α-Tropomyosin from rat cardiac muscle was shown by two-dimensional gel electrophoresis to become phosphorylated when tissue slices were incubated in Eagle's medium supplemented with 32Pi. In the adult rat and mouse heart the level of phosphorylation was ∼30%, but the level was much higher in the foetal heart (60–70%). A similar developmental trend was observed in skeletal muscle from the rat and mouse, where phosphorylated forms of both α- and β-tropomyosins were observed. When rat cardiac cells were grown in tissue culture in the presence of 32Pi, radioactivity was incorporated into the region of the gel containing tropomyosin.
