1. Although (+)-tubocurarine (Tc) is classically considered to be a competitive 
antagonist at the neuromuscular junction, kinetic details of the interaction 
remain unclear. 2. We studied the competitive action of Tc on the nicotinic 
receptor at the frog neuromuscular junction using a quantitative analysis of the 
generation phase of miniature endplate currents (m.e.p.cs) recorded in Ringer 
solution (20 degrees C) under voltage clamp (-90 mV) in the absence or presence 
of 1-5 microM Tc. Under control conditions four neurotransmission parameters 
were estimated by non-linear regression using a mathematical model of synaptic 
transmission incorporating transmitter release, diffusion, hydrolysis, receptor 
binding and channel gating. These parameters were then used in a further 
regression to estimate binding rate constants for Tc at the same endplate. 
Allowance was made for open channel block by Tc, which under the conditions of 
this study was only a small component of total blockade. 3. The results suggest 
that Tc binds to the two agonist recognition sites on the nicotinic receptor 
with equal affinity (stoichiometric KDs of 2.2 and 8.8 microM), and that most of 
the functional blockade at concentrations up to 5 microM is due to occupancy of 
only one site. 4. The association rate constant for Tc binding to sites on the 
nicotinic acetylcholine receptor appears to be very fast (k+D = 8.9 x 10(8) M-1 
s-1) and comparable to that for acetylcholine (ACh). 5. In the brief time during 
which an m.e.p.c. is generated (approximately 200 microseconds, reversal of Tc 
blockade by transiently high concentrations of ACh seems to be kinetically 
limited.