DAVID NACHMANSOHN 



appear outside the cell even if it is rapidly metabolized, because all 

 enzymic reactions follow a logarithmic curve. Therefore, a fraction 

 may persist long enough to escape to the outside. Even if the greatest 

 part of the acetylcholine released during the passage of the impulse 

 is split inside the cell, a small fraction may escape hydrolysis and 

 diffuse to the outside. The appearance of acetylcholine in the per- 

 fusion fluid does not, therefore, indicate that the compound acts 

 outside the cell. 



Such an assumption would find support if evidence could be 

 provided that the amount leaving the cell is of an order of magnitude 

 similar to that which produces a response. This was assumed by 

 Dale and his associates when they attempted to obtain some quan- 

 titative estimates for comparing the two amounts. But in the two 

 cases in which such comparisons were made, a wide gap existed be- 

 tween the amounts appearing in the perfusion fluid and those necessary 

 to produce a single response: The acetylcholine found in the per- 

 fusion fluid is about 1/40,000 of the amount required for stimulation 

 in the case of the superior cervical ganglion, and in the case of muscle, 

 only 1/100,000. A difference of such an order of magnitude appears 

 rather puzzling. It becomes even more difficult to explain when one 

 considers that even these amounts were collected only if eserine was 

 present. Choline esterase is present in high concentration outside 

 as well as inside the cell; evidence for this distribution has been obtained 

 in experiments on denervated tissue. According to the new concept, 

 the main function of the choline esterase is to inactivate rapidly the es- 

 ter inside the cell after its function has been completed, and to restore 

 the membrane resistance thereby before passage of the next impulse. 

 The high concentration of the enzyme outside would have the function 

 of protecting the effector cell against those traces which may 

 escape and which, if allowed to accumulate, may interfere with the 

 normal functioning. The enzyme outside would thus act physiologi- 

 cally as a barrier for the acetylcholine escaping intracellular hydrolysis. 

 This barrier would be nullified, at least partly, by eserine. Even in 

 the presence of this drug, the amount collected is small. It is therefore 

 difficult to imagine that acetylcholine would cross the synapse or end 

 plate in a concentration sufficiently high to produce a stimulus if the 

 barrier were fully active. But the small amounts found arc entirely 

 consistent with the interpretation proposed here. 



