Mechanisms of preserved baseline cardiac systolic function in rats with adrenergic inotropic downregulation

Maternal protein restriction compromises myocardial contractility in the young adult rat by changing proteins involved in calcium handling

Maternal protein restriction (MPR) during pregnancy is associated with increased cardiovascular risk in the offspring in adulthood. In this study we evaluated the cardiac function of young male rats born from mothers subjected to MPR during pregnancy, focusing on the myocardial mechanics and calcium-handling proteins. After weaning, rats received normal diet until 3 mo old, when the following parameters were assessed: arterial and left ventricular hemodynamics and in vitro cardiac contractility in isolated papillary muscles. The body weight was lower and arterial pressure higher in the MPR group compared with young adult offspring of female rats that received standard diet (controls); and left ventricle time derivatives increased in the MPR group. The force developed by the cardiac muscle was similar; but time to peak and relaxation time were longer, and the derivatives of force were depressed in the MPR. In addition, MPR group exhibited decreased post-pause potentiation of force, suggesting reduced reuptake function of the sarcoplasmic reticulum. Corroborating, the myocardial content of SERCA-2a and phosphorylated PLB-Ser16/total PLB ratio was decreased and sodium-calcium exchanger was increased in the MPR group. The contraction dependent on transsarcolemmal influx of calcium was higher in MPR if compared with the control group. In summary, young rats born from mothers subjected to protein restriction during pregnancy exhibit changes in the myocardial mechanics with altered expression of calcium-handling proteins, reinforcing the hypothesis that maternal malnutrition is related to increased cardiovascular risk in the offspring, not only for hypertension, but also cardiac dysfunction.

Impaired epicardial activation-repolarization coupling contributes to the proarrhythmic effects of hypokalaemia and dofetilide in guinea pig ventricles

AIM

Activation-repolarization coupling refers to the inverse relationship between action potential duration and activation time in myocardial regions along the path of ventricular excitation. This study examined whether the activation-repolarization coupling plays a role in coordinating repolarization times between the right ventricular (RV) and left ventricular (LV) chambers, and if impaired coordination contributes to electrical instability produced by hypokalaemia or dofetilide, a blocker of the delayed rectifier K(+) current.

METHODS

In Langendorff-perfused, isolated guinea pig hearts, six monophasic action potential recording electrodes were attached to RV and LV epicardium. Local activation time and action potential duration (APD90 ) were determined during spontaneous beating, regular pacing and extrasystolic excitation.

RESULTS

In regularly beating hearts, the RV epicardial sites had longer APD90 , but exhibited earlier activation times, as compared to LV sites, which minimized the interventricular difference in repolarization time. Upon extrasystolic stimulation, the APD90 was reduced to a greater extent in RV compared with LV, which translated to a reversed slope of APD90 -to-activation time relationship, and increased spatial repolarization gradients. Hypokalaemia and dofetilide prolonged APD90 , with the effect being greater in LV compared with RV. In hypokalaemic hearts, LV activation was delayed. These changes contributed to increased asynchrony in repolarization times in the LV and RV in both regular and extrasystolic beats, and enhanced susceptibility to tachyarrhythmia.

CONCLUSION

Impaired RV-to-LV activation-repolarization coupling is an important determinant of electrical instability in the setting of non-uniformly prolonged epicardial APD90 or slowed interventricular conduction.

Dofetilide promotes repolarization abnormalities in perfused Guinea-pig heart

PURPOSE

Dofetilide is class III antiarrhythmic agent which prolongs cardiac action potential duration because of selective inhibition of I (Kr), the rapid component of the delayed rectifier K(+) current. Although clinical studies reported on proarrhythmic risk associated with dofetilide treatment, the contributing electrophysiological mechanisms remain poorly understood. This study was designed to determine if dofetilide-induced proarrhythmia may be attributed to abnormalities in ventricular repolarization and refractoriness.

METHODS

The monophasic action potential duration and effective refractory periods (ERP) were assessed at distinct epicardial and endocardial sites along with volume-conducted ECG recordings in isolated, perfused guinea-pig heart preparations.

RESULTS

Dofetilide was found to produce the reverse rate-dependent prolongation of ventricular repolarization, increased the steepness of action potential duration rate adaptation, and amplified transepicardial variability in electrical restitution kinetics. Dofetilide also prolonged the T peak-to-end interval on ECG, and elicited a greater prolongation of endocardial than epicardial ERP, thereby increasing transmural dispersion of refractoriness. At epicardium, dofetilide prolonged action potential duration to a greater extent than ERP, thus extending the critical interval for ventricular re-excitation. This change was associated with triangulation of epicardial action potential because of greater dofetilide-induced prolonging effect at 90 % than 30 % repolarization. Premature ectopic beats and spontaneous short-lasting episodes of monomorphic ventricular tachycardia were observed in 44 % of dofetilide-treated heart preparations.

CONCLUSIONS

Proarrhythmic potential of dofetilide in guinea-pig heart is attributed to steepened electrical restitution, increased transepicardial variability in electrical restitution kinetics, amplified transmural dispersion of refractoriness, increased critical interval for ventricular re-excitation, and triangulation of epicardial action potential.

Quinidine elicits proarrhythmic changes in ventricular repolarization and refractoriness in guinea-pig

Quinidine is a class Ia Na+ channel blocker that prolongs cardiac repolarization owing to the inhibition of IKr, the rapid component of the delayed rectifier current. Although quinidine may induce proarrhythmia, the contributing mechanisms remain incompletely understood. This study examined whether quinidine may set proarrhythmic substrate by inducing spatiotemporal abnormalities in repolarization and refractoriness. The monophasic action potential duration (APD), effective refractory periods (ERPs), and volume-conducted electrocardiograms (ECGs) were assessed in perfused guinea-pig hearts. Quinidine was found to produce the reverse rate-dependent prolongation of ventricular repolarization, which contributed to increased steepness of APD restitution. Throughout the epicardium, quinidine elicited a greater APD increase in the left ventricular chamber compared with the right ventricle, thereby enhancing spatial repolarization heterogeneities. Quinidine prolonged APD to a greater extent than ERP, thus extending the vulnerable window for ventricular re-excitation. This change was attributed to increased triangulation of epicardial action potential because of greater APD lengthening at 90% repolarization than at 30% repolarization. Over the transmural plane, quinidine evoked a greater ERP prolongation at endocardium than epicardium and increased dispersion of refractoriness. Premature ectopic beats and monomorphic ventricular tachycardia were observed in 50% of quinidine-treated heart preparations. In summary, abnormal changes in repolarization and refractoriness contribute greatly to proarrhythmic substrate upon quinidine infusion.

Chronic sympathetic activation promotes downregulation of β-adrenoceptor-mediated effects in the guinea pig heart independently of structural remodeling and systolic dysfunction

It is uncertain if downregulation of β-adrenoceptor signaling pathway is promoted by an enhanced adrenergic tone at an early stage of cardiac disease, or it develops secondary to detrimental local myocardial changes in advanced heart failure. We examined the integrity of β-adrenoceptor signaling pathway upon chronic infusion of isoproterenol, a β-adrenoceptor agonist, at a dose producing no structural left ventricular (LV) remodeling and systolic dysfunction. Subcutaneous isoproterenol infusion (400 μg kg(-1) h(-1) over 16 days) to guinea pigs using osmotic minipumps produced no change in cardiac weights, LV internal dimensions, myocyte cross-sectional area, extent of interstitial fibrosis, and basal contractile function. Isolated, perfused heart preparations from isoproterenol-treated guinea pigs exhibited attenuated responsiveness to acute β-adrenoceptor stimulation, as evidenced by reduced LV developed pressure increase, less shortening of LV epicardial monophasic action potential and effective refractory period, and less myocardial cyclic adenosine monophosphate elevation, in response to isoproterenol exposure, when compared to saline-treated controls. Pharmacological responses to forskolin, an activator of the adenylate cyclase catalytic subunit, were well preserved in isoproterenol-treated hearts. Downregulation of β-adrenoceptor-mediated effects upon chronic isoproterenol infusion was associated with markedly reduced stimulatory G-protein α-subunit (G(sα)) myocardial expression levels. No change in expression levels of β-adrenoceptors, G-protein-coupled receptor kinase 2, inhibitory G-protein α-subunit (G(iα2)), and Ca(v)1.2 and K(v)7.1 ion channels was determined in isoproterenol-treated hearts. We therefore conclude that sustained adrenergic overstimulation may promote downregulation of myocardial β-adrenoceptor-mediated effects independently of structural LV remodeling and systolic failure, an effect attributed to β-adrenoceptor uncoupling from adenylate cyclase due to reduced G(sα)-protein expression.

Temporal changes in myocardial adrenergic regulation with the progression to pump dysfunction after chronic β-adrenoreceptor activation in rats

We evaluated the relationship between myocardial norepinephrine release or inotropic responsiveness to adrenergic stimulation and intrinsic myocardial function after the progression to pump dysfunction induced by chronic beta-adrenoreceptor activation (isoproterenol [ISO], 0.1 mg/kg/day for 1 month or 6 months) in rats. Left ventricular (LV) systolic chamber dysfunction occurred after 6 months, but not after 1 month of beta-adrenoreceptor activation, as evidenced by reduced LV endocardial fractional shortening determined by echocardiography and a decrease in the slope of the LV systolic pressure-volume relations assessed in isolated, perfused heart preparations. A reduced pump function at 6 months of ISO administration was associated with chamber dilatation, while LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relations (isolated heart preparations), indices of intrinsic myocardial function, were unchanged. After 1 month of ISO administration, reduced beta1- and beta2-adrenoreceptor-mediated and sustained alpha-adrenoreceptor-mediated inotropic responses were noted. Nevertheless, increased inotropic potency of beta-adrenoreceptor agonists and upregulation of alpha-adrenoreceptor-mediated contractile responses were noted after 6 months of ISO administration. Increased adrenergic-inotropic responsiveness after 6 months of ISO administration was associated with depleted LV norepinephrine stores, as evidenced by reduced desipramine-stimulated norepinephrine concentrations in the coronary effluent. In conclusion, in the progression from compensated cardiac hypertrophy to pump dysfunction after chronic sympathetic activation, a preserved intrinsic myocardial contractility is accounted for by paradoxical upregulation of adrenergic-mediated contractile responses.

Cardiac dilatation and pump dysfunction without intrinsic myocardial systolic failure following chronic β-adrenoreceptor activation

There is no direct evidence to indicate that pump dysfunction in a dilated chamber reflects the impact of chamber dilatation rather than the degree of intrinsic systolic failure resulting from myocardial damage. In the present study, we explored the relative roles of intrinsic myocardial systolic dysfunction and chamber dilatation as mediators of left ventricular (LV) pump dysfunction. Administration of isoproterenol, a β-adrenoreceptor agonist, for 3 mo to rats (0.1 mg·kg−1·day−1) resulted in LV pump dysfunction as evidenced by a reduced LV endocardial fractional shortening (echocardiography) and a decrease in the slope of the LV systolic pressure-volume relation (isolated heart preparations). Although chronic β-adrenoreceptor activation induced cardiomyocyte damage (deoxynucleotidyl transferase-mediated dUTP nick-end labeling) as well as β1- and β2-adrenoreceptor inotropic downregulation (attenuated contractile responses to dobutamine and salbutamol), these changes failed to translate into alterations in intrinsic myocardial contractility. Indeed, LV midwall fractional shortening (echocardiography) and the slope of the LV systolic stress-strain relation (isolated heart preparations) were unchanged. A normal intrinsic myocardial systolic function, despite the presence of cardiomyocyte damage and β-adrenoreceptor inotropic downregulation, was ascribed to marked increases in myocardial norepinephrine release, to upregulation of α-adrenoreceptor-mediated contractile effects as determined by phenylephrine responsiveness, and to compensatory LV hypertrophy. LV pump failure was attributed to LV dilatation, as evidenced by increased LV internal dimensions (echocardiography), and a right shift and increased volume intercept of the LV diastolic pressure-volume relation. In conclusion, chronic sympathetic stimulation, despite reducing β-adrenoreceptor-mediated inotropic responses and promoting myocyte apoptosis, may nevertheless induce pump dysfunction primarily through LV dilatation, rather than intrinsic myocardial systolic failure.

Rat strain-related differences in myocardial adrenergic tone and the impact on cardiac fibrosis, adrenergic responsiveness and myocardial structure and function

Sprague-Dawley (SD) rats have been reported to have a higher sympathetic activity than Wistar-Kyoto (WKY) rats. In the present study we sought to determine if these rat strain-related differences in sympathetic activity exist at a myocardial level and whether they translate into changes in cardiac fibrosis, contractile responsiveness to adrenergic agonists, and cardiac structure and function. Coronary effluent noradrenaline concentrations, as determined in isolated, perfused heart preparations, were higher in 5-month-old SD as compared to age-matched WKY male rats. This difference was accompanied by higher resting heart rates in SD rats as assessed in vivo. However, increases in myocardial noradrenaline release in SD rats did not translate into enhanced myocardial fibrosis, cardiac hypertrophy or remodeling, changes in basal ventricular systolic and diastolic function, or to down-regulation of inotropic responses to the beta-adrenoreceptor agonists, noradrenaline, isoproterenol and dobutamine. Although age-matched male SD rats were heavier, no differences in absolute heart weights were noted between rat strains. Moreover, left ventricular (LV) posterior wall thickness as assessed by echocardiography, as well as cardiac myocyte dimensions as determined by laser scanning confocal microscopy were similar between rat strains. Furthermore, LV internal diameters as determined in vivo, as well as LV diastolic volume intercept determined in isolated, perfused heart preparations were similar between rat strains. Increases in myocardial noradrenaline release in SD rats also did not translate into differences in LV systolic chamber and myocardial function as assessed in vivo (LV endocardial and midwall fractional shortening) and at controlled loads and heart rates ex vivo (the slope of the LV developed pressure-volume relation determined). Likewise, neither myocardial hydroxyproline content nor LV chamber stiffness as assessed by the slope of the LV end-diastolic pressure-volume relation were different in SD and WKY rats. In conclusion, rat strain-related differences in cardiac adrenergic tone do indeed exist, but in young animals these differences do not translate into cardiac phenotypes known to contribute to progressive cardiac dysfunction.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Impact of chronic beta-adrenoceptor activation on neurotensin-induced myocardial effects in rats

In heart failure chronic sympathetic activation results in contractile dysfunction in part through down-regulation of the beta-adrenoceptor-cAMP system. However, the impact of chronic adrenergic activation on cardiac sympathetic neuromodulator systems is unclear. In this study, we sought to determine whether chronic adrenergic activation modifies myocardial norepinephrine release and contractile responses elicited by neurotensin, a neuropeptide found in cardiovascular system. Chronic administration of isoproterenol, a beta-adrenoceptor agonist, to rats (0.05 mg/kg daily for 1 month, i.p.), produced cardiac hypertrophy with preserved baseline ventricular systolic function, but reduced contractile responses to exogenous norepinephrine as shown in isolated, isovolumically-contracting heart preparations. Neurotensin produced a marked increase in coronary effluent norepinephrine release, an effect abolished by SR 48692, a specific neurotensin receptor antagonist. In isoproterenol-treated rats, neurotensin has no significant impact on myocardial norepinephrine release. Consistently, concentration-dependent positive inotropic responses elicited by neurotensin in control rat hearts were blunted over a wide range of neurotensin concentrations (10(-10)-10(-5.5) M) in isoproterenol-treated rats. In conclusion, these data indicate that following chronic beta-adrenoceptor activation, neurotensin-induced effects on norepinephrine release and subsequent contractile changes are markedly down-regulated.