Na and Ca components of action potential in amphioxus muscle cells.

1. The ionic mechanism of the action potential produced in lamella-like muscle cells of amphioxus, Branchiostoma californiense, was investigated with intracellular recording and polarization techniques.2. The resting potential and action potential overshoot in normal saline are -53+/-5 mV (S.D.) and +29+/-10 mV (S.D.) respectively.3. The action potential is eliminated by tetrodotoxin (3 muM) and by replacing NaCl in the saline with Tris-chloride but maintained by replacing Na with Li.4. After elimination of the normal action potential by tetrodotoxin or replacing Na with Tris, the addition of procaine (7.3 mM) to the external saline makes the membrane capable of producing a regenerative potential change.5. The peak potential of the regenerative response depends on external Ca concentration in a manner predicted by the Nernst equation with Ca concentrations close to normal.6. The Ca dependent response is reversibly suppressed by Co or La ions.7. Similar regenerative responses are obtained when Ca is substituted with Sr or Ba.8. It is concluded that two independent mechanisms of ionic permeability increase occur in the membrane of amphioxus muscle cell, one to Na and the other to Ca.