title: A re-examination of calcium activation in the Huxley cross-bridge model.
abstract: This paper investigates mathematical relations between models of calcium activation kinetics and Huxley-type models of cross-bridge dynamics in muscle. It is found that different calcium-activation schemes lead to the same form of generalized Huxley rate equation with calcium activation. [formula: see text] if it is assumed that calcium-troponin interaction rates are fast compared to the rates of transition associated with force-generating cross-bridge states. Calcium affects cross-bridge dynamics by modifying the bonding rate f, but does not affect the number of interacting cross bridges a or the unbonding rate g; this occurs through the appearance in the equation of an activation factor, r, which is a pure function of sarcoplasmic free calcium concentration. In particular, it is shown that both the "tight-coupling" and "loose-coupling" calcium-activation schemes introduced by Zahalak and Ma [1] lead to the same rate equation with the same activation factor; the difference between them appears in the calcium mass-balance equation. While both of these activation models can be made to fit simple twitch and force-velocity data equally well, experimentally observed load-dependent shifts in the free calcium concentration are compatible with the right-coupling scheme, but not with loose coupling.
authors.affiliation: Department of Mechanical Engineering, Washington University, St. Louis, MO 63130, USA.
authors.forename: G I
authors.initials: GI
authors.lastname: Zahalak
authors.forename: I
authors.initials: I
authors.lastname: Motabarzadeh
mesh_terms: D000199:Actins / Q000502:physiology*; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000502:physiology*; D008956:Models, Chemical; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D009206:Myocardium / Q000378:metabolism; D009218:Myosins / Q000502:physiology*; D011485:Protein Binding; D013997:Time Factors; D019209:Troponin C / Q000502:physiology*
chemical_list: D000199:Actins; D019209:Troponin C; D009218:Myosins; D002118:Calcium