Alterations in cardiac oxygen consumption under chronic pressure overload

Summary Hypertension and resulting left ventricular hypertrophy was induced in young male Wistar rats (60 to 70 days old) by narrowing of one renal artery (Goldblatt II). 8 and 24 weeks after operation, myocrdial oxygen consumption was measured on a modified in situ heart-lung preparation with nearly isovolumetric left ventricular contractions. Measured myocardial oxygen consumption was related to left ventricular wall stress. The myosin isoenzyme pattern of each heart was determined with pyrophosphate gel electrophoresis.Oxygen consumption related to wall stress averaged over the entire heart cycle amounted to 15 moles O₂/g·min 8 weeks after operation, and 24.4 moles O₂/g·min in age-matched controls ( 38%, p<0.0005). When wall stress was averaged over systole, oxygen consumption of the hypertrophied hearts amounted to 0.112 moles O₂/g·beat, and 0.149 moles O₂/g·beat in the controls ( 25%, p<0.05). The proportion of VM-3 (the cardiac myosin isoenzyme of lowest ATPase activity) increased from 26.3% in the controls to 30.1% in the Goldblatt hearts ( 14%, n.s.).24 weeks after operation, oxygen consumption related to wall stress averaged over the entire heart cycle amounted to 16.1 moles O₂/g·min, in age-matched controls 20.5 moles O₂/g·min ( 21%, p<0.05). When wall stress was averaged over systole, oxygen consumption of the Goldblatt hearts amounted to 0.080 moles O₂/g·beat, and in the controls 0.107 moles O₂/g·beat ( 25%, p<0.005). The proportion of VM-3 increased from 33.5% in the controls to 43.2% in the hypertrophied hearts ( 29%, p<0.05).The present findings indicate that the reduced oxygen consumption of the pressure-loaded heart should be attributed to a redistribution of myosin isoenzymes. The transformation of myocardium into a slower, but more efficiently working muscle due to an increase in VM-3 can be interpreted as an adaptational process.Supported by the Deutsche Forschungsgemeinschaft     

压力超负荷对大鼠左室心肌CaMKⅡ和pCREB表达的影响

目的探讨钙调素激酶Ⅱ（CaMKⅡ）和磷酸化核转录因子CREB（pCREB）在压力超负荷大鼠左室心肌中表达的变化。方法100只健康雄性SD大鼠随机分为对照组（n=50）和心肌肥厚组（n=50）,制备腹主动脉缩窄大鼠心肌肥厚模型,手术后4周以免疫组化法观察左室心肌CaMKⅡ及pCREB蛋白表达及分布,RT-PCR法检测左室心肌组织bcl2-mRNA含量。结果CaMKⅡ分布于细胞核和细胞浆,pCREB主要分布于细胞核,心肌肥厚组CaMKⅡ和pCREB吸光值显著高于对照组（CaMKⅡ:21.0±0.6vs15.5±0.9;pCREB:16.4±0.7vs11±0.7;P<0.05）;心肌肥厚组bcl-2的mRNA表达显著低于对照组（0.52±0.07vs0.68±0.09,P<0.05）。结论压力超负荷时CaMKⅡ激活,核转录因子CREB磷酸化增加,可能通过下调抗凋亡基因bcl-2表达参与心肌肥厚的发生。     

The effects of cardiac myocytes on interstitial fibroblasts in toxic iron overload

Iron deposits preferentially in myocytes in mixed cultures of cardiac myocytes and nonmyocytic fibroblasts. In vivo, iron overload is associated with cardiac fibrosis. Therefore, we examined whether iron loading of cardiac myocytes in culture could trigger a response in nonmyocytes characteristic of a fibrogenic phenotype. We found that the nonmyocytes adopted a myofibroblast phenotype in culture. The rate of DNA synthesis (measured by [³H]thymidine incorporation) by the nonmyocytes was decreased by the myocyte-conditioned medium, compared to that of the unconditioned medium, and this activity was retained in >10-kDa fractions. The rate was partially restored when the medium was obtained from iron-loaded myocytes, and in this medium, the >10-kDa fraction was even more effective in reversing the suppression of proliferation. This suppression suggests a decreased secretion of a growth-inhibitory substance in the iron-loaded myocytes, and this effect was partially reversed when the iron-loaded cells were treated with the iron chelator, deferoxamine. This indicates that cardiac myocytes may play a paracrine role in suppressing the proliferation of myofibroblasts that is partially overcome when the myocytes are iron overloaded. The myocyte-conditioned medium also affects the myofibroblast phenntype, increasing the cells' fibronectin mRNA content and decreasing α-smooth-muscle actin mRNA. The myocyte-conditioned medium increases transforming growth factor-β (TGF-β) secretion by myofibroblasts, but the TGF-β content of the conditioned medium was found to play, at most, a minor role in determining the response of the myofibroblast.     

Role of alpha 1-adrenoceptor activity in progression of cardiac hypertrophy in guinea pig hearts with pressure overload

To elucidate the role of alpha 1- and beta-adrenergic activities in pressure overload hypertrophy, changes of alpha 1- and beta-adrenoceptors were measured by radioligand binding assay, and the preventive effects of alpha 1- and beta-adrenoceptor blockade on cardiac hypertrophy were assessed in guinea pigs after aortic banding. Five days after banding, dry weight of left ventricle had not increased, though wet weight increased due to marked intercellular oedema. In this period, the maximum binding capacity of [3H] prazosin increased to 31.1 (SEM 2.2) fmol.mg-1 from (sham operation) 17.0(2.1) fmol.mg protein-1, p less than 0.01, whereas the maximum binding capacity of [3H]dihydroalprenolol did not increase: 143(16) fmol.mg-1 (banded) v 153(13) fmol.mg-1 (sham). Three weeks after aortic banding, the maximum binding capacity of both ligands increased to 45.6(5.5) fmol.mg-1 and 232(21) fmol.mg-1, respectively, accompanied by a significant increase in left ventricular dry weight, from 0.46(0.02) mg.g-1 (sham) to 0.62(0.08) mg.g-1 (banded), p less than 0.01. Continuous subcutaneous administration of the alpha 1-blocker bunazosin (0.1 mg.kg-1.d-1) significantly attenuated the increase in left ventricular dry weight whereas the beta-blocker propranolol (5 mg.kg-1.d-1) did not: 0.55(0.03) v 0.66(0.04) mg.g-1 respectively, after 3 weeks. These results show that pressure overload elicited an increase in myocardial alpha 1-adrenoceptors before the onset of cardiac hypertrophy, and that an alpha 1-blocker could prevent the development of hypertrophy in the pressure overloaded heart.     

Diastolic characteristics and cardiac energetics of isolated hearts exposed to volume and pressure overload

The relation between diastolic dimensions and systolic function was studied in isolated rat hearts exposed to pressure (primary hypertension) and to volume load (hyperthyroidism). The hearts were arrested in diastole and cardiac dimensions analysed. Cardiac function was determined using an anterograde working heart preparation. Both models of cardiac overload produced significant left ventricular hypertrophy, however with different design. One concentric and one eccentric form of left ventricular hypertrophy were found in the spontaneously hypertensive rat obtained from two different breeders. An even more pronounced form of eccentric left ventricular hypertrophy was seen in hyperthyroid rats. The hearts with an eccentric form of hypertrophy had a higher maximal stroke volume when perfused at a high aortic pressure than those with a concentric form. At a low aortic pressure and hence a limited coronary perfusion cardiac performance was diminished in both types of spontaneously hypertensive rats but remained normal in hyperthyroid rats. Furthermore, oxygen consumption was reduced in all hypertrophied hearts compared with non-hypertrophied hearts. These data therefore suggest (a) that hearts having an eccentric type of hypertrophy may be in a more favourable situation, delivering a higher stroke volume for a given degree of myocardial fibre shortening; and (b) that the development of coronary vascular structural changes in spontaneously hypertensive rats, thereby increasing vascular resistance, leads to a decrease in left ventricular function at a low coronary perfusion pressure, whereas no such reduction was observed in hyperthyroid hearts, probably owing to a normal vascular resistance.     

Targeted deletion of matrix metalloproteinase 2 ameliorates myocardial remodeling in mice with chronic pressure overload

Matrix metalloproteinases (MMPs) play an important role in the extracellular matrix remodeling. Experimental and clinical studies have demonstrated that MMP 2 and 9 are upregulated in the dilated failing hearts and involved in the development and progression of myocardial remodeling. However, little is known about the role of MMPs in mediating adverse myocardial remodeling in response to chronic pressure overload (PO). We, thus, hypothesized that selective disruption of the MMP 2 gene could ameliorate PO-induced cardiac hypertrophy and dysfunction in mice. PO hypertrophy was induced by transverse aortic constriction (TAC) in male MMP 2 knockout (KO) mice (n=10) and sibling wild-type (WT) mice (n=9). At 6 weeks, myocardial MMP 2 zymographic activity was 2.4-fold increased in WT+TAC, and this increase was not observed in KO+TAC, with no significant alterations in other MMPs (MMP 1, 3, 8, and 9) or tissue inhibitors of MMPs (1, 2, 3, and 4). TAC resulted in a significant increase in left ventricular (LV) weight and LV end-diastolic pressure (EDP) with preserved systolic function. KO+TAC mice exerted significantly lower LV weight/body weight (4.2+/-0.2 versus 5.0+/-0.2 mg/g; P<0.01), lung weight/body weight (4.9+/-0.2 versus 6.2+/-0.4 mg/g; P<0.01), and LV end-diastolic pressure (4+/-1 versus 10+/-2 mm Hg; P<0.05) than WT+TAC mice despite comparable aortic pressure. KO+TAC mice had less myocyte hypertrophy (cross-sectional area; 322+/-14 versus 392+/-14 microm2; P<0.01) and interstitial fibrosis (collagen volume fraction; 3.3+/-0.5 versus 8.2+/-1.0%; P<0.01) than WT+TAC mice. MMP 2 plays an important role in PO-induced LV hypertrophy and dysfunction. The inhibition of MMP 2 activation may, therefore, be a useful therapeutic strategy to manage hypertensive heart disease.     

Hemodynamic and adrenergic effects of combined alpha/beta-receptor blockade versus combined beta-receptor and slow channel calcium blockade in patients with ischemic heart disease

The effects of combined alpha/beta-adrenoceptor blockade and of beta-receptor/slow channel calcium blockade on systemic and pulmonary hemodynamics and on adrenergic activity were compared in 2 matched groups of men suffering from ischemic heart disease. They were studied at rest supine and during ischemia-inducing exercise in the seated posture using invasive percutaneous techniques. Fourteen patients received 200 mg labetalol as a single oral dosis, 15 metoprolol (100 mg) plus nifedipine (10 mg). Both regimens reduced pressures in the systemic and pulmonary circulation under all conditions. At rest, stroke volume and cardiac output slightly decreased after labetalol and increased after metoprolol/nifedipine. During exercise the changes induced by the two regimens were virtually identical: heart rates and vascular resistances were reduced, stroke volume increased, cardiac output was not significantly changed. Plasma renin activity was lowered by labetalol, unchanged by metoprolol/nifedipine. Plasma adrenaline increased after metoprolol/nifedipine only, noradrenaline with both regimens. Both combinations significantly lowered stroke work and the rate pressure product and had similar beneficial effects on the onset and the duration of angina. It is concluded that both combinations attenuate or offset the potential adverse hemodynamic effects of beta-receptor blockade alone without loss but rather enhancement of symptomatic efficacy.     

Effects of early and late chronic pressure overload on extracellular matrix remodeling

The left ventricle (LV) remodels with age and in response to pressure overload. While aging and pressure overload are superimposed in the clinical context, the structural and functional consequences of the individual processes are not well-understood. Accordingly, the objective of this study was to compare the effects of both early and late chronic hypertension on extracellular matrix (ECM) remodeling. The following groups of Dahl rats were studied: 1) young salt-resistant (control, n=6); 2) young salt-sensitive (early phase of chronic hypertension, n=6); 3) middle-aged salt-resistant (aging, n=5); and 4) middle-aged salt-sensitive (late phase of chronic hypertension, n=6). We measured LV mass (LVM) and body weight (BW) and immunoblotted a panel of matrix metalloproteinases (MMPs), tissue inhibitors of metalloproteinases (TIMPs), and ECM proteins. Total collagen increased, several MMPs decreased, and TIMP-1 increased in the early phase of hypertension, consistent with fibrosis. Active MMP-8 decreased from 8,010+/-81 U in young salt-resistant to 5,260+/-313 U in young salt-sensitive (p<0.05) rats. During the late phase, chronic hypertension decreased total collagen levels and increased MMP-8 and MMP-14 (all p<0.05). Based on good-fit modeling analysis, MMP-14 (45 kDa) correlated positively with changes in LVM/BW during the early phase. In conclusion, this is the first study to evaluate MMP levels during both early and late chronic phases of hypertension. Our results highlight that ECM remodeling in response to pressure overload is a dynamic process involving excessive ECM accumulation and degradation.     

Vascular endothelial growth factor blockade promotes the transition from compensatory cardiac hypertrophy to failure in response to pressure overload

Cardiac hypertrophy is associated with upregulation of vascular endothelial growth factor (VEGF) in the myocardium. Here, we evaluated the effects of a decoy VEGF receptor on heart morphology and function to a murine model of pressure overload hypertrophy. Mice were administered adenoviral vector encoding a decoy VEGF receptor (Ad-Flk), and their hearts were subjected to pressure overload by transverse aortic constriction (TAC). Treatment with Ad-Flk led to a net reduction in capillary density in hearts subjected to TAC. Ad-Flk also led to a reduction in TAC-induced cardiac hypertrophy and promoted left ventricle dilatation and a loss in contractile function. Treatment with Ad-Flk markedly increased myocardial fibrosis and collagen gene upregulation. In contrast, Ad-Flk had no effect on any of these parameters in sham-treated mice. Administration of a VEGF trap reagent diminished pressure overload cardiac hypertrophy and promoted the progression to heart failure but had no effect on sham-treated animals. These findings suggest that VEGF is required to maintain myocardial capillary density and that reductions in the vascular bed are associated with the transition from compensatory hypertrophy to failure.     

cFLIP在压力超负荷诱导的心肌肥厚及纤维化中的作用

目的利用基因工程小鼠研究细胞型Fas相关死亡区域蛋白样β白介素-1转换酶抑制蛋白基因(cFLIP)在压力超负荷诱导的心肌肥厚及心肌纤维化中的作用。方法 cFLIP杂合子小鼠(heterozygote,HZ)和cFLIP野生型小鼠(wild type,WT)各20只分别随机分为模型组和假手术组,每组10只。主动脉缩窄术建立小鼠心肌肥厚模型,术后4周超声检测小鼠心室腔大小及左室短轴缩短率,组织病理学方法评价心肌肥厚及纤维化程度,实时定量聚合酶链式反应在mRNA水平检测心肌肥厚标志物心钠肽,脑钠肽,β-MHC以及心肌纤维化标志物结缔组织生长因子(CTGF),Ⅰ、Ⅳ型胶原(collagenⅠ,Ⅳ),TGF-β1。结果术后4周,HZ模型组较WT模型组和HZ假手术组心室腔增大,左室短轴缩短率下降(P<0.05,n=10);心肌肥厚及纤维化组织病理学检测及相关标志物表达水平模型组较假手术组升高(P<0.05,n=10),而HZ模型组表达增加程度较WT模型组更为显著(P<0.05,n=10)。结论cFLIP可抑制心肌肥厚及纤维化,对心功能起保护作用。     

Direct cardiac effects in isolated perfused rat hearts of fentanyl and remifentanil

Opioids are widely used as analgesics to supplement general anaesthesia or as adjunct to anaesthetic agents and for long term analgesia and sedation in intensive care patients. Some clinical studies have suggested that opioids may have different and deleterious haemodynamic effects that remain incompletely examined. We compared the direct cardiac effects of fentanyl and remifentanil in isolated Wistar rat hearts. Twenty rats were randomly assigned to two groups. Hearts were perfused with modified Krebs Henseleit solution and were exposed to 1 x 10(-6) moles(M)/L fentanyl (n=10) in Group I and 1 x 10(-6) M/L remifentanil (n=10) in Group II. Heart rates, contractile force and coronary perfusion were recorded continuously during the study. There was a significant decrease in heart rate and increase in contractility and coronary perfusion in two groups (p<0.001). Fentanyl had less depressant effects on heart rate than remifentanil. We conclude that in isolated rat heart, fentanyl and remifentanil cause direct negative chronotropic and positive inotropic effect. Remifentanil had more depressant effects on heart rate than fentanyl in isolated rat heart.     

Effects of acute and chronic beta-receptor blockade on ventricular repolarisation in man.

The right ventricular repolarisation phase was studied electrophysiologically after an injection of 15 mg metoprolol in 16 healthy volunteers. Eight of them were restudied after chronic treatment with 400 mg metoprolol daily for five weeks. The assessment of the repolarisation time included ventricular effective refractory periods, monophasic action potential duration, and the QT interval measured during atrial stimulation at different driving frequencies. The acute administration of 15 mg metoprolol intravenously had no detectable effect on the repolarisation phase, while chronic treatment caused a significant increase of the ventricular effective refractory periods, monophasic action potential duration, and the QT interval during atrial stimulation. Thus the study confirmed the contrasting effect of acute and chronic beta-receptor blockade on the ventricular repolarisation time in man.     

Effects of magnesium on matrix metalloproteinase-2 production in cultured rat cardiac fibroblasts

Abstract. The precise correlation between magnesium and cardiac disease remains to be established. Matrix metalloproteinases (MMPs) are important in cardiac disease such as heart failure. Cardiac fibroblasts are the most abundant cell type in the heart and play an important role in the regulation of collagen degradation by MMPs. To assess the association between magnesium and MMPs, we examined the effects of different extracellular magnesium concentrations (0-3.0 mmol/L) on MMP-2 production in cultured rat cardiac fibroblasts. Using gelatin zymography and western blotting, we found that magnesium reduced MMP-2 production dose-dependently, and this effect was inhibited by the tyrosine kinase inhibitors, genistein or herbimycin A. The results of this study indicated that the beneficial effect of magnesium supplementation on the cardiac disease may be due, at least in part, to the inhibitory effect of magnesium on production of MMPs in cardiac fibroblasts, which appears to be mediated by a protein tyrosine phosphorylation related signal transduction pathway.     

Cardiac mast cell regulation of matrix metalloproteinase-related ventricular remodeling in chronic pressure or volume overload

The chronic elevation in ventricular wall stress secondary to ventricular volume or pressure overload leads to structural remodeling of the muscular, vascular and extracellular matrix components of the myocardium. While initially a compensatory response, the progressive hypertrophy and ventricular dilatation induced by this condition ultimately have a detrimental effect on ventricular function, resulting in heart failure. Fibrillar collagen provides the skeletal framework which interconnects the cardiomyocytes, thereby maintaining ventricular shape and size and contributing to tissue stiffness. Accordingly, these myocardial collagen fibers must be disrupted for ventricular dilatation, sphericalization and wall thinning to occur. The presence of an abundant, latent matrix metalloproteinase (MMP) population which coexists with myocardial fibrillar collagen has been documented. Thus, the potential for collagen degradation to exceed synthesis exists should there be significant activation of this latent MMP system. Mast cells are known to store and release a variety of biologically active mediators including TNF-alpha and proteases such as tryptase and chymase, which can induce MMP activation. Increased cardiac mast cell density has been implicated in the pathophysiology of human end-stage cardiomyopathy and experimental myocardial infarction, hypertension and chronic volume overload secondary to mitral regurgitation and aorto-caval fistula. The potential role of cardiac mast cells in activating MMPs, which then results in fibrillar collagen degradation and adverse myocardial remodeling secondary to chronic volume and pressure overload will be the subject of this review.