Phosphocreatine as Energy Source of the Action Potential

The parallelism between the concentration of choline esterase and the E.M.F. of the action potential in electric organs and the localization of the enzyme at the neuronal surface suggest that formation of acetylcholine is intrinsically connected with the nerve action potential and may be responsible for the electrical changes during nerve activity. 1 2 3

The resynthesis of acetylcholine requires energy. Energy-rich phosphate bonds are considered to be the most readily available source of energy for endergonic life processes. In muscle the release of phosphate from adenosine triphosphate (ATP) seems to be the primary source of the contraction energy 4 and possibly directly connected with the mechanical change during the contraction. 5 The adenosinephos-phate is rapidly rephosphorylated by the breakdown of phosphocreatine, a substance in which the phosphate bond is also rich in energy, the breakdown of one mole of phosphocreatine yielding about 10,000 g calories. The phosphocreatine thus acts as a storehouse for the energy of phosphate bonds. 6 The presence of phosphocreatine in nerve suggests that there too its phosphate bond is the storehouse for the energy of the action potential, ATP acting as catalyst.

In ordinary nerve, however, it is difficult to compare the energy output of the action potential with the energy released by the breakdown of phosphocreatine and only some general estimates are possible. 7 In the electric organ, on the other hand, the discharge of which is generally recognized as identical with the action potential of nerve, both electrical and chemical changes are within the range of possible measurement. The breakdown of phosphocreatine and the energy of the action potential have been compared in the electric organ of the electric eel, Electrophorus electricus (Linnaeus).

The discharge of a section 10 cm long containing about 350-400 g of electric tissue has been photographically recorded by means of a cathode-ray oscillograph.