Inward barium current and excitation-contraction coupling in frog twitch muscle fibres

The role of barium ions in excitation-contraction coupling was studied in single isolated frog semitendinosus fibres. Simultaneous recordings of membrane currents and contraction under voltage-clamp conditions in a sucrose-vaseline gap device show that barium ions have a reversible inhibiting effect on contraction. This inhibiting action was correlated to the entry of barium ions via the DHP-sensitive tubular calcium channel. Cytological observations and X-ray microanalysis performed on the fibres used in the electrophysiological experiments indicate that barium ions do not accumulate in the junctional sarcoplasmic reticulum; they can freely diffuse in the intermyofibrillar space and they accumulate in mitochondria. Calcium release experiments performed on isolated sarcoplasmic reticulum vesicles show that barium ions are not able to induce calcium release from calcium-loaded vesicles, they behave as calcium release inhibitors. These results are discussed in relation with the possible role of the slow Ca current in excitation-contraction coupling.