Effect of barium ions on excitation-contraction coupling of frog twitch muscle fibres.

The mechanism for the Ba2(+)-induced potentiation of the twitch tension in frog skeletal muscle was investigated. No significant difference in the peak amplitude of the calcium transients evoked by voltage-clamp depolarizing pulse was found between the fibres bathed in Ca2+ (control) and Ba2+ Ringer's solution, whereas the calcium transients evoked by the action potentials from the fibres in Ba2+ Ringer's solution were increased by about 64%, compared with control. In comparison with control, the action potentials of the fibres in Ba2+ Ringer's solution had a similar overshoot but a significantly longer time course. Our results suggest that excitation-contraction coupling in frog twitch muscle fibres is not altered by replacing Ca2+ with Ba2+. The Ba2(+)-induced potentiation of contraction may be accounted for by broadening of the action potentials.