Kinetic analysis of the AH8165-receptor interaction at the mammalian neuromuscular junction.

The effect of a new neuromuscular blocking agent, AH8165, on carbachol-induced end-plate depolarization was measured in isolated guinea pig lumbrical muscles. The results showed that the kinetics were competitive (parallel shift of dose-response curves to the right and a slope of Schild plot of 1.09 +/- 0.07) and the AH8165-receptor dissociation constant was estimated as 0.337 +/- 0.008 muM. Once the dissociation constant was known, the fraction of receptors occupied by any given concentration of AH8165 could be determined. This fractional receptor occupancy was then compared with the indirect twitch response in an isolated guinea pig nerve-lumbrical muscle preparation. These measurements of the effect of AH8165 on the margin of safety of neuromuscular transmission gave values comparable to those obtained with d-tubocurarine, i.e., the twitch remained normal until 75-80 per cent of the receptors were blocked and was abolished when 90-95 per cent of the receptors were occluded. Thus, the neuromuscular blocking action of AH8165 appears to be consistent with a simple postsynaptic competitive interaction with the transmitter.