There is a different adaptation of β-AR and adrenergic signalling pathways in children with HF compared with adults. Our results begin to address the disparities in cardiovascular research specific to children and suggest that age-related differences in adaptation could influence the response to therapy. These findings could lead to a paradigm shift in the contemporary management of children with HF.

Background Cardiolipin, a unique phospholipid in the inner mitochondrial membrane, is critical for optimal mitochondrial function. CL abnormalities have been demonstrated in the failing rodent and adult human heart. The aim of this study was to determine whether abnormalities in CL content and the CL biosynthesis and remodeling pathways are present in pediatric idiopathic dilated cardiomyopathy (IDC). Methods and Results A cross-sectional analysis of myocardial tissue from 119 IDC and non-failing (NF) control samples was performed. Electrospray ionizing mass spectrometry was used to measure total CL and CL species content in LV tissue. RT-PCR was employed to measure gene expression of the enzymes in the CL biosynthesis and remodeling pathways in both the adult and pediatric heart. Significantly lower total and (18:2)4CL (the beneficial species) content was demonstrated in myocardium from pediatric patients with IDC compared to NF controls. Analysis of mitochondrial gene transcripts was used to demonstrate that there is no decrease in mitochondrial content. Expression of two biosynthesis enzymes and one remodeling enzyme was significantly lower in pediatric IDC compared to NF controls. Expression of two phospholipases involved in CL degradation were also altered, one up and one down-regulated. Except for one remodeling enzyme, these changes are unique from those in the failing adult heart. Conclusion Similar to what has been seen in adults and in a rat model of IDC, total and (18:2)4CL are lower in pediatric IDC. Unique CL species profiles are seen in heart tissue from children with IDC compared to adults. Differences in CL biosynthesis and remodeling enzyme expression likely explain the differences in CL profiles observed in IDC and implicate unique age-related mechanisms of disease.

Decreases in cardiac connexin43 (Cx43) play a critical role in abnormal cell to cell communication and have been linked to the resistance of the female heart to arrhythmias. We therefore hypothesized that Cx43 expression would be greater in female cardiomyocytes than male cardiomyocytes under pathologic conditions. Adult ventricular myocytes were isolated from male and female rats and treated with phenylephrine, a well established pathologic stimulus. Cx43 gene and protein expression was determined. microRNA-1 expression, a microRNA known to control Cx43 protein expression in cardiomyocytes, was also determined. Cx43 mRNA and protein levels were significantly higher in the female cardiomyocytes than the male cardiomyocytes (mRNA: 1.4-fold; Protein: 5-fold, both p< 0.05) under both basal and pathologic conditions. Phenylephrine treatment increased Cx43 expression only in female cardiomyocytes. Cx43 phosphorylation, a marker of preserved Cx43 function, was also higher (P<0.05) and microRNA-1 expression was lower (P<0.05) in the female cardiomyocytes after phenylephrine treatment. microRNA-1 expression was unchanged by phenylephrine treatment in male cardiomyocytes. Thus, a sex-difference in microRNA-1 may be responsible for the sex-difference in Cx43 expression in cardiomyocytes under pathologic conditions. Taken together our results demonstrate a sex-difference in Cx43 expression and site specific phosphorylation that favors cardioprotection in female cardiomyocytes.

Children with heart failure are treated with similar medical therapy as adults with heart failure. In contrast to adults with heart failure, these treatment regiments are not associated with improved outcomes in children. Recent studies have demonstrated age-related pathophysiological differences in the molecular mechanisms of heart failure between children and adults. There are no animal models of pediatric cardiomyopathy to allow mechanistic studies. The purpose of the current experiments was to develop a mouse model of pediatric heart disease and test whether the influence of β-adrenergic receptor (β-AR) antagonism could be modeled in this system. We hypothesized that isoproterenol treatment of young mice would provide a model system of cardiac pathology, and that nonselective β-AR blockade would provide benefit in adult, but not young, mice, similar to clinical trial data. We found that isoproterenol treatment (through osmotic minipump implantation) of young and adult mice produced similar degrees of cardiac hypertrophy and recapitulated several age-related molecular abnormalities in human heart failure, including phospholamban phosphorylation and β-AR expression. We also found that nonselective β-AR blockade effectively prevented pathological cardiac growth and collagen expression in the adult but not young mice, and that selective β1-AR blockade was effective in both young and adult isoproterenol-treated mice. In conclusion, we have developed the first model system for β-AR-mediated pediatric heart disease. Furthermore, we have generated novel data suggesting beneficial effects of selective β1-AR blockade in the pediatric heart.

BACKGROUND The purpose of the current study was to define the myocellular changes and adaptation of the β-adrenergic receptor (β-AR) system that occur in the systemic right ventricle (RV) of children with hypoplastic left heart syndrome (HLHS). METHODS Explanted hearts from children with HLHS and non-failing controls were used for this study. HLHS patients were divided into 2 groups: “compensated” (C-HLHS), infants listed for primary transplant with normal RV function and absence of heart failure symptoms, and “decompensated” (D-HLHS), patients listed for transplant after failed surgical palliation with RV failure and/or refractory protein-losing enteropathy or plastic bronchitis. RESULTS Compared with non-failing control RVs, the HLHS RV demonstrated decreased sarcoplasmic reticulum calcium-adenosine triphosphatase 2a and α-myosin heavy chain (MHC) gene expression, decreased total β-AR due to down-regulation of β1-AR, preserved cyclic adenosine monophosphate levels, and increased calcium/calmodulin-dependent protein kinase II (CaMKII) activity. There was increased atrial natriuretic peptide expression only in the C-HLHS group. Unique to those in the D-HLHS group was increased β-MHC and decreased α-MHC protein expression (MHC isoform switching), increased adenylyl cyclase 5 expression, and increased phosphorylation of the CaMK target site on phospholamban, threonine 17. CONCLUSIONS The HLHS RV has an abnormal myocardial gene expression pattern, downregulation of β1-AR, preserved cyclic adenosine monophosphate levels, and increased CaMKII activity compared with the non-failing control RV. There is MHC isoform switching, increased adenylyl cyclase 5, and increased phosphorylation of phospholamban threonine 17 only in the D-HLHS group. Although abnormal gene expression and changes in the β-AR system precede clinically evident ventricular failure in HLHS, additional unique adaptations occur in those with HLHS and failed surgical palliation.