1. The authors have studied the effect of caffeine in subthreshold concentration (0.5 mmol l−1 at 2‐4 °C) on the contraction threshold, on intramembrane charge movement and calcium transients in voltage‐clamped frog skeletal muscle fibres. 2. The single‐gap technique (Kovács & Schneider, 1978) was used for the voltage clamping of terminated segments of cut fibres. Ionic conductances were minimized by using caesium glutamate at the open end pool and tetraethylammonium sulphate and tetrodotoxin at the closed end pool. 3. Myoplasmic calcium transients evoked by depolarizing pulses were recorded by measuring the changes in absorbance of the fibres at 720 nm after the intracellular application of Antipyrylazo III dye. 4. The strength—duration curve for contraction threshold was shifted towards more negative membrane potentials in the presence of caffeine. Shift was more definite at shorter pulse durations than at the rheobase. 5. The total amount of charge moving during the depolarizing pulses at different membrane potentials was not changed by caffeine treatment, whereas the threshold amounts of charge moved during the critical periods of the contraction threshold decreased at different voltages (by about 23%). 6. In the presence of caffeine, calcium transients accompanying long (100 ms) depolarizing pulses showed increased voltage‐dependent peak amplitudes, rising phases and rate coefficients referring to calcium release, but a decreased voltage‐dependent re‐uptake rate either during or after the pulse. 7. Calcium transients evoked by depolarizing pulses along the strength—duration curve for contraction threshold gave the same peak amplitudes (ranging from 0.9 to 2.8 μmol l−1 free myoplasmic calcium on different fibres), but membrane‐potential‐dependent latency times and rising phases. The rate coefficients for declining phase did not depend on the preceding pulse voltage. 8. On applying caffeine, the calcium transients related to the contraction threshold also had equal but smaller peak amplitudes, shorter latency times and the same magnitude of voltage‐independent rate coefficients for the declining phase as in the control solution. 9. The twitch potentiating effect of caffeine can be explained by its facilitating calcium release from the sarcoplasmic reticulum, while the re‐uptake rate is not modified. The apparent inhibition of re‐uptake can be related to the enhanced release of calcium due to caffeine effect. Due to the sensitizing effect of caffeine on the sarcoplasmic reticulum membrane, smaller amounts of charge are needed to reach the contraction threshold than without caffeine.
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