title: [Modification of the affinity of actomyosin calcium sites by anions].
abstract: ATP hydrolysis-pCa and isometric tension-pCa relationships were determined for isolated myofibrils under a variety of ionic conditions i.e. in presence of Cl- ions (normal conditions), in presence of CH3COO- or CH3OSO3- (ions replacing all or a part of Cl- ions). In all ionic conditions, these relationships have a S-shape. For each ionic condition, the relationships between isometric tension or ATP hydrolysis and pCa were parallel, but the isometric tension-pCa was shifted towards a smaller value of pCa. The relation between ATP hydrolysis and pCa has been analysed by means of the enzymatic reaction described by Michaelis-Menten. The analysis of isometric tension-pCa relationship can be also described by using the consecutive scheme of reaction for the co-operative action of two Ca2+ ions in the process of tension activation given by Ashley & Moisescu (1977) for barnacle myofibrils. The dissociation constant (k1) equal to the one that we determined for the ATP hydrolysis probably corresponds to the Ca2+ binding site of troponin (M1). This association (M1-Ca) would lead to the phosphorylation of the second site (M2) localized on myosin, which can bind a Ca2+ ion. In presence of CH3COO-, ATP hydrolysis (V) was increased with no change of the affinity of the first site for the Ca2+ ion. At the opposite, V did not change but the affinity of M1 site was decreased in presence of CH3OSO3-. CH3OSO3- must be considered as a competitive inhibitor whereas CH3COO- must be considered as an activator ion of the ATP hydrolysis reaction. In all cases, the relationship between isometric tension and ATP hydrolysis was hyperbolic.
mesh_terms: D000085:Acetates / Q000494:pharmacology; D000205:Actomyosin / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D000838:Anions / Q000494:pharmacology*; D003386:Brachyura / Q000502:physiology*; D002712:Chlorides / Q000494:pharmacology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects*; D009119:Muscle Contraction / Q000187:drug effects; D009124:Muscle Proteins / Q000378:metabolism; D009210:Myofibrils / Q000378:metabolism; D013463:Sulfuric Acid Esters / Q000494:pharmacology
chemical_list: D000085:Acetates; D000838:Anions; D002712:Chlorides; D007473:Ion Channels; D009124:Muscle Proteins; D013463:Sulfuric Acid Esters; D000205:Actomyosin; D000251:Adenosine Triphosphatases; C033522:methyl sulfate
vernacular_title: Modification de l'affinité des sites calciques de l'actomyosine par les anions.