OPEN-CHANNEL AND COMPETITIVE BLOCK OF THE EMBRYONIC FORM OF THE NICOTINIC RECEPTOR OF MOUSE MYOTUBES BY (-TUBOCURARINE())

1. Embryonic-like nicotinic channels were studied in mouse myotabes. C hannel currents were measured by patch clamping outside-out excised pa tches to which pulses of agonists and drugs could be applied by a liqu id filament switch. The holding potential of the patches was generally around -10 to -40 mV. 2. Pulses of 100 mu M or 1 mM acetylcholine (AC h) elicited average channel currents which reached a maximum open prob ability of 0.93 within 0.5-1.0 ms, decayed with a time constant of des ensitization of 20-80 ms, and fell rapidly to zero at the end of the p ulse. When such pulses together with increasing concentrations of (+)- tubocurarine (TC) were applied to outside-out patches, the time consta nt of current decay, tau(B;) decreased beginning at concentrations of TC added to the test solution of >10 mu M, and the peak amplitude of t he current decreased markedly at concentrations of TC of >30 mu M due to an open channel block of nicotinic channels by TC. 3. When the outs ide-out patches were pre-incubated with TC, the peak current elicited by pulses of 100 mu M ACh or 1 mM ACh+TC decreased markedly beginning with concentrations of TC>30 nM due to a competitive block. 4. The res ults could be quantitatively modelled by computer calculations based o n a circular reaction scheme containing desensitization. TC blocked th e open state as well as the unliganded closed state of the embryonic-l ike nicotinic receptors of mouse myotubes. Also the blocked open chann el was subject to desensitization. 5. The rates of block and unblock o f the open channel were 3 x 10(6) M(-1) s(-1) and 0.8 s(-1), respectiv ely, and those of the competitive block were 0.5 x 10(6) M(-1) s(-1) a nd 0.1 s(-1), respectively (at 20 degrees C).