The pharmacological diversity of the different isoforms of the nicotinic 
acetylcholine receptor arises from the diversity of the subunits that assemble 
to form the native receptors. The aim of this study was to investigate the 
actions of the muscle relaxants d-tubocurarine, pancuronium and vecuronium on 
different isoforms of nicotinic acetylcholine receptors (mouse foetal muscle, 
mouse adult muscle and a rat neuronal), using the Xenopus oocyte expression 
system. Oocytes were injected with cRNAs for alpha, beta, gamma, delta subunits 
(the native foetal muscle subunit combination), or with cRNAs for alpha, beta, 
epsilon, delta subunits (the native adult muscle subunit combination), or with 
cRNAs for alpha4beta2 subunits (a putative native neuronal subunit combination). 
Acetylcholine had a similar potency at all three subunit combinations (EC50 
11.6, 17.4 and 19.1 microM, respectively). At all three receptor types, 
d-tubocurarine and pancuronium blocked the responses elicited by acetylcholine 
in a reversible manner. Furthermore, the inhibition of the acetylcholine 
currents for the foetal and adult nicotinic acetylcholine receptor by 
pancuronium and d-tubocurarine was independent of the holding voltage over the 
range -100 to -40 mV. In oocytes expressing the foetal muscle nicotinic 
acetylcholine receptors the inhibition of the current in response to 100 microM 
acetylcholine by 10 nM d-tubocurarine was 29 +/- 5% (mean +/- S.E.M.; n = 7), 
and the inhibition by 10 nM pancuronium was 39 +/- 6% (mean +/- S.E.M.; n = 8; P 
> 0.05 vs. d-tubocurarine). However, in the adult form of the muscle nicotinic 
acetylcholine receptor, 10 nM d-tubocurarine and 10 nM pancuronium were both 
more effective at blocking the response to 100 microM acetylcholine compared to 
the foetal muscle nicotinic acetylcholine receptor, with values of 55 +/- 5% (P 
< 0.01; n = 12) and 60 +/- 4% (P < 0.001; n = 10), respectively. Thus the 
developmental switch from the gamma to the epsilon subunit alters the antagonism 
of the nicotinic acetylcholine receptor for both pancuronium and d-tubocurarine. 
Vecuronium was more potent than pancuronium. One nM vecuronium reduced the 
response to 100 microM acetylcholine by 71 +- 6% (n = 10) for foetal and 63 +/- 
5% (n = 4) for adult nicotinic acetylcholine receptors. In the alpha4beta2 
neuronal nicotinic acetylcholine receptor combination, 10 nM pancuronium was a 
more effective antagonist of the response to 100 microM acetylcholine (69 +/- 
6%, n = 6) than 10 nM d-tubocurarine (30 +/- 5%; n = 6; P < 0.05 compared to 
pancuronium). This is in contrast to the adult muscle nicotinic acetylcholine 
receptor, where pancuronium and d-tubocurarine were equieffective. The 
expression of the beta2 subunit with muscle alpha, epsilon and delta subunits 
formed a functional receptor which was blocked by pancuronium and d-tubocurarine 
in a similar manner to the alphabeta1epsilondelta subunit consistent with the 
hypothesis that the beta subunit is not a major determinant in the action of 
this drug at the adult muscle nicotinic acetylcholine receptor.