Regulatory proteins of the myocardium. Atrial and ventricular tropomyosin and troponin-I in the developing and adult bovine and human heart.

The myofibrillar regulatory proteins, troponin-I and tropomyosin were isolated from human and bovine atria and ventricles and studied in the adult and during foetal development. A number of analytical electrophoretic procedures were employed to detect possible atrial, ventricular or foetal specific forms of these proteins. Biological activity of the regulatory protein complex during development was monitored by measuring the calcium sensitivity of the myofibrillar Mg2+ activated ATPase. No evidence was obtained for unique or foetal specific forms of either troponin I or the alpha and beta subunits of tropomyosin. Although the atria and ventricles possess markedly different contractile properties, no difference was observed between the two chambers at any developmental stage in the relative amounts of alpha and beta tropomyosin present. However, the relative amount of beta tropomyosin increased by 50% in both atria and ventricles during the transition from foetus to adult. A strong inverse correlation existed (r = -0.92) between beta tropomyosin content and heart rate in different species and at different developmental stages in both cardiac chambers. The relative invariance of tropomyosin and troponin I forms in the myocardium was reflected in the high and constant level of calcium sensitivity of myofibrillar Mg2+ ATPase retained in the atria and ventricles throughout development. The implications of these results in relation to control of cardiac contraction are discussed.