TY - JOUR
T1 - Muscle force and stiffness during activation and relaxation
T2 - Implications for the actomyosin ATPase
AU - Brozovich, Frank V.
AU - Yates, Lawrence D.
AU - Gordon, Albert M.
PY - 1988/3/1
Y1 - 1988/3/1
N2 - Isolated skinned frog skeletal muscle fibers were activated (increasing [Ca2+]) and then relaxed (decreasing [Ca2+]) with solution changes, and muscle force and stiffness were recorded during the steady state. To investigate the actomyosin cycle, the biochemical species were changed (lowering [MgATP] and elevating [H2PO4−]) to populate different states in the actomyosin ATPase cycle. In solutions with 200 µM [MgATP], compared with physiological [MgATP], the slope of the plot of relative steady state muscle force vs. stiffness was decreased. At low [MgATP], crossbridge dissociation from actin should be reduced, increasing the population of the last cross-bridge state before dissociation. These data imply that the last cross-bridge state before dissociation could be an attached low-force-producing or non-force-producing state. In solutions with 10 mM total Pi, compared to normal levels of MgATP, the maximally activated muscle force was reduced more than muscle stiffness, and the slope of the plot of relative steady state muscle force vs. stiffness was reduced. Assuming that in elevated Pi, Pi release from the cross-bridge is reversed, the state(s) before Pi release would be populated. These data are consistent with the conclusion that the cross-bridges are strongly bound to actin before Pi release. In addition, if Ca2+ activates the ATPase by allowing for the strong attachment of the myosin to actin in an A.M.ADP.Pi state, it could do so before Pi release. The calcium sensitivity of muscle force and stiffness in solutions with 4 mM [MgATP] was bracketed by that measured in solutions with 200 µM [MgATP], where muscle force and stiffness were more sensitive to calcium, and 10 mM total Pi, where muscle force and stiffness were less sensitive to calcium. The changes in calcium sensitivity were explained using a model in which force-producing and rigor cross-bridges can affect Ca2+ binding or promote the attachment of other cross-bridges to alter calcium sensitivity.
AB - Isolated skinned frog skeletal muscle fibers were activated (increasing [Ca2+]) and then relaxed (decreasing [Ca2+]) with solution changes, and muscle force and stiffness were recorded during the steady state. To investigate the actomyosin cycle, the biochemical species were changed (lowering [MgATP] and elevating [H2PO4−]) to populate different states in the actomyosin ATPase cycle. In solutions with 200 µM [MgATP], compared with physiological [MgATP], the slope of the plot of relative steady state muscle force vs. stiffness was decreased. At low [MgATP], crossbridge dissociation from actin should be reduced, increasing the population of the last cross-bridge state before dissociation. These data imply that the last cross-bridge state before dissociation could be an attached low-force-producing or non-force-producing state. In solutions with 10 mM total Pi, compared to normal levels of MgATP, the maximally activated muscle force was reduced more than muscle stiffness, and the slope of the plot of relative steady state muscle force vs. stiffness was reduced. Assuming that in elevated Pi, Pi release from the cross-bridge is reversed, the state(s) before Pi release would be populated. These data are consistent with the conclusion that the cross-bridges are strongly bound to actin before Pi release. In addition, if Ca2+ activates the ATPase by allowing for the strong attachment of the myosin to actin in an A.M.ADP.Pi state, it could do so before Pi release. The calcium sensitivity of muscle force and stiffness in solutions with 4 mM [MgATP] was bracketed by that measured in solutions with 200 µM [MgATP], where muscle force and stiffness were more sensitive to calcium, and 10 mM total Pi, where muscle force and stiffness were less sensitive to calcium. The changes in calcium sensitivity were explained using a model in which force-producing and rigor cross-bridges can affect Ca2+ binding or promote the attachment of other cross-bridges to alter calcium sensitivity.
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U2 - 10.1085/jgp.91.3.399
DO - 10.1085/jgp.91.3.399
M3 - Article
C2 - 2967885
AN - SCOPUS:0023912104
SN - 0022-1295
VL - 91
SP - 399
EP - 420
JO - Journal of General Physiology
JF - Journal of General Physiology
IS - 3
ER -