ATPase Activity Of Cardiac Myofibrils Research Articles

Different phosphorylation patterns of cardiac myosin light chains using ATP and ATPλS as substrates

Controversial views have been reported regarding the role of myosin light chain phosphorylation in the regulation of cardiac contractility (for review see. In the past, adenosine 5'-(-thio)triphosphate) (ATP gamma S) instead of ATP has frequently been used to study mechanical and biochemical consequences of myosin P-light chain (P-LC, LC-2) phosphorylation since thiophosphorylated sites are not significantly attacked by phosphatases. Unlike thiophosphorylation phosphorylation of myosin by myosin light chain kinase did neither decrease maximal (unloaded) shortening velocity of cardiac skinned fibres nor ATPase activity of cardiac myofibrils. We have accordingly investigated the phosphorylation pattern of purified cardiac myosin light chains using radioactive labeled ATP gamma S and ATP. We found that both the 28 kDa myosin light chain (LC-1) and the 18 kDa myosin light chain (LC-2, P-LC) were phosphorylated when ATP gamma S was present. In the presence of ATP, however, only LC-2 was found to be phosphorylated.

Multiple cardiac contractile protein abnormalities in myopathic syrian hamsters (BIO 53:58)

Hearts of genetically myopathic male hamsters (BIO 53 : 58) were studied at 1 month, 2 months, 3 months, 4 to 5 months and 7 months of age. The time course of alterations in the cardiac myofibrillar ATPase activity, the relationship of myofibrillar ATPase activity to free [Ca2+], myosin ATPase activity and the distribution of heavy chain myosin isoenzymes were evaluated. Mg2+-Ca2+ ATPase activity of cardiac myofibrils in myopathics was increased in 4 month and 7 month-old hamsters. Elevated Mg2+ ATPase activity was found as early as in 2-month-old hamster. However, there was no loss in the regulation of the myopathic myofibrillar assembly as measured by the PCa response (10(-7) M to 10(-4) M Ca2+). Scans of SDS electrophoresis slab gels of cardiac myofibrillar proteins from control (C) and myopathic animals (M) did not show any differences at any age group (1, 4 and 7 months). There was a significant decrease in myosin Ca2+ ATPase activity and actin activated Mg2+-ATPase activity at 4 to 5 months and 7 months of age in the myopathic hearts. At all ages in normal and myopathic animals cardiac myosin consisted of three isoenzymes, V1, V2 and V3. At all ages in controls and at 1 to 3 months in myopathics, V1 predominated and the isoenzyme distribution was V1 greater than V2 greater than V3. However, in myopathics at 4 to 5 months, the distribution was V1 = V3 greater than V2 and at 7 months was V3 greater than V2 greater than V1. Our experiments suggest alterations in different components of the contractile protein system that occur at different stages of myopathy.

The differential effect of Ca2+ and Mg2+ on the ATPase activity of cardiac myofibrils

The differential effect of Ca2+ and Mg2+ on the ATPase activity of cardiac myofibrils

Myofibrillar ATPase activity in rat heart after chronic propranolol administration

A previous study has shown that chronic chemical sympathectomy brought about by 6-hydroxydopamine injections results in a dpression in myocardial contractile function which is accompanied by reduced myofibrillar ATPase activity. To determine whether chronic beta-adrenergic receptor blockade elicits similar alterations in cardiac contractile-protein ATPase activity, adult rats were given twice-daily injections of propranolol 7 days/wk for 2 wk. Effective beta-adrenergic receptor blockade was verified by the lack of hemodynamic responsiveness to isoproterenol infusion. Myofibrils were prepared from left ventricular tissue and analyzed for ATPase activity. Myofibrillar ATPase activity was 295 +/- 8 nmol Pi.mg-1.min-1 in controls. Enzyme activity was not significantly different in propranolol-injected rats. The results demonstrate that chronic propranolol administration does not alter the ATPase activity of cardiac myofibrils. Therefore, it seems likely that the altered contractile-protein enzymatic properties resulting from chronic chemical sympathectomy do not occur as the result of a reducted level of cardiac beta-adrenergic receptor stimulation.

Myofibrillar ATPase activity in the transplanted canine heart.

Myofibrils were prepared from the left ventricles of normal dogs, of dogs after cardiac denervation, and of dogs in low output cardiac failure after cardiac homotransplantation. The ATPase activity of the myofibrils in the presence and absence of calcium was measured. It was found that cardiac denervation and cardiac homotransplantation leading to cardiac failure did not alter the intrinsic ATPase activity of myofibrils in either the presence or absence of calcium. Although this study does not eliminate a defect in excitation-contraction coupling in cardiac failure secondary to transplantation, it indicates that there is no primary defect in the intrinsic ATPase activity of cardiac myofibrils from transplanted hearts in failure.

Cardiac myofibrillar ATPase: A comparison with that of fast skeletal actomyosin in its native and in an altered conformation

Some enzymatic properties of cardiac myofibrils that were low in mitochondrial contamination are compared with those of skeletal actomyosin. We have observed with skeletal actomyosin two different types of enzymatic behavior: (a) the native behavior typical for skeletal myofibrils and (b) a modified behavior, found sometimes in reconstituted skeletal actomyosin preparations. Such modified skeletal actomyosin had acquired some enzymatic properties similar to those of cardiac myofibrils. The rate of hydrolysis of ATP by modified actomyosin was greatly reduced although that of ITP remained unchanged. As a result ITPase activity was much greater than ATPase activity just as in cardiac myofibrils. However, with respect to ATPase activity as a function of MgATP concentration there was no resemblance between cardiac myofibrillar and modified reconstituted skeletal actomyosin. Whereas the ATPase activity of cardiac myofibrils shows Michaelis-Menten kinetics similar to that of skeletal myofibrils, with a K m of about 10 μM, the modified actomyosin was inhibited by higher ATP concentrations. The Ca 2+ concentration for half maximal ATPase activation ranged between 0.7 and 1 · 10 −6 M, a range similar to that reported by others for reconstituted cardiac actomyosin and similar to that of skeletal actomyosin.