title: Effects of magnesium pyrophosphate on mechanical properties of skinned smooth muscle from the guinea pig taenia coli.
abstract: Effects of the non-hydrolyzable nucleotide analogue magnesium pyrophosphate (MgPPi) on cross-bridge properties were investigated in skinned smooth muscle of the guinea pig Taenia coli. A "high" rigor state was obtained by removing MgATP at the plateau of an active contraction. Rigor force decayed slowly towards an apparent plateau of approximately 25-35% of maximal active force. MgPPi markedly increased the rate of force decay. The initial rate of the force decay depended on [MgPPi] and could be described by the Michaelis-Menten equation with a dissociation constant of 1.6 mM. The decay was irreversible amounting to approximately 50% of the rigor force. Stiffness decreased by 20%, suggesting that the major part of the cross-bridges were still attached. The results can be interpreted as "slippage" of PPi-cross-bridges to positions of lower strain. The initial rate of MgPPi-induced force decay decreased with decreasing ionic strength in the range 45-150 mM and was approximately 25% lower in thiophosphorylated fibers. MgADP inhibited the MgPPi-induced force decay with an apparent Ki of 2 microM. The apparent Km of MgATP for the maximal shortening velocity in thiophosphorylated fibers was 32 microM. This low Km of MgATP suggests that steps other than MgATP-induced detachment are responsible for the low shortening velocity in smooth muscle. No effects were observed of 4 mM MgPPi on the force-velocity relation, suggesting that cross-bridges with bound MgPPi do not constitute an internal load or that binding of MgPPi is weaker in negatively strained cross-bridges during shortening.
authors.affiliation: Department of Physiology and Biophysics, University of Lund, Sweden.
authors.forename: H
authors.initials: H
authors.lastname: Arheden
authors.forename: A
authors.initials: A
authors.lastname: Arner
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology; D000818:Animals; D011756:Diphosphates / Q000494:pharmacology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects*; D008274:Magnesium / Q000494:pharmacology*; D017616:Magnesium Compounds; D009119:Muscle Contraction / Q000187:drug effects*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009994:Osmolar Concentration; D010782:Photolysis
chemical_list: D011756:Diphosphates; D017616:Magnesium Compounds; D000244:Adenosine Diphosphate; C034732:magnesium pyrophosphate; D008274:Magnesium