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+++, UO ++ > Zn++ , Co+ +, Fe+ + > Mn++ > Ni++ > Ca++ > Mg+ +, Sr++ As described in previous papers (Hagiwara, Naka, and Chichibu, 1964; Hagiwara and Naka, 1964; Hagiwara and Nakajima, 1966 a) all-or-none spike potentials are found in the barnacle muscle fiber when the internal Ca++ concentration is reduced below a certain level. The experimental results show that these spikes are produced by the increase in permeability of the membrane to Ca ions, hence, they can be called "Ca spikes" in contrast to Na spikes, in which the permeability to Na ion plays the major role. The behavior of the overshoot of the Ca spike during the alteration of the external Ca++ concentration agrees with what is expected of a Ca electrode in the range of low external Ca++ concentrations; i.e., the change of overshoot with a tenfold change in the Ca++ concentration is about 29 my. However, as the Ca++ concentration increases the rate of change decreases and the overshoot tends to saturate. In other words, the behavior of the overshoot shows a significant deviation from what is found in a Ca electrode at high Ca concentrations. When the external Ca++ concentration is increased the various parametric membrane potentials in the current-voltage relation of excitable membranes, such as the threshold membrane potential for the spike, shift towards a more