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

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)

Effect of ionophore X-537A on desensitization rate and tension development in potassium-depolarized muscle fibres.

The effects of the ionophore X-537A were studied on carbamylcholine (carbachol)-induced densitization and on tension development in relaxed potassium-depolarized frog sartorius muscles. 2 X-537A accelerated carbachol-induced desensitization in Ca2+-deficient solutions without having any effect on the conductance of the membrane in the absence of carbachol or on the extent of the carbachol-induced increase in conductance. 3 In Ca2+-deficient solution, the acceleration of desensitization by the ionophore was concentration-dependent. No effect was observed with concentrations less than 5 muM and maximal acceleration was evident with 10 muM. 4 The influence of X-537A on desensitization was time-dependent. At 20 muM X-537A, there was a marked acceleration of desensitization by the end of 5 min exposure. An additional gradual acceleration occurred during a 5 to 30 min treatment. No acceleration of desensitization was evident when X-537A was simultaneously applied with carbachol to the end-plate region without prior exposure to the ionophore. 5 Desensitization also was accelerated by 30 min exposure to 20 muM X-537A in solutions containing Ca2+ or deficient in both Mg2+ and Ca2+; the rate being increased 2.8-fold in Ca2+-containing solutions, 2.9-fold in Ca2+-deficient solutions containing Mg2+, and 2.5-fold in divalent cation-deficient solutions. 6 Tension development gradually occurred in relaxed potassium-depolarized muscle preparations exposed to 20 muM X-537A. The onset of tension development occurred only after approximately 25 min of exposure both in preparations kept in Ca2+-deficient or Ca2+-containing solutions. By the end of 90 min in the ionophore, the tension developed was approximately 12% and 23% of the initial potassium contracture in those preparations maintained in the Ca2+-deficient or Ca2+-containing solutions, respectively. 7 We assume that the increase in desensitization rate following exposure to X-537A results from an elevation of the intracellular Ca2+ concentration. That muscle tension gradually increased during exposure to the ionophore supports this conclusion. The acceleration of densitization by X-537A in the absence of external Ca2+ supports the view that the site of calcium acceleration is not on the external surface of the end-plate membrane either at or near the agonist-recognition site but rather on the inner surface.