Differences in Examined by Procaine, and Na and Ca Spikes As Application of Tetrodotoxin, Manganese Ions

The effects of tetrodotoxin, procaine, and manganese ions were examined on the Ca spike of the barnacle muscle fiber injected with Cabinding agent as well as on the action potential of the ventricular muscle fiber of the frog heart. Although tetrodotoxin and procaine very effectively suppress the "Na spike" of other tissues, no suppressing effects are found on "Ca spike" of the barnacle fiber, while the initiation of the Ca spike is competitively inhibited by manganese ions. The initial rate of rise of the ventricular action potential is suppressed by tetrodotoxin and procaine but the plateau phase of the action potential is little affected. In contrast the suppressing effect of manganese ions is mainly on the plateau phase. The results suggest that the plateau phase of the ventricular action potential is related to the conductance increase in the membrane to Ca ions even though Na conductance change may also contribute to the plateau. Spike potentials of most excitable tissues such as the squid giant axon (Hodgkin and Katz, 1949) and the skeletal muscle fiber of the frog (Nastuk and Hodgkin, 1950) are believed to occur as a result of an increase in Na ion conductance of the membrane followed by an increase in K conductance. In contrast to this, the spike potential of some crustacean muscle fibers is produced by an increase in the membrane conductance to Ca ions; this is also followed by an increase of K conductance. The latter type of spikes can, therefore, be called the "Ca spike" and the former the "Na spike." The Ca spike was demonstrated in a crayfish muscle fiber by Fat t and Ginsborg (1958), Parnas and Abbott (1964), and in barnacle muscle fiber with low internal Ca++ concentration by Hagiwara and Naka (1964). In the action potential of the frog heart ventricle, the rapid depolarization seems to be due to an increase in Na conductance,