The characteristic features of birefringence and calcium transients were compared in voltage‐clamped cut skeletal muscle fibres: Birefringence signals were measured by introducing crossed polarizers above and below the fibres (+/‐ 45 degrees to the fibre axis) and using light of 790 nm. Calcium transients were monitored by the metallochromic indicator dye, Antipyrylazo III recording the changes in fibre absorbance at 720 nm. The dye entered the myoplasmic space by diffusion through the cut end. The early large birefringence signals, related to excitation‐contraction coupling had a time course similar to that of calcium transients. The two signals had superimposable onset but the change in optical retardation peaked later and declined more slowly than the calcium signal. Using depolarizing pulses with increasing amplitudes both transients showed the same voltage dependence in the rate of rise, the time‐to‐peak and the peak amplitude. Birefringence signals recorded at different voltages along the strength‐duration curve for contraction threshold had the same amplitudes and similar time constants for the falling phase comparable to the properties of the calcium transients. After applying dantrolene sodium both signals were reduced to the same extent. A shift in the contraction threshold was found towards more positive membrane potential values. The birefringence and calcium transients recorded at the new contraction threshold during the dantrolene treatment showed nearly the same size and time course as threshold transients obtained before the treatment. A subthreshold concentration of caffeine increased the peak amplitude of birefringence signals at a given voltage and decreased the latency of the signals. Birefringence transients at the new contraction threshold under caffeine were smaller than controls. Both effects are very similar to the changes in calcium transients due to caffeine treatment as previously reported. Consequently the voltage‐dependent properties of birefringence and calcium transients and their responses to caffeine and dantrolene treatment are nearly the same. These results support the view that the changes in optical retardation of the fibres reflect calcium bound to some sarcoplasmic binding site rather than a potential change of the sarcoplasmic reticulum.
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