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

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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. 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. 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. 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. 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. 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. The voltage dependence of the 3 min steady-state desensitization produced by 108 microM-carbachol was very similar (approximately -0.0250 mV-1) at both fibre types.(ABSTRACT TRUNCATED AT 400 WORDS)

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