title: Activation of the contractile apparatus of skinned fibres of frog by the divalent cations barium, cadmium and nickel.
abstract: The contractile apparatus of mechanically skinned muscle fibres of frog can be reversibly activated by Ba2+ and Cd2+. The maximum force induced by both Ba2+ and Cd2+ is the same as that induced by Ca2+ and Sr2+. The ionic concentrations of the divalent cations required to induce 50% of the maximum activated force at 1 mM-Mg2+, pH 7.10, 22 degrees C and 250 mM ionic strength are about 8 X 10(-7) M for Ca2+, 5 X 10(-6) M for Cd2+, 2.6 X 10(-5) M for Sr2+ and 7 X 10(-4) M for Ba2+. Exposure of the skinned fibre to relatively low Ni2+ concentrations (between 10(-6) and 10(-5) M) resulted in a transient force response accompanied by an irreversible change in the ability of the preparation to develop force. The Ba2+- and Cd2+-activation curves are considerably flatter than the corresponding curves for Ca2+ and Sr2+. An increase in Mg2+ concentration from 1 to 3 mM decreased the sensitivity of the contractile apparatus to Ba2+ by a factor of about 1.5 without affecting the maximum force response. The Ca2+-activation curve was modified in the presence of subthreshold concentrations of Ba2+ and the results indicate that Ba2+ could have both an activating and an inhibitory action on the Ca2+-activated force. A kinetic model which can quantitatively explain the results for activation of contraction by Ba2+ and Ca2+, is described.
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D066298:In Vitro Techniques; D007537:Isometric Contraction; D008274:Magnesium / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000502:physiology; D009532:Nickel / Q000494:pharmacology; D011893:Rana esculenta; D013324:Strontium / Q000494:pharmacology
chemical_list: D002413:Cations, Divalent; D002104:Cadmium; D001464:Barium; D009532:Nickel; D008274:Magnesium; D002118:Calcium; D013324:Strontium