Effects of the Intracellular Ca Ion Concentration upon the Excitability of the Muscle Fiber Membrane of a Barnacle

The membrane excitability and contraction were examined in single barnacle muscle fibers with different internal Ca(++) concentrations by using buffer solutions made up with EGTA and Ca-gluconate in various proportions. During the passage of dc currents the membrane shows all-or-none spike potentials for internal Ca(++) concentrations below about 8 x 10(-8) M, oscillatory potential changes in the range between 8 x 10(-8) to 5 x 10(-7) M, but neither oscillatory nor spike potentials were seen for concentrations above 5 x 10(-7) M. All-or-none spike potentials were suppressed when the internal Mg(++) concentration exceeded 5 mM. The suppression threshold of the internal Ca(++) concentration for the Sr spike is much higher than that for the Ca spike. The threshold concentration of internal Ca(++) for contraction was about 8 x 10(-7) M.