Simultaneous expression of skeletal muscle and heart actin proteins in various striated muscle tissues and cells. A quantitative determination of the two actin isoforms.

A procedure was developed to determine the percentage of skeletal muscle actin and cardiac actin present in different striated muscle tissues. The method was applied to 2 mg of actin mixtures isolated from various origins. All samples show simultaneous expression of both striated muscle isoactins, with the cardiac actin being the major form (congruent to 80%) in 11-day-old chick embryonic leg muscle, decreasing to approximately 50% values in the late fetal stage of chicken, mouse, and in fused mouse muscle cell cultures and becoming the minor species (less than 5%) in adult skeletal muscle tissues. We also find a significant amount (up to 20%) of the skeletal muscle isoform in adult heart (ventricle) of porcine, bovine, and human origin and no differences in muscle actin ratios in human atrium and ventriculum cells. Similarly, no significant variation in the actin ratios was observed between a normal heart and a heart from a patient with hereditary obstructive myopathy. For those cells and tissues where comparison with levels of mRNA was possible we mostly find a good correlation between the relative ratios of expression of cardiac and skeletal actin proteins and mRNAs.