The thermodynamic properties of the actin filaments prepared from cardiomyocytes were investigated with differential scanning calorimetry. This method could distinguish between the alpha-cardiac and alpha-skeletal components of the actin filaments polymerised from ADP-actin monomers by their different melting temperatures (T(m)). Similar separation was not possible with filaments polymerised from ATP-actin monomers. Further analyses revealed that the activation energy (E(act)) was greater for filaments of alpha-skeletal actin than for alpha-cardiac actin monomers when the filaments were polymerised from ADP-actin monomers. These results showed that the alpha-cardiac actin filaments were thermodynamically less stable than the filaments of alpha-skeletal actin and their difference was nucleotide dependent. Based on these results and considering previous observations it was concluded that the existence of two actin isoforms and their nucleotide dependent conformational differences are part of the tuning regulatory mechanism by which the cardiac muscle cells can maintain their biological function under pathological conditions.