title: Effects of Ca2+ on the kinetics of phosphate release in skeletal muscle.
abstract: The process of phosphate dissociation during the muscle cross-bridge cycle has been investigated by photoliberation of inorganic phosphate (Pi) within skinned fibers of rabbit psoas muscle. This permitted a test of the idea that Ca2+ controls muscle contraction by regulating the Pi release step of the cycle. Photoliberation of Pi from structurally distinct "caged" Pi precursors initiated a rapid tension decline of up to 12% of active tension, and this was followed by a slower tension decline. The apparent rate constant of the fast phase, kPi, depended on both [Pi] and [Ca2+], whereas the slow phase generally occurred at 2-4 s-1. At maximal Ca2+, kPi increased in a nonlinear manner from 43 +/- 2 s-1 to 118 +/- 7 s-1, as Pi was raised from 0.9 to 12 mM. This was analyzed in terms of a three-state kinetic model in which a force-generating transition is coupled to Pi dissociation from the cross-bridge. As Ca(2+)-activated tension was reduced from maximal (Pmax) to 0.1 Pmax, (i) kPi decreased by up to 2.5-fold, (ii) the relative amplitude of the rapid phase increased 2-fold, and (iii) the relative amplitude of the slow phase increased about 6-fold. Changes in the rapid phase are compatible with Ca2+ influencing an apparent equilibrium constant for the force-generating transition. By comparison, kPi was faster than the rate constant of tension redevelopment, ktr, and was influenced less by Ca2+. Ca2+ effects on the caged Pi transient cannot account for the large effects of Ca2+ on actomyosin ATPase rates or cross-bridge cycling kinetics but may be a manifestation of reciprocal interactions between the thin filament and force-generating cross-bridges, and may represent Ca2+ regulation of the distribution of cross-bridges between non-force-and force-generating states.
authors.affiliation: Department of Physiology, University of Wisconsin, Madison 53706.
authors.forename: J W
authors.initials: JW
authors.lastname: Walker
authors.forename: Z
authors.initials: Z
authors.lastname: Lu
authors.forename: R L
authors.initials: RL
authors.lastname: Moss
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism* / Q000502:physiology; D010710:Phosphates / Q000378:metabolism*; D010782:Photolysis; D011817:Rabbits
chemical_list: D010710:Phosphates; D002118:Calcium