in presence of Rp-cAMPS, a protein kinase A inhibitor, indicating an involvement of cAMP in this response. PTH-rP(1-34) also increased the cellular accumulation of cAMP. It is concluded that PTH-rP exert direct effects on adult cardiomyocytes by activating either PKC via a functional domain covered by amino acids 107-111 or by activation of cAMP-dependent protein kinase via a functional domain covered by amino acids 7-34. Since these parts of PTH-rP have either no homology [PTH-rP(107-111)] or only a limited structural similarity [PTH-rP(7-34)] to parathyroid hormone, these activities of PTH-rP have to be clearly distinguished from those described for parathyroid hormone.

ANP protects against reoxygenation-induced hypercontracture in adult cardiomyocytes

It was investigated whether atrial natriuretic peptide (ANP) or the related peptide urodilatin can be used for protecting cardiomyocytes against reoxygenation-induced hypercontracture. Isolated ventricular cardiomyocytes (from adult rats) were used as the experimental model. When the cells were submitted to substrate-free anoxia (135 min) and subsequent reoxygenation (30 min), the onset of reoxygenation provoked their hypercontracture. It was studied whether the temporary presence of ANP or urodilatin (1 nM to 1 microM) or 8-bromo-guanosine 3',5'-cyclic monophosphate (8-BrcGMP; 1 microM to 1 mM) during the last 15 min of anoxia and the first 15 min of reoxygenation prevented hypercontracture. It was found that ANP (1 microM) prevented hypercontracture in 82 +/- 8% (SD), urodilatin (1 microM) in 80 +/- 9%, and 8-BrcGMP (1 mM) in 72 +/- 10% of the cells (n = 40 cells). When ANP (1 microM) was added during the last 15 min of anoxia and the first 15 min of reoxygenation, the cellular concentration of cGMP increased from 0.41 +/- 0.04 to 2.80 +/- 0.81 pmol/mg protein (n = 6 cultures). The results show that the reoxygenation-induced hypercontracture in cardiomyocytes can be attenuated by the temporary presence of the stimulators of particulate guanylate cyclase, ANP or urodilatin.

Hypertrophic responsiveness to beta 2-adrenoceptor stimulation on adult ventricular cardiomyocytes

The aim of the present study was to characterize the receptor subtype and the second messenger involved in the newly discovered hypertrophic effect of beta-adrenoceptor stimulation in cultures of adult ventricular cardiomyocytes. Cardiomyocytes isolated from adult rats and cultured for 6 days in presence of 20% fetal calf serum (FCS) were used as experimental model. Hypertrophic responsiveness of cardiomyocytes was characterized by rate of protein synthesis, increase in protein mass, and increase in RNA content. The hypertrophic effect of the non-specific beta-adrenoceptor agonist isoprenaline was abolished in presence of a specific beta 2-adrenoceptor antagonist (ICI 118,551), could be mimicked by use of a beta 2-adrenoceptor agonist (procaterol) or direct stimulation of adenylate cyclase (forskolin) or addition of a cell-permeable analogue of cAMP (dibuytyrylcyclo-AMP). In presence of Rp-cAMPS, an inhibitor of protein kinase A, the hypertrophic effect of isoprenaline was abolished. The results indicate that the hypertrophic effect of beta-adrenoceptor stimulation is due to stimulation of beta 2-adrenoceptors and activation of adenylate cyclase and protein kinase A.

Protection of reoxygenated cardiomyocytes against osmotic fragility by nitric oxide donors

In ischemic-reperfused myocardium, myocardial cells are jeopardized not only by reoxygenation-induced hypercontracture but also by the development of a transsarcolemmal osmotic gradient. Here the question of whether osmotic fragility of cardiomyocytes can be reduced by interventions during reoxygenation was addressed. Isolated ventricular cardiomyocytes (from adult rats), exposed to 120 min of hypoxia and subsequent reoxygenation, were used as model. With reoxygenation, medium osmolarity was reduced from 270 to 80 mosM. Loss of sarcolemmal integrity was characterized by enzyme loss from cells (creatine kinase and lactate dehydrogenase). Cardiomyocytes reoxygenated after 120 min of hypoxia hypercontracted, but enhanced enzyme loss was observed only at 80 mosM. The nitric oxide (NO) donors 3-morpholinosydnonimine (10 mM), sodium nitroprusside (10 mM), S-nitroso-N-acetyl-DL-penicillamine (100 microM), and the antilipid peroxidant diphenylphenylenediamine (DPPD, 2.5 microM) reduced enzyme loss with hyposmolar reoxygenation. Agents activating guanosine 3',5'-cyclic monophosphate (cGMP)-dependent pathways [atrial natriuretic peptide (1 microM), urodilatin (1 microM), and 8-bromo-cGMP (10 mM)], the contractile inhibitor 2,3-butanedione monoxime (10 mM), and the SIN-1 metabolite SIN-1C (10 mM) did not protect cardiomyocytes against osmotic fragility. The results show that increased osmotic fragility of isolated adult rat cardiomyocytes can be prevented at the time of reoxygenation by NO donors and DPPD in a cGMP-independent way.

Parathyroid hormone-related protein antagonizes the action of parathyroid hormone on adult cardiomyocytes

Ventricular cardiomyocytes have been identified as target cells for parathyroid hormone (PTH). A structurally related peptide hormone, parathyroid hormone-related peptide (PTH-rP), is expressed in the heart. In the present study, it was investigated whether PTH-rP can mimic or modify effects of PTH on cardiomyocytes. The investigated effect was induction of creatine kinase (CK) activity, which is associated with cardiac hypertrophy. PTH and PTH-rP have a similar secondary structure within the active domain 28 34, with exception of amino acid 29. At this position the hydrophilic glutamine in the PTH molecule corresponds to hydrophobic alanine in the PTH-rP molecule. Synthetic PTH or PTH-rP peptides covering domain 28 34 and recombinant full-length PTH(1 84) were used. PTH(28 48) (100 nm) induced CK activity within 24 h (123 +/- 3%; means +/- S.D., n = 4). PTH-rP(7-34) (1 nm to 1 microm) failed to induce CK activity in cardiomyocytes. Given simultaneously, PTH-rP (1 mum) reduced the stimulation of CK activity by PTH(1-84), PTH(1-34), and PTH(28-48) by 94 +/- 9, 79 +/- 8, and 69 +/- 14%, respectively (means +/- S.D., n = 4). In contrast, PTH-rP(7-34) was sufficient to stimulate proliferation of chicken chondrocytes. Thus, PTH-rP exerts different effects on cardiomyocytes and classical target cells for PTH. A synthetic hybrid peptide was synthesized, [Ala29]PTH(28-48), in which alanine replaced glutamine at position 29, as in the PTH-rP molecule. In contrast to PTH(28-48), this mutated peptide [Ala29]PTH(28-48) had no intrinsic activity but antagonized the effect of PTH(1-84) and PTH(28-48) on cardiomyocytes. The results demonstrate that on cardiomyocytes the effect of PTH can be antagonized by PTH-rP. This antagonism seems due to a hydrophobic replacement at position 29.

Myocardial protection during reperfusion

After prolonged periods of energy depletion, myocardial cells may rapidly deteriorate during the early stage of reperfusion. It has now been clearly demonstrated that this kind of acute lethal reperfusion injury is due to specific processes elicited by cellular re-energization. The most prominent single cause of acute harm to the reoxygenated myocardial cells is myofibrillar hypercontraction. Hypercontraction is caused by a resupply of energy of the myofibrils at excessive cytosolic Ca2+ concentrations. Additionally, the ability of the cytoskeleton to withstand large mechanical forces seems to be weakened after a prolonged period of energy depletion. Intracellular acidosis during the early stage of reperfusion represents a natural mechanism of protection against acute reperfusion injury. The reperfused myocardial cell may also suffer from uncontrolled water uptake and increased sarcolemmal fragility, favoring osmotic damage of cell membranes. As yet therapeutical interventions trying to specifically interfere with these pathomechanisms of reperfusion injury have only been tested experimentally. It seems promising to evaluate their utility for myocardial protection in cardio-surgical operations.

Low increase in cGMP induced by organic nitrates and nitrovasodilators improves contractile response of rat ventricular myocytes

Whether organic nitrates are bioactivated to NO in cardiac muscle cells and may thus directly affect cardiac contractile function has remained an open question. Therefore, we determined the effects of the organic nitrates glyceryl trinitrate (100 mumol/L), pentaerythritol tetranitrate (10 mumol/L), and isosorbide-5-mononitrate on electrically stimulated contractile response (CR) and cAMP and cGMP content of isolated adult rat ventricular cardiomyocytes compared with different concentrations of the spontaneous NO donors S-nitroso-N-acetyl-d,1-penicillamine (SNAP) and 2,2-diethyl-1-hydroxy-1-nitroso-hydrazine (DEA/NO). A high concentration of spontaneous NO donors (100 mumol/L caused a large increase in cGMP content that was accompanied by a decrease in CR to 73.8 +/- 6.7% (SNAP) and 80.9 +/- 6.1% (DEA/NO) of the control values. Inhibition of cGMP-dependent protein kinase by 10 mumol/L KT 5822 converted this effect into a pronounced improvement of CR (163.5 +/- 14.0%) By contrast, the organic nitrates caused a small but significant increase in cGMP, which was accompanied by an increase in cAMP and CR identical to that induced by 10 nmol/L isoprenaline (141.6 +/- 6.4%) A similar effect was observed with a low concentration (1 mumol/L of SNAP and DEA/NO. All increases in CR induce by nitrates were abolished after inhibition of cAMP-dependent protein kinase by Rp-cAMPS (10 mumol/L). The positive contractile effect of isoprenaline was enhanced by 1 mumol/L SNAP. This effect was also demonstrated in isolated rat papillary muscles. These results indicate that in cardiac muscle (1) organic nitrate are bioactivated to NO; (2) this results in a moderate increase in cGMP, which causes an improved CR by increasing cAMP and activating cAMP-dependent protein kinase; and (3) a large increase in cGMP, produced by high doses of NO donors, reduces CR because of the activation of CGMP-dependent protein kinase.

Regulation of protein synthesis and degradation in adult ventricular cardiomyocytes

For studies on the regulation of myocardial protein metabolism, isolated adult cardiomyocytes were introduced as an experimental model about a decade ago. When used shortly after isolation, this model represents a tool for studying the properties of normal and diseased myocardium on the cellular level. The influence of various peptide hormones, neurotransmitters, and mechanical stimulation on protein synthesis and degradation in isolated cardiomyocytes has been studied. It has been demonstrated, for example, that alpha 1-adrenoceptor stimulation increases protein synthesis in newly isolated cardiomyocytes, independently of any mechanical effects. Other potential growth stimuli require appropriate conditions to induce cellular responsiveness. Neuropeptide Y, for example, does not stimulate cellular protein synthesis in newly isolated cells, whereas it does so in cells that have been cultured for a week in the presence of serum. Mechanical stretch also represents a growth stimulant. It seems that its signal transduction involves an autocrine loop. Thus different mechanisms, by which exogenous influences can modify cellular protein synthesis and degradation, have been identified on the cellular level, with the use of isolated adult cardiomyocytes.

Hypertrophic effects of calcitonin gene-related peptide (CGRP) and amylin on adult mammalian ventricular cardiomyocytes

Calcitonin gene-related peptide (CGRP), a neuropeptide localized in the cardiac autonomic nervous supply, shares 46% similarity in sequence of amino acids with amylin, a peptide synthesized in pancreatic beta-cells. In the present study, the question was addressed whether these peptides could exert hypertrophic effects in cardiomyocytes isolated from the ventricles of adult rats and maintained in short-term, serum-free primary culture. FCS (10% v/v), employed as a positive control, increased the incorporation of l-[14C]phenylalanine into cellular protein, total content of cellular RNA and total mass of cellular protein significantly. CGRP and amylin also increased each of these parameters significantly and in a concentration-dependent manner; maximum responses occurred at 100 pM and 10 nM for CGRP and amylin, respectively. The selective antagonist at CGRP1-receptors, CGRP8-37(100 nM), inhibited significantly the incorporation of l-[14C] phenylalanine into cellular protein in response to CGRP and amylin. The selective inhibitor of protein kinase C (PKC), bisindolylmalemide (BIM) (5 microM), reduced significantly the incorporation of l-[14C] phenylalanine into cellular protein in response to phenylephrine (1 microM), employed as a positive control, but did not inhibit the response to insulin (1 unit/ml), employed as a negative control. BIM (5 microM) reduced significantly the responses to FCS (10% v/v), amylin (10 nM) and CGRP (10 pM), but did not inhibit the response to CGRP (100 pM). The activity of protein kinase C in membranes prepared from intact myocytes pre-treated for 10 min with the phorbol ester, phorbol 12-myristate 13-acetate (PMA) (100 nM), employed as a positive control, and CGRP (10 pM) was significantly greater than in membranes prepared from cardiomyocytes not subjected to agonist stimulation. Phenylephrine (1 microM) increased significantly the specific activity of creatine kinase but not of lactate dehydrogenase in day 1 cultures of freshly isolated cardiomyocytes. Significant induction of creatine kinase, but not lactate dehydrogenase, was also stimulated by CGRP and amylin; the maximum responses occurred at 100 pM and 100 nM CGRP and amylin, respectively. In conclusion, CGRP and amylin exert hypertrophic effects directly on ventricular cardiomyocytes from the hearts of adult rats in vitro. These effects are: (1) due to de novo protein synthesis since total content of cellular RNA and incorporation of l-[14C]phenylalanine into cellular protein were also increased; (2) mediated by a common population of CGRP1-preferring receptors at which amylin binds with lower potency: (3) mediated, at least partly, by the activation of PKC; (4) may be associated with a fetal shift in gene expression, characterized by selective induction of creatine kinase.

Induction of hypertrophic responsiveness to isoproterenol by TGF-beta in adult rat cardiomyocytes

In a previous publication we reported that hypertrophic responsiveness to beta-adrenoceptor stimulation can be induced in isolated cardiomyocytes when these are cultured for 6 days in presence of fetal calf serum (FCS; Pinson et al., J. Mol. Cell. Cardiol.. 25: 477-490, 1993). The role of transforming growth factor-beta (TGF-beta) in this induction process has now been investigated. Isolated cardiomyocytes from adult rats were cultured for 6 days in presence of 20% FCS. It was found that induction of hypertrophic responsiveness to beta-adrenoceptor stimulation was abolished when a neutralizing anti-TGF-beta 1 antibody was added to FCS-containing culture medium. In culture media with FCS contents (5%) too low to induce hypertrophic responsiveness to beta-adrenoceptor stimulation, addition of 1 ng/ml TGF-beta 1,2 induces this responsiveness. It was demonstrated that cardiomyocytes already release TGF-beta into culture media on day 1 of culture and that they continue to do so in presence of FCS supplements of > 5%. The results demonstrate that hypertrophic responsiveness to beta-adrenoceptor stimulation is induced in cardiomyocytes by an autocrine mechanism involving TGF-beta 1 as mediator.

The new NO donor SPM3672 increases cGMP and improves contraction in rat cardiomyocytes and isolated heart

Recent evidence indicates that organic nitrate esters may directly affect heart muscle. In the present study we investigated the effects of the new organic nitrate ester, N-(3-nitratopivaloyl)-1-cysteineethylester (SPM3672), on isolated adult rat ventricular myocytes and on Langendorff preparations of spontaneously beating rat hearts perfused in a volume-constant manner. In cardiomyocytes SPM3672 (100 microM) induced a significant increase in the basal level of cGMP to 232 +/- 44% (n=8) indicating its metabolism to nitric oxide. This was associated with an enhanced contractile response to electrical field stimulation (to 174 +/- 9%, n=108). In isolated hearts SPM3672 elicited a slight reduction of coronary perfusion pressure (-15 +/- 8%) and a significant increase in maximal left ventricular pressure (LVPmax), dp/dtmax and dp/dtmin amounting to 18 +/- 7%, 18 +/- 6% and 21 +/- 7% (n=7), respectively. Oxygen consumption and heart rate remained constant. Thus, SPM3672 improved the contractile response of cardiomyocytes and of isolated heart. This is probably due to the metabolism of SPM3672 to nitric oxide in ventricular cardiomyocytes.

Parathyroid hormone induces protein kinase C but not adenylate cyclase in adult cardiomyocytes and regulates cyclic AMP levels via protein kinase C-dependent phosphodiesterase activity

Adult ventricular cardiomyocytes have been identified as target cells for parathyroid hormone (PTH) but little is known about its signal transduction in these cells. In the present study the influence of PTH on cyclic AMP accumulation and the activity of protein kinase C (PKC) in cardiomyocytes was evaluated. A mid-regional synthetic fragment of PTH, PTH-(28-48), which exerts a hypertrophic effect on cardiomyocytes, increased the activity of membrane-associated PKC in a dose-dependent manner (1-100 nM). Activated membranous PKC was dependent on Ca2+ and sensitive to an inhibitor of Ca(2+)-dependent isoforms of PKC. When adenylate cyclase was stimulated by the addition of isoprenaline, a beta-adrenoceptor agonist, PTH-(28-48) antagonized cyclic AMP accumulation. This antagonistic effect of PTH-(28-48) could be mimicked by activation of PKC with a phorbol ester and inhibited by isobutylmethylxanthine, a phosphodiesterase inhibitor. An N-terminal synthetic fragment, PTH-(1-34), which includes an adenylate cyclase-activating domain, did not stimulate the accumulation of cyclic AMP in cardiomyocytes. The results demonstrate that in adult cardiomyocytes PTH (1) is able to stimulate PKC, (2) is not able to cause accumulation of cyclic AMP and (3) functionally antagonizes the effect of beta-adrenoceptor stimulation to increase cellular cyclic AMP concentrations via PKC-dependent phosphodiesterase activity.

NO donor SIN-1 protects against reoxygenation-induced cardiomyocyte injury by a dual action

It was investigated whether morpholinosydnonimine (SIN-1), which spontaneously decomposes into NO and 3-morpholinoiminoacetonitrile (SIN-1C), can be used for protection of cardiomyocytes against reoxygenation-induced hypercontracture. Isolated ventricular cardiomyocytes (from adult rats) were used as the experimental model. SIN-1 [concentration with half-maximal effect (EC50) 2.5 x 10(-4) M] and SIN-1C (EC50 8.3 x 10(-3) M) inhibited the contractile response of electrically paced cardiomyocytes. When the cells were submitted to substrate-free anoxia (135 min) and subsequent reoxygenation (30 min), the onset of reoxygenation provoked their hypercontracture. It was studied whether the temporary presence of the test agents during the last 15 min of anoxia and the first 15 min of reoxygenation prevented hypercontracture. At 10 nM, SIN-1 prevented hypercontracture in 96% of the cells and SIN-1C in 72% of the cells. The protective effect of SIN-1 was reduced to that of SIN-1C by simultaneous presence of methylene blue (50 microM). Methylene blue had no influence on the protective action of SIN-1C. SIN-1C (10 mM) plus sodium nitroprusside (another NO donor, 250 microM) provided the same degree of protection as SIN-1 (10 mM). The results show that reoxygenation-induced hypercontracture can be prevented or attenuated by the temporary presence of high concentrations of SIN-1 or SIN-1C. SIN-1 acts through a dual mechanism, protecting through the generation of NO and SIN-1C.

Neuropeptide Y stimulates hypertrophy of adult ventricular cardiomyocytes

It was investigated whether neuropeptide Y (NPY) could exert a trophic effect on ventricular myocytes isolated from the adult rat heart. Two different culture models were used: day 1 and 7 cultures of cardiomyocytes. In day 1 and 7 cultures, NPY caused an increase in cellular protein mass. In day 1 cultures, NPY (10 nM) increased the protein-to-DNA ratio within 24 h by 10.1 +/- 2.8% (P < 0.01), but did not stimulate the incorporation of [14C]phenylalanine into cell proteins. The degradation of proteins was retarded in presence of NPY, revealed by pulse-chase experiments. In day 7 cultures, NPY (10 nM) increased the protein-to-DNA ratio within 24 h by 33.9 +/- 5.0% (P < 0.01), increased the RNA-to-DNA ratio by 19.2 +/- 6.4%, and stimulated the incorporation of [14C]phenylalanine by 45.5 +/- 4.5% (P < 0.01). As in day 1 cultures, protein degradation was retarded. The specific activities of cytosolic creatine kinase and lactate dehydrogenase were increased in presence of NPY. This study demonstrates for the first time that NPY is a trophic factor for cardiomyocytes. NPY can cause an increase in cellular mass of protein, i.e., hypertrophy, by two mechanisms: 1) reduction of degradation of protein, found in day 1 and 7 cultures, and 2) stimulation of protein synthesis, observed only in day 7 cultures. The responsiveness of protein synthesis to NPY stimulation is induced during prolonged incubation in culture.

Calcium and the oxygen paradox

When myocardial cells are reoxygenated after a prolonged period of energy depletion, they rapidly hypercontract. In tissue, hypercontracture induced by reoxygenation is accompanied by cytolysis ("oxygen paradox"). Recent studies have indicated that severe cytosolic Ca2+ overload and reactivation of energy production represent the causal key factors for the deleterious hypercontracture, through the following mechanism: prolonged energy depletion leads to a progressive cytosolic Ca2+ overload in cardiomyocytes; when oxidative phosphorylation is then resumed with the resupply of oxygen, activation of the myofibrils at (still) increased cytosolic Ca2+ concentrations provokes a sustained maximal force development and consecutive mechanical cell injury. This injury can largely be prevented when the contractile machinery is inhibited during the initial phase of reoxygenation. In the model of isolated cells it has been shown that a normal cytosolic Ca2+ control can be reestablished upon reoxygenation. This seems to explain why contractile blockade is needed only temporarily for the prevention of reoxygenation induced hypercontracture and cellular deterioration. Temporary contractile blockade at the onset of reperfusion has also been shown to protect the heart in vivo against lethal reperfusion injury.

Calcium and sodium control in hypoxic-reoxygenated cardiomyocytes

When oxygen-deprived cardiomyocytes become energy depleted, they accumulate Na+ and Ca2+ in the cytosol. Influx of Ca2+ via the Na+/Ca2+ exchange mechanism seems to contribute to the development of Ca2+ overload, but Ca2+ overload may eventually also occur when this route is blocked. Hypoxic-reoxygenated cardiomyocytes in a state of severe overload of Na+ and Ca2+ can rapidly re-establish a normal cation control when oxidative energy production is re-initiated. The recovery of cellular Ca2+ control may be divided into three stages: first, sequestration of large amounts of Ca2+ into the sarcoplasmic reticulum; second, oscillatory movement of Ca2+ from and back into the sarcoplasmic reticulum and gradual extrusion across the sarcolemma; third, re-establishment of constant low cytosolic Ca2+ concentrations. When the Na+/Ca2+ exchanger is inhibited, extrusion of Ca2+ from the cells' interior is impaired and oscillatory Ca2+ movements between cytosol and sarcoplasmic reticulum continue for long time. Thus, the functions of the sarcoplasmic reticulum and the Na+/Ca2+ exchanger are of crucial importance for the recovery of Ca2+ control in reoxygenated cardiomyocytes. In re-energized cardiomyocytes, a persistent elevation of the cytosolic Ca2+ concentration provokes maximal force development and consecutive mechanical cell injury ("oxygen paradox"). This injury can be prevented when the contractile machinery is inhibited during the initial phase of reoxygenation as long as necessary for the re-establishment of a normal cytosolic Ca2+ control.

Alpha- and beta-adrenergic stimulation of protein synthesis in cultured adult ventricular cardiomyocytes

The effect of the alpha 1-adrenoceptor agonist phenylephrine (PE, 1-10 microM) and the beta-adrenoceptor agonist isoprenaline (ISO, 1-10 microM) on protein synthesis and ultrastructure of ventricular cardiomyocytes from adult rat in culture (6 days in medium 199 plus 20% fetal calf serum) was studied. In these cultures cardiomyocytes were spread, but not spontaneously contractile. ISO and PE significantly increased total cell protein and incorporation of (14C)-phenylalanine within 24 h of exposure. These effects were inhibited by the antagonists propranolol and prazosin, respectively. The incorporation of (14C)-uridine was stimulated only by PE but not ISO. Induction of fetal BB-isoform of cytosolic creatine kinase was also caused only by PE but not ISO. The ultrastructure of PE-treated cardiomyocytes was altered as compared to controls, by a greater number of Golgi complexes, denser myofibrillar structures and the appearance of paracrystalline bands in mitochondrial matrices. In conclusion, in this culture model the protein synthesis of cardiomyocytes can be stimulated, independently of the contractility, by either alpha 1- or beta-adrenoceptor agonists. Catecholamines differ, however, in their effects on specific cellular proteins and structures. Only alpha 1-adrenergic stimulation leads to a "fetal shift" in the expression of CK-isoforms.

Trophic effects of catecholamines and parathyroid hormone on adult ventricular cardiomyocytes

Trophic effects of isoproterenol (Iso), norepinephrine (NE), phenylephrine (PE), and biologically active fragments of parathyroid hormone (PTH), PTH-(1-34) and PTH-(28-48), were investigated in mechanically quiescent, isolated ventricular cardiomyocytes from adult rat. In 24-h incubations in modified serum-free medium 199 incorporation of [14C]phenylalanine, changes in total protein and specific activities of cytosolic enzymes, creatine kinase (CK) and lactate dehydrogenase (LDH) were monitored. NE and PE (10 microM), but not Iso, distinctly increased phenylalanine incorporation, total cell protein, and specific activity of CK but not LDH. Induction of CK, but not LDH, was also produced by phorbol 12-myristate 13-acetate (10 nM) but not dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP, 1 mM). It was abolished by copresence of cycloheximide (35 microM) or actinomycin D (5 microM). CK-BB was the only induced isoform of CK, as shown for PE incubations. PTH-(1-34) and PTH-(28-48) (30-300 nM) had effects comparable to NE and PE. They increased phenylalanine incorporation and total protein content and induced CK but not LDH. In summary, distinct trophic effects on adult cardiomyocytes were found with alpha 1-adrenergic agonists, fragments of PTH containing the midregional amino acids 28-34, and direct activation of protein kinase C but neither beta-adrenergic agonists nor DBcAMP.

Prevention of the oxygen paradox in hypoxic-reoxygenated hearts

Reoxygenation after 60 min substrate-free hypoxic perfusion (modified Tyrode solution, 37 degrees C) caused isolated Langendorff hearts (from rats) to rapidly develop hypercontracture and sarcolemmal disruptions indicated by massive and sudden loss of enzymes ("oxygen paradox"). Reoxygenation (30 min) caused an augmented loss of creatine kinase by 25.8% (lactate dehydrogenase by 40.1%) of the initial total tissue activity. It was investigated whether a temporary contractile blockade by 2,3-butanedione monoxime (BDM; 20 mM) can prevent reoxygenation-induced injury. In the presence of BDM, reoxygenation no longer caused hypercontracture or increased enzyme release. Instead, ultrastructure recovered, and contents of creatine phosphate (CrP) were partially restored (60 min hypoxia: 0.4 mumol CrP/g dry wt; after subsequent 60 min reoxygenation in presence of BDM: 7.8 mumol CrP/g dry wt). When BDM was eluted after first 20 min of reoxygenation, an attenuated but distinct increase in enzyme release was still observed. When BDM was eluted after 60 min of reoxygenation, ultrastructure did not deteriorate and increase of enzyme release remained virtually absent. During first 30 min after removal of BDM, the increased loss of creatine kinase amounted to only 5.7% (lactate dehydrogenase to 6.9%) of the initial total tissue activity. The results demonstrate that the oxygen paradox can be prevented in the hypoxic-reoxygenated heart when the contractile apparatus is temporarily paralyzed during the initial phase of reoxygenation.