Atrial and ventricular myosin during development and senescence of the rat

Myosin was isolated from rat atrial and ventricular myocardium and examined during post-natal development and senescence. The post-natal increase of Ca2+-ATPase activity of myosin from rat atria did not run in parallel with changes of ATPase activity of myosin from the ventricles. Ca2+-ATPase of both atrial and ventricular myosin was activated at pH 9.5, when compared with the assay performed at pH 7.5. The myosin light-chain subunits in the ventricles were different from the light-chain subunits in the atria, when characterized by SDS-polyacrylamide gel electrophoresis and the pattern remained practically unchanged during development, with the exception of atrial myosin from new-born and very old rats which contained an additional protein of low molecular weight.