Functional changes in troponin T by a splice donor site mutation that causes hypertrophic cardiomyopathy

Hypertrophic Cardiomyopathy Mutations of Troponin Reveal Details of Striated Muscle Regulation

Striated muscle contraction is inhibited by the actin associated proteins tropomyosin, troponin T, troponin I and troponin C. Binding of Ca²⁺ to troponin C relieves this inhibition by changing contacts among the regulatory components and ultimately repositioning tropomyosin on the actin filament creating a state that is permissive for contraction. Several lines of evidence suggest that there are three possible positions of tropomyosin on actin commonly called Blocked, Closed/Calcium and Open or Myosin states. These states are thought to correlate with different functional states of the contractile system: inactive-Ca²⁺-free, inactive-Ca²⁺-bound and active. The inactive-Ca²⁺-free state is highly occupied at low free Ca²⁺ levels. However, saturating Ca²⁺ produces a mixture of inactive and active states making study of the individual states difficult. Disease causing mutations of troponin, as well as phosphomimetic mutations change the stabilities of the states of the regulatory complex thus providing tools for studying individual states. Mutants of troponin are available to stabilize each of three structural states. Particular attention is given to the hypertrophic cardiomyopathy causing mutation, Δ14 of TnT, that is missing the last 14 C-terminal residues of cardiac troponin T. Removal of the basic residues in this region eliminates the inactive-Ca²⁺-free state. The major state occupied with Δ14 TnT at inactivating Ca²⁺ levels resembles the inactive-Ca²⁺-bound state in function and in displacement of TnI from actin-tropomyosin. Addition of Ca²⁺, with Δ14TnT, shifts the equilibrium between the inactive-Ca²⁺-bound and the active state to favor that latter state. These mutants suggest a unique role for the C-terminal region of Troponin T as a brake to limit Ca²⁺ activation.

Stepwise C-Terminal Truncation of Cardiac Troponin T Alters Function at Low and Saturating Ca2

Activation of striated muscle contraction occurs in response to Ca²⁺ binding to troponin C. The resulting reorganization of troponin repositions tropomyosin on actin and permits activation of myosin-catalyzed ATP hydrolysis. It now appears that the C-terminal 14 amino acids of cardiac troponin T (TnT) control the level of activity at both low and high Ca²⁺. We made a series of C-terminal truncation mutants of human cardiac troponin T, isoform 2, to determine if the same residues of TnT are involved in the low and high Ca²⁺ effects. We measured the effect of these mutations on the normalized ATPase activity at saturating Ca²⁺. Changes in acrylodan tropomyosin fluorescence and the degree of Ca²⁺ stimulation of the rate of binding of rigor myosin subfragment 1 to pyrene-labeled actin-tropomyosin-troponin were measured at low Ca²⁺. These measurements define the distribution of actin-tropomyosin-troponin among the three regulatory states. Residues SKTR and GRWK of TnT were required for the functioning of TnT at both low and high Ca²⁺. Thus, the effects on forming the inactive B-state and in retarding formation of the active M-state require the same regions of TnT. We also observed that the rate of binding of rigor subfragment 1 to pyrene-labeled regulated actin at saturating Ca²⁺ was higher for the truncation mutants than for wild-type TnT. This violated an assumption necessary for determining the B-state population by this kinetic method.

GSK-3β heterozygous knockout is cardioprotective in a knockin mouse model of familial dilated cardiomyopathy

Glycogen synthase kinase-3β (GSK-3β) plays a central role in both cardiac physiology and pathology. Herein we want to clarify the role of GSK-3β in familial dilated cardiomyopathy. We generated a mouse model carrying a heterozygous knockout mutation of GSK-3β (GSK-3β(+/-) KO), together with a ΔK210 knockin mutation in cardiac troponin T (ΔK210 cTnT KI), which was proved to be one of the genetic causes of familial dilated cardiomyopathy (DCM). GSK-3β(+/-) KO prevented the slow and rapid deterioration in left ventricular systolic function accompanying heart failure (HF) in DCM mice with heterozygous and homozygous ΔK210 cTnT KI mutations, respectively. GSK-3β(+/-) KO also prevented cardiac enlargement, myocardial fibrosis, and cardiomyocyte apoptosis and markedly reduced the expression of cardiac β-myosin heavy chain isoform, indicative of HF, in DCM mice with homozygous ΔK210 cTnT KI mutation. GSK-3β(+/-) KO also extended the life span of these DCM mice. This study suggests that the inhibition of GSK-3β is cardioprotective in familial DCM associated with ΔK210 cTnT mutation.

Stage-dependent benefits and risks of pimobendan in mice with genetic dilated cardiomyopathy and progressive heart failure

BACKGROUND AND PURPOSE

The Ca(2+) sensitizer pimobendan is a unique inotropic agent that improves cardiac contractility with less of an increase in oxygen consumption and potentially fewer adverse effects on myocardial remodelling and arrhythmia, compared with traditional inotropes. However, clinical trials report contradictory effects of pimobendan in patients with heart failure (HF). We provide mechanistic experimental evidence of the efficacy of pimobendan using a novel mouse model of progressive HF.

EXPERIMENTAL APPROACH

A knock-in mouse model of human genetic dilated cardiomyopathy, which shows a clear transition from compensatory to end-stage HF at a fixed time during growth, was used to evaluate the efficacy of pimobendan and explore the underlying molecular and cellular mechanisms.

KEY RESULTS

Pimobendan prevented myocardial remodelling in compensated HF and significantly extended life span in both compensated and end-stage HF, but dose-dependently increased sudden death in end-stage HF. In cardiomyocytes isolated from end-stage HF mice, pimobendan induced triggered activity probably because of early or delayed afterdepolarizations. The L-type Ca(2+) channel blocker verapamil decreased the incidence of triggered activity, suggesting that this was from over-elevated cytoplasmic Ca(2+) through increased Ca(2+) entry by PDE3 inhibition under diminished sarcoplasmic reticulum Ca(2+) reuptake and increased Ca(2+) leakage from sarcoplasmic reticulum in end-stage HF.

CONCLUSIONS AND IMPLICATIONS

Pimobendan was beneficial regardless of HF stage, but increased sudden cardiac death in end-stage HF with extensive remodelling of Ca(2+) handling. Reduction of cytoplasmic Ca(2+) elevated by PDE3 inhibition might decrease this risk of sudden cardiac death.

Survival benefit of ghrelin in the heart failure due to dilated cardiomyopathy

Although ghrelin has been demonstrated to improve cardiac function in heart failure, its therapeutic efficacy on the life expectancy remains unknown. We aim to examine whether ghrelin can improve the life survival in heart failure using a mouse model of inherited dilated cardiomyopathy (DCM) caused by a deletion mutation ΔK210 in cardiac troponin T (cTnT). From 30 days of age, ghrelin (150 μg/kg) was administered subcutaneously to DCM mice once daily, control mice received saline only. The survival rates were compared between the two groups for 30 days. After 30-day treatment, functional and morphological measurements were conducted. Ghrelin-treated DCM mice had significantly prolonged life spans compared with saline-treated control DCM mice. Echocardiography showed that ghrelin reduced left ventricular (LV) end-diastolic dimensions and increased LV ejection fraction. Moreover, histoanatomical data revealed that ghrelin decreased the heart-to-body weight ratio, prevented cardiac remodeling and fibrosis, and markedly decreased the expression of brain natriuretic peptide. Telemetry recording and heart rate variability analysis showed that ghrelin suppressed the excessive cardiac sympathetic nerve activity (CSNA) and recovered the cardiac parasympathetic nerve activity. These results suggest that ghrelin has therapeutic benefits for survival as well as for the cardiac function and remodeling in heart failure probably through suppression of CSNA and recovery of cardiac parasympathetic nerve activity.

In vivo effects of propyl gallate, a novel Ca(2+) sensitizer, in a mouse model of dilated cardiomyopathy caused by cardiac troponin T mutation

AIMS

We have previously demonstrated that propyl gallate has a Ca(2+) sensitizing effect on the force generation in membrane-permeabilized (skinned) cardiac muscle fibers. However, in vivo beneficial effects of propyl gallate as a novel Ca(2+) sensitizer remain uncertain. In the present study, we aim to explore in vivo effects of propyl gallate.

MAIN METHODS

We compared effects of propyl gallate on ex vivo intact cardiac muscle fibers and in vivo hearts in healthy mice with those of pimobendan, a clinically used Ca(2+) sensitizer. The therapeutic effect of propyl gallate was investigated using a mouse model of dilated cardiomyopathy (DCM) with reduced myofilament Ca(2+) sensitivity due to a deletion mutation ΔK210 in cardiac troponin T.

KEY FINDINGS

Propyl gallate, as well as pimobendan, showed a positive inotropic effect. Propyl gallate slightly increased the blood pressure without changing the heart rate in healthy mice, whereas pimobendan decreased the blood pressure probably through vasodilation via inhibition of phosphodiesterase and increased the heart rate. Propyl gallate prevented cardiac remodeling and systolic dysfunction and significantly improved the life-expectancy of knock-in mouse model of DCM with reduced myofilament Ca(2+) sensitivity due to a mutation in cardiac troponin T. On the other hand, gallate, a similarly strong antioxidant polyphenol lacking Ca(2+) sensitizing action, had no beneficial effects on the DCM mice.

SIGNIFICANCE

These results suggest that propyl gallate might be useful for the treatment of inherited DCM caused by a reduction in the myofilament Ca(2+) sensitivity.

Role of brain serotonin dysfunction in the pathophysiology of congestive heart failure

Inherited or non-inherited dilated cardiomyopathy (DCM) patients develop varied disease phenotypes leading to death after developing congestive heart failure (HF) or sudden death with mild or no overt HF symptoms, suggesting that environmental and/or genetic factors may modify the disease phenotype of DCM. In this study, we sought to explore unknown genetic factors affecting the disease phenotype of monogenic inherited human DCM. Knock-in mice bearing a sarcomeric protein mutation that causes DCM were created on different genetic backgrounds; BALB/c and C57Bl/6. DCM mice on the BALB/c background showed cardiac enlargement and systolic dysfunction and developed congestive HF before died. In contrast, DCM mice on the C57Bl/6 background developed no overt HF symptoms and died suddenly, although they showed considerable cardiac enlargement and systolic dysfunction. BALB/c mice have brain serotonin dysfunction due to a single nucleotide polymorphism (SNP) in tryptophan hydroxylase 2 (TPH2). Brain serotonin dysfunction plays a critical role in depression and anxiety and BALB/c mice exhibit depression- and anxiety-related behaviors. Since depression is common and associated with poor prognosis in HF patients, we examined therapeutic effects of anti-depression drug paroxetine and anti-anxiety drug buspirone that could improve the brain serotonin function in mice. Both drugs reduced cardiac enlargement and improved systolic dysfunction and symptoms of severe congestive HF in DCM mice on the BALB/c background. These results strongly suggest that genetic backgrounds involving brain serotonin dysfunction, such as TPH2 gene SNP, may play an important role in the development of congestive HF in DCM.

Hypertrophic cardiomyopathy in childhood

Hypertrophic cardiomyopathy has important differences in children compared with adults, particularly with regard to the range of causes and the outcomes in infants. Survival is highly dependent on etiology, particularly in the youngest patients, and pursuit of the specific cause is therefore necessary. The clinical utility of defining the genotype in children with familial hypertrophic cardiomyopathy exceeds that at other ages and has a highly favorable cost/benefit ratio. Although most of the available information concerning treatment and prevention of sudden death is derived in adults, management of children requires consideration of the differences in age-specific risk/benefit ratios.

Role of phosphodiesterases in adult-onset pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is characterized by increased mean pulmonary artery pressure (mPAP) due to vasoconstriction and structural changes in the small pulmonary arteries (PAs); proliferation of pulmonary artery smooth muscle cells (PASMCs) contributes to the remodeling. The abnormal pathophysiology in the pulmonary vasculature relates to decreased cyclic nucleotide levels in PASMCs. Phosphodiesterases (PDEs) catalyze the hydrolysis of cAMP and cGMP, thereby PDE inhibitors are effective in vasodilating the PA and decreasing PASMC proliferation. Experimental studies support the use of PDE3, PDE5, and PDE1 inhibitors in PAH. PDE5 inhibitors such as sildenafil are clinically approved to treat different forms of PAH and lower mPAP, increase functional capacity, and decrease right ventricular hypertrophy, without decreasing systemic arterial pressure. New evidence suggests that the combination of PDE inhibitors with other therapies for PAH may be beneficial in treating the disease. Furthermore, inhibiting PDEs in the heart and the inflammatory cells that infiltrate the PA may offer new targets to reduce right ventricular hypertrophy and inhibit inflammation that is associated with and contributes to the development of PAH. This chapter summarizes the advances in the area and the future of PDEs in PAH.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Hypertrophic cardiomyopathy: from genetics to treatment

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is the prototypic form of pathological cardiac hypertrophy. HCM is an important cause of sudden cardiac death in the young and a major cause of morbidity in the elderly.

DESIGN

We discuss the clinical implications of recent advances in the molecular genetics of HCM.

RESULTS

The current diagnosis of HCM is neither adequately sensitive nor specific. Partial elucidation of the molecular genetic basis of HCM has raised interest in genetic-based diagnosis and management. Over a dozen causal genes have been identified. MYH7 and MYBPC3 mutations account for about 50% of cases. The remaining known causal genes are uncommon and some are rare. Advances in DNA sequencing techniques have made genetic screening practical. The difficulty, particularly in the sporadic cases and in small families, is to discern the causal from the non-causal variants. Overall, the causal mutations alone have limited implications in risk stratification and prognostication, as the clinical phenotype arises from complex and often non-linear interactions between various determinants.

CONCLUSIONS

The clinical phenotype of 'HCM' results from mutations in sarcomeric proteins and subsequent activation of multiple cellular constituents including signal transducers. We advocate that HCM, despite its current recognition and management as a single disease entity, involves multiple partially independent mechanisms, despite similarity in the ensuing phenotype. To treat HCM effectively, it is necessary to delineate the underlying fundamental mechanisms that govern the pathogenesis of the phenotype and apply these principles to the treatment of each subset of clinically recognized HCM.

Rescue of familial cardiomyopathies by modifications at the level of sarcomere and Ca2+ fluxes

Cardiomyopathies are a heterogeneous group of diseases of the myocardium associated with mechanical and/or electrical dysfunction that frequently show inappropriate ventricular hypertrophy or dilation. Current data suggest that numerous mutations in several genes can cause cardiomyopathies, and the severity of their phenotypes is also influenced by modifier genes. Two major types of inherited cardiomyopathies include familial hypertrophic cardiomyopathy (FHC) and dilated cardiomyopathy (DCM). FHC typically involves increased myofilament Ca(2+) sensitivity associated with diastolic dysfunction, whereas DCM often results in decreased myofilament Ca(2+) sensitivity and systolic dysfunction. Besides alterations in myofilament Ca(2+) sensitivity, alterations in the levels of Ca(2+)-handling proteins have also been described in both diseases. Recent work in animal models has attempted to rescue FHC and DCM via modifications at the myofilament level, altering Ca(2+) homeostasis by targeting Ca(2+)-handling proteins, such as the sarcoplasmic reticulum ATPase and phospholamban, or by interfering with the products of different modifiers genes. Although attempts to rescue cardiomyopathies in animal models have shown great promise, further studies are needed to validate these strategies in order to provide more effective and specific treatments.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

Experimental therapies in hypertrophic cardiomyopathy

The quintessential clinical diagnostic phenotype of human hypertrophic cardiomyopathy (HCM) is primary cardiac hypertrophy. Cardiac hypertrophy is also a major determinant of mortality and morbidity including the risk of sudden cardiac death (SCD) in patients with HCM. Reversal and attenuation of cardiac hypertrophy and its accompanying fibrosis is expected to improve morbidity as well as decrease the risk of SCD in patients with HCM.The conventionally used pharmacological agents in treatment of patients with HCM have not been shown to reverse or attenuate established cardiac hypertrophy and fibrosis. An effective treatment of HCM has to target the molecular mechanisms that are involved in the pathogenesis of the phenotype. Mechanistic studies suggest that cardiac hypertrophy in HCM is secondary to activation of various hypertrophic signaling molecules and, hence, is potentially reversible. The hypothesis is supported by the results of genetic and pharmacological interventions in animal models. The results have shown potential beneficial effects of angiotensin II receptor blocker losartan, mineralocorticoid receptor blocker spironolactone, 3-hydroxy-3-methyglutaryl-coenzyme A reductase inhibitors simvastatin and atorvastatin, and most recently, N-acetylcysteine (NAC) on reversal or prevention of hypertrophy and fibrosis in HCM. The most promising results have been obtained with NAC, which through multiple thiol-responsive mechanisms completely reversed established cardiac hypertrophy and fibrosis in three independent studies. Pilot studies with losartan and statins in humans have established the feasibility of such studies. The results in animal models have firmly established the reversibility of established cardiac hypertrophy and fibrosis in HCM and have set the stage for advancing the findings in the animal models to human patients with HCM through conducting large-scale efficacy studies.

Therapeutic effect of {beta}-adrenoceptor blockers using a mouse model of dilated cardiomyopathy with a troponin mutation

AIMS

Extensive clinical studies have demonstrated that beta-adrenoceptor blocking agents (beta-blockers) are beneficial in the treatment of chronic heart failure, which is due to various aetiologies, including idiopathic dilated cardiomyopathy (DCM) and ischaemic heart disease. However, little is known about the therapeutic efficacy of beta-blockers in the treatment of the inherited form of DCM, of which causative mutations have recently been identified in various genes, including those encoding cardiac sarcomeric proteins. Using a mouse model of inherited DCM with a troponin mutation, we aim to study the treatment benefits of beta-blockers.

METHODS AND RESULTS

Three different types of beta-blockers, carvedilol, metoprolol, and atenolol, were orally administered to a knock-in mouse model of inherited DCM with a deletion mutation DeltaK210 in the cardiac troponin T gene (TNNT2). Therapeutic effects were examined on the basis of survival and myocardial remodelling. The lipophilic beta(1)-selective beta-blocker metoprolol was found to prevent cardiac dysfunction and remodelling and extend the survival of knock-in mice. Conversely, both the non-selective beta-blocker carvedilol and the hydrophilic beta(1)-selective beta-blocker atenolol had no beneficial effects on survival and myocardial remodelling in this mouse model of inherited DCM.

CONCLUSION

The highly lipophilic beta(1)-selective beta-blocker metoprolol, known to prevent ventricular fibrillation via central nervous system-mediated vagal activation, may be especially beneficial to DCM patients showing a family history of frequent sudden cardiac death, such as those with a deletion mutation DeltaK210 in the TNNT2 gene.

Role of phosphodiesterases in modulation of BKCa channels in hypertensive pulmonary arterial smooth muscle

BKCa channels regulate pulmonary arterial pressure, and protein kinase C (PKC) inhibits BK(Ca) channels, but little is known about PKC-mediated modulation of BKCa channel activity in pulmonary arterial smooth muscle. Studies were carried out to determine mechanisms of PKC modulation of BKCa channel activity in pulmonary arterial smooth muscle cells (PASMC) of the fawn-hooded rat (FHR), an animal model of pulmonary hypertension. Forskolin opened BKCa channels in FHR PASMC, which was blocked by PKC activation, and reversed by the phosphodiesterase (PDE) inhibitors IBMX, milrinone, and zaprinast. PDE inhibition also blocked the vasoconstrictor response to PKC activation in FHR pulmonary arteries. These results indicate that PKC inhibits cAMP-induced activation of BKCa channels and causes pulmonary vasoconstriction in hypertensive pulmonary arterial smooth muscle via PDE, which further suggests PDE inhibitors for treatment of pulmonary hypertension.

Downregulation of angiopoietin-1 and Tie2 in chronic hypoxic pulmonary hypertension

BACKGROUND

Angiopoietins, newly discovered vascular-specific growth factors, and vascular endothelial growth factors (VEGF) play distinct and complementary roles in angiogenesis and vascular maturation. However, the exact roles of angiogenic factors in the adult pulmonary vasculature remain unclear.

OBJECTIVE

To elucidate possible roles of angiopoietins and VEGF in the development of hypoxic pulmonary hypertension (PH), changes in the expression of angiogenic factors were examined.

METHODS

The cellular distribution and expression of angiopoietins and their receptor Tie2 and VEGF were investigated by RT-PCR, immunoblot, and immunohistochemical methods in rat lung under normal and hypoxic conditions.

RESULTS

During the development of PH with vascular remodeling characterized by a decrease in vessel density of intrapulmonary arteries, protein expression of angiopoietin-1 (Ang-1), Tie2, and VEGF significantly decreased in the pulmonary arteries, and Tie2 receptor was inactivated in the lung. The expression of angiopoietin-3 (Ang-3), an endogenous antagonist of Ang-1, significantly increased in the intima under hypoxic conditions.

CONCLUSIONS

Since both Ang-1/Tie2 and VEGF promote angiogenesis and vascular survival, and play protective roles in the adaptation of microvascular changes during the onset of PH, the downregulation of both Ang-1/Tie2 and VEGF and upregulation of Ang-3 appear to be associated with vascular rarefaction and the development of hypoxic PH.

BAY 41-2272, a direct activator of soluble guanylate cyclase, reduces right ventricular hypertrophy and prevents pulmonary vascular remodeling during chronic hypoxia in neonatal rats

Exposure to hypoxia during the first weeks of life in newborn rats decreases vascular growth and alveolarization and causes pulmonary hypertension (PH). BAY 41-2272 is a novel direct activator of soluble guanylate cyclase independent of nitric oxide, effective as an acute pulmonary vasodilator in an animal model of persistent pulmonary hypertension of the newborn, but whether prolonged BAY 41-2272 therapy is effective in the setting of chronic PH is unknown. We hypothesize that BAY 41-2272 would prevent PH induced by chronic exposure to neonatal hypoxia. At 2 days of age, newborn rats were randomly exposed to hypoxia (FiO2, 0.12) or room air, and received daily intramuscular treatment with BAY 41-2272 (1 mg/kg) or saline. After 2 weeks, rats were killed for assessment of right ventricular hypertrophy (RVH), wall thickness of small pulmonary arteries, vessels density, radial alveolar counts and mean linear intercepts. In comparison with control, hypoxia increased RVH and artery wall thickness, reduced vessels density, decreased radial alveolar counts and increased mean linear intercepts. In comparison with hypoxic controls, prolonged BAY 41-2272 treatment during chronic hypoxia reduced RVH (0.67 +/- 0.03 vs. 0.52 +/- 0.05; p < 0.05), and attenuated artery wall thickness (48.2 +/- 2.8% vs. 35.7 +/- 4.1 microm; p < 0.01). However, BAY 41-2272 did not change vessels density, radial alveolar counts or mean linear intercepts. We conclude that BAY 41-2272 prevents the vascular structural effects of PH and reduces RVH but does not protect from hypoxia-induced inhibition of alveolarization and vessel growth. We speculate that BAY 41-2272 may provide a new therapy for chronic PH.

Genistein, a Phytoestrogen, Attenuates Monocrotaline-Induced Pulmonary Hypertension

BACKGROUND

Pulmonary hypertension is characterized by high pulmonary blood pressure, vascular remodeling, and right ventricular hypertrophy. Although recent studies suggest that an imbalance between endothelial mediators on pulmonary vasculature may contribute to the development of pulmonary hypertension, the pathogenesis is not fully understood and the treatment of pulmonary hypertension is still unresolved.

OBJECTIVE

The purpose of this study was to investigate whether genistein, a phytoestrogen derived from soybean, would prevent the development of monocrotaline (MCT)-induced pulmonary hypertension in rats. Hemodynamic parameters of catheterized rats and morphological feature of lungs were evaluated among MCT-treated rats receiving or not receiving genistein. Furthermore, examination of expression in endothelial nitric oxide synthase and endothelin-1 peptide level was performed.

METHODS

Daily supplementation with either genistein (0.2 mg/kg) or vehicle was started 2 days prior to a single-dose injection of MCT (60 mg/kg). On day 28, rats underwent catheterization, and right ventricular hypertrophy and morphological features were assessed. Furthermore, endothelial nitric oxide synthase and endothelin-1 were examined by Western blot analysis and radioimmunoassay, respectively, in homogenated lungs.

RESULTS

In rats that received daily supplementation of genistein, mean pulmonary arterial pressure was significantly reduced, whereas mean systemic arterial pressure and heart rate were unaltered compared with MCT control rats on day 28 after MCT injection. Right ventricular hypertrophy, medial wall thickness of pulmonary arteries corresponding to the terminal bronchioles, and the degree of neo-muscularization of more distal arteries were less severe in genistein-treated rats. Genistein supplementation improved MCT-induced downregulation of expression of endothelial nitric oxide synthase in the lungs. However, endothelin-1 peptide levels did not differ among all groups of lungs.

CONCLUSIONS

We conclude that daily supplementation of genistein potently attenuates MCT-induced pulmonary hypertension, right ventricular hypertrophy, and pulmonary vascular remodeling in rats. The underlying mechanism responsible for this effect may be partly related to the restoration of a decreased expression of endothelial nitric oxide synthase.

Drastic Ca2+ sensitization of myofilament associated with a small structural change in troponin I in inherited restrictive cardiomyopathy

Six missense mutations in human cardiac troponin I (cTnI) were recently found to cause restrictive cardiomyopathy (RCM). We have bacterially expressed and purified these human cTnI mutants and examined their functional and structural consequences. Inserting the human cTnI into skinned cardiac muscle fibers showed that these mutations had much greater Ca2+-sensitizing effects on force generation than the cTnI mutations in hypertrophic cardiomyopathy (HCM). The mutation K178E in the second actin-tropomyosin (Tm) binding region showed a particularly potent Ca2+-sensitizing effect among the six RCM-causing mutations. Circular dichroism and nuclear magnetic resonance spectroscopy revealed that this mutation does not extensively affect the structure of the whole cTnI molecule, but induces an unexpectedly subtle change in the structure of a region around the mutated residue. The results indicate that the K178E mutation has a localized effect on a structure that is critical to the regulatory function of the second actin-Tm binding region of cTnI. The present study also suggests that both HCM and RCM involving cTnI mutations share a common feature of increased Ca2+ sensitivity of cardiac myofilament, but more severe change in Ca2+ sensitivity is associated with the clinical phenotype of RCM.

Lung vasodilatory response to inhaled iloprost in experimental pulmonary hypertension: amplification by different type phosphodiesterase inhibitors

Inhaled prostanoids and phosphodiesterase (PDE) inhibitors have been suggested for treatment of severe pulmonary hypertension. In catheterized rabbits with acute pulmonary hypertension induced by continuous infusion of the stable thromboxane analogue U46619, we asked whether sildenafil (PDE1/5/6 inhibitor), motapizone (PDE3 inhibitor) or 8-Methoxymethyl-IBMX (PDE1 inhibitor) synergize with inhaled iloprost. Inhalation of iloprost caused a transient pulmonary artery pressure decline, levelling off within <20 min, without significant changes in blood gases or systemic hemodynamics. Infusion of 8-Methoxymethyl-IBMX, motapizone and sildenafil caused each a dose-dependent decrease in pulmonary artery pressure, with sildenafil possessing the highest efficacy and at the same time selectivity for the pulmonary circulation. When combining a per se ineffective dose of each PDE inhibitor (200 μg/kg × min 8-Methoxymethyl-IBMX, 1 μg/kg × min sildenafil, 5 μg/kg × min motapizone) with subsequent iloprost nebulization, marked amplification of the prostanoid induced pulmonary vasodilatory response was noted and the area under the curve of P_PA reduction was nearly threefold increased with all approaches, as compared to sole iloprost administration. Further amplification was achieved with the combination of inhaled iloprost with sildenafil plus motapizone, but not with sildenafil plus 8MM-IBMX. Systemic hemodynamics and gas exchange were not altered for all combinations. We conclude that co-administration of minute systemic doses of selective PDE inhibitors with inhaled iloprost markedly enhances and prolongs the pulmonary vasodilatory response to inhaled iloprost, with maintenance of pulmonary selectivity and ventilation perfusion matching. The prominent effect of sildenafil may be operative via both PDE1 and PDE5, and is further enhanced by co-application of a PDE3 inhibitor.

Inherited cardiomyopathies as a troponin disease

Troponin, one of the sarcomeric proteins, plays a central role in the Ca(2+) regulation of contraction in vertebrate skeletal and cardiac muscles. It consists of three subunits with distinct structure and function, troponin T, troponin I, and troponin C, and their accurate and complex intermolecular interaction in response to the rapid rise and fall of Ca(2+) in cardiomyocytes plays a key role in maintaining the normal cardiac pump function. More than 200 mutations in the cardiac sarcomeric proteins, including myosin heavy and light chains, actin, troponin, tropomyosin, myosin-binding protein-C, and titin/connectin, have been found to cause various types of cardiomyopathy in human since 1990, and more than 60 mutations in human cardiac troponin subunits have been identified in dilated, hypertrophic, and restrictive forms of cardiomyopathy. In this review, we have focused on the mutations in the genes for human cardiac troponin subunits and discussed their functional consequences that might be involved in the primary mechanisms for the pathogenesis of these different types of cardiomyopathy.

The Delta 14 mutation of human cardiac troponin T enhances ATPase activity and alters the cooperative binding of S1-ADP to regulated actin

The complex of tropomyosin and troponin binds to actin and inhibits activation of myosin ATPase activity and force production of striated muscles at low free Ca(2+) concentrations. Ca(2+) stimulates ATP activity, and at subsaturating actin concentrations, the binding of NEM-modified S1 to actin-tropomyosin-troponin increases the rate of ATP hydrolysis even further. We show here that the Delta14 mutation of troponin T, associated with familial hypertrophic cardiomyopathy, results in an increase in ATPase rate like that seen with wild-type troponin in the presence of NEM-S1. The enhanced ATPase activity was not due to a decreased incorporation of mutant troponin T with troponin I and troponin C to form an active troponin complex. The activating effect was more prominent with a hybrid troponin (skeletal TnI, TnC, and cardiac TnT) than with all cardiac troponin. Thus it appears that changes in the troponin-troponin contacts that result from mutations or from forming hybrids stabilize a more active state of regulated actin. An analysis of the effect of the Delta14 mutation on the equilibrium binding of S1-ADP to actin was consistent with stabilization of an active state of actin. This change in activation may be important in the development of cardiac disease.

Synergistic effects of ANP and sildenafil on cGMP levels and amelioration of acute hypoxic pulmonary hypertension

We hypothesized that the phosphodiesterase 5 inhibitor, sildenafil, and the guanosine cyclase stimulator, atrial natriuretic peptide (ANP), would act synergistically to increase cGMP levels and blunt hypoxic pulmonary hypertension in rats, because these compounds act via different mechanisms to increase the intracellular second messenger. Acute hypoxia: Adult Sprague-Dawley rats were gavaged with sildenafil (1 mg/ kg) or vehicle and exposed to acute hypoxia with and without ANP (10(-8)-10(-5) M ). Sildenafil decreased systemic blood pressure (103 +/- 10 vs. 87 +/- 6 mm Hg, P < 0.001) and blunted the hypoxia-induced increase in right ventricular systolic pressure (RVSP; percent increase 73.7% +/- 9.4% in sildenafil-treated rats vs. 117.2% +/- 21.1% in vehicle-treated rats, P = 0.03). Also, ANP and sildenafil had synergistic effects on blunting the hypoxia-induced increase in RVSP (P < 0.001) and on rising plasma cGMP levels (P < 0.05). Chronic hypoxia: Other rats were exposed to prolonged hypoxia (3 weeks, 0.5 atm) after subcutaneous implantation of a sustained-release pellet containing lower (2.5 mg), or higher (25 mg) doses of sildenafil, or placebo. Higher-dose, but not lower-dose sildenafil blunted the chronic hypoxia-induced increase in RVSP (P = 0.006). RVSP and plasma sildenafil levels were inversely correlated in hypoxic rats (r(2) = 0.68, P = 0.044). Lung cGMP levels were increased by both chronic hypoxia and sildenafil, with the greatest increase achieved by the combination. Plasma and right ventricular (RV) cGMP levels were increased by hypoxia, but sildenafil had no effect. RV hypertrophy and pulmonary artery muscularization were also unaffected by sildenafil. In conclusion, sildenafil and ANP have synergistic effects on the blunting of hypoxia-induced pulmonary vasoconstriction. During chronic hypoxia, sildenafil normalizes RVSP, but in the doses used, sildenafil has no effect on RV hypertrophy or pulmonary vascular remodeling.

DA-8159, a new PDE5 inhibitor, attenuates the development of compensatory right ventricular hypertrophy in a rat model of pulmonary hypertension

This study evaluated the effect of DA-8159, a new phosphodiesterase 5 inhibitor, on the compensatory development of right ventricular hypertrophy in monocrotaline (MCT)-induced pulmonary hypertension (PH). Rats treated with subcutaneous MCT were divided into three groups, which received DA-8159 1 mg/kg, DA-8159 5 mg/kg or saline-vehicle orally, twice daily for 21 days. The vehicle group demonstrated increased right ventricular weight, pulmonary artery medial wall thickening, myocardial fibrosis, increased plasma cyclic guanosine monophosphate (cGMP) concentration and reduced body weight gains. DA-8159, however, markedly attenuated the compensatory development of right ventricular hypertrophy and pulmonary artery medial wall thickening, amplified the increase in plasma cGMP levels and increased lung cGMP concentrations. In addition, DA-8159 prevented myocardial fibrosis induced by MCT. These results demonstrate that DA-8159 attenuates the compensatory development of right ventricular hypertrophy in a rate model of PH. DA-8159 might, therefore, be a useful treatment option for PH, but its efficacy in humans needs evaluating.

Cardiac troponin T mutation R141W found in dilated cardiomyopathy stabilizes the troponin T-tropomyosin interaction and causes a Ca2+ desensitization

A missense mutation R141W in the strong tropomyosin-binding region of cardiac troponin T (cTnT) has recently been reported to cause dilated cardiomyopathy (DCM), following the first report of a DCM-causing deletion mutation DeltaK210. To clarify the molecular mechanism for the pathogenesis of DCM caused by this novel mutation in cTnT gene, functional analyses were made on the recombinant human cTnT mutant proteins. Exchanging human wild-type and mutant cTnTs into rabbit skinned cardiac muscle fibers revealed that R141W mutation resulted in a decrease in the Ca(2+) sensitivity of force generation, as in the case of DeltaK210 mutation lying outside the strong tropomyosin-binding region. In contrast, a missense mutation R94L in the vicinity of the strong tropomyosin-binding region associated with hypertrophic cardiomyopathy (HCM) resulted in an increase in the Ca(2+) sensitivity of force generation, as in the case of the other HCM-causing mutations in cTnT reported previously. An assay using a quartz-crystal microbalance (a very sensitive mass-measuring device) revealed that R141W mutation increased the affinity of cTnT for alpha-tropomyosin by approximately three times, whereas an HCM-causing mutation DeltaE160 in the strong tropomyosin-binding region, as well as DeltaK210 and R94L mutations, had no effects on the interaction between cTnT and alpha-tropomyosin. Since cTnT has an important role in structurally integrating cardiac troponin I (cTnI) into the thin filaments via its two-way interactions with cTnI and tropomyosin, the present results suggest that R141W mutation in the strong tropomyosin-binding region in cTnT strengthens the integrity of cTnI in the thin filament by stabilizing the interaction between cTnT and tropomyosin, which might allow cTnI to inhibit the thin filament more effectively, leading to a Ca(2+) desensitization.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Expression of cardiac troponin T with COOH-terminal truncation accelerates cross-bridge interaction kinetics in mouse myocardium

Transgenic mice expressing an allele of cardiac troponin T (cTnT) with a COOH-terminal truncation (cTnT(trunc)) exhibit severe diastolic and mild systolic dysfunction. We tested the hypothesis that contractile dysfunction in myocardium expressing low levels of cTnT(trunc) (i.e., <5%) is due to slowed cross-bridge kinetics and reduced thin filament activation as a consequence of reduced cross-bridge binding. We measured the Ca(2+) sensitivity of force development [pCa for half-maximal tension generation (pCa(50))] and the rate constant of force redevelopment (k(tr)) in cTnT(trunc) and wild-type (WT) skinned myocardium both in the absence and in the presence of a strong-binding, non-force-generating derivative of myosin subfragment-1 (NEM-S1). Compared with WT mice, cTnT(trunc) mice exhibited greater pCa(50), reduced steepness of the force-pCa relationship [Hill coefficient (n(H))], and faster k(tr) at submaximal Ca(2+) concentration ([Ca(2+)]), i.e., reduced activation dependence of k(tr). Treatment with NEM-S1 elicited similar increases in pCa(50) and similar reductions in n(H) in WT and cTnT(trunc) myocardium but elicited greater increases in k(tr) at submaximal activation in cTnT(trunc) myocardium. Contrary to our initial hypothesis, cTnT(trunc) appears to enhance thin filament activation in myocardium, which is manifested as significant increases in Ca(2+)-activated force and the rate of cross-bridge attachment at submaximal [Ca(2+)]. Although these mechanisms would not be expected to depress systolic function per se in cTnT(trunc) hearts, they would account for slowed rates of myocardial relaxation during early diastole.

DA-8159, a potent cGMP phosphodiesterase inhibitor, attenuates monocrotaline-induced pulmonary hypertension in rats

In this study, we evaluated the effects of oral administration of DA-8159, a selective phosphodiesterase-5 inhibitor, on the development of pulmonary hypertension (PH) induced by monocrotaline (MCT). Rats were administered either MCT (60 mg/kg) or saline. MCT-treated rats were divided into three groups and received orally administered vehicle, or 1 mg/kg or 5 mg/kg of DA-8159, twice a day for twenty-one days. The MCT group demonstrated increased right ventricular weights, medial wall thickening in the pulmonary arteries, myocardial fibrosis and the level of plasma cyclic guanosine monophosphate (cGMP), along with decreased body weight gains. However, DA-8159 markedly and dose-dependently reduced the development of right ventricular hypertrophy and medial wall thickening. DA-8159 also amplified the increase in plasma cGMP level and significantly increased the level of lung cGMP, compared with the MCT group. Although the body weight gain was still lower from the saline-treated control group, DA-8159 demonstrated a significant increase in body weight gains, in both 1 mg/kg and 5 mg/kg groups, when compared with the MCT group. In myocardial morphology, MCT-induced myocardial fibrosis was markedly prevented by DA-8159. These results suggest that DA-8159 may be a useful oral treatment option for PH.

Phosphodiesterase type 5 as a target for the treatment of hypoxia-induced pulmonary hypertension

BACKGROUND

Phosphodiesterase type 5 (PDE5) is a novel therapeutic target for the treatment of pulmonary hypertension. This study examined the distribution of PDE5 in normal and hypoxic lung and the effect of chronic PDE5 inhibition with sildenafil, initiated before and during exposure to hypoxia, on pulmonary artery pressure (PAP) and structure.

METHODS AND RESULTS

Sprague-Dawley rats were exposed to hypoxia (10% O2) for up to 42 days. PAP, measured continuously by telemetry, increased gradually by 20 to 40 mm Hg, reaching a plateau between 10 and 14 days, and declined to normal levels on return to normoxia. PDE5 immunoreactivity was localized to smooth muscle cells in the medial layer of pulmonary arteries and veins in the normal lung and in distal muscularized arteries (<25 microm diameter) after hypoxia-induced pulmonary hypertension. Sildenafil (25 or 75 mg x kg(-1) x d(-1)) given before hypoxia produced marked dose-dependent inhibition in the rise of PAP (60% to 90% reduction; P<0.0001) and vascular muscularization (28.4+/-5.0% reduction; P<0.001). When begun after 14 days of hypoxia, sildenafil significantly reduced PAP (30% reduction; P<0.0001) and partially reversed pulmonary artery muscularization (39.9+/-4.9% reduction; P<0.001).

CONCLUSIONS

PDE5 is found throughout the muscularized pulmonary vascular tree, including in newly muscularized distal pulmonary arteries exposed to hypoxia. PDE5 inhibition attenuates the rise in PAP and vascular remodeling when given before chronic exposure to hypoxia and when administered as a treatment during ongoing hypoxia-induced pulmonary hypertension.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

Beneficial effects of phosphodiesterase 5 inhibition in pulmonary hypertension are influenced by natriuretic Peptide activity

BACKGROUND

Phosphodiesterase type 5 (PDE5) inhibitors (eg, sildenafil) are a novel, orally active approach to the treatment of pulmonary arterial hypertension. The role of natriuretic peptides in the response to sildenafil was examined in mice lacking NPR-A, a guanylyl cyclase-linked natriuretic peptide receptor, in which pulmonary hypertension was induced by hypoxia.

METHODS AND RESULTS

Mice homozygous for NPR-A (NPR-A+/+) and null mutants (NPR-A-/-) were studied. Sildenafil inhibited the pressor response to acute hypoxia in the isolated perfused lungs of both genotypes. This effect was greater in the presence of atrial natriuretic peptide in the perfusate in NPR-A+/+ mice but not NPR-A-/- animals. In vivo, NPR-A mutants had higher basal right ventricular (RV) systolic pressures (RVSPs) than did NPR-A+/+ mice, and this was not affected by 3 weeks of treatment with sildenafil (25 mg x kg(-1) x d(-1)). Both genotypes exhibited a rise in RVSP and RV weight with chronic hypoxia (10% O2 for 21 days); RVSP and RV weight were reduced by continuous sildenafil administration in NPR-A+/+ mice, but only RVSP showed evidence of a response to the drug in NPR-A-/- mice. The effect of sildenafil on hypoxia-induced pulmonary vascular muscularization and cyclic GMP levels was also blunted in NPR-A-/- mice.

CONCLUSIONS

The natriuretic peptide pathway influences the response to PDE5 inhibition in hypoxia-induced pulmonary hypertension, particularly its effects on RV hypertrophy and vascular remodeling.

Acute and chronic effects of T-1032, a novel selective phosphodiesterase type 5 inhibitor, on monocrotaline-induced pulmonary hypertension in rats

We examined the hemodynamic property of T-1032 (methyl 2-(4-aminophenyl)-1,2-dihydro-1-oxo-7-(2-pyridylmethoxy)-4-(3,4,5-trimethoxy-phenyl)-3-isoquinoline carboxylate sulfate), a novel selective phosphodiesterase type 5 (PDE5) inhibitor, and evaluated the chronic effect of T-1032 on cardiac remodeling and its related death in monocrotaline (MCT)-induced pulmonary hypertensive rats. T-1032 (1, 10, 100 micro g/kg, i.v.) significantly reduced mean arterial pressure (MAP) and right ventricular systolic pressure (RVSP) without a change in heart rate. The change in RVSP was more potent than that in MAP with 1 micro g/kg T-1032 treatment (RVSP: -8.2+/-1.2%, mean arterial pressure: -5.7+/-1.2%), and reductions in RVSP and MAP reached a peak at doses of 1 and 10 micro g/kg, respectively. In contrast, nitroglycerin (0.1, 1, 10 micro g/kg, i.v.) and beraprost (0.1, 1 micro g/kg, i.v.) did not cause a selective reduction in RVSP at any dose. When T-1032 (300 ppm in diet) was chronically administered, it delayed the death, and significantly suppressed right ventricular remodeling (T-1032-treated: 0.318+/-0.021 g, control: 0.401+/-0.013 g, p<0.05). Our present results suggest that T-1032 selectively reduces RVSP, and resulting in the suppression of right ventricular remodeling with a delay of the death in MCT-induced pulmonary hypertensive rats.

Relaxant effects of selective phosphodiesterase inhibitors on U46619 precontracted human intralobar pulmonary arteries and role of potassium channels

We examined the influence of K+ channel antagonists on the vasorelaxation induced by theophylline (non selective PDEI), siguazodan (PDE3I), rolipram (PDE4I) and zaprinast (PDE5I) in human intralobar pulmonary arteries. All PDEI tested induced a concentration-dependent relaxation with theophylline being significantly (p < 0.05) more efficient and rolipram more potent than PDE5I and PDE3I (Emax values, expressed as a percentage of maximal relaxation by papaverine 10(-4)M, were 92% +/- 2%, 84% +/- 8%, 90% +/- 4% and 99% +/- 1%, and pD2 values were 7.30 +/- 0.35, 6.14 +/- 0.25, 5.86 +/- 0.17, and 4.85 +/- 0.47 for rolipram, siguazodan, zaprinast and theophylline, respectively). 4-Aminopyridine (4-AP, Kv, voltage dependent channel blocker, 1 mM) induced a significant increase (+17% p < 0.05) of U46619-induced vasoconstriction whereas the other K+-channels blockers, glibenclamide (KATP channels, 1 microM) charybdotoxin (predominant BKCa, large conductance Ca2+-sensitive K+ channels, 0.1 microM) and apamine (SKCa, small conductance, 0.3 microM) were without effect. The concentration response curves (CRC) for rolipram were significantly shifted to the right by glibenclamide (1 microM), charybdotoxin (0.1 microM) and 4-AP (1 mM). The CRC for siguazodan was significantly displaced to the right by 4-AP. None of the potassium channel blockers displaced the CRC for zaprinast and theophylline. Apamine was without effect on the CRC for all the PDEI used in this study. (1) PDE3, 4 and 5 are functionally present in human intralobar pulmonary arteries; (2) the vasoconstriction induced by U46619 is downregulated by 4-aminopyridine sensitive-K+ channels; (3) the relaxant effects of rolipram (PDE4I) are partly mediated through KATP, BKCa, and Kv potassium channels and those of siguazodan (PDE3I) by Kv potassium channels.

Ca(2+)-desensitizing effect of a deletion mutation Delta K210 in cardiac troponin T that causes familial dilated cardiomyopathy

A deletion mutation Delta K210 in cardiac troponin T (cTnT) was recently found to cause familial dilated cardiomyopathy (DCM). To explore the effect of this mutation on cardiac muscle contraction under physiological conditions, we determined the Ca(2+)-activated force generation in permeabilized rabbit cardiac muscle fibers into which the mutant and wild-type cTnTs were incorporated by using our TnT exchange technique. The free Ca(2+) concentrations required for the force generation were higher in the mutant cTnT-exchanged fibers than in the wild-type cTnT-exchanged ones, with no statistically significant differences in maximal force-generating capability and cooperativity. Exchanging the mutant cTnT into isolated cardiac myofibrils also increased the free Ca(2+) concentrations required for the activation of ATPase. In contrast, a deletion mutation Delta E160 in cTnT that causes familial hypertrophic cardiomyopathy (HCM) decreased the free Ca(2+) concentrations required for force generation, just as in the case of the other HCM-causing mutations in cTnT. The results indicate that cTnT mutations found in the two distinct forms of cardiomyopathy (i.e., HCM and DCM) change the Ca(2+) sensitivity of cardiac muscle contraction in opposite directions. The present study strongly suggests that Ca(2+) desensitization of force generation in sarcomere is a primary mechanism for the pathogenesis of DCM associated with the deletion mutation Delta K210 in cTnT.

Post-beta-receptor impairment in the regulation of myofibrillar Ca2+ sensitivity in tachypacing-induced canine failing heart

Although one of the salient abnormalities in signal transduction of failing myocardium is downregulation of the beta-adrenergic receptor, the extent of presentation of downstream pathways distal to beta-receptors is misunderstood. We addressed this question in tachypacing-induced canine failing heart by assessing changes in myofibrillar Ca2+ sensitivity and troponin I phosphorylation. At a basal state, no significant difference in myofibrillar Ca2+ sensitivity was found between normal and failing hearts. Isoproterenol 8-bromo-cylic adenosine monophosphate (cAMP), and 8-bromo-cAMP isobutylmethylxantine all significantly decreased the Ca2+ sensitivity in the normal, but not in the failing, heart. EMD57033 (10 microM ), a myofibrillar Ca2+ sensitizer increased the Ca2+ sensitivity to a similar extent in both groups. The troponin I phosphorylation levels were significantly decreased in the failing heart. These results suggest that abnormalities of the beta-adrenergic signaling system exist not only at the receptor level but also at downstream steps after cAMP production.

Functional consequences of the mutations in human cardiac troponin I gene found in familial hypertrophic cardiomyopathy

Functional consequences of the six mutations (R145G, R145Q, R162W, DeltaK183, G203S, K206Q) in cardiac troponin I (cTnI) that cause familial hypertrophic cardiomyopathy (HCM) were studied using purified recombinant human cTnI. The missense mutations R145G and R145Q in the inhibitory region of cTnI reduced the intrinsic inhibitory activity of cTnI without changing the apparent affinity for actin. On the other hand, the missense mutation R162W in the second troponin C binding region and the deletion mutation DeltaK183 near the second actin-tropomyosin region reduced the apparent affinity of cTnI for actin without changing the intrinsic inhibitory activity. Ca(2+) titration of a fluorescent probe-labeled human cardiac troponin C (cTnC) showed that only R162W mutation impaired the cTnC-cTnI interaction determining the Ca(2+) affinity of the N-terminal regulatory domain of cTnC. Exchanging the human cardiac troponin into isolated cardiac myofibrils or skinned cardiac muscle fibers showed that the mutations R145G, R145Q, R162W, DeltaK183 and K206Q induced a definite increase in the Ca(2+)-sensitivity of myofibrillar ATPase activity and force generation in skinned muscle fibers. Although the mutation G203S also showed a tendency to increase the Ca(2+) sensitivity in both myofibrils and skinned muscle fibers, no statistically significant difference compared with wild-type cTnI could be detected. These results demonstrated that most of the HCM-linked cTnI mutations did affect the regulatory processes involving the cTnI molecule, and that at least five mutations (R145G, R145Q, R162W, DeltaK183, K206Q) increased the Ca(2+) sensitivity of cardiac muscle contraction.

[Molecular mechanisms of calcium regulation of striated muscle contraction and its genetic disorder]

Contraction of skeletal and cardiac muscles is regulated by Ca2+ through a specific Ca(2+)-receptive protein, troponin, regularly distributed along the thin filaments. This protein consists of three different subunits, troponins C, I and T. In this article, studies on the structural and biochemical aspects of the molecular mechanisms of Ca(2+)-regulation were first reviewed with particular reference to the regulatory role of troponin T. Several properties of the isoforms of troponin from fast and slow skeletal and cardiac muscles were discussed, based on the findings obtained by the use of troponin-exchange techniques under physiological conditions. Recent findings on the functional consequence of mutations in human cardiac troponins T and I found in familial hypertrophic cardiomyopathy were also presented. The results clarified the increase in Ca(2+)-sensitivity of contraction to be the critical consequence due to this genetic disorder.

Hypoxic vasoconstriction of rat main pulmonary artery: role of endogenous nitric oxide, potassium channels, and phosphodiesterase inhibition

This study investigated the influence of NO, potassium (K+) channel blockade, and the phosphodiesterase inhibitors (PDEIs) theophylline (non-selective PDEI), siguazodan (PDE3I), rolipram (PDE4I), and zaprinast (PDE5I) on rat isolated main pulmonary artery hypoxic (95% N2 and 5% CO2) vasoconstriction. Hypoxic vasoconstriction increased by 27% (p < 0.01) in the presence of the NO synthase inhibitor L-NAME (10(-4) M), and by 15% (p < 0.05) in the presence of the K(ATP) channel blocker glibenclamide (10(-6) M), without potentiation by the combination of these two drugs. Hypoxic vasoconstriction decreased by 28% (p < 0.01) in presence of the Kv,-voltage-dependent channel blocker 4-aminopyridine (10(-3) M), whereas the other K+ channel blockers, charybdotoxin (BKCa, large-conductance Ca2+-sensitive K+ channels) and apamin (SKCa, small-conductance Ca2+-sensitive K+ channels) had no effect. The nonselective PDEI theophylline induced a concentration-dependent relaxation (pD2 = 4.05, Emax = 90% [expressed as a percentage of maximal relaxation induced by papaverine 10(-4) M]). Among the selective PDEIs, siguazodan was significantly (p < 0.01) more efficient than rolipram and zaprinast (Emax values were 84%, 67%, and 58%, respectively) and significantly (p < 0.05) more potent than zaprinast (pD2 values were 6.48, 6.34, and 6.16 for siguazodan, rolipram, and zaprinast). Glibenclamide and L-NAME significantly (p < 0.05) shifted the concentration-response curve (CRC) for zaprinast to the right, and L-NAME shifted the CRC significantly to the right for siguazodan. In the presence of L-NAME, glibenclamide had no effect on the CRC of zaprinast. We conclude that (a) NO exerts a permanent inhibitory effect against hypoxic vasoconstriction that might be mediated in part by an activation of K(ATP) channels; (b) a 4-aminopyridine-sensitive K+ channel is involved in vasoconstriction under hypoxic conditions; (c) PDEs 3 and 5 are the predominant PDE isoforms in rat pulmonary artery relaxation; and (d) NO and K(ATP), but neither BK(Ca), SK(Ca), nor Kv channels, are involved in the relaxant effect of PDEIs.

Sildenafil inhibits hypoxia-induced pulmonary hypertension

BACKGROUND

This study investigated the effect of the phosphodiesterase 5 inhibitor sildenafil on the pulmonary vascular response to hypoxia in humans and mice.

METHODS AND RESULTS

In a randomized, double-blind study, sildenafil 100 mg or placebo was given orally to 10 healthy volunteers 1 hour before breathing 11% O(2) for 30 minutes. Pulmonary artery pressure (PAP) was measured with an indwelling right heart catheter. The acute 56% increase in mean PAP produced by hypoxia during placebo treatment (mean PAP [mean+/-SD mm Hg]: normoxia 16.0+/-2.1 versus hypoxia 25.0+/-4.8) was almost abolished by sildenafil (normoxia 16.0+/-2.1 versus hypoxia 18.0+/-3.6), with no significant effect on systemic blood pressure. In the isolated perfused lung of wild-type and endothelial nitric oxide synthase (eNOS)-deficient mice, sildenafil markedly blunted acute hypoxic pulmonary vasoconstriction. Wild-type mice dosed orally with the drug (25 mg. kg(-1). d(-1)) throughout 3 weeks of exposure to hypoxia (10% O(2)) exhibited a significant reduction in right ventricular systolic pressure (placebo versus sildenafil: 43.3+/-9.9 versus 29.9+/-9.7 mm Hg, P<0.05) coupled with a small reduction in right ventricular hypertrophy and inhibition of pulmonary vascular remodeling. In eNOS mutant mice, sildenafil attenuated the increase in right ventricular systolic pressure but without a significant effect on right ventricular hypertrophy or vascular remodeling.

CONCLUSIONS

Sildenafil attenuates hypoxia-induced pulmonary hypertension in humans and mice and offers a novel approach to the treatment of this condition. The eNOS-NO-cGMP pathway contributes to the response to sildenafil, but other biochemical sources of cGMP also play a role. Sildenafil has beneficial pulmonary hemodynamic effects even when eNOS activity is impaired.