title: Determinants of relaxation rate in rabbit skinned skeletal muscle fibres.
abstract: The influence of Ca(2+)-activated force, the rate of dissociation of Ca(2+) from troponin C (TnC) and decreased crossbridge detachment rate on the time course of relaxation induced by flash photolysis of diazo-2 in rabbit skinned psoas fibres was investigated at 15 degrees C. The rate of relaxation increased as the diazo-2 chelating capacity (i.e. free [diazo-2]/free [Ca(2+)]) increased. At a constant diazo-2 chelating capacity, the rate of relaxation was independent of the pre-photolysis Ca(2+)-activated force in the range 0.3-0.8 of maximum isometric force. A TnC mutant that exhibited increased Ca(2+) sensitivity caused by a decreased Ca(2+) dissociation rate in solution (M82Q TnC) also increased the Ca(2+) sensitivity of steady-state force and decreased the rate of relaxation in fibres by approximately twofold. In contrast, a TnC mutant with decreased Ca(2+) sensitivity caused by an increased Ca(2+) dissociation rate in solution (NHdel TnC) decreased the Ca(2+) sensitivity of steady-state force but did not accelerate relaxation. Decreasing the rate of crossbridge kinetics by reducing intracellular inorganic phosphate concentration ([P(i)]) slowed relaxation by approximately twofold and led to two phases of relaxation, a slow linear phase followed by a fast exponential phase. In fibres, M82Q TnC further slowed relaxation in low [P(i)] conditions by approximately twofold, whereas NHdel TnC had no significant effect on relaxation. These results are consistent with the interpretation that the Ca(2+)-dissociation rate and crossbridge detachment rate are similar in fast-twitch skeletal muscle, such that decreasing either rate slows relaxation, but accelerating Ca(2+) dissociation has little effect on relaxation.
authors.affiliation: Department of Physiology and Cell Biology, Ohio State University, 1645 Neil Ave., Columbus, OH 43210 USA.
authors.forename: Ye
authors.initials: Y
authors.lastname: Luo
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Davis
authors.forename: Lawrence B
authors.initials: LB
authors.lastname: Smillie
authors.forename: Jack A
authors.initials: JA
authors.lastname: Rall
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism; D002614:Chelating Agents / Q000494:pharmacology; D003979:Diazonium Compounds; D005260:Female; D006652:Histological Techniques; D007537:Isometric Contraction / Q000502:physiology; D007700:Kinetics; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D009126:Muscle Relaxation / Q000187:drug effects; D018482:Muscle, Skeletal / Q000502:physiology*; D009154:Mutation; D010642:Phenoxyacetates; D010782:Photolysis; D011817:Rabbits; D013997:Time Factors; D019209:Troponin C / Q000235:genetics / Q000378:metabolism
chemical_list: D002614:Chelating Agents; D003979:Diazonium Compounds; D010642:Phenoxyacetates; D019209:Troponin C; C063685:diazo-2; D002118:Calcium