Inhibitory factor of actomyosin ATPase in the cardiac conduction system.

ATPase activation and superprecipitation of natural actomyosin in the cardiac conduction system (specialized myocardium) and ventricle were compared. The ATPase activity and superprecipitation of natural actomyosin from the specialized myocardium were much lower than those from the ventricle. The sensitivity of natural actomyosin ATPase to calcium was higher in the specialized myocardium than in the ventricle, whereas the maximum activity of the specialized myocardium was lower than that of the ventricle. The Ca2+-desensitized actomyosin of the specialized myocardium presented scarcely any superprecipitation. Addition of the 0-40% ammonium sulfate fraction (mainly containing myosin and actin) of natural actomyosin from the specialized myocardium to ventricular natural actomyosin inhibited superprecipitation of the latter. On the other hand, addition of the 40-60% ammonium sulfate fraction (mainly containing troponins and tropomyosin) of natural actomyosin from the specialized myocardium enhanced superprecipitation of ventricular natural actomyosin. These results suggested that the specialized myocardium contains some factor(s) that inhibits actin-myosin interaction.