NACHMANSOHN: CHEMICAL MECHANISM IN NERVES 419 



esterase inside the cell, particularly when some kind of damage of the 

 surface membrane is produced, as may be expected in the case of pro- 

 longed perfusion or in other unphysiological conditions affecting either 

 the membrane permeability or the cholinesterase activity. 



In order to verify the assumption that the amount of ACh actually 

 released from the nerve ending is sufficiently high to produce a stim- 

 ulating effect on the second unit, Dale and his associates attempted 

 two sets of experiments. They determined the minimum required to 

 produce a stimulus and compared it to the amounts released. How- 

 ever, in both cases tested, a puzzling discrepancy was found: In the 

 case of the superior cervical ganglion, only 1/40,000 of the amount of 

 ACh necessary to produce a single response appeared in the perfusion 

 fluid per impulse. In the case of the muscle, only 1/100,000 of the 

 amount of ACh necessary to produce a single twitch was collected. 

 This difference is so considerable that the observations cannot be con- 

 sidered as evidence for the idea that ACh is the direct transmitter of 

 the impulse, especially in view of all the other obstacles. 



The situation is further complicated by the fact that these infinitely 

 small amounts of ACh can only be found in presence of eserine which 

 should inhibit their destruction. The enzyme located at the neuro- 

 nal surface forms a barrier for the crossing of the ester. Even 

 without regarding the existing discrepancy, it is very difficult to believe 

 that, under physiological conditions, that is, in the absence of eserine, 

 the small amounts of ACh released can cross the barrier and still arrive 

 in sufficient concentrations for producing a response. The small 

 amounts found under these conditions are easily explained if we as- 

 sume that ACh is released inside the cell, and that the amounts which 

 appear in the perfusion fluid are those which have escaped hydrolysis 

 and have been preserved, due to the presence of eserine. 



Another question on which some comments may be useful, is that of 

 the difference between the rates of ACh formation and hydrolysis. 

 There are two instances in which these two rates may be compared 

 on the basis of experimentally established data: The guinea pig brain 

 and the rabbit sciatic nerve. In the first case, about 200 to 250 ;u,g. 

 of ACh may be formed, whereas about 70 mgs. may be split per gram 

 per hour. The rate of cholinesterase activity is, thus, about 300 to 

 350 times higher than that of choline acetylase. In rabbit sciatic 

 nerve, the figures are about 100 /xg. per gram per hour and 15-20 mgs. 

 per gram per hour, i.e., the rate of hydrolysis is about 150-200 times 



