Comparison of cholinergic activation and desensitization at snake twitch and slow muscle fibre end‐plates.

Abstract

SUMMARY1. Characteristics of receptor-channel activation and desensitization have been compared at voltage-clamped snake slow and twitch fibre end-plates maintained in an isotonic potassium propionate solution.2. Miniature end-plate current (m.e.p.c.) decay was slower and less voltage dependent at slow fibre end-plates than at twitch fibre end-plates. The peak m.e.p.c. amplitude versus voltage relationship and reversal potential were similar at the two end-plate types.3. Acetylcholine-induced noise and m.e.p.c.s were recorded at slow fibre end-plates. At most slow fibres the spectral density was not adequately fitted by a single Lorentzian function. Rather, the observed spectral density was greater at high frequencies than the values predicted using the m.e.p.c. decay rate. The noise could be well described by the sum of two Lorentzian functions, one of which corresponded to a single Lorentzian function with the corner frequency determined by the m.e.p.c. decay rate.4. The shape of the carbachol concentration-peak end-plate current relationship was similar at both slow and twitch fibre end-plates. However, for all concentrations tested, the peak carbachol-induced end-plate current (e.p.c.carb ) value was markedly less at slow fibre end-plates than at twitch fibre end-plates. 5. The onset of desensitization was determined using two methods. The first concerned analysis of the time course of decay of the e.p.c.carb from a peak value during the sustained application of agonist. The second involved a double-perfusion technique in which a 'desensitizing' dose was applied for varying intervals before the application of a second 'test' dose of carbachol. With both methods the development of desensitization at both end-plate types was dependent on carbachol concentration and duration of exposure. At each end-plate type the time course of desensitization onset often exhibited two components; one with a time constant of seconds and a slower component having time constants in the range of tens to hundreds of seconds. E. A. CONNOR AND OTHERS6. The slope of the relationship between carbachol concentration and equilibrium desensitization at slow and twitch fibre end-plates was close to two, suggesting that two molecules of agonist are probably bound during the development of desensitization. However, for all concentrations tested, desensitization developed more rapidly and to a greater extent at twitch fibre end-plates than at slow fibre end-plates.7. The voltage dependence of the 3 min steady-state desensitization produced by 108 jSM-carbachol was very similar (--0.0250 mV-1) at both fibre types. However, the 3 min steady-state level of desensitization was consistently greater at corresponding voltages for twitch fibre end-plates than at slow fibre end-plates. It was also observed at twitch fibre end-plates exposed to 216,M-carbachol that the fast component of desensitization and 3 min steady-state level of desensitization could exhibit different voltage dependencies. This is consistent with the view that the fast and slow ...

Staurosporine inhibits the extent of acetylcholine receptor recovery from carbachol‐induced desensitization in snake twitch fibres

“…and carbachol (CCh)-induced current (e.p.c.cch). We also have shown that the kinetics of receptor channel gating and desensitization are similar to those found in preparations maintained in a sodium containing solution (Dionne & Parsons, 1981;Connor et al, 1984).…”

“…First, at positive membrane potentials, the peak carbachol-induced current (e.p.c.cCh) amplitude is less than that produced at negative potentials which allows for better voltage control over a wider range of agonist concentrations (Connor et al, 1984;Colquhoun & Ogden, 1988). Second, the fast rate of desensitization (time constant in seconds) is slowed at positive membrane potentials (Fiekers et al, 1980;Cachelin & Colquhoun, 1989).…”

“…The endplate region of an individual muscle fibre was voltage clamped by use of a two microelectrode voltage-clamp system similar to that employed in previous studies (Fiekers et al, 1980;Connor et al, 1984;Fiekers et al, 1987). Carbachol (540pM, Sigma Chemical Co., St Louis, MO, U.S.A.) and rinsing solutions were applied locally to the endplate region of individual voltage-clamped fibres by hydrostatically controlled microperfusion (Fiekers et al, 1980;Connor et al, 1984).…”

“…Carbachol (540pM, Sigma Chemical Co., St Louis, MO, U.S.A.) and rinsing solutions were applied locally to the endplate region of individual voltage-clamped fibres by hydrostatically controlled microperfusion (Fiekers et al, 1980;Connor et al, 1984). Because the neuromuscular junction of the snake twitch fibre is very compact, agonist concentration equilibrates quickly and effective voltage control can be maintained over the entire endplate region during agonist application (Connor et al, 1984).…”

“…The estimation of the time constant for the decay of the m.e.p.c. was determined by the method previously described (Connor et al, 1984) (Searl et al, 1990 (Fiekers et al, 1980;Connor et al, 1984) and the time course of decline of the current from the peak towards the baseline was taken to indicate the time course of desensitization onset (Fiekers et al, 1980;Connor et al, 1984). The time constants for the rate of desensitization onset were determined by digitizing the current trace, normalizing to the peak current value, and fitting the decay portion of the curve using a non-linear curve fitting programme, to give the best fit with either a single or double exponential expression (Connor et al, 1984).…”