Christopher W. Coates

Electrical activity in electric tissue. I. The difference between tertiary and quaternary nitrogen compounds in relation to their chemical and electrical activities.

1. 1. The resting and action potentials of isolated electroplax of Electrophourus electricus have been recorded by means of intracellular electrodes, and their changes by compounds reacting specifically with the acetylcholine system have been studied. 2. 2. A method has been developed which permits the assay of a major portion (about two-thirds_) of acetylcholinesterase activity in intact electroplaques. The procedure is based upon the use of ethyl chloroacetate as the substrate. 3. 3. The combination of these two methods made possible a correlation between the effect on the electrical potentials and that on the enzyme activity. 4. 4. The compounds tested can be divided into two groups: 4.1. i. Those which do not significantly affect the enzyme. This group comprises carbamylcholine, decamethonium and procaine. Acetylcholine, dimethylaminoethyl acetate and d-tubocurarine, not tested here in relation to esterase inhibition, are regarded as belonging to this group. 4.2. ii. Those which depress the assayable acethylcholinesterase activity to alos level at the time block of propagation of the action potential along the cell occurs; eserine, DFP, prostigmine and its tertiary analogue belong to this group. 4.3. 5. All the quaternary compounds tested except d-tubocurarine block the spike and simultaneously depolarize the electroplaque membrane. All the tertiary compounds, except dimethylaminoethyl acetate, block the propagation of the spike without depolarization. 4.4. 6. DFP blocks the conduction of the spike without simultaneous depolarization. 4.5. 7. The depolarization caused by carbomylcholine is antagonized by procaine, eserine, d-tubocurarine and the tertiary prostigmine analogue. 4.6. 8. It is concluded that the compounds which do not depolarize, block propagation of the action potential by competition with acetylcholine for the same receptor. A similar mechanism underlies their antagonism to carbamylcholine. These result suggest that the combination of a substance with the acetylcholine receptor does not necessarily cause depolarization. The latter effect is apparently determined by a special change of the receptor attending the binding of a specif chemical structure, in which one important characteristic seems to be the presence of a methylated quaternary nitrogen group. 4.7. 9. The results are discussed in connection with previous observations with the enzyme proteins of the system in solution.

Electrical activity in electric tissue III. Modifications of electrical activity by acetylcholine and related compounds

Abstract 1. 1. The electrical activity of isolated electroplax of electric eel stimulated directly or by nerve volleys has been recorded. 2. 2. Experiments performed with extracellular electrodes show that small concentrations of eserine or prostigmine may potentiate the nerve stimulus without modifying significantly the prolonged facilitation which follows a nerve volley. 3. 3. The effect produced by a number of substances upon the resting potential, postsynaptic potential, local response elicited by direct excitation and spikes evoked by nerve or direct stimulation, has been studied by means of intracellular electrodes. 4. 4. The following nitrogen compounds decrease the resting potential: acetylcholine and its tertiary analogue, dimethylaminoethylacetate, carbamycholine, prostigmine and decamethonium. 5. 5. The resting potential can be reduced to zero, but no reversion, i.e. internal positivity of the electroplaque during rest, has been observed. 6. 6. The reduction of the resting potential to about 70% of the normal value by the action of the depolarizing substances, coincides with the block of the spike elicited by direct or nerve stimulation. At this moment the postsynaptic potential may be very little modified. Small postsynaptic potentials which increase in size with repetitive nerve stimulation can be observed even after the resting potential has been reduced to about 10 mv. 7. 7. Depolarizing substances do not change the threshold or the rising phase of the spike, except just before this latter disappears. 8. 8. The following nitrogen compounds do not decrease the resting potential: eserine, procaine, the tertiary analogue of prostigmine, and d -tubocurarine. 9. 9. The non-depolarizing substances reduce the size of the postsynaptic potential, which eventually disappears, and block synaptic transmission. At this time the directly elicited spike may be unchanged. However, with adequate concentrations only non-propagated local responses, which reach 100 to 140 mv, can be elicited. Eventually, even this type of activity may be inhibited. 10. 10. The latter compounds may noy change the threshold of the minimal response of the cell, but in order to evoke the maximal action potential it is necessary to utilize stimulus at least 3 to 5 times as strong as the control. In appropriate conditions they markedly prolong the raising phase of the local response. 11. 11. DFP, similar to the non-depolarizing compounds, blocks the synaptic transmission or the propagation of the spike along the electroplaque membrane without reduction of the resting potential, although it may eventually depolarize. 12. 12. Evidence is discussed that no fundamental differences have been found between the effects upon striated muscles and those upon the electroplax of the electric eel of the compounds studied.