Structural and enzymic properties of myosins from atrial and ventricular cardiac muscle of the chicken were investigated and compared with myosins from the fast skeletal pectoralis and the slow skeletal anterior latissimus dorsi muscle. 2. The Ca2+-ATPase activity, both in function of pH and [K+], of atrial myosin closely resembled that of the fast pectoralis myosin, whereas the enzymic properties of ventricular myosin were similar to those of slow skeletal myosin. 3. By sodium dodecyl sulphate polyacrylamide gel electrophoresis on gradient gel and two-dimensional electrophoresis, involving isoelectric focusing in the first dimension and SDS gel electrophoresis in the second dimension, no difference could be demonstrated in the light-chain pattern of atrial and ventricular myosin. Complete identity was also found between anterior latissimus dorsi and cardiac light chains. 4. Electrophoretic analysis of soluble peptides released by tryptic digestion of myosin and electron microscopic study of light meromyosin paracrystals showed significant differences between the heavy chains of atrial and ventricular myosins, as well as between the heavy chains of cardiac and skeletal myosins. 5. The results confirm previous immunochemical findings and provide direct biochemical evidence for the existence of a new, unique type of myosin in the chicken atrial tissue.
