Cross-bridge scheme and force per cross-bridge state in skinned rabbit psoas muscle fibers.
The rate and association constants (kinetic constants) which comprise a seven state cross-bridge scheme were deduced by sinusoidal analysis in chemically skinned rabbit psoas muscle fibers at 20 degrees C, 200 mM ionic strength, and during maximal Ca2+ activation (pCa 4.54-4.82). The kinetic constants were then used to calculate the steady state probability of cross-bridges in each state as the function of MgATP, MgADP, and phosphate (Pi) concentrations. This calculation showed that 72% of available cross-bridges were (strongly) attached during our control activation (5 mM MgATP, 8 mM Pi), which agreed approximately with the stiffness ratio (active:rigor, 69 +/- 3%); active stiffness was measured during the control activation, and rigor stiffness after an induction of the rigor state. By assuming that isometric tension is a linear combination of probabilities of cross-bridges in each state, and by measuring tension as the function of MgATP, MgADP, and Pi concentrations, we deduced the force associated with each cross-bridge state. Data from the osmotic compression of muscle fibers by dextran T500 were used to deduce the force associated with one of the cross-bridge states. Our results show that force is highest in the AM*ADP.Pi state (A = actin, M = myosin). Since the state which leads into the AM*ADP.Pi state is the weakly attached AM.ADP.Pi state, we confirm that the force development occurs on Pi isomerization (AM.ADP.Pi --> AM*ADP.Pi). Our results also show that a minimal force change occurs with the release of Pi or MgADP, and that force declines gradually with ADP isomerization (AM*ADP -->AM.ADP), ATP isomerization (AM+ATP-->AM*ATP), and with cross-bridge detachment. Force of the AM state agreed well with force measured after induction of the rigor state, indicating that the AM state is a close approximation of the rigor state. The stiffness results obtained as functions of MgATP, MgADP, and Pi concentrations were generally consistent with the cross-bridge scheme.
Kawai
M
M
Department of Anatomy, College of Medicine, University of Iowa, Iowa City 52242.
Zhao
Y
Y
D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology; D004548:Elasticity; D066298:In Vitro Techniques; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D009119:Muscle Contraction; D009132:Muscles / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D010710:Phosphates / Q000378:metabolism / Q000494:pharmacology; D011817:Rabbits; D013314:Stress, Mechanical
D010710:Phosphates; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D004492:Edetic Acid; D002118:Calcium