Accelerated Growth, Differentiation, and Ploidy with Reduced Proliferation of Right Ventricular Cardiomyocytes in Children with Congenital Heart Defect Tetralogy of Fallot

The myocardium of children with tetralogy of Fallot (TF) undergoes hemodynamic overload and hypoxemia immediately after birth. Comparative analysis of changes in the ploidy and morphology of the right ventricular cardiomyocytes in children with TF in the first years of life demonstrated their significant increase compared with the control group. In children with TF, there was a predominantly diffuse distribution of Connexin43-containing gap junctions over the cardiomyocytes sarcolemma, which redistributed into the intercalated discs as cardiomyocytes differentiation increased. The number of Ki67-positive cardiomyocytes varied greatly and amounted to 7.0–1025.5/106 cardiomyocytes and also were decreased with increased myocytes differentiation. Ultrastructural signs of immaturity and proliferative activity of cardiomyocytes in children with TF were demonstrated. The proportion of interstitial tissue did not differ significantly from the control group. The myocardium of children with TF under six months of age was most sensitive to hypoxemia, it was manifested by a delay in the intercalated discs and myofibril assembly and the appearance of ultrastructural signs of dystrophic changes in the cardiomyocytes. Thus, the acceleration of ontogenetic growth and differentiation of the cardiomyocytes, but not the reactivation of their proliferation, was an adaptation of the immature myocardium of children with TF to hemodynamic overload and hypoxemia.

Late effects of radiofrequency ablation lesions resulting in a progressive mitral valve perforation in a 2-month-old infant

There has been great concern with the use of radiofrequency ablation in infants since radiofrequency ablation lesions were shown to have a progressing nature in immature myocardium of animals. In this report, we present a 2-month-old infant with life-threatening medically refractory supraventricular tachycardia. Radiofrequency ablation successfully cured arrhythmia; however, late effects of radiofrequency ablation lesions resulted in a progressive mitral valve perforation requiring surgical repair.

Cardiotoxicity of β-mimetic catecholamines during ontogenetic development — possible risks of antenatal therapy

Catecholamines are involved in the regulation of a wide variety of vital functions. The β-adrenergic receptor (β-AR) – adenylyl cyclase system has been identified early in embryogenesis before the heart has received adrenergic innervation. The structure of β-receptors in the immature myocardium is similar to that in adults; there are, however, significant quantitative developmental changes in the inotropic and chronotropic responsiveness. Information on the toxic effect of the β-AR agonists in the immature heart is surprisingly scarce, even though these agents are used in clinical practice both during pregnancy and in early postnatal development. Large doses of β-AR agonists induce malformations of the cardiovascular system; the type of change depends upon the time at which the β-AR agonist was administered during embryogenesis. During postnatal ontogeny, the cardiotoxicity of β-AR agonists increased from birth to adulthood. It seems likely that despite interspecies differences, developmental changes in the cardiac sensitivity to β-AR agonists may exist in all mammals, depending on the degree of maturation of the system involved in β-adrenergic signaling. All the existing data draw attention to the possible harmful consequences of the clinical use of β-AR agonists during early phases of cardiac development. Late effects of the early disturbances of the cardiac muscle cannot be excluded.

Abstract 366: Engineered Human Cardiac Tissue from Skeletal Muscle Derived Cells and Induced Pluripotent Stem Cell Derived Cardiomyocytes

We have previously shown that rat skeletal muscle derived stem cells differentiate into an immature cardiomyocyte (CM) phenotype within a 3-dimensional collagen gel engineered cardiac tissue (ECT). Here, we investigated whether human skeletal muscle derived progenitor cells (skMDCs) can differentiate into a CM phenotype within ECT similar to rat skeletal muscle stem cells and compared the human skMDC-ECT properties with ECT from human induced pluripotent stem cell (iPSc) derived CMs. SkMDCs differentiated into a cardiac muscle phenotype within ECT and exhibited spontaneous beating activity as early as culture day 4 and maintained their activity for more than 2 weeks. SkMDC-ECTs stained positive for cardiac specific troponin-T and troponin-I, and were co-localized with fast skeletal muscle myosin heavy chain (sk-fMHC) with a striated muscle pattern similar to fetal myocardium. The iPS-CM-ECTs maintained spontaneous beating activity for more than 2 weeks from ECT construction. iPS-CM stained positive for both cardiac troponin-T and troponin-I, and were also co-localized with sk-fMHC while the striated expression pattern of sk-fMHC was lost similar to post-natal immature myocardium. Connexin-43 protein was expressed in both engineered tissue types, and the expression pattern was similar to immature myocardium. The skMDC-ECT significantly upregulated expression of cardiac-specific genes compared to conventional 2D culture. SkMDC-ECT displayed cardiac muscle like intracellular calcium ion transients. The contractile force measurements demonstrated functional properties of fetal type myocardium in both ECTs. Our results suggest that engineered human cardiac tissue from skeletal muscle progenitor cells mimics developing fetal myocardium while the engineered cardiac tissue from inducible pluripotent stem cell-derived cardiomyocytes mimics post-natal immature myocardium.

Cytotoxic effects of cardioplegic solutions and cytoprotective effects of insulin on immature cardiac myocytes during hypothermic preservation

The purpose of this study was to evaluate the functional and biochemical effects of cardioplegic solutions on immature cardiac myocytes incubated under hypothermic conditions. In addition, the effects of insulin as an additive were evaluated in each solution. Cardiac myocytes were isolated from neonatal rat ventricles and cultured for four days; 12.5 x 105myocytes/flask were then incubated at 4 °C for three, six and 12 hours in three types of cardioplegic solutions—glucose-potassium solution (glucose: 50 gm/l, NaHCO3: 20 mEq, KCl: 20 mEq), lactated Ringer's solution (KCl: 20 mEq) and St. Thomas' Hospital solution. After each hypothermic incubation, enzymes were measured in the incubation solutions. The myocytes were then cultured for an additional 24 hours at 37 °C to evaluate the recovery of the myocyte beating rate. In the Ringer's group, the recovery ratio of the myocyte beating rate was complete at three hours, then decreased to 48.8 percent of control (beating rate prior to hypothermic incubation) at 12 hours. The glucose-potassium and St. Thomas' groups had significantly lower recovery ratios than the Ringer's group, beginning at three hours (63.4, 72.9, 95.6 percent, respectively). Release of enzymes (CPK and LDH) in the Ringer's group gradually increased and at 12 hours was 29.0 mIU/flask and 260.0 mIU/flask, respectively. The St. Thomas' group, in contrast, had significantly increased values for CPK at 12 hours to 116.0 mIU/flask, and the greatest increases of both enzymes were observed in the glucose-potassium group at 12 hours (CPK: 115.5, LDH: 1163.9). By addition of 20 IU/l insulin, marked improvements were observed in the Ringer's and glucose-potassium groups both functionally and biochemically. Thus, the lactated Ringer's solution had the least cytotoxic effects that might be suitable for a basic solution of various cardioplegic solutions during the neonatal period, and insulin may have beneficial effects on immature myocardium under hypothermic conditions.

Na+/Ca2+ exchange and cell contraction in isolated neonatal and adult rabbit cardiac myocytes

Recent studies have demonstrated a relative deficiency in voltage-gated Ca2+ currents (ICa) in immature myocardium. We hypothesized that contraction in developing heart results in part from Ca2+ influx via “reverse” Na+/Ca2+ exchange current (INa/Ca). Accordingly, INa/Ca and cell contraction amplitude were measured in single neonatal and adult rabbit ventricular myocytes. INa/Ca was dependent on Ca2+ concentration, Na+ concentration, and membrane potential and was blocked by 5 mM Ni2+ but not by the Ca(2+)-channel blocker nifedipine. In neonatal cells, contraction amplitude reached a plateau for depolarizations positive to 0 mV. In adult myocytes, contraction amplitude was maximal at 0 mV and decreased at positive membrane potentials. Inhibition of ICa with nifedipine did not affect maximal contraction amplitude in neonatal myocytes but almost completely suppressed contraction of adult cells. These data suggest that Ca2+ influx via ICa is not required for contraction of neonatal rabbit cardiac myocytes. Moreover, Ca2+ influx via reversal of the Na+/Ca2+ exchange mechanism may provide a significant portion of the Ca2+ regulating cell contraction, especially during depolarization to positive membrane potentials.