Activation of the contractile apparatus of skinned fibres of frog by the divalent cations barium, cadmium and nickel.

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.