Xestoquinone, a novel cardiotonic agent activates actomyosin ATPase to enhance contractility of skinned cardiac or skeletal muscle fibers.
Xestoquinone (XQN), a novel cardiotonic principle from the sea sponge Xestospongia sapra, enhanced Ca+(+)-induced tension development of chemically skinned fibers from guinea pig cardiac muscle, even at both free Ca++ concentrations as low as -log molar free Ca++ (pCa) 9 to 8. In skinned fibers from guinea pig skeletal muscle, XQN (10 microM) also increased developed tension with a similar Ca++ dependence to that for cardiac fibers. In contrast to the unique Ca+(+)-dependence of XQN effects, the reference drug sulmazole enhanced Ca+(+)-induced tension development of skinned cardiac fibers at pCa 6.6 but did not affect it at pCa 8. In natural actomyosin from canine cardiac muscle, as well as in that from rabbit skeletal muscle, XQN (1-30 microM) enhanced the rate and extent of superprecipitation. Moreover, XQN produced a concentration-dependent increase in the myofibrillar ATPase activity of canine cardiac muscle, even at very low free Ca++ concentrations below the normal threshold for ATPase activation (pCa 9-8). The natural actomyosin ATPase activity of chicken smooth muscle was not influenced by XQN (up to 30 microM). In cardiac myofibrils, no significant difference was observed between the bound 45Ca+(+)-pCa relationship curves in the presence and absence of XQN (10 microM). Furthermore, XQN (30 microM) did not cause or potentiate Ca+(+)-induced Ca++ release from cardiac sarcoplasmic reticulum vesicles. These observations suggest that XQN directly activates actomyosin ATPase activity of cardiac and skeletal myofibrils, thus producing an enhanced superprecipitation activity as well as an increase in skinned fiber contractility.(ABSTRACT TRUNCATED AT 250 WORDS)