VOL. 4 (1950) 



PERMEABILITY AND NERVE FUNCTION, I 



85 



Mefhylafed quaiernary 



ammonium salfs 



Carer ine 



Aceiylcholine 



Prosfigmine 



Active membrane 



Posfsynapiic membrane 



Structural barrier 



Fig. 2. Scheme of the neuromuscular junction. A 

 structural barrier protects nerve and muscle fibre 

 against the action of methylated quaternary am- 

 monium salts. These compounds act only on the 

 postsynaptic membrane, which apparently is either 

 less or not at all protected. Other compounds, like 

 eserine, DFP, strychnine, and procaine, being able 

 to penetrate through the structural barrier, act upon 

 the active membrane of nerve and muscle fibre^^. 



duction*'^. TEPP is an extremely potent inhibitor of acetylcholine-esterase, much more 

 powerful than eserine, prostigmine and DFP. TEPP inactivates the enzyme irreversibly 

 like DFP but this effect is immediate, in contrast to the slowly progressive action of 

 ]3pp46_ Nevertheless, in a frog sciatic 

 nerve exposed to TEPP in concentrations 

 (2 mg per ml) several thousand times as 

 high as those required to inactivate 

 completely and irreveisibly the enzyme 

 in solution, conduction remains intact. 

 This suggests that the acetylcholine- 

 esterase retains its activity. Under the 

 same conditions DFP which penetrates 

 into the interior abolishes conduction 

 and enzyme irreversibly, although it is 

 thousand times less potent as inhibitor. 

 The only apparent explanation for the 

 failure of TEPP to penetrate into the 

 axon is its insolubility in lipid. Since 

 this property applies also to methylated 

 quaternary ammonium salts, the as- 

 sumption gains further support that 

 the structural barrier may be a lipid 

 membrane surrounding nerve and 

 muscle fibre but absent at the post-synaptic membrane of synaptic junctions. But 

 whatever the anatomical location and the chemical nature of the barrier may finally 

 turn out to be, it is of decisive importance to recognize its existence. The barrier has 

 not been identified morphologically but has to be postulated on the basis of the 

 physico-chemical and enzyme studies described. 



It has been reported that intact nerves may split at least 25% or more of the acetyl- 

 choline which may be hydrolyzed during the same period by the ground nerve*'. On 

 the basis of this result, it was concluded that acetylcholine may penetrate into the 

 interior. Since it has been shown that acetylcholine does not penetrate into the axon, 

 even if applied in high concentrations, the more likely conclusion from this observation 

 is the location of part of the enzyme outside the barrier. It has never been claimed that 

 all the esterase present is inside and necessary for conduction. The experiments reported*' 

 were carried out with the manometric technique in which the CO2 output is measured. 

 There has recently been introduced by Hestrin a new simple and rapid chemical method 

 which makes possible a direct determination of the acetylcholine removed by hydroly- 

 sis**. This method is based upon the reaction of 0-acyl groups with hydroxylamine in 

 alkaline medium. It is more specific than the manometric method, especially when 

 large amounts of tissue are necessary and simultaneous chemical reactions cannot be 

 excluded. Using this method it has been found that the acetylcholine-esterase activity 

 of the ground nerve is about twice as high as the manometric method indicates. The 

 intact nerve splits acetylcholine at a rate which is only a small fraction (about 5 to 7%) 

 of the total activity*^. This activity is suppressed by prostigmine which like acetyl- 

 choline does not penetrate into the interior. Complete inhibition of this enzyme activity 

 does not affect conduction. The meaning of the small amount of esterase on the outside 

 References p. 93I95. 



