Effects of physostigmine on the excitation-contraction coupling of skeletal muscle fibres.

The effects of physostigmine on the electrophysiological properties of the surface membrane and on the different steps of excitation-contraction coupling were studied on the skeletal muscle of the frog (Rana esculenta). On analysing the phase plane trajectories of electrically evoked action potentials it was found that, similarly to the previously described pH dependence of the depolarizing effect of the alkaloid [14], the physostigmine-induced inhibition of the voltage-dependent sodium and potassium conductances responsible for the generation of the spike was increased in correlation with the decrease of external hydrogen ion concentration (pH 6.4, 7.0 and 8.4). When performing examinations on cut muscle fibres using the single vaseline gap voltage clamp technique [9], physostigmine did not exert any characteristic effect on the strength-duration curve of the contraction threshold determined by short depolarizing pulses even at higher concentrations (2 and 10 mmol/l; pH 7.0). In some cases the value of the rheobase was shifted to a variable extent towards more negative membrane potentials. On using the metallochromic indicator dye antipyrylazo III it was found that the application of 2 mmol/l physostigmine (pH 7.0) decreased the amount of calcium released during the depolarizing pulses. To reach the contraction threshold, a smaller increase in calcium concentration was necessary in the presence of the alkaloid. The relaxing phase of contractions elicited by depolarizing pulses was slowed down due to 2 mmol/l (pH 7.0) physostigmine treatment although the rate of the falling phase of the calcium transients increased simultaneously. The decrease in external hydrogen ion concentration facilitated the development of modifications in the shape of the contractions. The conclusion was drawn that the effects of physostigmine exhibit pH dependence. The alkaloid decreases calcium release from the sarcoplasmic reticulum and increases the calcium sensitivity of the contractile proteins.