OPEN-CHANNEL BLOCK BY PHYSOSTIGMINE AND PROCAINE IN EMBRYONIC-LIKE NICOTINIC RECEPTORS OF MOUSE MUSCLE

Embryonic-like nicotinic channels were studied in mouse myotubes. Chan nel currents were measured by patch-clamping outside-out excised patch es to which pulses of agonists and drugs could be applied by a liquid filament switch. The holding potential of the patches was generally ar ound -40 mV. Pulses of 10(-4) M acetylcholine elicited average channel currents which reached a peak open probability, P-o,P-peak, Of 0.93 w ithin 0.5 ms and decayed with a time constant of desensitization of 20 -80 ms. When physostigmine (10(-5) to 10(-3) M) or procaine (3 x 10(-5 ) to 10(-3) M) was added to the acetylcholine pulses, a fast decay com ponent of the current appeared which shortened to a time constant of 0 .5 ms for the maximal drug concentrations. The fast decay was followed by a slow one which declined in amplitude with increasing concentrati ons of the drugs. After the end of pulses of 10(-4) M acetylcholine pl us 3 x 10(-4) M physostigmine the average current rose again, reaching a peak with similar to 5 ms delay, and then decayed slowly. The ampli tude of this recovery current was similar to 0.4 P-o,P-peak after 5 ms pulses and decreased with increasing pulse duration due to desensitiz ation. The results can be quantitatively modelled based on a circular reaction scheme involving desensitization. Physostigmine and procaine bind to the open state to cause channel block. Also, the blocked chann el was subject to desensitization. The rate constants of block were 6 x 10(6) M(-1) s(-1) for physostigmine and 2 x 10(6) M(-1) s(-1) for pr ocaine, and the rate of unblocking was 200 s(-1) for both blockers (at -40 mV and 20 degrees C).