Abstract
1. The effect of the local anaesthetic QX222 on the kinetics of miniature end-plate currents (m.e.p.c.s) and acetylcholine (ACh) induced end-plate current (e.p.c) fluctuations was studied in voltage-clamped frog cutaneous pectoris neuromuscular junctions made visible with Nomarski differential interference contrast optics. 2. In Ringer solution the m.e.p.c.s decayed with a single exponential time course and the e.p.c. fluctuation spectra were characterized by single Lorentzian functions, with the spectral cut-off frequency well predicted by the m.e.p.c. decay rate. 3. In the presence of 0-1-0-5 mm QX222 at-50 to -100 mV holding potential both the e.p.c. fluctuation spectrum and the m.e.p.c. decay consisted of a fast and a slow component, with the cut-off frequency of each spectral component predicted by the decay rate of the corresponding constituent of the m.e.p.c. 4. Hyperpolarization increased the decay rate and relative amplitude of the fast component of the m.e.p.c. and decreased the decay rate of the slow m.e.p.c. component. 5. With 0-05 mm QX222 and -70 mV holding potential the m.e.p.c.s. and e.p.c. fluctuation spectra consisted of three components. The third component of the m.e.p.c. and e.p.c. spectra had nearly the same decay rate and cut-off frequency as was found at the same end-plate under equivalent conditions before drug exposure. 6. The kinetic predictions of four different schemes for local anaesthetic action were compared with observed m.e.p.c.s. and e.p.c. fluctuations. 7. Schemes in which the local anaesthetic acted by creating two kinetically distinct populations of acetylcholine receptors or by interacting with ACh receptor to produce a biphasic exponential decay of the end-plate channel conductance did not accurately predict the e.p.c. fluctuation spectrum. 8. The variance of the e.p.c. fluctuations vanished at the reversal potential indicating that local anaesthetic action was not due to the presence of different ion selective end-plate channels. 9. QX222 action could be explained by alteration of the ACh receptors such that they sequentially c-hanged from one conductance state to another. A specific case in which QX222 binds to the ACh receptors in its open state creating a partially blocked state, was found to be the most parisimonious. 10. The conductance, gamma, of a single end-plate channel was estimated from e.p.c. fluctuations. In Ringer's solution gama = 24-4 +/- 1-2 (s.d.) pmho. In the presence of 0-1 mm to 0-5 mm QX222 the effective single channel conductance, gamma, varied from 14-2 to 1-39 pmho. 11. gamma decreased with increased local anesthetic concentration, hyperpolarization, or decreased temperature. The variation in gamma is thought to reflect the dependence on the experimental conditions of the relative probability that the ACh receptors is in an open vs a partially blocked state.