Thyroid hormone (TH) induces marked changes in the biochemical and physiological 
functioning of cardiac muscle affecting its bioenergetics, contractility and 
structure. Using a time-course analysis of in vitro treatment of neonatal rat 
cardiomyocytes with triiodothyronine (T3), mitochondrial biogenesis, functional 
bioenergetics and cardiomyocyte hypertrophic phenotype were assessed. Activity 
of respiratory complexes II, IV, V and citrate synthase (CS), levels of 
mitochondrial enzyme subunits (e.g. COXI, COXIV) and nuclear-encoded 
transcription factors, involved in mitochondrial biogenesis (e.g. PGC-1, mtTFA 
and PPAR-alpha), were significantly elevated with 72 h T3 treatment. A 
time-course analysis showed an early increase (between 3 and 12 h) in activity 
and levels of subunits of complex IV and V, mitochondrial Ca2+ accumulation and 
a late increase (at 72 h) in complex II and CS activities, mitochondrial protein 
content and mitochondrial respiration. Based on overall protein content and 
specific peptide levels (e.g. actin or myosin) only mild cardiomyocyte 
hypertrophy was detected. T3 mediates an early stimulation of enzymes containing 
mtDNA encoded subunits (e.g. complex IV and V) in contrast to a different 
regulatory pattern for the entirely nuclear-encoded enzymes (e.g. CS and complex 
II). T3-regulation was similar in both neonatal and young adult cardiomyocytes 
(ARCM) but absent in the senescent cardiomyocytes. This model offer an 
opportunity to study the rapid timing of events involved in myocardial cell 
signaling, bioenergetics and growth dynamics in a timeframe not available with 
whole animal studies.