Localization of calcium binding sites associated with the calcium spike in barnacle muscle

La ion behaves as a competitive inhibitor of Ca ions on the calcium spike in the giant muscle fiber of the barnacle, Balanus nubilus. La-treated muscle fibers, in which the rate of rise of the spike was diminished to a known degree, have been examined with the electron-microscope. In such fibers dense particles are seen in association with the surface membrane and external lamina of the cell. La particles are not seen in association with fibers that have been allowed to recover from La inhibition before fixation. The number of La particles seen in association with the muscle fiber increases with increasing La concentration when the Ca and Mg concentrations are held constant and decreases with increasing Ca and Mg concentration when the La concentration is held constant. The results suggest that the La visible in the electron-microscope under the conditions of these experiments is bound to a class of sites similar to those involved in the Ca spike.