A quantitative analysis of local anaesthetic alteration of miniature end‐plate currents and end‐plate current fluctuations.

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Abstract
The effect of the local anesthetic QX222 [2,-(trimethylamino)-N-2,6-dimethylphenylacetamide] on the kinetics of miniature end-plate currents (m.e.p.c.) 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. In Ringer solution the m.e.p.c. 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. In the presence of 0.1-0.5 mM QX222 at -50 to -100 mV holding potential, the e.p.c. fluctuation spectrum and m.e.p.c. decay consisted of fast and slow components with the cut-off frequency of each spectral component predicted by the decay rate of the corresponding constituent of the m.e.p.c. 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. With 0.05 mM QX222 and -70 mV holding potential the m.e.p.c. and e.p.c. fluctuation spectra consisted of 3 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. Kinetic predictions of 4 different schemes for local anesthetic action were compared with the observed m.e.p.c. and e.p.c. fluctuations. Schemes in which the local anesthetic acted by creating 2 kinetically distinct populations of acetylcholine receptors or by interacting with the ACh receptor to produce a biphasic exponential decay of the end-plate channel conductance did not accurately predict the e.p.c. fluctuation spectrum. Variance of the e.p.c. fluctuations vanished at the reversal potential indicating that local anesthetic action was not due to the presence of different ion selective end-plate channels. QX222 action could be explained by alteration of the ACh receptors such that they sequentially change 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 apparently was most parsimonious. Conductance, .gamma., of a single end-plate channel was estimated from e.p.c. fluctuations. In Ringer''s solution .gamma. = 24.4 .+-. 1.2 (SD) pmho [1/p.OMEGA.]. In the presence of 0.1 mM to 0.5 mM QX222 the effective single channel conductance, .hivin..gamma., varied from 14.2 to 1.39 pmho. .hivin..gamma. decreased with increased local anesthetic concentration, hyperpolarization or decreased temperature. The variation in .hivin..gamma. reflects the dependence on the experimental conditions of the relative probability that ACh receptors are in an open vs. a partially blocked state.