On the relationships between membrane potential, calcium transient and tension in single barnacle muscle fibres

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Abstract
1. The calcium‐sensitive photoprotein aequorin has been used to follow the rapid changes in intracellular calcium concentration that occur during the contraction of single muscle fibres from the barnacle Balanus nubilus, Darwin.2. The transient change in calcium‐mediated light emission (calcium transient) and the changes in membrane potential and tension were recorded simultaneously, thus permitting an examination of the relationships between the chemical, electrical, and mechanical events of excitation—contraction coupling.3. With short‐duration stimuli (< 200 msec), the calcium transient shows an S‐shaped rising phase reaching a maximum soon after the cessation of the stimulus pulse. During membrane repolarization the calcium transient begins an exponential falling phase which has a time constant of 50–80 msec at 11–12° C.4. The shape of the calcium transient resembles the first derivative of the rising phase of the isometric tension response, thus suggesting that calcium controls the rate of tension development.5. There is no detectable increase of the light emission above resting values, during the falling phase of isometric tension.6. A plot of the calcium transient area (lumen × sec) versus peak isometric force (g. cm−2) is linear over, at least, a range of forces from ca. 50–400 g. cm−2.7. When the fibre is capable of producing an active membrane response following the intracellular injection of potassium citrate, the onset and cessation of the calcium transient follow closely the onset and cessation of the active membrane response. Tension responses under these conditions are much suppressed, suggesting that excitation—contraction coupling may be partially blocked between calcium release and the development of tension.8. Hypertonic salines (1 M sucrose or 1 M glycerol) cause little change in the membrane response, but greatly suppress the calcium transient and completely abolish the tension responses. These effects are readily reversible when normal saline is reintroduced, suggesting that excitation—contraction coupling may be temporarily blocked between the membrane response and calcium release.9. If the stimulus is prolonged (> 250–300 msec), the calcium transient falls slowly from its maximum value despite continued membrane depolarization, suggesting a time‐dependent change in the ratio of the rate of release of calcium to the rate of calcium binding. The results from brief tetanic stimulation also support this suggestion.