Abstract 150: Determining Changes in Contractility, Myofilament Expression and Myofilament Sensitivity in the Developing Human Ventricle

Abstract

As pediatric cardiac surgery is increasingly performed in the first year of life, the need for understanding contractility regulation becomes increasingly important. We examined contractility and myofilament changes in ventricular biopsies removed as part of the surgical correction of congenital heart defects from newborns (NB) (hypoplastic left heart syndrome, < 1 wk old) and infants (IF) (tetralogy of Fallot, 3-12 mo old). Sarcomere shortening and calcium (Ca) transients were measured in isolated ventricular cells stimulated at 0.5 and 1 Hz. Increasing pacing frequency caused a significant increase in sarcomere shortening (p< 0.05)in only the IF, but the Ca transient amplitude was relatively unchanged in both groups. Consistent with work in the developing rat heart, we found an isoform switch with increasing developmental age in myofilament proteins, troponin T (TnT) and troponin I (TnI). Western blot analysis revealed that total TnI (the sum of cardiac TnI (cTnI) and slow skeletal TnI (ssTnI)) was not significantly different between the two age groups. However, when comparing only the cTnI isoforms, we found that the NB had a significantly lower levels compared to the IF (11556±789 a.u. vs. 21770±1700 a.u., p<0.001). Analysis of the RT-PCR revealed nearly significant lower levels of TnT isoform 1 in the IF group than the NB group (p=0.078), no significant difference in the levels of TnT isoform 2 (p= 0.536), and significantly higher levels of TnT isoform 3 in the IF group than the NB group (p= 0.039). We also observed developmental changes in myofilament sensitivity to calcium as assessed by measuring the gradient of the fura-2 cell length trajectory on phase-plane diagrams during the late relaxation of the twitch contraction. These results collectively suggest that contractility and myofilament changes, or the lack of changes, may serve as targets for clarifying elements associated with cardiac dysfunction in the developing human ventricle.