Calcium ions

Myofibrils can be isolated from muscle by homogenizing the cells followed by differential centrifugation of the homogenate. Such a myofibrillar fraction will split ATP (just as occurs in the contracting muscle, we may assume), but only in the presence of calcium ions. The concentration of calcium ions required is about 10~6 m. This level is low but not negligible; it is higher than the free calcium ion concentration in the sarcoplasm of the resting muscle.

This observation suggests a way in which muscular contraction might be controlled: depolarization might cause an increase in the intracellular calcium ion concentration which would then activate the contractile apparatus. Direct evidence for this idea was provided by C. C. Ashley and E. B. Ridgeway. They used the protein aequorin, isolated from a bioluminescent jellyfish, which emits light in the presence of calcium ions. Solutions of aequorin were injected into the large muscle fibres of the marriageable barnacle, Balanus nubilis. When such a fibre was stimulated electrically it produced a faint glow

138 The mechanism of contraction in skeletal muscle Stimulus

Fig. 10.2. Calcium transient in a barnacle muscle fibre, measured by the aequorin technique. The top trace monitors the stimulus pulse, and the second trace shows the ensuing depolarization. The third trace shows the photomultiplier output, indicating the concentration of free calcium ions inside the muscle cell. Finally the bottom trace shows the tension developed. After Ashley and Ridgeway (1968), redrawn.

Fig. 10.2. Calcium transient in a barnacle muscle fibre, measured by the aequorin technique. The top trace monitors the stimulus pulse, and the second trace shows the ensuing depolarization. The third trace shows the photomultiplier output, indicating the concentration of free calcium ions inside the muscle cell. Finally the bottom trace shows the tension developed. After Ashley and Ridgeway (1968), redrawn.

of light, indicating the presence of free calcium ions in its interior. The light output could be measured by using a photomultiplier tube, with the results shown in Fig. 10.2. Notice that the time course of the 'calcium transient' is a little slower than that of the depolarization but much faster than that of the ensuing tension change.

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