Muscle force and stiffness during activation and relaxation. Implications for the actomyosin ATPase

F. V. Brozovich, L. D. Yates, A. M. Gordon

Research output: Contribution to journalArticle

32 Citations (Scopus)

Abstract

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], cross-bridge 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 P(i), 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 P(i), P(i) release from the cross-bridge is reversed, the state(s) before P(i) release would be populated. These data are consistent with the conclusion that the cross-bridges are strongly bound to actin before P(i) release. In addition, if Ca2+ activates the ATPase by allowing for the strong attachment of the myosin to actin in an A·.M·ADP·P(i) state, it could do so before P(i) 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 P(i), 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.

Original languageEnglish (US)
Pages (from-to)399-420
Number of pages22
JournalJournal of General Physiology
Volume91
Issue number3
StatePublished - 1988
Externally publishedYes

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Myosins
Adenosine Triphosphate
Muscles
Calcium
Actins
Actomyosin
Calcium-Transporting ATPases
Skeletal Muscle Fibers
Anura
Population

ASJC Scopus subject areas

  • Physiology

Cite this

Muscle force and stiffness during activation and relaxation. Implications for the actomyosin ATPase. / Brozovich, F. V.; Yates, L. D.; Gordon, A. M.

In: Journal of General Physiology, Vol. 91, No. 3, 1988, p. 399-420.

Research output: Contribution to journalArticle

Brozovich, F. V. ; Yates, L. D. ; Gordon, A. M. / Muscle force and stiffness during activation and relaxation. Implications for the actomyosin ATPase. In: Journal of General Physiology. 1988 ; Vol. 91, No. 3. pp. 399-420.
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