Temperature-dependence of tension development by glycerinated muscle fibers of rabbit psoas in Mn (II)-ATP and Mg-ATP solutions.

The tension of single fibers isolated from glycerinated rabbit psoas muscle was measured at various temperatures using Mg-ATP or Mn-ATP as a substrate. The tension developed in Mn-ATP was 80-89% of that in Mg-ATP at 4 degrees-16 degrees, and both tensions decreased as the temperature was reduced. Myosin forms the myosin-product complex predominantly generated on admixture with ADP (and Pi) at the burst site during Mn(II)-ATP hydrolysis below 10 degrees, while it forms the myosin-product complex predominantly formed via ATP hydrolysis upon hydrolysis above 10 degrees, as it does during Mg-ATP hydrolysis (Hozumi & Tawada (1975) Biochim. Biophys. Acta 376, 1; Tawada & Yoshida (1975) J. Biochem. 78, 293; Yazawa & Morita (1973) J. Biochem. 74, 1107). Since the cycle time of cross-bridge attachment to and detachment from actin in muscle is about 1-10 sec and because the spontaneous decay time of the myosin-product complex into the myosin-product complex in the absence of actin is less than 1 sec, then if most of the cross-bridges are detached from actin as suggested by X-ray data, it can be inferred that most of the cross-bridges detached from actin may form the predominant myosin-product complex in Mn-ATP below 10 degrees. If this is so, the tension development in Mn-ATP below 10 degrees cannot be compatible with some muscle models which assume the formation of the myosin-product complex by the cross-bridges prior to combination with actin during contraction.