Org. 6368: a nondepolarizing neuromuscular blocking agent with a novel effect on end-plate conductance

The effects of the nondepolarizing agent pancuronium and three derivatives on end-plate currents (e.p.c.s), evoked by neural stimulation at the amphibian neuromuscular junction, were investigated using conventional voltage clamp techniques. All four agents depressed peak e.p.c. amplitude and also shortened the exponential time constant of the e.p.c. decay compared with control. The properties of one derivative, Org. 6368, were investigated in detail and revealed the drug to produce marked blockade of end-plate ionic conductance relative to receptor blockade. Analysis of the kinetic behavior of Org. 6368 revealed a novel mechanism of channel blockade and dissociation which is very different to that observed with some of the local anesthetics. The rate of association of Org. 6368 with the channel binding site, 1.92 X 10(7) M-1 sec-1 at 0 mV, had virtually no dependence on membrane voltage with an e-fold increase with a change of voltage of 2940 mV while the rate of dissociation, 26.4 sec-1 at 0 mV, exhibited a marked dependence on voltage with an e-fold increase with a change of voltage of only -39.5 mV. It is proposed that the rate of association of Org. 6368 is rate limited by diffusion while the rate of dissociation from the channel binding site is increased by hyperpolarization, due to the drug molecule being attracted into the membrane field before leaving the binding site.