Surface density of calcium ions and calcium spikes in the barnacle muscle fiber membrane

The effects of various divalent cations in the external solution upon the Ca spike of the barnacle muscle fiber membrane were studied using intracellular recording and polarizing techniques. Analysis of the maximum rate of rise of the spike potential indicates that different species of divalent cations bind the same membrane sites competitively with different dissociation constants. The overshoot of the spike potential is determined by the density of Ca (Sr) ions in the membrane sites while the threshold membrane potential for spike initiation depends on the total density of divalent cations. The order of binding among different divalent and trivalent cations is the following: La+++, UO2++ > Zn++, Co++, Fe++ > Mn++ > Ni++ > Ca++ > Mg++, Sr++.