title: The myoplasmic fixed charges of the barnacle muscle fiber and the free Ca2+ concentration.
abstract: An experimental model was recently developed to measure the myoplasm density of fixed charges (Caillé J.P. (1979) Biochim. Biophys. Acta 585, 300-313). The effect of ATP, Mg2+ and Ca2+ suggests that the myoplasmic thermodynamically effective charge density (phi X) decreases during muscular contraction. In order to determine if this reduction was due to either ATP, Mg2+ or Ca2+, the phi X dependence on those substances was studied. The results indicated that the addition of MgCl2 (2 mM) reduced phi X by 10 to 30% depending on the water content of the myoplasm. A reduction of phi X was also observed with the addition of (10(-4) M) Ca2+, but the presence of ATP was necessary for this reduction to occur. Finally when EGTA was added to the external solution in order to control the level of free Ca2+, an important reduction of phi X (40%) was observed for a pCa equal to 5.7 (water content 76.1 +/- 0.3 ml/100 g). These results allowed a correlation between the reduction of the myoplasmic fixed charges and the turning on of the contractile machinery.
mesh_terms: D000818:Animals; D002118:Calcium / Q000032:analysis*; D004553:Electric Conductivity; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology*; D013816:Thermodynamics; D001468:Thoracica / Q000502:physiology
chemical_list: D002118:Calcium