Inotropic and energetic effects of altering the force-calcium relationship: Mechanisms, experimental results, and potential molecular targets

Prostanoid-mediated inotropic responses are attenuated in failing human and rat ventricular myocardium

Prostanoid-modulatory approaches in heart failure patients have displayed effects which may seem to be mutually incompatible. Both treatment with prostanoids and inhibition of prostanoid synthesis have resulted in increased mortality in heart failure patients. Currently, it is unknown if prostanoids mediate contractile effects in failing human heart and if this can explain some of the clinical effects seen after prostanoid modulatory treatments. Therefore, the objectives of this study were to determine if prostanoids could elicit direct inotropic responses in human ventricle, and if so to determine if they are modified in failing ventricle. Contractile force was measured in left ventricular strips from non-failing or failing human and rat hearts. The ratio of phosphorylated to non-phosphorylated myosin light chain 2 (MLC-2) was measured by Western blotting in myocardial strips, and the levels of prostanoid FP receptor mRNA and protein were measured in rat by real-time RT-PCR and receptor binding assays. In non-failing human hearts, prostanoids evoked a positive inotropic effect and an increase of MLC-2 phosphorylation which was absent in failing human hearts. In failing rat heart, the prostanoid FP receptor-mediated inotropic response and prostanoid FP receptor-density was reduced by ~40-50% compared to non-failing rat heart. Prostanoids mediate a sustained positive inotropic response in non-failing heart, which appears to be down regulated in failing heart. The pathophysiological significance of changes in prostanoid-mediated inotropic support in the failing heart remains to be determined.

Left ventricular torsion, energetics, and diastolic function in normal human aging

This study determined, for the first time, whether the effects of normal aging on systolic and diastolic left ventricular function in subjects without cardiovascular disease are related to underlying energetic defects. Cardiac magnetic resonance imaging with tissue tagging and (31)P spectroscopy was used to determine global structure, function, myocardial strains, and the phosphocreatine-to-ATP ratio (PCr/ATP) in 49 healthy subjects aged 20-69 yr. The three major abnormalities that developed with increasing age were the early filling percentage (EFP, the left ventricular volume increase from end systole to mid-diastole divided by stroke volume × 100), which decreased with age, indicating impaired early diastolic filling (r = -0.72, P < 0.0001), the torsion-to-shortening ratio (TSR, measure of subepicardial torsion exerting mechanical advantage over subendocardial shortening), which increased with age indicating relative subendocardial dysfunction (r = 0.44, P < 0.02), and the PCr/ATP (decreased with increasing age, r = -0.52, P < 0.003). EFP and TSR were strongly correlated (r = -0.63, P < 0.0001), although they were not related to PCr/ATP [EFP vs. PCr/ATP: r = 0.34, not significant (NS) and TSR vs. PCr/ATP: r = -0.3, P = NS]. In normal aging, changes in EFP and TSR likely share the same pathophysiology, although it is unlikely that energetics have a major role in the functional effects of aging.

Acute hemodynamic effects of intravenous sildenafil citrate in congestive heart failure: comparison of phosphodiesterase type-3 and -5 inhibition

BACKGROUND

The reversibility of elevated pulmonary vascular resistance in heart failure bears an important relation to outcome after cardiac transplantation. The phosphodiesterase 3 (PDE3) and PDE5 inhibitors both increase levels of cyclic nucleotides in the vascular smooth muscle, causing vasodilatation. PDE3 inhibitors also have direct inotropic effects. We contrasted the acute hemodynamic responses to intravenous PDE3 and PDE5 inhibitors in patients with congestive cardiac failure to assess their relative suitability for reversibility testing in this setting.

METHODS

Thirty patients undergoing assessment for cardiac transplantation underwent right heart catheterization. Patients were randomized to receive an intravenous bolus of milrinone (0.05 mg/kg) or sildenafil citrate at a high (0.43 mg/kg) or low dose (0.05 mg/kg).

RESULTS

Differences between low- and high-dose sildenafil were not significant. Both agents caused similar reductions in systemic and pulmonary vascular resistance. Milrinone caused significantly greater reductions in pulmonary artery wedge and mean pulmonary artery pressure, and increases in heart rate. In all study groups, greater increases in cardiac index (>25%) were seen in patients with a higher pulmonary artery wedge pressure at baseline (29 +/- 1 vs 20 +/- 2 mm Hg; p < 0.001).

CONCLUSIONS

In end-stage congestive cardiac failure, intravenous milrinone and sildenafil both cause similar reductions in systemic and pulmonary vascular resistance; however, milrinone has more cardiac selective effects on left ventricular filling and heart rate. Both agents appear to have a suitable hemodynamic profile for testing of reversibility of secondary pulmonary hypertension in congestive cardiac failure. Larger studies are needed to confirm these results.

The myofilament force-calcium relationship as a target for positive inotropic therapy in congestive heart failure

To-date positive inotropic therapy in the treatment of congestive heart failure has resulted in adverse effects on long term survival. These agents increase calcium cycling through beta-adrenergic stimulation or phosphodiesterase inhibition. An alternative method of producing positive inotropy is to increase the myofilament sensitivity to calcium. This can occur at several levels within the myofilament, and has potential benefits with respect to avoiding increased calcium cycling and producing a more favourable energy efficient positive inotropy. A potential adverse effect of increasing calcium sensitivity is slowed relaxation and diastolic dysfunction. We have learnt a considerable amount about the function of specific sites within the myofilament by the use of genetically engineered mouse models, which have shown diverse effects of various myofilament sites on global left ventricular function. Levosimendan is a novel inotropic agent that has several mechanisms of action including calcium sensitization, and is undergoing clinical trials at present. This review article will provide a comprehensive molecular, biophysical and physiological insight into the concepts underlying the myofilament force-calcium relationship and its potential as a target for positive inotropic therapy in heart failure.

Cardiac transgenesis with the tetracycline transactivator changes myocardial function and gene expression

The cardiac-specific tetracycline-regulated gene expression system (tet-system) is a powerful tool using double-transgenic mice. The cardiac alpha-myosin heavy chain promoter (alphaMHC) drives lifetime expression of a tetracycline-inhibited transcription activator (tTA). Crossing alphaMHC-tTA mice with mice containing a tTA-responsive promoter linked to a target gene yields double-transgenic mice having tetracycline-repressed expression of the target gene in the heart. Using the tet-system, some studies use nontransgenic mice for the control group, whereas others use single-transgenic alphaMHC-tTA mice. However, previous studies found that high-level expression of a modified activator protein caused cardiomyopathy. Therefore, we tested whether cardiac expression of tTA was associated with altered function of alphaMHC-tTA mice compared with wild-type (WT) littermates. We monitored in vivo and in vitro function and gene expression profiles for myocardium from WT and alphaMHC-tTA mice. Compared with WT littermates, alphaMHC-tTA mice had a greater heart-to-body weight ratio (approximately 10%), ventricular dilation, and decreased ejection fraction, suggesting mild cardiomyopathy. In vitro, submaximal contractions were greater compared with WT and were associated with greater myofilament Ca2+ sensitivity. Gene expression profiling revealed that the expression of 153 genes was significantly changed by >20% when comparing alphaMHC-tTA with WT myocardium. These findings demonstrate that introduction of the alphaMHC-tTA construct causes significant effects on myocardial gene expression and major functional abnormalities in vivo and in vitro. For studies using the tet-system, these results suggest caution in the use of controls, since alphaMHC-tTA myocardium differs appreciably from WT. Furthermore, the results raise the possibility that the phenotype conferred by a target gene may be influenced by the modified genetic background of alphaMHC-tTA myocardium.

Effects of levosimendan and milrinone on oxygen consumption in isolated guinea-pig heart

Levosimendan is a novel calcium sensitizer that increases contraction force without change in intracellular calcium ([Ca2+]i); milrinone is a phosphodiesterase inhibitor that exerts a positive inotropic effect by increasing [Ca2+]i. The effects of levosimendan and milrinone on oxygen consumption in the isolated guinea-pig heart were studied. Isolated guinea-pig hearts were paced (280 beats/min) and perfused according to the Langendorff technique. Levosimendan (0.01-1 microM) or milrinone (0.1-10 microM) were added cumulatively and changes from baseline for diastolic and systolic pressure (LVEDP and LVSP), contractility and relaxation (+dP/dt and -dP/dt), and coronary flow and oxygen consumption (CF and VO2) were calculated. Levosimendan was found to be 10 to 30 times more potent than milrinone as an inotropic agent. The effect on VO2 was markedly lower in levosimendan-perfused hearts than in milrinone-perfused hearts (P = 0.031 between the concentration-dependent effects of the two drugs). The maximum increase in VO2 was 10 +/- 4% in the levosimendan group and 38 +/- 15% in the milrinone group. The economy of the contraction was more advantageous in levosimendan-perfused hearts (P </= 0.005 vs. milrinone group on both VO2/+dP/dt and VO2/LVSP). It was concluded that levosimendan exerts a positive inotropic effect without disturbing the energy balance of the heart.

New molecular insights into heart failure and cardiomyopathy: potential strategies and therapies

BACKGROUND

In the most severely affected patients the mortality for congestive heart failure exceeds that of many cancers. While therapies are largely aimed at attenuating neurohumoral responses recent molecular insights reveal other potential targets for therapy.

AIMS

To summarise some of the recent developments in the management of heart failure and provide the clinician who treats heart failure with new insights into emerging approaches.

METHODS

A literature review was conducted of the recent literature together with personal research data.

RESULTS

Large randomised trials will provide a more comprehensive understanding of the interaction of beta-blockers and other heart failure therapies with gene polymorphisms. Cytokines are important in the progression of heart failure, yet therapy aimed at blocking cytokine effects has not been successful. More selective use of anti-cytokine therapy may have beneficial effects. Gene therapy to improve heart failure has not yet reached clinical trials. The molecular genetics of hypertrophic and dilated cardiomyopathy is rapidly improving our understanding so that genetic diagnostics and counselling may soon be performed for patients and families.

CONCLUSIONS

The emergence of a molecular based understanding of heart failure will hopefully improve therapy of this common condition.

Temperature-dependent postextrasystolic potentiation and Ca(2+) recirculation fraction in canine hearts

We have found that cardiac temperature proportionally changes O(2) cost of contractility, defined as O(2) consumption for myocardial total Ca(2+) handling normalized to contractility in terms of the end-systolic pressure-volume ratio (maximal elastance, E(max)), in the canine left ventricle (temperature sensitivity, Q(10) = 2). We have separately found that a decrease in the recirculation fraction (RF) of Ca(2+) within myocardial cells underlies an increased O(2) cost of E(max) in stunned hearts. We therefore hypothesized that a similar change in RF would underlie the Q(10) of O(2) cost of E(max). We tested this hypothesis by analyzing RF calculated from an exponential decay component of the transiently alternating postextrasystolic potentiation in the canine left ventricle. RF decreased from 0.7 to 0.5 as cardiac temperature increased from 33 to 38 degrees C with Q(10) of 0.5, reciprocal to that of O(2) cost of E(max). We conclude that Q(10) of ATP-consuming reactions involved in Ca(2+) handling and E(max) response to it could reasonably account for the reciprocal Q(10) of RF and O(2) cost of E(max).

Compensatory changes in Ca(2+) and myocardial O(2) consumption in beta-tropomyosin transgenic hearts

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.