title: The effect of phosphate and calcium on force generation in glycerinated rabbit skeletal muscle fibers. A steady-state and transient kinetic study.
abstract: The effect of varying concentrations of Pi and Ca2+ on isometric force and on the rate of force development in skinned rabbit psoas muscle fibers has been investigated. Steady-state results show that the three parameters that define the force-pCa relation (Po, pK, and n) all vary linearly with log [Pi]. As [Pi] increases, Po and pK decrease while n increases. The kinetics of force generation in isometrically contracting fibers were studied by laser flash photolysis of caged phosphate. The observed rate of the resulting tension transient, kPi, is 23.5 +/- 1.7 s-1 at 10 degrees C, 0.7 mM Pi, and is independent of [Ca2+] over the range pCa 4.5-7.2. By contrast, kTR, the rate of tension redevelopment following a period of isotonic shortening, is sensitive to [Ca2+] and is slower than kPi (kTR = 13.6 +/- 0.2 s-1 at pCa 4.5, 0.7 mM Pi). The results show that [Ca2+] does not directly affect the Pi release or force-generating steps of the cross-bridge cycle and show that the observed rate of force development depends on how the measurement is made. The data can be interpreted in terms of a model in which strong cross-bridges activate the thin filament, this activation being modulated by Ca2+ binding to troponin.
authors.affiliation: Department of Physiology, University of California, Los Angeles 90024.
authors.forename: N C
authors.initials: NC
authors.lastname: Millar
authors.forename: E
authors.initials: E
authors.lastname: Homsher
mesh_terms: D000205:Actomyosin / Q000378:metabolism; D000818:Animals; D005990:Glycerol; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D010710:Phosphates / Q000494:pharmacology*; D010782:Photolysis; D011188:Potassium / Q000494:pharmacology*; D011817:Rabbits; D012984:Software
chemical_list: D010710:Phosphates; D000205:Actomyosin; D005990:Glycerol; D011188:Potassium