Myosin. Cardiac myosin is composed of two heavy chains (HCs) and four light chains (LCs). The ventricles contain only two types of light chains (skeletal muscle myosin possess three) with estimated molecular mass of 19,000 dalton (regulatory light chain, RLC) and 27,000 dalton (essential light chain, ELC). The mass of entire cardiac myosin is about 480,000 and its length is approximately 1,700 A (Katz 1992), similar to these parameters in skeletal muscle myosin.(For structural organization see Figure M1)
The multigene families of HCs and LCs generate different isoforms that occur at various stages of heart development. Two HC isoforms are known as a- and p-isoforms. First it was thought that the functional properties of heart muscle are determined by the LC composition, but it turned out that HCs determine the myosin ATPase activity in vitro and the shortening velocity in the intact heart (Katz, 1992).
In the atria there are two myosin isoforms, Ai and A2, with high and low ATPase activity, respectively, whereas in the ventricles there are three myosin isoforms, composed of either homodimer or heterodimer HCs, Vt (a-a), V2 (a-p), and V3 (p-p), with high, intermediate, and low ATPase, respectively. A comparative study on the various ATPase activities of rabbit heart and skeletal muscle myosins showed that under all conditions the activities of the skeletal source are higher (Barany et al., 1964).
The atria and the ventricles contain different LC isoforms. The atrial LCs are also found in developing heart and fast skeletal muscle, and in adult slow skeletal muscle. The 19,000-dalton heart LC is phosphorylated and possibly participates in the regulation of heart contraction (Kopp and Barany, 1979).
Actin: Heart actin has not been studied in detail. It is generally believed that heart and skeletal actin have a very similar structure and their biological properties are also similar. That is, actin in the thin filaments is built of monomers, it binds TM and TN (see Fig. A6), upon contractile stimulus it binds to myosin and it activates myosin ATPase.
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